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Combustion and Fuels

2006;():1-9. doi:10.1115/GT2006-90015.

The oxidation of kerosene (Jet-A1) and that of surrogate mixtures have been studied experimentally in a jet-stirred reactor at 1 to 40 atm and constant residence time, over the high temperature range 800–1300 K, and for variable equivalence ratio 0.5<φ<2). Concentration profiles of the reactants, stable intermediates, and final products have been obtained by probe sampling followed by on-line and off-line GC analyses. The oxidation of kerosene in these conditions was modeled using a detailed kinetic reaction mechanism (209 species and 1673 reactions, most of them reversible). In the modeling, kerosene was represented by four surrogate model fuels: 100% n-decane, n-decane-n-propylbenzene (74% / 26% mole), n-decane-n-propylcyclohexane (74% / 26% mole), and n-decane-n-propylbenzene-n-propylcyclohexane (74% / 15% / 11% mole). The 3-components model fuel was the most appropriate for simulating the JSR experiments. It was also successfully used to simulate the structure of a fuel-rich premixed kerosene-oxygen-nitrogen flame and ignition delays taken from the literature.

Topics: Combustion , Modeling
Commentary by Dr. Valentin Fuster
2006;():11-21. doi:10.1115/GT2006-90055.

Large pressure oscillations due to thermoacoustic instabilities may occur in the modern gas turbine equipped with lean premixed burners. The Research Department of ENEL is studying this phenomenon using research methods that include plant supervision, laboratory experiments and modelling. This paper presents a new CFD modelling approach capable of simulating the time evolution of thermofluiddynamic fields during thermoacoustic instabilities in a whole annular combustor. Its peculiarity consists in the adoption of a very rough computational mesh. The use of the Very Rough Grid (VRG) approach allows all the resonant cavities involved in the acoustic oscillations to be considered, as well as to prolonging the computed transient until spontaneous thermoacoustic oscillations onset, with affordable computation time. The rationale of this approach is that thermoacoustic instabilities are more affected by acoustics than by fluiddynamics. KIEN, an in-house low diffusive URANS code capable of simulating 3D reactive flows, has been used. A 3D structured monoblock computational grid of an industrial annular combustor has been set up. It goes from the compressor outlet to the turbine inlet, including both the annular plenum and the annular combustion chamber, and extends over the entire circumferential angle. The results obtained by an exemplifying computed case are illustrated. They appear to be congruent with the real behaviour of thermoacoustic oscillation reported in literature. The type of information that can be extracted directly or by suitable post-processing from these results is shown and their usefulness in interpreting the real data obtained from functioning plants or experimental facilities is demonstrated.

Commentary by Dr. Valentin Fuster
2006;():23-30. doi:10.1115/GT2006-90059.

In this work the temperature field in a gas turbine combustion chamber is investigated through numerical computations. The combustion chamber under study is part of a 70 MW gas turbine from an operating combined cycle power plant. The simulation of combustion and flow dynamics is fully 3-dimensional. It addresses complex turbulence structure and temperature distribution inside the combustion chamber. The swirling effect is taken into account using a detailed gas-fuel-air mixing swirler. The combustion was simulated with proper gas-fuel-air flow ratio assuming stoichiometric equilibrium conditions. Based on previous results, pressure imbalance conditions of air flow between primary and secondary inlets is used to perturb the temperature distribution. In this work, a periodic function was used to produce pressure variation in the air flow, which in turn alter the temperature field and turbulence structures. First, characteristic temperature and pressure fields were obtained using steady state boundary conditions. The steady state solutions were perturbed using a periodic boundary condition (6 kPa per short periods of time) resulting in different results. The results are discussed and confirm previous 2-dimesional computations where excessive heating in regions other than the combustion chamber core occurred. The investigation is aimed to explain why overheating occurs, since it causes burning out of pipe materials, producing permanent damage to auxiliary cross flame pipes.

Commentary by Dr. Valentin Fuster
2006;():31-40. doi:10.1115/GT2006-90063.

The challenge of achieving a clean and stable combustion in modern power plants or aero engines leads to develop new technologies and new combustors. Pioneer work on ‘flameless’ combustion has shown the great potential of such a technology for meeting modern requirements in term of safety and low emissions. The idea behind this technique is to ensure low emissions by operating at very low fuel/air equivalence ratio but with high preheating to stabilize the combustion. In addition, a careful design of the combustor should ensure that fresh gases are diluted by hot exhaust gases. The result is a distributed but efficient oxidation region. This paper presents a new efficient model for simulating ‘flameless’ oxidation. The model is based on Large Eddy Simulation (LES) ensuring an accurate description of the mixing. In addition, a ‘low-cost’ technique for coupling the LES code with some complex chemistry is presented. This approach was used for simulating a reacting jet close to the ‘flameless’ regime. The simulation showed the capability of the present LES tool for understanding the flow dynamics and improving the design of ‘flameless’ combustors.

Commentary by Dr. Valentin Fuster
2006;():41-49. doi:10.1115/GT2006-90064.

Due to their excellent behaviour within the scope of mixing, ignition and burnout, swirl-flames are used within quite a manifold of scientific and industrial applications. The development of a swirl-induced inner recirculation zone, which provides heat and active chemical species to the ignition domain of the flame, plays an important role for stabilisation of these highly turbulent flames. Modern concepts for reducing thermal NOx emissions require high ignition stability even if very lean fuel/air-mixtures are in use. Therefore, there is a great demand for models which are able to predict lean blow out of turbulent, aerodynamically stabilised flames. In contrast to the integral approach of many stability models which mostly are based on global quantities, numerical models offer highest possible flexibility aiming at variation of geometry, operating conditions and further parameters. For solving the convective-diffusive problem, a RANS (Reynolds Averaged Navier Stokes) method based on a finite volume approach is applied, using the standard k-ε turbulence model. A joint-probability-density model with an assumed shape of the probability-density-function (presumed shape JPDF-model) describes the interaction of turbulence and chemical reaction. The latter is based on one single variable, describing the mixing state and one single variable, describing the state of reaction progress. The demand, to apply a chemical reaction mechanism, which is based on one single reaction progress variable, is solved by using the concept of the semi-global 2-domain-1-step chemical kinetics scheme. To predict lean blow out for confined diffusive swirl-flames makes it necessary to take into account the convective and radiative heat loss processes. To consider the influence of heat loss on the chemical reaction, the 2-domain-1-step chemical kinetics scheme had to be extended. The local distribution of heat loss inside the flow field is covered by a variable named “enthalpy-index”, which describes the normalised ratio of the local enthalpy and local enthalpy under adiabatic conditions for a given mixture composition. With this combined model LBO (Lean Blow Out) limits have been deduced for a Methane/Air-flame in a model gas turbine combustor. The results confirm, that lean blow out is predicted at much lower thermal loads if taking heat loss processes into account.

Commentary by Dr. Valentin Fuster
2006;():51-56. doi:10.1115/GT2006-90065.

Non-circular burner geometries have shown some promise of reducing pollutant emissions form combustion systems. The use of non-axisymmetric geometries has the potential to alter the behavior of a flame through modification of the flow field. To investigate these flow field effects on combustion performance, a study of the partially premixed flames emitted from a circular burner and a 3:1 aspect ratio (major axis / minor axis) elliptical burner of equal exit area was performed. For laminar conditions, the elliptical and circular burner produced similar global emissions of carbon monoxide and nitric oxide. In turbulent flames, the elliptical burner produced a larger amount of carbon monoxide, but reduced nitric oxide production. In turbulent flames, the enhanced mixing facilitated by elliptical burners froze the CO oxidation reaction and thus increased its emission. In laminar flames, the elasticity did not significantly affect mixing rates, and thus resulted in similar CO emissions between the burners. The hypothesis on CO reaction freezing was confirmed with inflame structure measurements of CO, OH, and temperature. The decreased NO production in turbulent flame was attributed to a reduction of the flame length of the 3:1 aspect ratio elliptical burner and thus a decrease of residence time compared to the circular burner.

Commentary by Dr. Valentin Fuster
2006;():57-66. doi:10.1115/GT2006-90072.

The concept of the periodic mixing and combustion process, which has been presented earlier [1,2], has been implemented in a nearly adiabatic combustor for investigations at atmospheric pressure. The objective of this combustion process is to achieve stable combustion at adiabatic flame temperatures being considerably lower than the lean blowout temperature of aerodynamically stabilized flames with low pressure drop in the combustor in order to reduce NOx emissions and to achieve CO emissions near the thermodynamic equilibrium. For preheat temperatures between 390 K and 790 K, the periodic mixing combustor can be operated near the lean blowout limit with adiabatic flame temperatures down to 1510 K – 1600 K. The test combustor yields over the entire operation range of 1:4 (thermal powers from 47 kW up to 175 kW) very low emissions of NOx below 1 ppm(v) (15% O2 , dry) and of CO below 8 ppm(v).

Commentary by Dr. Valentin Fuster
2006;():67-76. doi:10.1115/GT2006-90093.

Strong evidence is presented that entropy noise is the major source of external noise in aero-engine combustion. Entropy noise is generated in the outlet nozzles of combustors. Low frequency entropy noise — which was predicted earlier in theory and numerical simulations — was successfully detected in a generic aero-engine combustion chamber. It is shown that entropy noise dominates even in the case of thermo-acoustic resonances. In addition to this, a different noise generating mechanism was discovered that is presumably of even higher relevance to jet engines: There is strong evidence of broad band entropy noise at higher frequencies (1 kHz to 3 kHz in the reported tests). This unexpected effect can be explained by the interaction of small scale entropy perturbations (hot spots) with the strong pressure gradient in the outlet nozzle. The direct combustion noise of the flame zone seems to be of minor importance for the noise emission to the ambiance. The combustion experiments were supplemented by experiments with electrical heating. Two different methods for generating entropy waves were used, a pulse excitation and a sinusoidal excitation. In addition, high-frequency entropy noise was generated by steady electrical heating.

Commentary by Dr. Valentin Fuster
2006;():77-86. doi:10.1115/GT2006-90096.

A new approach for modeling combustion instabilities using acoustic energy conservation is proposed. This approach allows the flexibility to calculate contributions from amplification and/or attenuation across all frequencies, rather than predicting eigenmodes. It is likely that such a direct acoustic energy approach could be of benefit in determining the susceptibility of all frequencies to growth or decay as operating conditions in a gas turbine change. This approach may also give designers a method for improving passive damping in the initial design phase, with the ability to scan various frequencies and investigate the susceptibility to oscillatory instability at different operating conditions. A linear model, including linear approximations to nonlinear processes, is introduced whereby the mechanisms contributing to amplification and damping of acoustic energy are assessed independently to find a net amplification coefficient, a reciprocal time (or a rate). Stability for each frequency is assessed by examining a ratio between amplification and damping. It is anticipated that this effort may provide a useful new perspective on and enhanced prediction capability for combustion oscillatory instabilities.

Commentary by Dr. Valentin Fuster
2006;():87-95. doi:10.1115/GT2006-90119.

Future gas turbine engines are required to be more capable than their predecessors. This often implies severe demands on the engine that translate into increasing compressor and combustor exit temperatures, higher combustion pressures and higher fuel/air ratio combustors with greater turn-down ratios (wider operating limits between idle and maximum power conditions). Major advances in combustor technology are required to meet the conflicting challenges of improving performance, increasing durability and maintaining cost. Unconventional combustor configurations are one promising approach to address these challenges. Ultra-short combustors to minimize residence time, with special flame-holding mechanisms to cope with increased through-velocities are likely in the future. Engine cycles other than the standard Brayton cycle may also be used for special applications in order to avoid the use of excessive combustion temperatures, and to extract additional power from the system. This paper focuses on vortex-stabilized combustor technologies that can enable the design of compact, high-performance combustion systems. Compact combustors weigh less and take up less volume in space-limited turbine engine for aero applications. This paper presents a parametric design study of the Ultra-Compact Combustor (UCC), a novel design based on TVC work that uses high swirl in a circumferential cavity to enhance reaction rates via high cavity g-loading on the order of 3000 g’s. Increase in reaction rates translates to a reduced combustor volume. Three combustor geometric features were varied during experiments which included (1) high-g cavity flame-holding method, (2) high-g cavity to main airflow transport method, and (3) fuel injection method. Experimental results are presented for these combustor configurations and results have shown promise for advanced engine applications. Lean blowout fuel-air ratio limits at 25–50% the value of current systems were demonstrated. Combustion efficiency was measured over a wide range of UCC operating conditions. This data begins to build the design space required for future engine designs that may use these novel, compact, high-g combustion systems.

Commentary by Dr. Valentin Fuster
2006;():97-106. doi:10.1115/GT2006-90127.

This paper discusses the structural changes observed in oscillating premixed turbulent swirling flames and demonstrates the influence of modulated mass flows on the flame dynamics in a preheated atmospheric test rig with a natural gas fired swirl burner. The experimentally investigated self excited and forced combustion oscillations of swirl stabilized premixed flames show varying time delays between the acoustically driven mass flow oscillations and the integral heat release rate of the flame. High speed films of the OH*-chemiluminescence reveal how the flame structure changes with the oscillation frequency and the phase angle between the fuel mass flow oscillation and the total mass flow at the burner exit. These parameters are found determine the spatial and temporal heat release distribution and thus the net heat release fluctuation. Therefore, the spatial and temporal heat release distribution along the flame length has an influence on the thermoacoustic coupling, even in the case of acoustically compact flames. The observed phenomena are discussed further using an 1-d analytical model. It underscores that for swirl stabilized premixed turbulent flames the dynamics of the flow field perturbation play a major role in creating the effective heat release fluctuation.

Commentary by Dr. Valentin Fuster
2006;():107-115. doi:10.1115/GT2006-90150.

The continued development of a low swirl injector for ultra-low NOx gas turbine applications is described. An injector prototype for natural gas operation has been designed, fabricated and tested. The target application is an annular gas turbine combustion system requiring twelve injectors. High pressure rig test results for a single injector prototype are presented. On natural gas, ultra-low NOx emissions were achieved along with low CO. A turndown of approximately 100°F in flame temperature was possible before CO emissions increased significantly. Subsequently, a set of injectors was evaluated at atmospheric pressure using a production annular combustor. Rig testing again demonstrated the ultra-low NOx capability of the injectors on natural gas. An engine test of the injectors will be required to establish the transient performance of the combustion system and to assess any combustor pressure oscillation issues.

Commentary by Dr. Valentin Fuster
2006;():117-122. doi:10.1115/GT2006-90154.

A privately owned LNG plant was taken into service at the Tuha Oil Fields in western China during 2004. The plant is the first of its kind and will produce Liquefied Natural Gas (LNG) from associated gas from the oil fields. The LNG is delivered to Central China by trucks. The plant was delivered by Tractebel with Linde AG being responsible for the LNG process design. The compression set of the refrigeration cycle consists of a three-stage Ebara compressor driven by a 24 MW Siemens SGT-600 gas turbine operating on the off-gas from the LNG plant. The operation of the gas turbine integrated in this plant is associated with some special challenges: • the ambient conditions out in the desert; • the fuel, that varies from natural gas to a process gas consisting of methane diluted with up to 28% nitrogen; • the refrigeration medium, which is circulated by the gas turbine driven compressor, changes in composition dependent on load; • the starting procedure with the compressor in the refrigeration loop. A combustion test was performed to verify that the DLE combustion system could accept the variations in gas composition. The control system was modified to handle the variable gas qualities in the fuel and in the refrigeration loop. Since the gas turbine/compressor set is an integrated part of the LNG process the commissioning was a long process governed by the LNG plant commissioning. It included some unexpected events. Now all is working well. It has been shown that a standard SGT-600 DLE unit can start and operate reliably and with low emissions on very much diluted natural gases. The paper contains a brief description of the LNG plant, definition of the special requirements on the gas turbine, a description of the combustion verification test on diluted gas, some events during commissioning and finally the engine verification test.

Commentary by Dr. Valentin Fuster
2006;():123-135. doi:10.1115/GT2006-90174.

This paper presents the aerodynamic study of two premixing systems for gas turbine combustion chamber based on detailed CFD 3-D simulations. The work was carried out with the aim to describe the aerodynamic and the mixing process in two different premixing system schemes, typical for DLE gas turbine combustion chamber. Results from different numerical tools (CFD 3-D and 0/1-D) for the estimation of the fuel jet pathway were compared. Both the premixer configurations analysed are related to the cross-flow injection scheme. The first one considers the fuel injection orthogonal to a low swirled air stream while the second one considers the fuel injection directly from hole rows drilled on the suction and pressure side of the swirler blades. The aerodynamic analysis of the premixing devices was focused on the fuel injection in terms of the jets pathway and air/fuel mixing in steady-state conditions. The aerodynamic investigations were performed by CFD 3-D “full Navier-Stokes” codes. Calculations were repeated, on the same mesh, by an in-house developed code (HybFlow) and by commercial codes also. Some previous experimental results were exploited to tune and validate the calculations. Results of the simulation were post-processed in order to allow a quantitative evaluation of the air/fuel mixing. Moreover the calculations were used to verify the accuracy of 0/1-D models, taken from the literature, for the estimation of the maximum penetration and the trajectory for the cross-flow of gaseous fuel jet, considering typical working conditions for gas turbine premixing system. Finally, preliminary considerations related to the fuel injection schemes and to the influence of the main injection conditions on the mixing were carried out.

Commentary by Dr. Valentin Fuster
2006;():137-147. doi:10.1115/GT2006-90179.

An aero gas turbine engine has been proposed that uses a near-constant-temperature (NCT) cycle and an Inter-Turbine Burner (ITB) to provide large amounts of power extraction from the low-pressure turbine. This level of energy is achieved with a modest temperature rise across the ITB. The additional energy can be used to power a large geared fan for an ultra-high bypass ratio transport aircraft, or to drive an alternator for large amounts of electrical power extraction. Conventional gas turbines engines cannot drive ultra-large diameter fans without causing excessively high turbine temperatures, and cannot meet high power extraction demands without a loss of engine thrust. Reducing the size of the combustion system is key to make use of a NCT gas turbine cycle. Ultra-compact combustor (UCC) concepts are being explored experimentally. These systems use high swirl in a circumferential cavity about the engine centerline to enhance reaction rates via high cavity g-loading on the order of 3000 g’s. Any increase in reaction rate can be exploited to reduce combustor volume. The UCC design integrates compressor and turbine features which will enable a shorter and potentially less complex gas turbine engine. This paper will present experimental data of the Ultra-Compact Combustor (UCC) performance in vitiated flow. Vitiation levels were varied from 12–20% oxygen levels to simulate exhaust from the high pressure turbine (HPT). Experimental results from the ITB at atmospheric pressure indicate that the combustion system operates at 97–99% combustion efficiency over a wide range of operating conditions burning JP-8 +100 fuel. Flame lengths were extremely short, at about 50% of those seen in conventional systems. A wide range of operation is possible with lean blowout fuel-air ratio limits at 25–50% below the value of current systems. These results are significant because the ITB only requires a small (300°F) temperature rise for optimal power extraction, leading to operation of the ITB at near-lean-blowout limits of conventional combustor designs. This data lays the foundation for the design space required for future engine designs.

Commentary by Dr. Valentin Fuster
2006;():149-160. doi:10.1115/GT2006-90184.

The paper uses experimental measurements to characterise the extent that improved external aerodynamic performance (reduced total pressure loss, increased flow quality) of a gas-turbine combustion system may be achieved by adopting an integrated OGV/pre-diffuser technique. Two OGV/pre-diffuser combinations were tested. The first is a datum design corresponding to a conventional design approach, where OGV and pre-diffuser are essentially designed in isolation. The second is an ‘integrated’ design where the OGV blade shape has been modified, following recommendations of earlier CFD work ([14]), to produce a secondary flow/wake structure that allows the pre-diffuser to operate at a higher area ratio without boundary layer separation. This is demonstrated to increase static pressure recovery and reduce dump losses. Experimental measurements are presented on a fully annular rig. Several traverse planes are used to gather 5-hole probe data which allow the flow structure through the OGV, at inlet and exit of the pre-diffuser, and in the inner/outer annulus supply ducts to be examined. Both overall performance measures (loss coefficients) and measures of flow uniformity and quality are used to demonstrate that the integrated design is superior.

Commentary by Dr. Valentin Fuster
2006;():161-171. doi:10.1115/GT2006-90186.

This paper gives an overview of open and closed loop active control methodologies used to suppress symmetric and helical thermoacoustic instabilities in an experimental low-emission swirl-stabilized gas turbine combustor. The controllers were based on fuel (or equivalence ratio) modulations in the main pre-mixed combustion or alternatively in secondary pilot fuel. For the main premix fuel supply two methods of fuel injection modulations were tested: symmetric and asymmetric injection. The tests showed that the closed loop asymmetric modulations were more effective in the suppression of the symmetric mode instability than symmetric fuel excitation. Symmetric excitation was quite efficient in abating the symmetric mode as well, however, at a certain range of phase shift the combustion was destabilized to an extent that caused blow out of the flame. Using premixed open loop fuel modulations the symmetric instability mode responded to symmetric excitation only when the two frequencies matched. The helical fuel injection affected the symmetric mode only at frequencies that were much higher than that of the instability mode. The asymmetric excitation required more power to obtain the same amount of reduction as that required by symmetric excitation. Unlike the symmetric excitation which destabilized the combustion when the modulation amplitude was excessive, the asymmetric excitation yielded additional suppression as the modulation level increased. The NOx emissions were reduced to a greater extent by the asymmetric modulation. Secondary fuel injection in a pilot flame was used to control low frequency symmetric instability and high frequency helical instability. Adding a continuous flow of fuel into the pilot flame controlled both instabilities. However, modulating the fuel injection significantly decreased the amount of necessary fuel. The reduced secondary fuel resulted in a reduced heat generation by the pilot diffusion flame and therefore yielded lower NOx emissions. The secondary fuel pulsation frequency was chosen to match the time scales typical to the central flow recirculation zone which stabilizes the flame in the burner. Suppression of the symmetric mode pressure oscillations by up to 20 dB was recorded.

Commentary by Dr. Valentin Fuster
2006;():173-182. doi:10.1115/GT2006-90213.

A generic combustor was built, that gives wide optical access at higher pressure and shares typical features with aero engine combustors. A comprehensive data set for validation of RANS and LES codes was generated at isothermal as well as combusting conditions at 2 and 10 bars with 650 K preheat using natural gas as fuel. The velocity field was measured using LDA (Laser Doppler Anemometry) and DGV (Doppler Global Velocimetry) as well as PIV (Particle Image Velocimetry). Temperature data were acquired using CARS (Coherent Anti stokes Raman Scattering) and SRS (Spontaneous Raman Scattering). Major species concentrations as well as the mixture fraction in the primary zone of the combustor were also measured using SRS. Mean and RMS values of the temperature measured by CARS in the secondary zone illustrate the influence of the jet impingement on the unsteady mixing of the jets with the swirling primary air.

Commentary by Dr. Valentin Fuster
2006;():183-190. doi:10.1115/GT2006-90239.

The flow fields in and around two versions of a water-cooled gas-sampling probe, situated downstream of a gas turbine combustor, were numerically studied in an elevated pressure and temperature environment. The probes are of triple-walled stainless steel assembly, where the gas sample is transported through a centre tube, while preheated and pressurized cooling water flows through two surrounding annuli. Complex conjugate heat transfers amongst the exhaust mixture, cooling water and probe walls were modelled at a selected operating condition. The numerical results indicate over-heating and possible vaporization of water or cavitation in the upstream tip region of the probe with the original design. This is consistent with the evidence of damage observed in these probes from prolonged testing under similar conditions. For the modified probe, the effectiveness of cooling water is much improved, which is confirmed by long-term combustor rig testing. From this investigation, some recommendations for probe design and operation are provided. Moreover, the present study has proved that the numerical simulation is a valuable tool for probe design and trouble-shooting, and to accurately predict conjugate heat transfers in such flows, the laminar sub-layer in the near-wall region should be adequately resolved.

Commentary by Dr. Valentin Fuster
2006;():191-200. doi:10.1115/GT2006-90248.

An important question for future aero-engine combustors is how partial vaporization influences the NOx emissions of spray flames. In order to address this question an experimental study of the combustion of partially vaporized kerosene/air mixtures was conducted, which assesses the influence of the degree of fuel vaporization on the NOx emissions in a wide range of equivalence ratios covering the entire lean burning regime. The tests were performed at atmospheric pressure, inlet air temperatures of 313 to 376K, a reference mean air velocity of 1.35m/s, and equivalence ratios of 0.6, 0.7 and 0.9 using Jet A1 fuel. An ultrasonic atomizer was used to generate a fuel spray with a Sauter Mean Diameter of approximately 50μm. The spray and the heated air were mixed in a glass tube of 71mm diameter and a variable length of 0.5 to 1m. The temperature of the mixing air and the length of the preheater tube were used for the control of the degree of vaporization. Downstream of the vaporizing section, the mixture was ignited and the flame was stabilized with a hot wire ring that is electrically heated. For local exhaust measurements a temperature controlled suction probe in combination with a conventional gas analysis system were used. The vaporized ratio of the injected fuel was determined by a Phase Doppler Anemometer (PDA). In order to optimize the accuracy of these measurements, extensive validation tests with a patternator method were performed and a calibration curve was derived. The data collected in this study illustrates the effect of the vaporization rate Ψ upstream of the flame front on the NOx emissions, which changes with varying equivalence ratio and degree of vaporization. In the test case with low pre-vaporization, the equivalence ratio only has a minor influence on the NOx emissions. Experiments made with air preheat and higher degrees of vaporization show two effects: With increasing preheat air temperature, NOx emissions increase due to higher effective flame temperatures. However, with an increasing degree of vaporization, emissions become lower due to the dropping number and size of burning droplets, which act as hot spots. A correction for the effect of the preheat temperature was developed. It reveals the effect of the degree of pre-vaporization and shows that the NOx emissions are almost independent of Ψ for near-stoichiometric operation. At overall lean conditions the NOx emissions drop nonlinearly with Ψ. This leads to the conclusion that a high degree of vaporization is required in order to achieve substantial NOx abatement.

Commentary by Dr. Valentin Fuster
2006;():201-209. doi:10.1115/GT2006-90257.

Gas turbine related combustion systems often operate under conditions, where the turbulent time scales of the flow field and the chemical reaction times are of the same order of magnitude. To describe this regime of moderate and small Damköhler numbers, pdf-methods have become an established modeling technique with a thoroughly derived mathematical basis. In this paper a hybrid pdf/FANS method is presented. It is based on the transported pdf equation closed at the joint composition level, which is implemented into a commercial 3D CFD solver. The solution algorithm features a stochastic particle system in a Lagrangian framework. The test case under consideration is a H2 -stabilized turbulent premixed methane/air flame with co-flow. The set up was investigated using optical and conventional measurement techniques. Field measurements of velocity and composition were compared against calculations with the hybrid approach. The thermo-chemistry was described by a two domain-one step kinetic scheme. This semi-global kinetics was derived for use with CFD simulations of technical applications in the context of gas turbine combustion. For the numerical calculations the mean estimates are extracted from the stochastic particle field, which contains the complete one point statistics of the species composition vector. The knowledge of the composition pdf allows an evaluation of the structure of the turbulent reaction zone at any position of the flame.

Commentary by Dr. Valentin Fuster
2006;():211-220. doi:10.1115/GT2006-90281.

This paper describes the concept and benefits of the fuel moisturization system for the GE H System™ steam-cooled industrial gas turbine. The DLN2.5H combustion system and fuel moisturization system are both described, along with the influence of fuel moisture on combustor performance as measured during full-scale, full-pressure rig testing of the DLN2.5H combustion system. The lean, premixed DLN2.5H combustion system was targeted to deliver single-digit NOx and CO emissions from 40% to 100% combined cycle load in both the Frame 7H (60 Hz) and Frame 9H (50 Hz) heavy-duty industrial gas turbines. These machines are also designed to yield a potential combined-cycle efficiency of 60 percent or higher. Fuel moisturization contributes to the attainment of both the NOx and the combined-cycle efficiency performance goals, as discussed in this paper.

Commentary by Dr. Valentin Fuster
2006;():221-235. doi:10.1115/GT2006-90282.

A chemical reactor network (CRN) is developed and applied to a dry low emissions (DLE) industrial gas turbine combustor with the purpose of predicting exhaust emissions. The development of the CRN model is guided by reacting flow computational fluid dynamics (CFD) using the University of Washington (UW) eight-step global mechanism. The network consists of 31 chemical reactor elements representing the different flow and reaction zones of the combustor. The CRN is exercised for full load operating conditions with variable pilot flows ranging from 35% to 200% of the neutral pilot. The NOpilot. The NOx and the CO emissions are predicted using the full GRI 3.0 chemical kinetic mechanism in the CRN. The CRN results closely match the actual engine test rig emissions output. Additional work is ongoing and the results from this ongoing research will be presented in future publications.

Commentary by Dr. Valentin Fuster
2006;():237-245. doi:10.1115/GT2006-90300.

Non-intrusive laser-based and optical measurements were performed in a gas turbine model combustor with a lean premixed swirl-stabilized CH4 -air flame at atmospheric pressure. The main objective was to gain spatially and temporally resolved experimental data to enable the validation of numerical CFD-results of oscillating flames. The investigated flame was operated at 25 kW and φ = 0.70 and exhibited self-excited oscillations of more than 135 dB at about 300 Hz. The applied measurement techniques were 3-D LDV for velocity measurements, OH* chemiluminescence yielding information about the heat release, and point-wise laser Raman scattering for the determination of joint PDFs of the major species concentrations, temperature, and mixture fraction. Each of these techniques was applied with phase resolution with respect to the periodic fluctuation of the pressure in the combustion chamber that was measured with a microphone probe. The measurements finally revealed that the mixing of fuel and air in this technical premixing system was strongly affected by the pressure fluctuations leading to changes in equivalence ratio during an oscillation cycle which in turn induced the pressure fluctuations.

Commentary by Dr. Valentin Fuster
2006;():247-254. doi:10.1115/GT2006-90302.

This paper describes an experimental investigation of the flame sheet dynamics of an acoustically forced bluff body flame over a range of perturbation frequencies and amplitudes. When acoustically excited, the flame sheet displays well defined, periodic corrugations, presumably due to flame sheet perturbations created at its attachment point that convect downstream, as well as the rollup of shear layer instabilities into large scale coherent structures. The dynamics of the flame front response, such as its growth and decay in the bluff body wake, disturbance convection velocity, sub-harmonic response, and total flame area is discussed.

Topics: Acoustics , Flames
Commentary by Dr. Valentin Fuster
2006;():255-262. doi:10.1115/GT2006-90319.

An ultra lean-premixed Advanced Vortex Combustor (AVC) has been developed and tested. The natural gas fueled AVC was tested at the U.S. Department of Energy’s National Energy Technology Laboratory (USDOE NETL) test facility in Morgantown (WV). All testing was performed at elevated pressures and inlet temperatures and at lean fuel-air ratios representative of industrial gas turbines. The improved AVC design exhibited simultaneous NOx /CO/UHC emissions of 4/4/0 ppmv (all emissions are at 15% O2 dry). The design also achieved less than 3 ppmv NOx with combustion efficiencies in excess of 99.5%. The design demonstrated tremendous acoustic dynamic stability over a wide range of operating conditions which potentially makes this approach significantly more attractive than other lean premixed combustion approaches. In addition, a pressure drop of 1.75% was measured which is significantly lower than conventional gas turbine combustors. Potentially, this lower pressure drop characteristic of the AVC concept translates into overall gas turbine cycle efficiency improvements of up to one full percentage point. The relatively high velocities and low pressure drops achievable with this technology make the AVC approach an attractive alternative for syngas fuel applications.

Commentary by Dr. Valentin Fuster
2006;():263-270. doi:10.1115/GT2006-90326.

This paper reports results of experimental and numerical investigations of ethanol-air diffusion flames and partially premixed flames at an air-side strain rate of 100 s−1 , in a counterflow geometry. The diffusion flame consists of prevaporized fuel, with mole fraction of 0.3, diluted with nitrogen in the fuel stream, and plant air as the oxidizer stream. The partially premixed flame includes prevaporized fuel in air partially premixed to an equivalence ratio of 2.3 in the fuel stream, and plant air as the oxidizer stream. Temperature profiles were measured by thermocouple, and concentration profiles of the stable species C2 H5 OH, CO, CO2 , H2 , H2 O, O2 , N2 , CH4 , C2 H6 , and C2 H2 +C2 H4 were measured by gas chromatography of samples withdrawn by a fine probe. Computational studies involved numerical integration of the conservation equations, with detailed chemistry, transport and radiation effects included, to calculate the structures of the counterflow flames. A chemical-kinetic mechanism consisting of 235 elementary steps and 46 species with recently published reaction-rate parameters was developed and tested for these flames. The proposed mechanism, which produces reasonable agreement with previous measurements of ignition, freely propagating premixed flames and diffusion-flame extinction, also yields good agreement with much of the present data, although there are quite noticeable differences between predicted and measured peak C2 H6 concentrations. These differences and the desirability of additional tests of other predictions and of tests under other conditions motivate further research.

Topics: Ethanol , Flames
Commentary by Dr. Valentin Fuster
2006;():271-280. doi:10.1115/GT2006-90333.

An empirical modelling concept for the prediction of NOx emissions from Dry Low Emissions (DLE) gas turbines is presented. The approach is more suited to low emissions operation than are traditional approaches. The latter, though addressing key operating parameters, such as temperature and pressure drop, do not address issues such as variation in fuel/air distribution through the use of multi fuel stream systems, which are commonly applied in DLE combustors to enable flame stability over the full operating range. Additionally the pressure drop dependence of NOx in such systems is complex and the exponent of a simple pressure drop term can vary substantially. The present approach derives the NOx model from the equations that govern the NOx chemistry, the fuel/air distribution and the dependence of the main reaction zone upon its controlling parameters. The approach is evaluated through comparing its characteristics with data obtained from high pressure testing of a DLE combustor fuelled with natural gas. The data were acquired at a constant pressure and preheat temperature (14 Bara and 400°C) and a range of flame temperatures and flow rates. Though the model is configured to address both relatively fast and slow NOx formation routes, the present validation is conducted under conditions where the latter is negligible. The model is seen to reproduce key features apparent in the data, in particular the variable pressure drop dependence without any ad-hoc manipulation of a pressure drop exponent.

Commentary by Dr. Valentin Fuster
2006;():281-291. doi:10.1115/GT2006-90338.

Combustion of syngas with humid air is important for integrated coal gasification humid air turbine cycle for power generation. In contrast to hydrocarbon fuels, CO exists in bulky content in syngas. Thus the influence of air humidity on the oxidation of CO in syngas flames should be well understood. In the present study, the influence of air humidity on CO oxidation in syngas diffusion flames was investigated both experimentally and numerically. In the experiment, CO, CO2 , O2 and temperature profiles in a model combustor of a syngas turbulent jet diffusion flame were measured. It was found that as the mass flow rate of dry air and the exhaust temperature kept constant, the profile of CO was influenced by air humidity, but CO exhaust did not exhibit a monotone increase over humidity. There exist an inflexion at the case of absolute humidity of the air X = 6.0. In addition, CO emission decreases with the increase of the thermal load. And the influence of the humidity on CO oxidation was not obviously when the thermal load is high. In the numerical simulation, flow field in the combustor was calculated by applying the composition PDF transport model and Flamelet model respectively. The numerical results were compared with the experiment and the PDF model was verified that it is more suited to simulate the CO oxidation. From the numerical analysis, it was found that the concentration of OH wasn’t monotonously increased with the added H2 O and this directly affect the oxidation of CO via CO + OH → CO2 + H. In general, the influence of air humidity on the oxidation of CO in syngas diffusion flames depends on the coupling and competition of chemistry and fluid mechanics aspects. This helps to interpret the discrepancy of the results of gas turbine combustor test.

Commentary by Dr. Valentin Fuster
2006;():293-302. doi:10.1115/GT2006-90340.

Combustion near LBO involves the complex physical processes including turbulence, air/fuel mixing, and chemical kinetics. The goal of this paper was to identify the typical combustion behaviour near LBO of the burner and to develop an effective actuator that will have the necessary control authority without having adverse effects such as increased emissions. Early detection and effective extension of lean blowout (LBO) are the keys to ensure flight safety and low emissions for aero engines, and are of importance to industrial gas turbines for operation below regulated NOx limits. In addition, efficient actuation are crucial for effective active LBO control. An experimental investigation of LBO was carried out using a swirl-stabilized atmospheric combustor with separate pilot and premix gaseous fuel (natural gas) injection systems. Systematic tests were performed including measurements of pressure, OH chemiluminescence and emissions for different combustor lengths, fuel split ratios, preheat temperatures and air flow rates. Operation near LBO may involve excitation of undesired thermoacoustic instabilities that have to be mitigated. LBO was approached by reducing the fuel flow rate while keeping the air flow rate, the preheat temperature and the other parameters constant. Control of the LBO and thermoacoustic instabilities was achieved by generating periodic flame balls. The LBO could be extended by 13 % relative to the natural lean blowout limit at nearly 50% reduced NO emission in comparison to common pilot fuel modulation. A spark discharge system was installed at the pilot fuel injection location. The periodic spark discharge was synchronized with the pulsed fuel injection at a phase shift of 165° and an operating frequency of 22 Hz to produce flame balls that affected the main combustion region. The flame balls excitation provided an effective tool for controlling the premix combustion characteristics at the LBO.

Topics: Flames
Commentary by Dr. Valentin Fuster
2006;():303-309. doi:10.1115/GT2006-90344.

In order to investigate the combustion behavior of gas turbine flames fired with low-caloric syngases, a model combustor with good optical access for confined, non-premixed swirl flames was developed. The measuring techniques applied were particle image velocimetry, OH* chemiluminescence detection and laser-induced fluorescence of OH. Two different fuel compositions of H2 , CO, N2 and CH4 , with similar laminar burning velocities, were chosen. Their combustion behavior was studied at two different pressures, two thermal loads and two combustion air temperatures. The overall lean flames (equivalence ratio 0.5) burned very stably and their shapes and combustion behavior were hardly influenced by the fuel composition or by the different operating conditions. The experimental results constitute a data-base that will be used for the validation of numerical combustion models and form a part of a co-operative EC project aiming at the development of highly efficient gas turbines for IGCC (Integrated Gasification Combined Cycle) power plants.

Commentary by Dr. Valentin Fuster
2006;():311-318. doi:10.1115/GT2006-90358.

Currently, more than 1,500 gas turbines are in operation on natural gas transmission lines all over Europe. These turbines do not comply with the requirements for toxic substances content in exhaust gases. Therefore, an environmentally friendly update of these turbines is a hot topic now, especially because these turbines are supposed to remain in operation for another 10 or 15 years. Besides, environmentally friendly update is a specific issue that differs from the development of a new low-emission combustion chamber. The authors participated in environmentally friendly update of more than 500 gas turbines of this design in Russia, Ukraine, Slovakia, Czech Republic, Germany, and Hungary. As new emission limits are expected to be issued in the EU, a new low-emission burner was developed that makes use of a combination of kinetic and diffusion combustion to achieve low NOx and CO emissions. The burner operation in combustion chambers of gas turbines is characterized by a wide range of the coefficient of excess air from idle run to full performance. Therefore, the control of the quantity of primary air is necessary. The paper will describe the main stages of the burner research. Tests were performed on an atmospheric pressure test rig where the basic characteristics were gained. The influence of pressure was examined on a special test rig at 0.75–1.1 MPa of pressure. Tests have confirmed that the required NOx and CO emission limits can be achieved with the designed burner. The low emission burner was used for the combustion chamber of a 6 MW gas turbine. The tests performed on a part of a model burner will be presented and an analysis of measurement results will be given.

Commentary by Dr. Valentin Fuster
2006;():319-326. doi:10.1115/GT2006-90423.

Premixed combustion is the commonly adopted technique to reduce NOx emissions from gas turbine combustion chambers, but it has been proved to be susceptible to thermo-acoustic instabilities, known as humming. These self-excited oscillations can reduce the efficiency of the turbine and generate structural damage to the combustion chamber. One of the proposed suppression methods lies in the application of Helmholtz resonators to the combustion chambers. This passive technique is advantageous in carrying out appreciable oscillation damping with modest costs and long life, but it is effective only in a restricted range of frequency, close to resonator eigenfrequency. Therefore, in order to design effective resonators, it is necessary to know the eigenfrequencies of the annular combustion chamber, because combustion instabilities arise in correspondence of these frequencies. Acoustic analysis of combustion chamber and its connected components may be carried out by means of Finite Element Method, but it requires a considerable computational effort due to the complex geometry of the complete system, which needs to be meshed by a refined grid. A combined numerical and experimental technique allows the authors to increase computational efficiency by adopting coarser and more regular meshes. First acoustic behavior of annular combustion chamber has been studied by means of numerical simulations and, therefore, the influence of the burners has been taken into account by substituting burner geometries by experimentally measured acoustic impedances. Then some Helmholtz resonators, tuned to one eigenfrequency of the combustion chamber, have been designed and manufactured. Their acoustic impedances have been experimentally measured and applied as boundary conditions into FE simulations of the annular chamber. In this way the acoustic pressure field inside the damper-equipped combustion chamber has been analyzed. Numerical simulations of the annular chamber, with burner and damper impedances applied, show that Helmholtz resonators are effective in oscillation suppression in correspondence of their resonance frequency, but may produce the splitting of the acoustic pressure peak of the chamber into two new peaks, whose frequencies lie on either side of the original common eigenfrequency. The amplitudes of these two new pressure peaks appear lower than the amplitude of the baseline one. The proposed technique can be used as an effective design tool: acoustic analysis of annular combustion chamber, with burner impedance applied, produces accurate indications about its acoustic behavior and allows the design of new passive suppression systems and the evaluation of their performances.

Commentary by Dr. Valentin Fuster
2006;():327-336. doi:10.1115/GT2006-90430.

The preliminary design of a new combustion chamber requires the combination of many elements of know-how in terms of combustor design rules, aerothermal calculations and preliminary design tools. To use this knowledge more efficiently pre-competitive work on an automated knowledge-based combustor design methodology is done within the European project INTELLECT D.M. (Integrated Lean Low Emission Combustor Design Methodology) in order to set up a KBE (K nowledge B ased E ngineering) system. In the method presented here, the rules and calculation routines are implemented into an automated preliminary design system using an Excel-driven database to generate a parametric Unigraphics CAD model. The utilized design rules represent state-of-the-art combustor design and will be extended later by lean combustion design rules, which are currently developed within INTELLECT D.M.. The database contains all design parameters and rules to provide CAD, CFD and optimization tools with the required information. Based on a set of performance parameters the system automatically generates the parametric geometry of a combustor containing the liners with cooling devices (optionally Z-ring or effusion cooling) and mixing holes, heat shield, cowl, casings and (pre)diffusor. To estimate the required cooling air, one-dimensional heat transfer equations including convection, radiation and conduction are solved. The generated CAD model visualizes the calculated combustor geometry and forms the basis for an automated CFD mesh generation utilizing the grid generator ICEM CFD.

Commentary by Dr. Valentin Fuster
2006;():337-345. doi:10.1115/GT2006-90432.

Within the context of lean premixed prevaporized combustion (LPP) which is considered as most promising technology for the next generation of low emission combustors for aero engines, combustion instabilities are a major issue. These combustion instabilities may compromise the pollutant emissions and even cause damage to the combustion chamber structure. In the literature, numerous phenomenological studies on combustion oscillation are available, but a comprehensive theory is still missing. One potential excitation mechanism is the interaction of strong air velocity fluctuations and pressure oscillations with the airblast atomizer leading to temporal fluctuations of the spray characteristics. This phenomenon was investigated experimentally at the Institute of Thermal Turbomachinery (ITS) within a parametric study. A duct with a prefilming surface was set up as an abstraction of a prefilming airblast atomizer. A mean air velocity up to 65 m/s can be reached, and periodic oscillations can be superimposed by means of a siren with a frequency up to 570 Hz. The disintegration process of the liquid fuel was studied downstream the atomizing edge of a plain airblast nozzle. Several optical diagnostics like phase resolved LDV (Laser Doppler Velocimetry) and an improved PTV technique (Particle Tracking Velocimetry) were used. The mean air velocity, the film load, the kinematic viscosity and the surface tension of the fluid as well as the pulsation frequency and amplitude of the siren were varied, and their effect on the temporal evolution of the droplet size and droplet rate was studied. It was found that the amplitude of fluctuations of the droplet size and the droplet rate is almost proportional to the air velocity fluctuations at low frequencies. At higher frequencies, however, both are nearly unaffected. In addition, the fluctuations of droplet diameter and rate increase strongly if the mean air velocity is increased. The phase shift between particle diameter, particle rate and air velocity fluctuations was found to increase at higher excitation frequencies.

Commentary by Dr. Valentin Fuster
2006;():347-356. doi:10.1115/GT2006-90441.

The goal of this paper is to discuss the derivation and behaviour of non-reflecting boundary conditions in the framework of accurate calculations of combustion instabilities. Therefore, it is explained for the first time, how to modify the coefficients of the discrete pressure correction equation and of the discrete conservation equations, in order to apply non-reflecting boundary conditions to a pressure based SIMPLE-Algorithm. The theory and practical implementation of the boundary conditions, which are based on Poinsot & Lele’s [1] formulation, will be explained for inflow and outflow boundaries. The method will be validated based upon test cases which are relevant to the simulation of gas turbine combustion chambers. Moreover, the accuracy of non-reflecting boundary conditions is assessed for cases where combustion leads to inhomogeneous temperature and species fields. The impact of the acoustic wave propagation speed on the reflectivity of the non-reflecting boundary conditions is analysed.

Commentary by Dr. Valentin Fuster
2006;():357-368. doi:10.1115/GT2006-90470.

This paper reports experimental data on flashback and lean blowout characteristics of H2 /CO/CH4 mixtures. Data were obtained over a range of fuel compositions at fixed approach or burned flow velocity, reactant temperature, and combustor pressure at several conditions up to 4.4 atm and 470 K inlet reactants temperature. Consistent with prior studies, these results indicate that the percentage of H2 in the fuel dominates the mixture blowout characteristics. These blowout characteristics can be captured with classical Damköhler number scalings to predict blowoff equivalence ratios to within 10%. Counter-intuitively, the percentage of hydrogen had far less effect on flashback characteristics, at least for fuels with hydrogen mole fractions less than 60%. This is due to the fact that two mechanisms of “flashback” were noted: rapid flashback into the premixer, presumably through the boundary layer, and movement of the static flame position upstream along the centerbody. The former and latter mechanisms were observed at high and low hydrogen concentrations. In the latter mechanism, flame temperature, not flame speed, appears to be the key parameter describing flashback tendencies. We suggest that this is due to an alteration of the vortex breakdown location by the adverse pressure gradient upstream of the flame, similar to the mechanism proposed by Sattelmayer and co-workers [1]. As such, a key conclusion here is that classical flashback scalings derived from, e.g., Bunsen flames, may not be relevant for many parameter regimes found in swirling flames.

Topics: Syngas
Commentary by Dr. Valentin Fuster
2006;():369-388. doi:10.1115/GT2006-90478.

Pressure conditions under which chemical reactions proceed in gas turbine combustors impact the behavior of the combustion process by either increasing or decreasing the reaction rates depending on whether these reactions are unimolecular/recombination or chemically activated bimolecular reactions. Some reactions are pressure independent such as H-abstraction reactions, while others are conditionally pressure independent if they are not at their either low or high limits. The recombination and decomposition of kinetic reactions rate constants change relative to their limiting values as the pressure and/or temperature conditions vary and as a result the reactants concentrations and reactions pathways are also influenced. In this study, pressure-dependent kinetic rate parameters for 39 elementary reactions have been added to our detailed JP-8/Jet-A kinetic reaction mechanism, we have developed [1–3, 23, 58], to model ignition of JP-8 and Jet-A fuels behind a reflected shock wave. The main objective is to develop a detailed chemical kinetic reaction mechanism for low and high pressure combustion conditions, using a 6-component surrogate fuel blend considered to represent the actual (petroleum-derived) JP-8 and Jet-A fuels. The pressure-dependent kinetic rate parameters for 39 reactions have been incorporated into our low pressure detailed JP-8 chemical kinetic reaction mechanism to generate the fall-off curves for the Arrehnius rate parameters required for low and high pressure ignition analysis. The new JP-8 detailed mechanism has been evaluated, using a stoichiometric JP-8/02/N2 and Jet-A/air mixtures, over a temperature range of 968–1639 K and a pressure range of 10 to 34 atmosphere by predicting auto-ignition delay times and comparing them to the shock tube ignition data of Minsk, Sarikovskii, and Hanson [56]. The results indicated that the developed JP-8/Jet-A reaction mechanism is capable of reproducing the qualitative ignition trends of the measured ignition data behind a reflected shock wave. However, the detailed kinetic reaction mechanism overestimated the measured ignition delay times. The results also suggested that additional more reactions are high pressure-dependent under the conditions considered in this study and as such a need still exists for experimentally measured kinetic rate coefficients for high pressure ignition and combustion conditions. This study, therefore, warrants further experiments and detailed kinetic analysis.

Commentary by Dr. Valentin Fuster
2006;():389-396. doi:10.1115/GT2006-90488.

Shock-tube ignition delay time experiments and chemical kinetics model calculations were performed for several fuel blends of carbon monoxide and hydrogen in air at elevated pressures. Due to the interest in coal-derived fuels, namely syngas, these data are important for characterizing the ignition and oxidation of possible fuel blends used in gas turbines and for the validation of chemical kinetics models. Three lean, CO/H2 (80/20%, 90/10%, and 95/5% by volume) fuel blends in air were studied behind reflected shock waves at temperatures between 929 and 1304 K and pressures ranging from 1.7 to 15 atm. Ignition delay times were monitored using chemiluminescence emission from excited hydroxyl radicals. Results exhibit the second-explosion limit behavior from hydrogen oxidation kinetics at low temperatures and high pressures for all mixtures. In addition, comparisons of modeling results and experimental data show good agreement for the entire temperature range at high pressure and poor agreement with the data at low temperatures in the intermediate pressure regimes. Ignition and reaction sensitivity analyses indicate that the H + O2 + M = HO2 + M termination reaction is important at all conditions herein, and the early formation of HO2 suppresses the growth of the ignition-enhancing radicals H and OH.

Topics: Ignition
Commentary by Dr. Valentin Fuster
2006;():397-403. doi:10.1115/GT2006-90489.

The characteristics of the flow fields in a model combustor’s primary zone, which is confined by different swirl cups and the flame tube with primary holes and cooling air films, is experimentally investigated by a Particle Dynamic Analyzer (PDA). The spray of RP-3 jet fuel from the atomizer is used as the trace particles for laser detection. The inlet air pressure and temperature are 110 KPa and 300 K, respectively. The test sections start from 10 mm and end at 50 mm downstream the swirl cup, ensuring that the primary holes are located within the test range. The results show that the swirl cup with double radial swirlers causes a greater flow expansion than that with double axial swirlers, thus results in a greater recirculation zone. By comparison with the previously reported data for the flow field after a pure swirl cup, it can be concluded that the flow field, especially the recirculation zone, in the primary zone with primary hole jets, is no longer axisymmetrical, as it was after a pure swirl cup. The presence of primary hole air flows remarkably squeezes the recirculation zone, especially for double radial swirlers; thus resulting in a elliptic-like cross section, with the longer axis greater than the shorter one up to 2.3 times, and causing the diminish of the corner recirculation zone. The recirculation zone is always ended at the primary holes, resulting in a much shorter length of the recirculation zone, which is only about 1.3 times of the exit diameter of the swirl cup, instead of 2.5 times for the pure swirl cup structure without the primary holes.

Commentary by Dr. Valentin Fuster
2006;():405-412. doi:10.1115/GT2006-90490.

Flame stability is a crucial issue in low NOx combustion systems operating at extremely lean conditions. Hydrogen enrichment seems to be a promising option to extend lean blowout limits of natural gas combustion. This experimental study addresses flame stability enhancement and NOx reduction in turbulent, high-pressure, lean premixed methane/air flames in a generic combustor, capable of a wide range of operating conditions. Lean blowout limits (LBO) and NOx emissions are presented for pressures up to 14 bars, bulk velocities in the range of 32–80 m/s, two different preheating temperatures (673 K, 773 K), and a range of fuel mixtures from pure methane to 20% H2 /80% CH4 by volume. The influence of turbulence on LBO limits is discussed, too. In addition to the investigation of perfectly premixed H2 -enriched flames, LBO and NOx are also discussed for hydrogen piloting. Experiments have revealed that a mixture of 20% hydrogen and 80% methane, by volume, can typically extend the lean blowout limit by roughly 10% compared to pure methane. The flame temperature at LBO is approximately 60 K lower resulting in the reduction of NOx concentration by ≈ 35% (0.5 → 0.3 ppm/15% O2 ).

Commentary by Dr. Valentin Fuster
2006;():413-421. doi:10.1115/GT2006-90495.

Most gas turbine premix burners without centrebody employ the breakdown of a swirling flow at the transition between the mixing section and the combustor for aerodynamic flame stabilization. As the formation of the desired vortex breakdown pattern depends very sensibly on the shape of the axial and azimuthal velocity profiles in the mixing section, the design of suitable swirlers is typically a cumbersome process and requires an iterative approach consisting of numerical as well as experimental development steps to be iteratively applied until a geometry is found, that provides a spatially as well as temporarily stable vortex breakdown in the primary zone of the combustion chamber without backflow on the centerline of the vortex into the swirler. These difficulties stem from the lack of generally applicable aerodynamic design criteria. The paper attempts to contribute to the development of such design guidelines, which lead quickly to successful swirler designs without need for an excessive number of iterations. For this purpose a family of swirl profiles was generated and the corresponding axial velocity profiles were calculated assuming several radial total pressure distributions. In the next step, the flows were calculated using CFD in order to find out, which velocity profiles produce stable vortex breakdown bubbles at the burner exit. This study reveals that the stable breakdown of the vortex can be achieved for a wide range of velocity distributions, if the radial total pressure distribution is properly selected. However, the radial total pressure distribution in the vortex core is essential for the robustness of the design. Interestingly, velocity profiles with constant total pressure do not show a stable transition of the velocity field at the cross-sectional area change at the entrance of the combustion chamber. In addition, theoretical considerations reveal that an increase of the azimuthal velocity in the vortex core in streamwise direction avoids backflow on the centreline as well as flame flashback. This increase can be achieved using a slightly conical nozzle and introducing a swirl free jet on the centreline upstream of the mixing zone. All effects are explained using the vorticity transport equation.

Topics: Design , Vortices
Commentary by Dr. Valentin Fuster
2006;():423-431. doi:10.1115/GT2006-90497.

Most gas turbine premix burners without centrebody employ the breakdown of a swirling flow at the transition between the mixing section and the combustor for aerodynamic flame stabilization [1]. As the formation of the desired vortex breakdown pattern depends very sensibly on the distribution of axial and azimuthal velocity in the mixing section, the design of suitable swirlers is usually a cumbersome iterative process. The presented burner design was found through the implementation of design guidelines derived from CFD-calculations and on the basis of analytical considerations [5]. The swirling flow is generated by a radial swirler with tangential inlets. In order to stabilize the flow pattern, the swirling flow confines a slow non-swirling flow on the centreline. The centre flow being set into azimuthal motion creates increasing azimuthal velocity in streamwise direction in the vortex core. This process is reinforced by a conical nozzle and leads to the production of positive azimuthal vorticity inside the nozzle which stabilizes the flow field. First atmospheric test runs and Large Eddy Simulations of the isothermal as well as reactive flow field prove that the design goals have been reached: The burner creates stable vortex breakdown in the primary zone of the combustion chamber without flame flashback or backflow on the centreline over the entire operating range and even for difficult fuels like hydrogen containing gases. This finding indicates that reliable vortex breakdown burners with remarkable fuel flexibility can be designed using the guidelines presented in [5].

Topics: Design , Vortices
Commentary by Dr. Valentin Fuster
2006;():433-444. doi:10.1115/GT2006-90510.

Medium- and low-LHV fuels are receiving a continuously growing interest in stationary power applications. Besides that, since in many applications the fuels available at a site can be time by time of significantly different composition, fuel flexibility has become one of the most important requirements to be taken into account in developing power systems. A test campaign, aimed to provide a preliminary assessment of a small power gas turbine’s fuel flexibility, was carried over a full-scale GE10 prototypical unit, located at the Nuovo-Pignone manufacturing site, in Florence. The engine is a single shaft, simple cycle gas turbine designed for power generation applications, rated at 11 MW electrical power and equipped with a silos-type combustor. A variable composition gas fuel was obtained by mixing natural gas with CO2 to about 40% by vol. at engine base-load condition. Tests involved two different diffusive combustion systems: the standard version, designed for operation with natural gas, and a specific system designed for low-LHV fuels. Tests performed aimed to investigate both ignition limits and combustors’ performances, focusing on hot parts’ temperatures and pollutant emissions. Regarding NOx emissions, data collected during standard combustor’s tests were matched a simple scaling law (as a function of cycle parameters and CO2 concentration in the fuel mixture), which can be used in similar applications as a NOx predictive tool. In a following step, a CFD study was performed in order to verify in detail the effects of LHV reduction on flame structure and to compare measured and calculated NOx . STAR-CD™ code was employed as main CFD solver while turbulent combustion and NOx models were specifically developed and implemented using STAR’s user-subroutine features. Both models are based on classical laminar-flamelet approach. Three different operating points were considered at base-load conditions, varying CO2 concentration (0%, 20% and 30% vol. simulated). Numerical simulations point out the flexibility of the GE10 standard combustor to assure flame stabilization even against large variation of fuel characteristics. Calculated NOx emissions are in fairly good agreement with measured data confirming the validity of the adopted models.

Commentary by Dr. Valentin Fuster
2006;():445-452. doi:10.1115/GT2006-90521.

Flashback is one of the major problems in lean premixed combustion of gas turbine combustor. Due to the effulgent future of co-product system and IGCC, lean premixed combustion, one of the approaches to ultra low NOx for rich hydrogen syngas fuel need farther research on anti-flashback and low pressure drop combustor. Mechanism and characteristics of methane and syngas flashback for 2 types of flame holders, i.e. ring shape and rod shape have been researched through experiment as well as numerical simulation. The partial premix model has been selected to simulate premixed combustion flashback process since it combined the advantage of PDF model and TFC model. Experiments demonstrate that, the flashback velocity of different fuel compositions or flame holder size generally can be correlated to the same dimensionless function by using Peclet number model if the structures of flame holders are the same. Peclet function curves were used to compare the anti-flashback performance of the 2 types of flame holders mentioned above with swirl holder. Boundary coaxial jet can change flashback through the wall into flashback in the core flow and significantly improve the anti-flashback performance of the ring-type flame holder on condition that the velocity of the boundary coaxial jet is in an optimal range. As the result, ring shape holder shows the best while swirl holder the worst on anti-flashback performance.

Topics: Fuels , Syngas , Methane , Mechanisms
Commentary by Dr. Valentin Fuster
2006;():453-465. doi:10.1115/GT2006-90530.

A combustor is a crucial unit of gas turbine engine because it should work reliably at high temperatures; provide a suitable temperature distribution at entry to the turbine and supply a low emission level of harmful substances. An operational development of combustors is a very complex process, involving a great volume of design and experimental work. The application of computational fluid dynamics (CFD) methods allows to decrease the volume of experimental works on operational development of combustors and to make changes to the design of combustors on early stages. This paper describes development and validation of CFD-based analysis methodology, used to predict NOx emission level for different types of gas turbine combustors. This methodology includes comprehensive modeling of physical and chemical processes that take place in gas turbine combustors: turbulent flow of reacting gases, heat transfer, chemical kinetics and formation of nitric oxide. To simulate these processes the following mathematical models were used and validated: • Navier-Stockes equations; k-ε RNG, k-ε RSM, k-ω SST turbulence models; • Flamelet and Flamefront combustion models; • Different chemical kinetics mechanisms, describing methane and aviation kerosene oxidation processes; • Diffusion radiation model and discrete ordinates method to calculate radiation heat fluxes; • Extended n-heptane oxidation mechanism to simulate PAH and soot formation; • Prompt and thermal NO formation mechanisms; • Wide band exponential model for gases and empirical correlation for soot to calculate radiation properties of medium. Different factors that affect NOx formation process are considered. They include O and OH prediction methods, influence of radiation heat transfer, and choice of combustion and turbulence models. Developed methodology was used to simulate combustion process in gas turbine combustors that use RQL, LPP, wet NO technologies of low NOx combustion. Merits, demerits and peculiarities of considered low NOx combustion technologies are discussed. According to the results of the analysis, the most efficient technology for NOx reduction was selected.

Commentary by Dr. Valentin Fuster
2006;():467-476. doi:10.1115/GT2006-90536.

We study the role of jet-induced wake and vortical structures on the transport of drops formed by mass stripping and jet breakup due to crossflowing gas. Gas and liquid phases are computed as a single-fluid Eulerian continuum by tracking the gas-liquid interface with a Volume of Fluid (VOF) methodology. Droplets (numerically treated as parcels) are injected at fixed locations near the jet, based on near-field experimental observations. Their trajectories are evolved in time so that patterns of volumetric flux, axial velocity and Sauter Mean Diameter (SMD) can be compared with experimental measurements in the far field. We find that vortical structures due to aerodynamic blockage are maintained up to several orifice diameters downstream of the jet, affecting the droplet trajectories and ultimately the spray distribution. We also demonstrate that an interface-tracking algorithm for capturing the jet bulk features allows the prediction of far field spray characteristics when coupled to a simple set of rules for spray distribution after primary breakup.

Topics: Sprays
Commentary by Dr. Valentin Fuster
2006;():477-486. doi:10.1115/GT2006-90540.

In order to assess the stability of gas turbine combustors measured flame transfer functions are frequently used in thermoacoustic network models. Although many combustion systems operate at high pressure, the measurement of flame transfer functions was essentially limited to atmospheric conditions in the past. With the test rig employed in the study presented in the paper transfer function measurements were made for a wide range of combustor pressures. The results show similarities of the amplitude response in the entire pressure range investigated. However, the increase of the pressure leads to a considerable amplitude gain at higher frequencies. In the low frequency regime the phase is also independent of pressure, whereas above this region the pressure increase results in a considerably smaller phase lag. These observations are particularly important when evaluating Rayleigh’s criterion: Interestingly, the choice of the operating pressure can render a system stable or unstable, so that the common procedure of applying flame transfer functions measured at ambient pressure for the high pressure engine case may not always be appropriate. The detailed analysis of high speed camera images, which were recorded to get locally resolved information on the flame response reveal different regions of activity within the flame that change in strength, size and location with changing operating conditions. The observed transfer function phase behavior is explained by the interaction of those regions and it is shown that the region of highest dynamic activity dominates the phase.

Commentary by Dr. Valentin Fuster
2006;():487-494. doi:10.1115/GT2006-90565.

In this paper combustion of propane under gas turbine conditions is investigated with a focus on the chemistry and chemical kinetics in turbulent flames. The work is aimed at efficient and accurate modeling of the chemistry of heavy hydrocarbons, ie. hydrocarbons with more than one carbon atom, as occurring in liquid fuels for gas turbine application. On the basis of one dimensional laminar flame simulations with detailed chemistry, weight factors are determined for optimal projection of species concentrations on one or several composed concentrations, using the Computational Singular Perturbation (CSP) method. This way the species concentration space of the detailed mechanism is projected on a one dimensional space spanned by the reaction progress variable for use in a turbulent simulation. In the projection process a thermochemical database is used to relate with the detailed chemistry of the laminar flame simulations. Transport equations are formulated in a RaNS code for the mean and variance of the reaction progress variable. The turbulent chemical reaction source term is calculated by presumed shape probability density function averaging of the laminar source term in the thermochemical database. The combined model is demonstrated and validated in a simulation of a turbulent premixed prevaporized swirling propane/air flame at atmospheric pressure. Experimental data are available for the temperature field, the velocity field and the unburnt hydrocarbon concentrations. The trends produced by CFI compare reasonable to the experiments.

Commentary by Dr. Valentin Fuster
2006;():495-501. doi:10.1115/GT2006-90567.

In regular operation all gas turbine combustors have a significant noise level induced by the turbulent high power flame. This noise is characteristic for the operation as it is the result of the interaction between turbulence and combustion. Pressure fluctuations may also be generated by thermoacoustic instabilities induced by amplification by the flame of the acoustic field in the combustor. This paper focuses on prediction of the former process of the noise generation in a premixed natural gas combustor. In order to predict noise generated by turbulent combustion, a model is proposed to calculate the power spectrum of combustion noise in a turbulent premixed natural gas flame on the basis of a steady state RaNS CFD analysis. The instantaneous propagation of acoustic pressure fluctuations is described by the Lighthill wave equation, with the combustion heat release acting as a monopole source term. For a semi infinite tube the solution can be written as a volume integral over the acoustic domain using a Green’s function. The source term is written as a function of a reaction progress variable for combustion. Finite chemical kinetics is taken into account by using the TFC model, and turbulence is described by the k-ε model. Subsequently the volume integral for the noise field is evaluated for the turbulent situation on basis of the calculated steady state combustion solution and presumed shape probability density function weighting. The k- ε model provides the parameters for the presumed spectrum shape. Experiments have been performed in a 100 kW preheated premixed natural gas combustor. Comparison of predicted sound spectra with experimental results shows that the model is capable of prediction of the Sound Pressure Level. The modeled spectrum agrees well with the trends observed in the measured spectra.

Commentary by Dr. Valentin Fuster
2006;():503-512. doi:10.1115/GT2006-90571.

This study outlines the development of a new chemical kinetic surrogate aviation fuel air reaction mechanism which models up to four ring Polycyclic Aromatic Hydrocarbon (PAH) growth. A sensitivity analysis has been conducted to guide us in improving the correlation with modelled and measured species’ profiles in an n-decane – air combustion environment. It was reassuring that the mechanism could be successfully applied to an out of sample set of experimental profiles for acetylene combustion and showed a noticeable improvement over a previous reaction model. In order to calculate the soot volume fraction, a previously developed soot model was employed that accounts for soot particle coagulation, aggregation and surface growth. The impact of pressure, equivalence ratio and residence time on soot formation for a surrogate aviation fuel-air combustion in a Perfectly Stirred Reactor was also investigated. Generally speaking, the level of soot increased with increasing pressure, residence time and equivalence ratio.

Commentary by Dr. Valentin Fuster
2006;():513-521. doi:10.1115/GT2006-90573.

The reduced kinetic mechanism for syngas/methane developed in the present work consists of a global reaction step for fuel decomposition in which the fuel molecule breaks down into CH2 O and H2 . A detailed CH2 O/H2 /O2 elementary reaction sub-set is included as the formation of intermediate combustion radicals such as OH, H, O, HO2 , and H2 O2 is essential for accurate predictions of non-equilibrium phenomena such as ignition and extinction. Since the chemical kinetics of H2 and CH2 O are the fundamental building blocks of any hydrocarbon oxidation, the inclusion of detailed kinetic mechanisms for CH2 O and H2 oxidation enables the reduced mechanism to predict over a wide range of operating conditions provided the reaction rate parameters of fuel-decomposition reaction is optimized over those conditions. Therefore, the rate coefficients for the fuel-decomposition step are estimated and optimized for the ignition delay time measurements of CH4 , H2 , CH4 /H2 , CH4 /CO and CO/H2 mixtures available in the literature over a wide range of pressures, temperatures and equivalence ratios that are relevant to gas turbine operating conditions. The optimized reduced mechanism, consisting of 15 species and around 40 reactions, is able to predict the ignition delay time and laminar flame speed measurements of CH4 , H2 , CH4 /H2 , CH4 /CO and CO/H2 mixtures fairly well over a wide range conditions. The model predictions are also compared with that of GRI3.0 mechanism. The reduced kinetic mechanism predicts the ignition delay time of CH4 and CH4 /H2 mixtures far better than GRI mechanism at higher pressures. To demonstrate the predictive capability of the model in reactive flow systems, the reduced mechanism was implemented in Star-CD/KINetics commercial code using a RANS turbulence model to simulate CH4 /air premixed combustion in a backward facing step. The CFD model predictions of the stable species in the exhaust gas agree well with the GRI mechanism predictions in a chemical reactor network modeling by approximating the backward facing step with a series of perfectly-stirred reactor and plug-flow reactor.

Commentary by Dr. Valentin Fuster
2006;():523-532. doi:10.1115/GT2006-90600.

A comparative experimental investigation has been performed, comparing the emissions from a synthetic jet fuel and from Jet A1. In the investigation, the unburned hydrocarbons were analyzed chemically and the regulated emissions of NOx , CO and HC were measured. All combustion tests were performed under elevated pressures in a gas turbine combustor rig. A Swedish company, Oroboros AB, has developed a novel clean synthetic jet fuel, LeanJet®. The fuel is produced synthetically from synthesis gas by a Fischer-Tropsch process. Except for the density, the fuel conforms to the Standard Specification for Aviation Turbine Fuels. The low density is due to the lack of aromatics and polyaromatics. Organic emissions from the gas turbine combustor rig were collected by adsorption sampling and analyzed chemically. Both the fuels and the organic emissions were analyzed by gas chromatography/flame ionization (GC/FID) complemented with gas chromatography/mass spectrometry (GC/MS). Under the operating conditions investigated, no significant differences were found for the regulated emissions, except for emission of CO from the synthetic fuel, which, at leaner conditions, was one-quarter of that measured for Jet A1. Detailed analysis of the organic compounds showed that the emissions from both fuels were dominated by fuel alkanes and a significant amount of naphthalene. It was also found that Jet A1 produced a much higher amount of benzene than the synthetic fuel.

Commentary by Dr. Valentin Fuster
2006;():533-540. doi:10.1115/GT2006-90635.

Performance tests of a gas turbine combustor are usually conducted at atmospheric or medium pressure which is quite different from its real operating condition. The effects of pressure on the performance of a gas turbine combustor for burning medium-heating-value syngas are researched by numerical simulation in this paper. The geometry of the combustor is modeled by coupling all its components including nozzle, combustor liner and sealant tube. In the simulation a laminar flamelet model and P-1 radiation model are adopted. The numerical results show that at the same fuel and air inlet temperature and the same equivalence ratio, the operation pressure has less effect on the flow fields, but its effect on the temperature distribution is obvious. Both the highest temperature in the combustor and the outlet temperature increase with increasing operating pressure because of the weakening of the dissociation of the H2 O, CO2 and so on. Moreover, as pressure increases, the concentration of H2 O and CO2 in the combustor increase, and so to does the absorption coefficient and the emissivity of gas inside the liner. As a result, the radiation heat transfer between the gas and the combustion liner wall is enhanced, and the wall temperature of the liner increases. The NOx emissions of the combustor are also distinctly higher at high pressure than at low pressure.

Commentary by Dr. Valentin Fuster
2006;():541-547. doi:10.1115/GT2006-90705.

Measurements of emitted radiation from the gases within the reaction zone of a combustor were performed. The combustor was a 30% flat (rectangular) model of an annular combustion chamber of a turbojet-engine. Nonpremixed, turbulent combustion was fueled by kerosene. The equivalence ratios were within the range of 0.15–0.75. The combustor had two quartz windows permitting optical observation of the combustion process. In an earlier work, the infrared emission from the efflux gases, the combustion products, just outside of the combustor exit plane, was investigated using an infrared camera, equipped with an interference bandpass filter. In the present study, infrared images of the combustion inside the chamber were obtained. The location of the high temperature recirculation zones can be identified in the infrared images obtained by the camera. In the visible spectral range, the emission of CH* radicals and C2 * molecules from within the combustion chamber was investigated through the quartz window. These species exist within the reaction zones and play an important role in the combustion mechanism. Their excitation is mainly due to the chemical reactions and so they can serve for diagnosis of combustion processes in reaction zones. The emission from the combustor, in the visible range, was recorded with the aid of a fiber-optic based spectrometer. Local measurements of the emissions of the Swan bands of C2 * molecules at 471 nm, 513 nm, 560 nm, vibronic band of CH* radicals at 431 nm and continuum emission of carbonaceous products of pyrolysis were recorded along the combustor centerline. The intensity is correlated with location of the combustion zones. The distribution of the emission was observed as being dependent on the global equivalence ratio.

Commentary by Dr. Valentin Fuster
2006;():549-556. doi:10.1115/GT2006-90711.

The performances of two types of miniature air-assist atomizers were investigated; one with air being directed to the liquid spray through radial-tangential air channels and the other with air supplied through a small axial swirler. The study has shown that droplet size is reduced significantly when the air velocity increases up to about 50 m/s. However, further increase in air velocity has only a weak effect on the droplet size. In the absence of air supply, elevating the liquid pressure causes a reduction in the droplet diameter. The maximum values of the droplet mass flux shifts to the spray periphery with increasing of air velocity. In the air-assist operational regime, the liquid pressure has a slight effect on SMD however; the spray cone angle is increased significantly and can achieve values of up to 120 degrees for low liquid pressure drop. The larger spray angle at comparable droplet size distribution makes the atomizers with the radial air swirlers more favorable for small jet engines.

Commentary by Dr. Valentin Fuster
2006;():557-564. doi:10.1115/GT2006-90725.

The ever increasing strain on traditional centralized power generation and distribution systems has led to an increase in the use of distributed generation (DG) technologies. DG technologies are commonly found in urban areas that are sensitive to criteria pollutants, and as a result, they are subject to increasingly stringent emission regulations. Paralleling the growth of installed DG is the ever-increasing interest in hydrogen as an alternative fuel to natural gas. As a hydrogen infrastructure is developed, a desire to use this new fuel for DG applications will evolve. Microturbine generators (MTGs) are one example of DG technology that has emerged in this paradigm and are the technology of interest in the present work. To evaluate the potential role for hydrogen fired MTGs in this paradigm, understanding of what emission levels can be expected from such a system is needed The current study retrofits a natural gas fired MTG for operation on hydrogen and characterizes the resulting operability and emissions performance. The results of implementing design changes to improve emissions performance while maintaining stability and safety of the MTG when operating on hydrogen fuel are presented. The results also show improved stability limits which are utilized to help attain lower emissions of NOx. Further optimization is needed to achieve the NOx levels necessary to meet current regulations.

Commentary by Dr. Valentin Fuster
2006;():565-575. doi:10.1115/GT2006-90727.

The deployment of small gas turbines at landfills and wastewater treatment plants is attractive due to the availability of waste fuel gases generated at these sites and the need for onsite power and/or heat. The fuel gases produced by these applications typically contain 35 to 75% of the heating value of natural gas and contain methane (CH4 ) diluted primarily with carbon dioxide (CO2 ) and sometimes nitrogen (N2 ). Demonstrations of 30 to 250 kW gas turbines operating on these waste fuels are underway, but little detailed information on the systematic effect of the gas composition on performance is available. Growth in the use of small gas turbines for these applications will likely require that they meet increasingly stringent emission regulations, creating a need to better understand and to further optimize emissions performance for these gases. The current study characterizes a modified commercial natural gas fired 60 kW gas turbine operated on simluated gases of specified composition and establishes a quantitative relationship between fuel composition, engine load, and emissions performance. The results can be used to determine the expected impact of gas composition on emissions performance.

Commentary by Dr. Valentin Fuster
2006;():577-588. doi:10.1115/GT2006-90730.

The emissions of liquid-fuel fired gas turbine engines are strongly affected by the fuel preparation process that includes atomization, evaporation and mixing. In the present paper, the effects of fuel atomization and evaporation on emissions from an industrial gas turbine engine were investigated. In the engine studied, the fuel injector consists of a co-axial plain jet airblast atomizer and a premixer, which consists of a cylindrical tube with four mixing holes and swirler slits. The goal of this device is to establish a fully vaporized, homogeneous fuel/air mixture for introduction into the combustion chamber and the reaction zone. In the present study, experiments were conducted at atmospheric pressure and room temperature as well as at actual engine conditions (0.34MPa, 740K) both with and without the premixer. Measurements included visualization, droplet size and velocity. By conducting tests with and without the premixing section, the effect of the mixing holes and swirler slit design on atomization and evaporation was isolated. The results were also compared with engine data and the relationship between premixer performance and emissions was evaluated. By comparing the results of tests over a range of pressures, the viability of two scaling methods was evaluated with the conclusion that spray angle correlates with fuel to atomizing air momentum ratio. For the injector studied, however, the conditions resulting in superior atomization and vaporization did not translate into superior emissions performance. This suggests that, while atomization and the evaporation of the fuel are important in the fuel preparation process, they are of secondary importance to the fuel/air mixing prior to, and in the early stages of the reaction, in governing emissions.

Commentary by Dr. Valentin Fuster
2006;():589-599. doi:10.1115/GT2006-90764.

In this paper; the non-reacting flow in a swirl burner is studied using Large Eddy Simulation. The configuration consists of two unconfined co-annular jets at a Reynolds number of 81500. The flow is characterized by a Swirl number of 0.93. Two cases are studied in the paper differing with respect to the axial location of the inner pilot jet. It was observed in a companion experiment (Bender and Büchner, 2005) that when the inner jet is retracted the flow oscillations are considerably amplified. This is also found in the present simulations. Large-scale coherent structures rotating at a constant rate are observed when the inner jet is retracted. The rotation of the structures leads to vigorous oscillations in the velocity and pressure time signals recorded at selected points in the flow. In addition, the mean velocities, the turbulent fluctuations and the frequency of the oscillations are in good agreement with the experiments. A conditional averaging procedure is used to perform a detailed analysis of the physics leading to the low-frequency oscillations.

Commentary by Dr. Valentin Fuster
2006;():601-615. doi:10.1115/GT2006-90770.

This paper addresses the impact of fuel composition on the operability of lean premixed gas turbine combustors. This is an issue of current importance due to variability in the composition of natural gas fuel supplies and interest in the use of syngas fuels. Of particular concern is the effect of fuel composition on combustor blowout, flashback, dynamic stability, and autoignition. This paper reviews available results and current understanding of the effects of fuel composition on the operability of lean premixed combustors. It summarizes the underlying processes that must be considered when evaluating how a given combustor’s operability will be affected as fuel composition is varied.

Commentary by Dr. Valentin Fuster
2006;():617-627. doi:10.1115/GT2006-90772.

The objective of this work is to develop a liquid fuel injector-mixer to provide a uniform mixture of vaporized fuel, steam and air to a fuel cell reformer. This effort supports the NASA fuel cell program, which has the goal of cleaner aerospace power plants. The demonstration project is sized for a 10 kW fuel cell. The Swirl Venturi Mixer (SVM) is the fuel injector-mixer concept explored in the present study. The SVM consists of: a capillary tube to inject the fuel; a venturi tube to maximize the effective air-assist atomization of fuel injected at the throat; and a controlled expansion of radial mixing at the diffuser portion of the venturi. A swirler upstream of the venturi tube enhances turbulent mixing and improves the diffuser performance by suppressing flow separation. Variables evaluated are: swirl angle, throat diameter, throat length and diffuser length. The test section has a 76 mm diameter and includes a quartz cylinder to allow laser-based flow measurements downstream of the injector. Raman spectroscopy is used to measure chemical species distribution across the flow field while particle image velocimetry (PIV) is used to determine the field velocity profile. Test conditions consisted of an inlet air temperature of 700K, inlet steam temperature of 480K, atmospheric pressure, Jet A fuel, and mixture velocities of 1 to 3 m/s.

Commentary by Dr. Valentin Fuster
2006;():629-637. doi:10.1115/GT2006-90874.

The drive to low emissions from GT combustors has pushed manufacturers towards leaner combustion systems. Lean combustion systems are susceptible to thermo acoustic or combustion instabilities, which can significantly limit the operation of the GT in terms of performance and emissions. Combustion instability is the result of coupling between fluctuations in the heat release rate and pressure waves. The occurrence of instability dependent on (a) satisfying the Rayleigh criterion and (b) the growth must exceed the losses of acoustic energy. The growth of instability can be controlled by increasing the level of acoustic damping via a Helmholtz resonator and through viscous damping. Design rules for a passive damper have been developed through the EU funded project called PRECCINSTA (Prediction and control of combustion instabilities in tubular and annular combustion systems) by the University of Cambridge. These design rules are for a doubled-skinned perforated liner where a biasing flow is used to dissipated sound energy. The sound dissipation mechanism is via vortex formation. These design rules were then validated against atmospheric and intermediate pressure combustion tests at Rolls-Royce for self-excited and forced excited oscillations. This paper summaries these tests and gives the results for a simple perforated liner as a passive acoustic damper.

Topics: Combustion , Dampers
Commentary by Dr. Valentin Fuster
2006;():639-647. doi:10.1115/GT2006-90875.

A technical gas turbine combustor has been studied in detail with optical diagnostics for validation of Large-Eddy Simulations (LES). OH* chemiluminescence, OH laser-induced fluorescence (LIF) and particle image velocimetry (PIV) have been applied to stable and pulsating flames up to 8 bar. The combination of all results yielded a good insight into the combustion process with this type of burner and forms a data base which was used for the validation of complex numerical combustion simulations. Large-Eddy Simulations (LES) including radiation, convective cooling and air cooling were combined with a reduced chemical scheme that predicts NOx emissions. Good agreement of the calculated flame position and shape with experimental data was found.

Commentary by Dr. Valentin Fuster
2006;():649-662. doi:10.1115/GT2006-90878.

The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultra-low emissions (< 5 ppm NOx and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling or costly materials and catalysis. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO2 . Using Particle Image Velocimetry (PIV), the non-reacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed have been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enable the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.

Commentary by Dr. Valentin Fuster
2006;():663-671. doi:10.1115/GT2006-90891.

In this paper, flow fields inside a premixed combustor have been investigated by CFD analysis and PIV measurement in a preheating, non-reacting condition. Four types of premixer are examined. The design of the premixer is determined by the combination of swirlers and mixing tubes. There are two variations of triple-concentric swirlers and three variations of mixing tubes. Comparisons are made among mean velocity distributions derived from CFD and PIV. PDF analysis is performed on the data from PIV to discuss the possibility of the occurrence of flashback. Combustion rig tests have been carried out also on similar condition to see combustion instabilities depending on the choice of premixers and operating conditions. Flame is directly observed from crystal windows placed on the side and downstream of the combustion chamber. A glass rod is installed on the wall of the mixing tube so as to see light emissions inside the tube, i.e. evidence of flashback. Pressure fluctuations at the combustor liner are measured in one position. The spectra of pressure fluctuations are computed to look at the possibility of combustion oscillations. Discussions are made on the relation between the global flame structure and pressure modes. Finally, proper premixer configurations to prevent combustion instabilities are proposed.

Commentary by Dr. Valentin Fuster
2006;():673-682. doi:10.1115/GT2006-90895.

This paper presents two different active combustion control systems (ACCS) for the reduction of NOx levels, and suppression of thermo-acoustic instabilities within stationary gas turbines. Dependent on the actual measurement the ACCSs steer the fuel split between different burner groups or within the burner itself in order to find an optimum operation point. In a first step, an active control system has been developed for the ALSTOM GT13E2 gas turbine where the fuel ratio between two burner groups has to be handled to optimise NOx and pulsation levels. Since perturbations in combustion operating conditions have a direct effect on combustion pulsation behaviour, it is possible to control combustor equivalence ratio by using combustion pulsation measurements as an input for the control system. By doing that, lower operating NOx emissions are achieved as operating safety margin to lean blow out (LBO) may be reduced by more than 50% due to a more accurate and controlled handling of combustor equivalence ratio. Although combustion instabilities due to lean blow out are of minor concern within the GT26, the success of this combustion control approach has led to the development of a more advanced method where both, NOx levels and pulsation amplitudes are feedback-controlled simultaneously in order to track the optimum operating point. By using two premixed stages in the burners fuel supply, the equivalence ratio within the combustor is adapted. Engine and single burner test results confirmed control model dynamics predictions. This paper illustrates the applied closed-loop controls concepts and the successful controller verification on single burner and on engine level.

Commentary by Dr. Valentin Fuster
2006;():683-690. doi:10.1115/GT2006-90910.

The thermo-acoustic behaviour of non-premixed turbulent syngas flames is investigated by means of transient RaNS Computational Fluid Dynamics simulations. Three cases with two different fuel compositions are considered. Both fuels are combusted in a turbulent non-premixed swirl stabilised mode, and are mixtures of hydrogen, carbon monoxide and nitrogen. One fuel contains methane in addition. The flame transfer function considered here, describes the relation between a perturbation of the fuel mass flow rate and the rate of heat release in the flame. The fuel mass flow is perturbed by an impulse excitation. The investigated geometry is a laboratory scale burner that is designed in the framework of the European Union sponsored HEGSA project. Experimental data are generated in tests at DLR (chemiluminescence and LIF). The CFD results show that methane addition to syngas has a significant influence on the flame transfer function. The addition of methane to syngas induces thermoacoustic damping for higher frequency (¿ 400 Hz) regions and increases amplification for low frequencies (¡ 200 Hz). The time delay of the transfer function is affected by the addition of methane due to both calorific value and chemical time scale effects. A decrease in inlet temperature also affects the flame transfer function. This is due to the slower chemistry and lower velocity.

Commentary by Dr. Valentin Fuster
2006;():691-700. doi:10.1115/GT2006-90916.

A typical stationary premixed turbulent flame is the developing flame, as indicated by the growth of mean flame thickness with distance from flame-stabilization point. The goal of this work is to assess the importance of modeling flame development for RANS simulations of confined stationary premixed turbulent flames. For this purpose, submodels for developing turbulent diffusivity and developing turbulent burning velocity, which were early suggested by our group (FSC model) and validated for expanding spherical flames [4], have been incorporated into the so-called Zimont model of premixed turbulent combustion and have been implemented into the CFD package Fluent 6.2. The code has been run to simulate a stationary premixed turbulent flame stabilized behind a triangular bluff body in a rectangular channel using both the original and extended models. Results of these simulations show that the mean temperature and velocity fields in the flame are markedly affected by the development of turbulent diffusivity and burning velocity.

Commentary by Dr. Valentin Fuster
2006;():701-707. doi:10.1115/GT2006-90943.

The NOx emissions of low NOx premix combustors are not only determined by the burner design, but also by the multi burner interaction and the related distribution of air and fuel flows to the individual burners. Often the factors that have a positive impact on NOx emission have a negative impact on the flame stability, so the main challenge is to find an optimum point with the lowest achievable NOx while maintaining good flame stability. The hottest flame zones are where most of the NOx is formed. Avoiding such zones in the combustor (by homogenization of the flame temperature) reduces NOx emissions significantly. Improving the flame stability and the combustion control allows the combustor to operate at a lower average flame temperature and NOx emissions. ALSTOM developed a combustion optimization package for the GT13E2. The optimization package development focused on three major issues: • Flame stability; • Homogenization of flame temperature distribution in the combustor; • Combustion control logic. The solution introduced consists of: • The reduction of cooling air entrainment in the primary flame zone for improved flame stability; • The optical measurement of the individual burner flame temperatures and their homogenization by burner tuning valves; • Closed loop control logic to control the combustion dependent on the pulsation signal. This paper shows how fundamental combustion research methods were applied to derive effective optimization measures. The flame temperature measurement technique will be presented along with results of the measurement and their application in homogenization of the combustor temperature distribution in an engine equipped with measures to improve flame stabilization. The main results achieved are: • Widening of the main burner group operation range; • Improved use of the low NOx operation range; • NOx reduction at the combustor pulsation limit and hence, large margins to the European emission limit (50 mg/m3 @ 15%O2).

Commentary by Dr. Valentin Fuster
2006;():709-714. doi:10.1115/GT2006-90944.

The homogeneity of the fuel/air mix entering the combustion chamber of a gas turbine is known to be a factor in both the emissions performance (with poor mixing resulting in local hotspots and the formation of thermal NOx ) and the generation of acoustic vibrations (humming). Obviously it is desirable to reduce both pollutants and unwanted acoustics as far as possible. The aim of this paper is to study the relationship between the local inlet conditions and the mixing of the fuel and air, specifically looking at the effects of fuel gas preheating and inlet air temperature on mixedness at the combustor inlet. A CFD model of the lean pre-mixed combustor for a Siemens v94.3A gas turbine was used to analyse the problem. The 3-dimensional model employs a structured mesh scheme and uses the symmetry of the burner to reduce computational effort. The model was solved using a 2nd order discretisation of the momentum and continuity equations along with the RNG k-ε turbulence model to provide closure. The boundary conditions for the model were taken from data obtained from in service measurements. Several runs were made using air inlet temperatures varying from −10°C to 30°C and gas inlet temperatures from 10°C to 450°C. The data obtained from the CFD simulations was processed to give an indication of the quality of the fuel/air mixing for each set of inlet conditions. This was then used to create a tool which can be used to determine the amount of gas pre-heat required to achieve the best possible mixing for a given set of ambient conditions. An estimation of the NOx produced at different conditions was derived from the mixing data. Analysis of the results showed that increasing the gas preheat produces an improvement in the mixing of the fuel and air in the burner. This improvement in mixing also resulted in a reduction in the estimated amount of NOx produced.

Commentary by Dr. Valentin Fuster
2006;():715-720. doi:10.1115/GT2006-90945.

The prediction of high-frequency acoustic oscillations in gas turbine combustors is an important issue, related to engine performance, NOx emissions, component lifetime and engine operational flexibility. Different methods with increasing complexity and predictive ability have been discussed in a number of papers. Application of these methods requires large computational capacity and long computational times. Therefore, a limited number of variants of small combustor models or small sectors can be analyzed in a reasonable time. This paper presents an approximate approach, applicable under certain specific conditions. It is based on an understanding that the acoustic pressure oscillations are tied to the oscillation in heat release rate. The interaction is taking place in the heat release zone, independent of the type of the feedback mechanism. For a typical gas turbine combustion chamber, many acoustic modes exist in the frequency range of interest. However, only a few of these modes are excited by the combustion process and thus are relevant. The mode excitation depends both on combustion noise (due to flame excitation contribution independent of the acoustic field) and combustion instability (acoustic mode made unstable by the flame transfer function). With a flame surface obtained from steady state CFD simulation, and with acoustic mode shapes obtained from a Finite Element package, the forced acoustic response of the combustion system to the flame excitation was calculated. In a first validation step, this method has been tested on a single burner atmospheric test facility. In a second step, the method will be applied to an annular SEV combustion chamber of a GT26 ALSTOM gas turbine. The strength of this approach is that large models can be analyzed quickly to show the influence of changes in a flame position and effect of the combustor geometry. The weakness is that combustion instabilities can not be addressed by such a method. Furthermore, the phase relation of the excitation between different parts of the flame is frequency dependant and needs to be given as an input, which requires an experience and expert knowledge.

Commentary by Dr. Valentin Fuster
2006;():721-729. doi:10.1115/GT2006-90950.

Gas turbine combustor design entails multiple, and often contradictory, requirements for the designer to consider. Multiobjective optimisation on a low-fidelity linear-network-based code is suggested as a way of investigating the design space. The ability of the Tabu Search optimiser to minimise NOx and CO, as well as several acoustic objective functions, is investigated, and the resulting “good” design vectors presented. An analysis of the importance of the flame transfer function in the model is also given. The mass flow and the combustion chamber width and area are shown to be very important. The length of the plenum and the widths of the plenum exit and combustor exit also influence the design space.

Commentary by Dr. Valentin Fuster
2006;():731-742. doi:10.1115/GT2006-90956.

The principal requirements of industry, with respect to the numerical simulation of gas turbine combustors, are computational efficiency and algorithm robustness, together with an accurate representation of the complex fundamental processes. This paper examines the performance of the premixed combustion models implemented in the commercial CFD package Fluent™, in order to validate the ability to model combustion in the context of a premixed gas turbine combustor. The predictions of the model are found to compare well with the experimental results available, demonstrating robustness and computational efficiency.

Commentary by Dr. Valentin Fuster
2006;():743-752. doi:10.1115/GT2006-90958.

In order to predict with CFD codes the ignition of kerosene including pollution formation it is necessary to develop a reduced mechanism, which can be incorporated into the code and applied without dramatically increasing the turn-around time. The practical global chemical schemes, which reflect the combustion properties for large a range of parameters in the combustion chamber, can be developed on the base of a skeletal mechanism. These mechanisms are established applying analytical reduction methods to detailed mechanisms. The first reduction of n-heptane detailed mechanism, which is a part of a reference model for practical fuels, was performed to model the heat release of kerosene for equivalent ratios of 0.5–2, initial temperatures of 700–1400 K and initial pressures of 6.5–55.0 bar. The final skeletal mechanism consists of 47 species and 378 irreversible reactions and predicts reasonably well the experimental data of ignition delay times for the given combustion parameters.

Commentary by Dr. Valentin Fuster
2006;():753-762. doi:10.1115/GT2006-90974.

An experimental study of a single, swirl cup burner is carried out to improve understanding of the lean reacting flow field near idle conditions for an annular spray combustor. The counter-swirler is mounted horizontally in a trapezoidal cross-section combustor with quartz plate walls. Liquid fuel, Jet-A, is initially atomized using a simplex nozzle, and then a designed re-atomization occurs from the swirler hardware. Measurements of non-reacting and reacting gas phase velocities enable the direct comparison of critical flow features at various power settings. Droplet diameter and exhaust composition measurements confirm that the initial droplet size is a key factor in emission levels. Smaller droplets in the spray periphery tend to evaporate and burn premixed, while larger droplets in the spray core convect downstream and burn with a sheath-type, non-premixed flame. The presence of small fuel droplets in the spray may ensure more complete combustion and improve combustor stability at lean, low power settings.

Commentary by Dr. Valentin Fuster
2006;():763-771. doi:10.1115/GT2006-90986.

Catalytic combustion has proven to be a suitable alternative to conventional flame combustion in gas turbines for achieving Ultra-Low Emission levels (ULE). In the process of catalytic combustion, it is possible to achieve a stable combustion of lean fuel/air mixtures which results in reduced combustion temperature in the combustor. The ultimate result is that almost no thermal-NOx is formed and the emissions of carbon monoxide and hydrocarbon emissions are reduced to single-digit limits. Successful development of catalytic combustion technology would lead to reducing pollutant emissions in gas turbines to ultra-low levels at lower operating costs. Since the catalytic combustion prevents the pollutant formations in the combustion there is no need for costly emission cleaning systems. High-quality experimental data of combustion catalyst operations at gas turbine working conditions and validated numerical models are essential tools for the design and development of catalytic gas turbine combustors. The prime objective of the work presented in this paper was to obtain catalytic operational data under said conditions. Experimental investigations were carried out to determine the operational data on different types of combustion catalysts against different fuel types at gas turbine operational conditions. A pilot-scale 100 k W high-pressure combustion test facility was used for the experimental investigations of catalytic combustion under real gas turbine conditions. Combustor pressure can be maintained at any desired level between 1 to 35 bars. The maximum combustion air supply is 100 g/s, which can be electrically preheated up to 600°C and humidified up to 30% of weight as required by test conditions. Catalysts used in the test facility are highly active noble metal catalysts for ignition purposes and thermally stable metal oxide catalysts for continuing reactions. Tests are conducted as the testing of single catalyst segments or combinations of several segments. The measurements taken are flow rates (air/fuel ratio) temperatures (inlet, surface and the outlet of each catalyst segment), pressure (combustor) and emissions of NOx, CO and UHC. This paper presents the design of the high-pressure catalytic combustion test facility and an experimental comparison of methane combustion over Pd on alumina and Pd/Pt (bi-metal) on alumina catalysts at varying pressure levels up to 20 bars. The catalysts concerned were cylindrical shaped (35 mm in diameter and 20 mm in height) honeycomb type fully coated catalysts. The results showed that the Pt/Pd on alumina catalysts is better in low temperature ignition and combustion stability over the Pd on alumina catalysts. Emission measurements showed that the fuel conversion over the tested Pt/Pd on alumina catalyst was around 10% while fuel conversion over a similar Pd on alumina catalyst (geometry and capacity) was only 4%. Fuel conversion rates showed the tendency to be further reduced (over the same catalysts) against increasing pressure.

Commentary by Dr. Valentin Fuster
2006;():773-780. doi:10.1115/GT2006-90988.

Low heating value of gasified biomass and its fuel bound nitrogen containing compounds challenge the efforts on utilizing gasified biomass on gas turbine combustor. Low heating value of the gas brings along combustion stability issues and pollutant emission concerns. The fuel bound nitrogen present in gasified biomass could completely be converted to NOx during the combustion process. Catalytic combustion technology, showing promising developments on ultra low emission gas turbine combustion of natural gas could also be the key to successful utilization of biomass in gas turbine combustor. Catalysts could stabilize the combustion process of low heating value gas while the proper design of the catalytic configuration could selectively convert the fuel bound nitrogen into molecular nitrogen. This paper presents preliminary results of the experimental investigations on combustion stability and nitrogen selectivity in selective catalytic oxidation of ammonia in catalytic combustion followed by a brief description of the design of catalytic combustion test facility. The fuel-NOx reduction strategy considered in this study was to preprocess fuel in the catalytic system to remove fuel bound nitrogen before real combustion reactions occurs. The catalytic combustion system studied here contained two stage reactor in one unit containing fuel preprocessor (SCO catalyst) and combustion catalysts. Experiments were performed under lean combustion conditions (λ value from 6 up to 22) using a simulated mixture of gasified biomass. The Selective Catalytic Oxidation approach was considered to reduce the conversion of NH3 into N2 . Results showed very good combustion stability, higher combustion efficiency and good ignition performances under the experimental conditions. However, the selective oxidation of fuel bound nitrogen into N2 was only in the range of 20% to 30% under the above conditions.

Commentary by Dr. Valentin Fuster
2006;():781-791. doi:10.1115/GT2006-91004.

Increasingly, more stringent emissions regulations have necessitated new gas turbine combustor designs with low pollutant emissions. Radically different modern designs have been developed to meet these requirements while maintaining high combustion efficiencies and good flame stability. While several published methodologies for conventional combustor design exist, none exist for modern ones. This paper describes the development of a new preliminary design algorithm for a modern lean premixed prevaporized (LPP) combustor. It also introduces a new LPP combustor concept. The approach used is multi-disciplinary in nature, applying empirical and semi-empirical models in the algorithm to capture complex processes such as droplet evaporation, chemical reaction, jet mixing, and heat transfer. The resulting set of procedures allows a designer to quickly define the detailed geometry of the combustor and provides an assessment of its performance. The preliminary design procedures were verified using the advanced numerical techniques of computational fluid dynamics (CFD). Reasonable agreement between predictions from the preliminary design and numerical analysis was achieved which indicated that the design procedures have been developed successfully.

Commentary by Dr. Valentin Fuster
2006;():793-804. doi:10.1115/GT2006-91051.

This paper presents experimental data, performed at atmospheric conditions, on a novel flameless combustor with application to gas turbine engines. Flameless combustion is characterized by distributed flame and even temperature distribution achieved at conditions of high preheat air temperature and sufficiently large amounts of recirculating low oxygen concentration exhaust gases. Extremely low emissions of NOx , CO, and UHC are reported in this paper for flameless combustion in a multiple jets premixed gas turbine combustor. Measurements of the flame chemiluminescence, CO and NOx emissions, acoustic pressure, temperature field, and velocity field reveal the influence of various parameters including: preheat temperature, inlet air mass flow rate, combustor exhaust nozzle contraction ratio, and combustor chamber diameter on emissions and combustion dynamics. The data indicate that greater air mass flow rates, thus larger pressure drop, promotes the formation of flameless combustion and lower NOx emissions for the same flame temperature. This flameless combustor is basically a premixed combustion in which NOx emissions is an exponential function of the flame temperature regardless of different air preheating temperatures. High preheat temperature and flow rates also help in forming stable combustion which is another advantageous feature of flameless combustion. The effects of the combustor exhaust contraction and the combustion chamber diameter on emissions and combustion dynamics are discussed.

Commentary by Dr. Valentin Fuster
2006;():805-814. doi:10.1115/GT2006-91061.

This work is aimed to investigate the fundamental combustion and reignition process in semi-intermittent pressure-gain combustors for gas turbine applications. A combustion-torch ignition method is used to simulate reignition in one tube of a pressure-gain combustor by employing burned gas produced in a pre-chamber combustor. Numerical flow and combustion simulations are performed to understand and guide preliminary experimental results. The computational fluid dynamics code StarCD® is used to predict internal flow and combustion upon attempted ignition by a hot gas jet. This study provides improved understanding of the complex, sub-millisecond processes involved: transient supersonic jet mixing, ignition, highly turbulent flame propagation, and shock-flame interaction in near-wall region. The results are useful for successful design of rotary pressure gain combustors or internal combustion wave rotors under various operating conditions.

Commentary by Dr. Valentin Fuster
2006;():815-822. doi:10.1115/GT2006-91112.

This paper describes an experimental investigation of fuel spray jet breakup mechanisms when it is injected across the high temperature air flow in low and high pressure jet engine augmentor-like conditions. Phase Doppler particle analyzer data and short exposure pulsed shadow graph images were taken of fuel jet injected into an air cross flow with liquid to air momentum ratios ranging from 5 to 180. Measured droplet diameters taken at atmospheric pressure and a flow Mach number of ∼0.15 show a progressive breakup of the droplets, gradually decreasing in size from 250μm to 150μm and finally to 25 μm as the spray moves downstream. The progressive breakup of droplets follows the classical Rayleigh-Helmholtz breakup mechanism. At higher pressure and Mach number tests, the fuel jet undergoes a different breakup process; i.e., the fuel jet breaks up instantaneously into a monodispurse spray of smaller droplets near the injector. High speed images of this process suggest that an aerodynamic breakup mechanism dominates this atomization process near the injector. In summary, the results of this study show the fuel jet breakup mechanism in augmentors varies significantly over the flight envelope.

Commentary by Dr. Valentin Fuster
2006;():823-833. doi:10.1115/GT2006-91119.

An understanding of the amplitude dependence of the flame response to acoustic excitation is required in order to predict and/or correlate combustion instability amplitudes. This paper describes an experimental investigation of the nonlinear response of a lean, premixed flame to imposed acoustic oscillations. Detailed measurements of the amplitude dependence of the flame response were obtained at approximately 100 test points, corresponding to different flow rates and forcing frequencies. It is observed that the nonlinear flame response can exhibit a variety of behaviors, both in the shape of the response curve and the forcing amplitude at which nonlinearity is first observed. The phase between the flow oscillation and heat release is also seen to have substantial amplitude dependence. The nonlinear flame dynamics appear to be governed by different mechanisms in different frequency and flowrate regimes. These mechanisms were investigated using phase-locked, two-dimensional OH PLIF imaging. From these images, two mechanisms, vortex rollup and unsteady flame liftoff, are identified as important in the saturation of the flame’s response to large velocity oscillations. Both mechanisms appear to reduce the flame’s area and thus its response at these high levels of driving.

Commentary by Dr. Valentin Fuster
2006;():835-842. doi:10.1115/GT2006-91156.

A series of experimental researches, including ignition and combustion tests at atmospheric pressure conditions, were conducted to develop a combustor for a small class aircraft engine (with pressure ratio about 20). Under restrictions of the combustor size and cost, in order to satisfy the requirement for ignition and blowout performance with sufficient combustion efficiency and NOx reduction for wide range of operating conditions, we applied single fuel nozzles and utilized the rich-burn-quick quench-lean-burn (RQL) combustion approach. Preliminary combustion tests were conducted to optimize the ignition and blowout characteristics, approximately determining positions of air holes and igniter, and selecting fuel nozzle parameters. Consequently, tubular combustor tests with exhaust gas analysis were also conducted to optimize the air mass flow ratio and the air holes’ positions to suppress NOx emissions. Obtained results showing the RQL characteristics of the combustion, decreasing NOx emissions at high equivalence ratio range, are presented in this report, and the optimized air mass flow ratio and position of air holes, which will be applied to a single sector combustor for the testing at practical pressure and temperature conditions, are also presented.

Commentary by Dr. Valentin Fuster
2006;():843-852. doi:10.1115/GT2006-91157.

Axisymmetric plumes of hydrogen, acetylene or n-heptane were formed by the continuous injection of (pure or nitrogen-diluted) fuel into turbulent co-flows of hot air. Autoignition and subsequent flame propagation was visualized with a high-speed intensified camera. The resulting phenomena include the statistically steady ‘Random Spots’ and the ‘Flashback’ regimes. It was found that with higher velocities and smaller injector diameters, the boundary between Flashback and Random Spots shifted to higher air temperatures. In the Random Spots regime, the autoignition regions moved closer to the injector with increasing air temperature and/or decreasing air velocity. After a localized explosive autoignition event, flames propagated into the unburnt mixture in all directions and eventually extinguished, giving rise to autoignition ‘spots’ of mean radius 2–5mm for hydrogen and 6–10mm for the hydrocarbons. The average flame propagation velocity in both the axial and radial directions varied between 0.5 and 1.2 times the laminar burning speed of the stoichiometric mixture, increasing as the autoigniting regions shifted upstream.

Commentary by Dr. Valentin Fuster
2006;():853-865. doi:10.1115/GT2006-91216.

This paper presents the results of numerical investigations of a turbulent, swirling and recirculating flow without combustion inside a reverse flow gas turbine combustor. In order to establish the characteristics of fuel distribution patterns of the fuel spray injected into swirling flows, flow fields are analyzed inside the swirl combustor for varying amount of swirl strength using a commercial CFD code fluent 6.1.22. Three Dimensional computations are performed to study the influence of the various parameters like injection pressure, flow Reynolds number and Swirl Strength on the fuel distribution patterns. The model predictions are compared against the experimental results, and its applicability over a wide range of flow conditions was investigated. It was observed from the CFD analysis, that the fuel decay along the axis is faster with low injection pressures compared to higher injection pressures. With higher Reynolds numbers the fuel patterns are spreading longer in the axial direction. The higher momentum of the air impedes the radial mixing and increases the constraint on the jet spread. The results reveal that an increase in swirl enhances the mixing rate of the fuel and air and causes recirculation to be more pronounced and to occur away form the fuel injector. The CFD predictions are compared with the experimental data from the phototransistor probe measurements, and good agreement has been achieved.

Commentary by Dr. Valentin Fuster
2006;():867-875. doi:10.1115/GT2006-91217.

The performance of dry, low NOx gas turbines, which employ lean premixed (or partially premixed) combustors, is often limited by static and dynamic combustor stability, power density limitations and expensive premixing hardware. To overcome these issues, a novel design, referred to as the Stagnation Point Reverse Flow (SPRF) combustor, has recently been developed. Various optical diagnostic techniques are employed here to elucidate the combustion processes in this novel combustor. These include simultaneous planar laser-induced fluorescence (PLIF) imaging of OH radicals and chemiluminescence imaging, and separate experiments with particle image velocimetry and elastic laser sheet scattering from liquid particles seeded into the fuel. The SPRF combustor achieves internal exhaust gas recirculation and efficient mixing, which eliminates local peaks in temperature. This results in low NOx emissions, limited by flame zone (prompt) production, for both premixed and non-premixed modes of operation. The flame is anchored in a region of reduced velocity and high turbulent intensities, which promotes mixing of hot products into the reactants, thus enabling stable operation of the combustor even at very lean equivalence ratios. Also, the flame structure and flow characteristics were found to remain invariant at high loadings, i.e., mass flow rates. Combustion in the non-premixed mode of operation is found to be similar to the premixed case, with the OH PLIF measurements indicating that nonpremixed flame burns at an equivalence ratio that is close to the overall combustor equivalence ratio. Similarities in emission levels between premixed and non-premixed modes are thus attributable to efficient fuel-air mixing in the nonpremixed mode, and entrainment of hot products into the reactant stream before burning occurs.

Commentary by Dr. Valentin Fuster
2006;():877-884. doi:10.1115/GT2006-91254.

A novel radical jet generator (RJG) was developed, whose purpose it is to supply concentrated, relatively low temperature radicals that penetrate into a flammable stream of reactants and trigger or modify a combustion process. The RJG is driven by a plasma whose power is only a fraction of a percent of the total power released in the combustor. In this approach, the plasma induces an incomplete combustion process in a small duct carrying a rich mixture of fuel and air. Results obtained using the developed RJG show that a jet, which consists of partially burnt reactants, some products and is, apparently, rich in radicals produced by the incomplete combustion process triggers extremely steady combustion in a fast moving combustible mixture whose flow rate far exceeds that of the RJG. Importantly, the results show that the jet, rich with radicals, that emerges from the RJG cavity at a temperature well below traditional ignition can ignite a fast moving stream of combustible mixture. Moreover, when injected normal to the main flow, this jet ignites the main stream at a location relatively far from the entrance point of the jet. This makes it possible to keep the combustion process away from solid walls while at the same time eliminating the need for solid flame holders. This in turn, provides an augmenter with reduced I.R signature. Finally, the results show a drastic effect of the RJG upon the flame dynamics in general and combustion instabilities in particular. Flames which displayed large, periodic pressure oscillations became completely stable when the plasma in the RJG was turned on. This suggests a novel use of the RJG to inhibit instabilities in combustors.

Commentary by Dr. Valentin Fuster
2006;():885-889. doi:10.1115/GT2006-91285.

Alzeta Corporation has developed surface-stabilized fuel injectors for use in lean-premixed low-emissions combustion systems. These injectors use a patented technique to form interacting high-flow and low-flow flame zones immediately above a selectively-perforated porous metal surface. This allows stable operation at low reaction temperatures conducive for preventing high NOx formation. Solar Turbines and Alzeta had previously worked together to evaluate single-injector and full-scale proof-of-concept test hardware. This paper presents results of a combustion system developed for evaluation on an engine. The next-generation hardware has evolved to include a pilot to handle low engine speeds, and flow circuits have been adjusted to meet low-pressure drop requirements. Screening tests of the full-scale system have been completed at simulated engine conditions in a full-scale rig. Single-digit NOx and CO emissions have been achieved without encountering combustion-driven instabilities. The combustion system demonstrated adequate power turndown with the assistance of the pilot module, and studies to predict the service life of burners have been initiated.

Commentary by Dr. Valentin Fuster
2006;():891-902. doi:10.1115/GT2006-91300.

An unstructured and massively parallel Reynolds-Averaged Navier-Stokes (RANS) code is used to simulate 3-D, turbulent, non-reacting, and confined swirling flow field associated with a single-element and a nine-element Lean Direct Injection (LDI) combustor. In addition, the computed results are compared with the Large Eddy Simulation (LES) results and are also validated against the experimental data. The LDI combustors are a new generation of liquid fuel combustors developed to reduce aircraft NOx emission to 70% below the 1996 International Civil Aviation Organization (ICAO) standards and to maintain carbon monoxide and unburned hydrocarbons at their current low levels at low power conditions. The concern in the stratosphere is that NOx would react with the ozone and deplete the ozone layer. This paper investigates the non-reacting aerodynamics characteristics of the flow associated with these new combustors using a RANS computational method. For the single-element LDI combustor, the experimental model consists of a cylindrical air passage with air swirlers and a converging-diverging venturi section, extending to a confined 50.8-mm square flame tube. The air swirlers have helical, axial vanes with vane angles of 60 degree. The air is highly swirled as it passes through the 60 degree swirlers and enters the flame tube. The nine-element LDI combustor is comprised of 9 elements that are designed to fit within a 76 mm 76 mm flametube combustor. In the experimental work, the jet-A liquid fuel is supplied through a small diameter fuel injector tube and is atomized as it exits the tip and enters the flame tube. The swirling and mixing of the fuel and air induces recirculation zone that anchors the combustion process, which is maintained as long as a flammable mixture of fuel and air is supplied. It should be noted that in the numerical simulation reported in this paper, only the non-reacting flow is considered. The numerical model encompasses the whole experimental flow passage, including the flow development sections for the air swirlers, and the flame tube. A low Reynolds number K-e turbulence model is used to model turbulence. Several RANS calculations are performed to determine the effects of the grid resolution on the flow field. The grid is refined several times until no noticeable change in the computed flow field occurred; the final refined grid is used for the detailed computations. The results presented are for the final refined grid. The final grids are all hexahedron grids containing approximately 861,823 cells for the single-element and 1,567,296 cells for the nine-element configuration. Fine details of the complex flow structure such as helical-ring vortices, re-circulation zones and vortex cores are well captured by the simulation. Consistent with the non-reacting experimental results, the computation model predicts a major re-circulation zone in the central region, immediately downstream of the fuel nozzle, and a second, recirculation zone in the upstream corner of the combustion chamber. Further, the computed results predict the experimental data with reasonable accuracy.

Commentary by Dr. Valentin Fuster
2006;():903-914. doi:10.1115/GT2006-91310.

Large-eddy simulation (LES) of a lean-direct injection (LDI) combustor is reported in this paper. The full combustor and all the six swirl vanes are resolved and both cold and reacting flow simulations are performed. Cold flow predictions with LES indicate the presence of a broad central recirculation zone due to vortex breakdown phenomenon near the dump plane and two corner recirculation zones at the top and bottom corner of the combustor. These predicted features compare well with the experimental non-reacting data. Reacting case simulated a liquid Jet-A fuel spray using a Lagrangian approach. A three-step kinetics model that included CO and NO is used for the chemistry. Comparison of mean velocity field predicted in the reacting LES with experiments shows reasonable agreement. Comparison with the non-reacting case shows that the centerline recirculation bubble is shorter but more intense in the reacting case.

Commentary by Dr. Valentin Fuster
2006;():915-925. doi:10.1115/GT2006-91325.

A technique for design optimization of a combustor is presented in this study. The technique entails the use of Computational Fluid Dynamics (CFD) and mathematical optimization to minimize the combustor exit temperature profile. The empirical and semi-empirical correlations commonly used for optimizing Combustor Exit Temperature profile do not guarantee optimum. As experimental approach is time consuming and costly, use is made of numerical techniques. Using CFD without mathematical optimisation on a trial-and-error basis, however, does not guarantee optimal solutions. A better approach that is viewed as too expensive is a combination of the two approaches, thereby, incorporating the influence of the variables automatically. In this study the combustor exit temperature profile is optimised. The optimum (uniform) combustor exit temperature profile depends on mainly the geometric parameters. The combustor exit temperature profile is affected as soon as flow enters the combustor. However, in gas turbine applications where care has been taken on the influence of upstream flow related conditions, the combustor exit temperature profile is changed by dilution hole pattern and size. In this study dilution hole parameters have been used as optimization variables. The combustor in the study is an experimental liquid fuelled atmospheric combustor with turbulent diffusion flame. The CFD simulations uses the Fluent code with Standard k-ε model. The optimisation is carried out with Snyman’s Dynamic-Q algorithm, which is specifically designed to handle constrained problems where the objective or constraint functions are expensive to evaluate. The optimization leads to a more uniform combustor exit temperature profile as compared to the original.

Commentary by Dr. Valentin Fuster
2006;():927-936. doi:10.1115/GT2006-91338.

This paper describes an investigation of the performance of the recently developed ultra low emissions, Stagnation-Point Reverse-Flow (SPRF ) Combustor when burning liquid fuels (Jet-A and heptane). This study has been undertaken because of the need to burn liquid fuels with low emissions in gas turbines that are used, for example, in aircraft engines, land-based power generation, and marine applications. In contrast with state of the art combustors, in which the reactants and products enter and leave the combustor through opposite ends of the combustor, the reactants and products enter and leave the SPRF combustor through the same plane opposite a closed end. The design of the SPRF combustor allows mixing of reactants with hot combustion products and radicals within the combustor, prior to combustion. Thus, no external premixing of fuel and air is required. Additionally, since the air and fuel enter opposite the closed end of the combustor, they must stagnate near the closed end, thus establishing a region of low velocity just upstream of the closed end that helps stabilize the combustion process. This apparently produces a low temperature, stable, distributed reaction zone. Previous studies with the SPRF combustor investigated its performance while burning natural gas. This paper presents the results of SPRF combustor studies using liquid fuels, both heptane and Jet-A. The performance of the combustor was investigated using an airblast fuel injector, which is suitable for the low fuel flow rates used in laboratory experiments. To reduce pressure losses across the injector, a diffuser was incorporated into the airblast injector. It was found that stable combustor operation was achieved burning Jet-A with emissions of less than 1 ppm NOx and 5 ppm CO, pressure losses less than 5 percent, and a power density on the order of 10 MW/m3 in atmospheric pressure. This power density would linearly scale to 300 MW/m3 in a combustor at a pressure of 30 atmospheres.

Commentary by Dr. Valentin Fuster

Education

2006;():937-952. doi:10.1115/GT2006-90105.

A new turbomachinery design system, T-AXI, is described and demonstrated. It is intended primarily for use by educators and students, although it is sophisticated enough for actual designs. The codes, example cases and a user’s manual are available through the authors’ web sites. The design system can be used to design multistage compressors and turbines from a small number of physical design parameters. Students can understand the connection between these physical parameters such as Mach number and flow angles to the cross sectional area and angular momentum. There is also a clear connection between the angular momentum, work and blade loadings. Loss models are built-in and results are compared against tested geometries. The code also has a built-in blade geometry generator and the geometry can be output for running the MISES blade-to-blade solver on each section or visualizing the blades. A single stage compressor from the US Air Force Stage Matching Investigation rig, the 10 stage NASA/GE EEE high pressure compressor and the NASA/GE EEE 5 stage low pressure turbine have been used to validate T-AXI as a design tool.

Commentary by Dr. Valentin Fuster
2006;():953-962. doi:10.1115/GT2006-90130.

The Department of Mechanical Engineering at the University of Bath has been conducting an undergraduate engine-related design exercise at Rolls-Royce, Bristol since 2000. Each year a team of six undergraduates complete an engine-related design project under supervision from the company between February and September. This work is coordinated and assessed at both the company and university, and counts overall as 20% of the student’s four-year degree. In addition to working at Rolls-Royce, the students submit reports and give seminars at the university. The design exercise is predominantly technical in nature but must include a significant business element. The students are paid as company employees, typically £7.2k for the six months. This paper describes the design exercise and how it is accommodated into the undergraduate programme of study at the University of Bath. The benefits to the university, the students and the company are discussed. In addition, the six students undertaking the 2005 exercise describe their projects. This year there were three projects, two of which were continuations from previous design exercises. The three projects are listed below. Aero-Engine Rotor-Dynamics (V Cheng and S Peet): An experimental and computation study of engine vibration using a rotor-dynamics rig, simulating the engine. The aim was to assess the accuracy and improve the modeling techniques used at Rolls-Royce. Implementing Design for Environment on Gas turbine engines using a Design Tool (W Mezzulo): A study to create a tool to enable the designer to evaluate the environmental aspects of the life of an engine component. Aero-thermodynamics of aero-engines (M Child, R Johnson and C Pattinson): Various design aspects of aero-engines, both computational and business. Note that M Child’s project is not discussed here for reasons of Rolls-Royce proprietary and confidentiality.

Commentary by Dr. Valentin Fuster
2006;():963-969. doi:10.1115/GT2006-90139.

This paper describes a jet engine simulation project assigned to mechanical engineering students in a senior level course in aircraft propulsion systems. The project introduces the students to the methods of engine system analysis and design based on computer simulation. The project statement provides the students with the design point operating characteristics such as engine inlet conditions, pressure ratios, and static thrust developed, which are necessary to develop a design point thermodynamic model. After topics in cycle analysis and engine component performance are introduced the students can develop the system of equations necessary to model the engine system and use this model to predict the off-design performance (speed, thrust, efficiency, etc.) of the engine. Off-design conditions resulting from changes in throttle setting, aircraft speed, altitude, and environmental conditions are considered.

Commentary by Dr. Valentin Fuster
2006;():971-979. doi:10.1115/GT2006-90357.

A software package created for educating engineering students on the principles of gas turbines is presented. It starts from the presentation of basic material on the principles of gas turbine components operation (turbomachinery, combustion chambers, inlets, exhausts). The usual textbook material is supported by audiovisual material that enhances the student’s ability to assimilate the principles taught. Computational tools are included, allowing the execution of design studies as well as performance simulations, for a wide range of gas turbine types. Both aircraft and land based gas turbines are covered. A user friendly interface allows the execution of calculations, whose results can be presented in a variety of formats, with the help of a flexible graphical user interface. A number of specific engines have been chosen to be represented, one of the reasons for this choice being that the package in its current form is primarily intended for use by air force and naval academy students, expected to come in contact with the specific engines employed by the corresponding organizations. Finally, a number of laboratory exercises are included. The exercises are performed in a way that is a reproduction of actual laboratory tests. The software employs audio-visual effects, including videos and other animations, to support the instruction of gas turbine principles, and is implemented in a classroom specifically designed for this purpose. The audiovisual effects are employed to transfer the actual physical reality into the classroom, creating thus the virtual environment.

Commentary by Dr. Valentin Fuster
2006;():981-989. doi:10.1115/GT2006-90477.

This paper describes various techniques employed in a novel approach to instruction and assessment of an undergraduate sequence in thermo-propulsion at the United States Air Force (USAF) Academy. Integrated motivational contexts aid development of foundations in thermodynamics, compressible gas dynamics, and propulsion while reinforcing engineering problem solving skills. Students are first oriented to the context of new material. Subsequent lessons fortify the context, giving students the opportunity to collaborate on team design projects and interact with industry and government guest speakers. Real-world, practical examples and homework further motivate and help students grasp key concepts. Tests are administered in both oral and written formats with open-ended, scenario-based questions to assess student understanding of fundamentals. Grading procedures focus on analytical methods as opposed to numerical results. Specific performance criteria validate the achievement of course educational outcomes. Student course critique scores and written comments further support the assertion that a contextual framework is highly effective in teaching fundamental thermo-propulsion concepts.

Commentary by Dr. Valentin Fuster
2006;():991-1004. doi:10.1115/GT2006-90696.

The up-to-date design of turbomachinery involves the use of three dimensional computational fluid dynamic analyses to match the challenge of the ever increasing speed of product development. Recently, the application of these analyses to turbomachinery, which in past years was confined to specialist fields, has become widespread in many industrial applications. Hence, besides the teaching of traditional methods for turbomachinery design, an important part of modern engineering education is to produce graduates with advanced skills in Computational Fluid Dynamics (CFD) techniques applied to turbomachinery design. Moreover, at this time, these skills should either match the needs of research laboratories or be compatible with industrial needs, since three dimensional numerical calculation is also coming to be seen as a feasible tool in ordinary applications. For these reasons, CFD courses for the training of graduate students have been developed. In this paper, the methodology followed and the work carried out by the students of the course in Fluid Dynamic Design of Turbomachinery held at the University of Ferrara is presented. In particular, three levels of investigation are taken into consideration and presented: (i) focus on the fundamentals to achieve a basic knowledge of a design process based on numerical simulation; (ii) extension to higher level of in-depth analyses of the specific models which could be used in every phase of a numerical simulation; (iii) insight into the specificity of the main thermodynamic and fluid dynamic characteristics of turbomachinery. To achieve this, the numerical analysis of a simple but exhaustive geometry of a centrifugal pump has been carried out and the results obtained are analyzed. In particular, special emphasis is devoted to: (i) the comparison among the numerical models which can be chosen throughout the simulations of turbomachinery (type of grid, turbulence models, rotor/stator interface models) and (ii) the analysis of some of the most important fluid dynamic phenomena, such as, in this case, velocity profiles and jet-and-wake structure.

Commentary by Dr. Valentin Fuster
2006;():1005-1014. doi:10.1115/GT2006-90867.

Two-dimensional compressor flow simulation software has always been a very valuable tool in compressor preliminary design studies, as well as in compressor performance assessment, operating under uniform and non-uniform inlet conditions. In this context, a new streamline curvature (SLC) software has been developed capable of analyzing the flow inside a compressor in two dimensions. The software was developed to provide great flexibility, in the sense that it can be used as: a) A performance prediction tool for compressors of a known design, b) A development tool to assess the changes in performance of a known compressor after implementing small geometry changes, c) A design tool to verify and refine the outcome of a preliminary compressor design analysis, d) A teaching tool to provide the student with an insight of the two-dimensional flow field inside a compressor and how this could be effectively predicted using the SLC method, combined with various algorithms and loss models, e) A 2-D compressor model that can be integrated into a conventional 0-D gas turbine engine cycle simulation code for the investigation of the influence of non-uniform radial pressure profiles on whole engine performance. Apart from describing in detail the design, structure and execution of the SLC software, this paper also stresses the importance of developing robust, well thought-out software and highlights the main areas a potential programmer should focus on in order to achieve this. This manuscript highlights briefly the programming features incorporated into the development of software before continuing to explain the internal workings of individual algorithms. The paper reviews in detail the set of equations used for the prediction of the meridional flow field. Numerical aspects of the application procedure of the full radial equilibrium equation are examined. The loss models incorporated for subsonic and supersonic flow are presented for design and off design operating conditions. Deviation angle rules are presented, together with the parameters for quantifying the diffusion process. Moreover, the methods used for the prediction of surge and choke are discussed in detail. Finally, the end wall boundary layer displacement thickness calculation is discussed briefly, in conjunction with the blockage factor computation. The code has been validated against experimental results which are presented in this paper together with the strong and weak points of this first version of the software and the potential for future development.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2006;():1015-1022. doi:10.1115/GT2006-91180.

Since the mid-1990’s the European Commission (EC) has provided funding for transnational access schemes that open up existing major research facilities to outside users. In the current 6th Framework Program, two out of 14 funded projects — SUSPOWER and ENGAS — are of prime interest to the gas turbine community. SUSPOWER (KTH, Stockhom, Sweden) encompasses unique large-scale experimental facilities within the area of sustainable thermal power generation. Topics of key interest include high-temperature air combustion, catalytic combustion, gasification, aeroelasticity of turbine/compressor blades, film cooling aerodynamics, and stator/rotor interactions. ENGAS (NTNU, Trondheim, Norway) includes a complex array of specialized laboratories in the topic of environmental gas management. Relevant research topics include combustion of hydrogen and hythane, biomass gasification, CO2 absorption and sequestration, membranes for hydrogen and CO2 separation, gas storage in rock caverns, and hydrogen production and storage. This paper presents information on these projects along with a brief overview of previous EC transnational access activities as related to gas turbine research and development.

Commentary by Dr. Valentin Fuster

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