ASME Conference Presenter Attendance Policy and Archival Proceedings

2014;():V01AT00A001. doi:10.1115/GT2014-NS1A.

This online compilation of papers from the ASME Turbo Expo 2014: Turbine Technical Conference and Exposition (GT2014) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Aircraft Engine

2014;():V01AT01A001. doi:10.1115/GT2014-25040.

This paper describes the research carried out in the European Commission co-funded project LEMCOTEC (Low Emission Core Engine Technology), which is aiming at a significant increase of the engine overall pressure ratio. The technical work is split in four technical sub-projects on ultra-high pressure ratio compressors, lean combustion and fuel injection, structures and thermal management and engine performance assessment. The technology will be developed at subsystem and component level and validated in test rigs up to TRL5. The developed technologies will be assessed using three generic study engines (i.e. regional turbofan, mid-size open rotor, and large turbofan) representing about 90% of the expected future commercial aero-engine market. Two additional study engines from the previous NEWAC project will be used for comparison. These are based on intercooled and intercooled-recuperated future engine concepts.

The compressor work is targeting efficiency, stability margin and flow capacity by improved aerodynamic design. High-pressure and intermediate-pressure compressors are addressed. The mechanical and thermo-mechanical functions, including the variable-stator-systems, will be improved. Axial-centrifugal compressors with impeller and centrifugal diffuser are under investigation too.

Three lean burn fuel injection systems are developed to match the technology to the corresponding engine pressure levels. These are the PERM (Partially Evaporating Rapid Mixing), the MSFI (Multiple Staged Fuel Injection) and the advanced LDI (Lean Direct Injection) combustion systems. The air flow and combustion systems are investigated. The fuel control systems are adapted to the requirements of the ultra-high pressure engines with lean fuel injection. Combustor-turbine interaction will be investigated. A fuel system analysis will be performed using CFD methods.

Improved structural design and thermal management is required to reduce the losses and to reduce component weight. The application of new materials and manufacturing processes, including welding and casting aspects, will be investigated. The aim is to reduce the cooling air requirements and improve turbine aerodynamics to support the high-pressure engine cycles.

The final objective is to have innovative ultra-high pressure-ratio core-engine technologies successfully validated at subsystem and component level. Increasing the thermal efficiency of the engine cycles relative to year 2000 in-service engines with OPR of up to 70 (at max. condition) is an enabler and key lever of the core-engine technologies to achieve and even exceed the ACARE 2020 targets on CO2, NOx and other pollutant emissions:

• 20 to 30 % CO2 reduction at the engine level, exceeding both, the ACARE 15 to 20% CO2 reduction target for the engine and subsequently the overall 50% committed CO2 and the fuel burn reduction target on system level (including the contributions from operations and airframe improvements),

• 65 to 70 % NOx reduction at the engine level (CAEP/2) to attain and exceed the ACARE objective of 80% overall NOx reduction (including the contributions from both, operational efficiency and airframe improvement), reduction of other emissions (CO, UHC and smoke/particulates) and

• Reduction of the propulsion system weight (engine including nacelle without pylon).

Topics: Engines
Commentary by Dr. Valentin Fuster
2014;():V01AT01A002. doi:10.1115/GT2014-25057.

This article presents a method for predicting contra rotating propellers individual and total performance which is fast and robust enough to be used in performance engine cycle and engine subsystems detailed design. The method is based on the use of single propeller maps and models mutual induced velocities thanks to one-dimensional theories. These velocities are responsible for interferences between propellers. This article goes through the assumptions on which stands the proposed method and shows that it is relevant compared against more complex methods such as lifting line theory and definitively provides a valuable easy-to-enforce preliminary design tool for open rotor propulsor controls sizing.

Topics: Design , Propellers
Commentary by Dr. Valentin Fuster
2014;():V01AT01A003. doi:10.1115/GT2014-25066.

A durability test rig for erosion-resistant gas turbine engine compressor blade coatings was designed and commissioned. Bare and coated 17-4PH steel modified NACA 6505-profile blades were spun at an average speed of 10 860 rpm and exposed to garnet sand-laden air for 5 hours at an average sand concentration of Display Formula2.5gm3of air and a blade leading edge (LE) Mach number of 0.50. The rig was designed to represent a first stage axial compressor. Two 16μm-thick coatings were tested: Titanium Nitride (TiN) and Chromium-Aluminum-Titanium Nitride (CrAlTiN), both applied using an Arc Physical Vapour Deposition technique. A composite scale, defined as the LAZ score, was devised to compare the durability performance of bare and coated blades based on mass-loss and blade dimension changes. The bare blades’ LAZ score was set as a benchmark of 1.00, with the TiN-coated and CrAlTiN-coated blades obtaining respective scores of 0.69 and 0.41. A lower score identified a more erosion-resistant coating. Major locations of blade wear included: trailing edge (TE), LE and rear suction surface (SS). TE thickness was reduced, the LE became blunt, and the rear SS was scrubbed by overtip and recirculation zone vortices. The erosion effects of secondary flows were found to be significant. Erosion damage due to reflected particles was absent due to a low blade solidity of 0.7. The rig is best suited for durability evaluation of erosion-resistant coatings after being proven worthy of consideration for gas turbine engines through ASTM standardized testing.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A004. doi:10.1115/GT2014-25068.

Arnold Engineering Development Complex (AEDC) is the most advanced and largest complex of flight simulation test facilities in the world. The center operates 43 aerodynamic and propulsion wind tunnels, rocket and turbine engine test cells, space environmental chambers, arc heaters, ballistic ranges and other specialized units.

Over the years since these facilities have been operational, there have been a series of operational issues that have arisen involving compression system performance or the effect of compressor operability on facility performance or structural integrity. This paper presents three cases[1] where ground test facilities have experienced some form of compression system performance issue or a test article’s abnormal operation such as surge cycles effect on facility structural or operational capability. In each case, the analysis engineer trying to figure out what might be the cause or the effect of these abnormal operations on facility operation is hampered by the lack of proper instrumentation that would provide insight into the issue. In these types of situation, it has been shown that some form of numerical simulation can provide great insight into possible causes and even suggest solutions to the observed anomaly. This paper highlights where a specific numerical simulation based upon one-dimensional physics and its application to parallel compressor theory has been applied to the analysis of three aerospace facility compression system issues.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A005. doi:10.1115/GT2014-25087.

The aviation industry has set ambitious reduction targets for future air traffic emissions to compensate for the environmental impact of increasing air transport. Besides technological innovation in the fields of aircraft and engine technology, alternative fuels from non-petroleum feedstock are often seen as having long-term potential to offer reductions in aviation’s greenhouse gas emissions. The current paper studies the effects of potential emission reduction measures up to the year 2050, taking technology developments as well as renewable fuel scenarios into account. The individual impacts of both contributions are assessed in terms of fuel burn, direct CO2 emissions, and life cycle CO2eq emissions. The results are compared to the agreed mid and long term reductions targets, i.e. carbon-neutral growth by 2020 and halving CO2 emissions by 2050.

Future air traffic emissions are evaluated based on one reference scenario and three technology scenarios, derived from the ICAO global fuel burn forecast and Flightpath 2050 technology improvement goals. Four renewable fuel penetration scenarios are considered and the impact of alternative renewable fuels in terms of Synthetic Paraffinic Kerosene (SPK) on fuel burn and direct CO2 emissions are taken into consideration. Moreover, to account for the fuels’ life cycle emissions three different life cycle CO2eq emission reduction potentials scenarios are applied.

This study provides a quantitative view on exponential growth of aviation fleet CO2 emissions. The potential of individual contributions are explicitly highlighted and potential developments in air traffic emissions are quantified. The results show that in addition to significant efficiency improvements in aircraft and engine technology, a high renewable fuel share will be required to compensate for steady air traffic growth and to achieve carbon neutral growth from 2020 onwards and a 50% CO2eq emission reduction by 2050.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A006. doi:10.1115/GT2014-25171.

Accumulation of ice on aeroengine components could cause serious aircraft accidents. An understanding of the adhesion characteristics of ice-substrate interfaces is essential in order to design reliable anti-icing and de-icing systems. The main purpose of this paper is on the application of a bilinear cohesive zone model to simulate the interface between ice and aluminum by using ANSYS software. A finite element model which coupled with the cohesive zone model is built and some factors that affect the Al/ice tensile strength are discussed. These factors include interface roughness, initial damage of the interface, which is caused by the existence of bubbles. The adhesion strength between ice and aluminum are predicted and analyzed. This model could be used to further study on the mechanisms responsible for the non-linear relationship between the surface roughness and ice adhesion strength.

Topics: Adhesion , Aluminum , Ice
Commentary by Dr. Valentin Fuster
2014;():V01AT01A007. doi:10.1115/GT2014-25204.

Auxiliary power units (APUs) are gas turbine engines that provide high-pressure air and electrical power to aircraft systems. They provide primary power while the aircraft is parked on the ramp, starting services for the main engines, and backup power while the aircraft is in flight. Many APUs employ inlet systems which include a “pop-up” door that allow for the capture of freestream ram pressure during flight. This results in increased inlet recovery and a corresponding improvement in the performance of the APU. This APU door, when open, is exposed to airflow instability inherent in the aircraft boundary layer in the aft section of the fuselage, where APUs are typically housed. Additionally, the pop-up nature of the inlet door produces a large region of separated airflow off of the back side of the door. Systematic vortex shedding is frequently a major component of this separated region.

As new APU doors are made with less rigid material to save weight, a need to better understand the unsteady aerodynamic excitations of the flow field around the door has arisen, as these new doors may be more susceptible to vibration during flight.

Recent advances in Computational Fluid Dynamics (CFD) meshing tools and transient modeling have enabled a CFD study to be performed which will investigate this time-dependent phenomenon. As transient CFD analysis is still a relatively new field for commercial CFD codes, a calibration was needed to verify the accuracy of the CFD predictions and to form any calibration correction terms.

Honeywell Aerospace owns a Boeing 757 flight test vehicle which is normally used to flight test propulsion engines. However, this aircraft also includes a pop-up APU inlet door that is similar to most other APU inlet door styles. This APU door was instrumented using high response pressure transducers placed on the forward and aft sides of the inlet door as well as upstream of the door to measure upstream instability. The aircraft was flown at a variety of flight conditions and APU operating points and the transient data was recorded.

After the completion of the flight test, a CFD model was constructed of the B757 flight test vehicle. Because aerodynamic instability can be generated anywhere on the aircraft, the entire airframe from nose to tail was modeled. The APU inlet door geometry was also created, meshed and added to the CFD model. This CFD model was run in a transient mode to simulate the exact same flight conditions and APU operating points as were tested during the flight test. Dynamic results in the time and frequency domains predicted by the CFD analyses were compared to flight test data and correlation and calibration factors were derived.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A008. doi:10.1115/GT2014-25255.

The proposed work corresponds to a comprehensive comparison of aerothermal flow field arising from the heat dissipation of aircraft equipment. Laboratory experiments based on real-scale geometry were conducted to fully depict heat transfer between the fluid and the equipment. In some particular regions of interest, temperature profiles in the sub-layer have been directly measured up to the wall in order to estimate convective heat transfer coefficient. This way of measurement is very challenging in such configuration representative of powerplant environment in terms of geometry, Reynolds number and temperature. In parallel, a Large Eddy Simulation coupled to a thermal solver were carried and compared to experimental data allowing a cross-validation of simulations and experiments. A simulation verification methodology based on the turbulent energy spectra decay is proposed in order to ensure result accuracy for such aerothermal problems.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A009. doi:10.1115/GT2014-25359.

The United States Department of Defense (DoD) is continually looking for ways to improve test and evaluation techniques to ensure systems meet military requirements prior to acquisition. Recently, the DoD has been pursuing the use of statistical methods to improve test and evaluation. This paper highlights statistical methodologies used by the Air Force Test Center to improve aircraft propulsion system Modeling and Simulation (M&S) efforts.

The US Air Force has a long history of using M&S (more than 55 years) during aircraft test and evaluation. In the past, M&S usage was primarily in the aircraft performance and flying qualities areas. Now advancing technology and complex integration are resulting in increased M&S use across broader spectrum of technical disciplines, including propulsion. During propulsion testing, models are used to increase system knowledge in T&E areas which include: Test Planning, Execution, Data Analysis and Evaluation.

This paper highlights the 412 Test Wing at Edwards AFB first steps to improve aircraft propulsion system T&E through the implementation of statistically defensible model development techniques. Specifically, this paper will provide an example of typical engineer model development strategies based on past experience, system knowledge, relevant physics and subjective evaluations to determine variables used and structure of the model. This paper will also provide insight into a number of statistics-based approaches including stepwise regression, backwards elimination, the inadequacy of using R-squared and an examination into the effects of mulit-collinearity. However, the focus of this paper is on how Information Theory and Akaike’s Information Criteria (AIC) can be easily applied to compare a variety of models and determine the best model available. This paper presents an example of these model development methods applied during a development of a predictive model used for evaluating thrust response of an aircraft engine with a new digital engine control. A case will be made that statistical approaches provide a more mathematically rigorous approach for model selection as compared to traditional approaches based on engineering judgment.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A010. doi:10.1115/GT2014-25448.

In order to reduce the infrared radiation, and to improve the survival rates of the fighter, it is necessary to further enhance the mixing of the nozzle exhaust jet with the ambient air. A novel technology known as air-tab is adopted to enhance the exhaust mixing with the environment air in high bypass turbofan nozzle model. Numerical results show that: 1)The technology can flexibly adapt to the every condition of nozzle and keep the best mixing effect all the time, with very little flowfield loss; 2)The parameters present zigzag distribution. 3) High temperature region area has been reduced nearly 50% by the air-tab with the second mass flow rate only about 3% of total mass flow rate.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A011. doi:10.1115/GT2014-25594.

Thrust performance, which can be obtained by indoor sea level test facility, plays a critical role in jet and turbofan engine performance. In this study, to obtain the aerodynamic characteristics in test cell and the final thrust correction coefficient, a standard turbofan engine is chosen as the testing engine, which is mounted separately on a fixed contracting or adjustable Laval nozzle. Experimental results show that the aerodynamic performance of intake is poor if no guide vanes are implemented. In this situation, inhomogeneity of intake flow is up to 97%. Besides, the flow would separate in the upstream of the engine, which is also proven by the CFD simulation results.

With the flow separating upstream of the test engine, the traditional intake momentum correction method by using ideal horizontal intake condition could result in greater correction error of intake momentum. Thus, a new formula is derived in the present paper to correct the intake momentum with flow separation by mechanic control system. Comparisons of thrust performance and specific fuel consumption are conducted between the result corrected by new formula suited for flow separation condition and those obtained from the performance data of the same engine tested in the sea level condition. And good agreement has been achieved. Thus it is proven that the new thrust correction method is correct and reliable.

Result of this study indicates that: 1) intake flow separation has a great influence on thrust measurement in jet engine test facility, which will result in the 1.4 to 2 times greater correction error. Thus, guide vanes should be implemented at inlet tower corner in preventing from the flow separation. 2) The new correction method derived in this paper is very effective to correct the engine thrust performance with the flow separation upstream of the test engine.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A012. doi:10.1115/GT2014-25686.

When gas turbine engines operate in environments where the intake air has some concentration of particles, the engine will experience degradation. Very few studies of microparticles at temperatures approaching the melting temperature of the particles are available in open literature. Coefficient of Restitution (COR), a measure of the particles’ impact characteristics, was measured for microparticles using a particle tracking technique. This study presents data taken using the Virginia Tech Aerothermal Rig and Arizona Road Dust (ARD) of 20–40μm size range. Data was taken at temperatures up to and including 1323 K, where significant deposition of the sand particles was observed. The velocity at which the particles impact the surface was held at a constant 70m/s for all of the temperature cases. The target on which the particles impacted was made of a nickel alloy, Hastelloy X. The particle angle of impact was also varied between 30° and 80°. The COR of the particles decreases slightly as some of the particles approach their glass transition point and start to become molten. Other particles, which do not become molten due to different particle composition, rebound and maintain a relatively high COR. Images were taken using a microscope to examine the particle deposition that occurs at various angles. A rebound ratio is formulated to give a measure of the number of particles which deposit on the surface. The results show an increase in deposition as the temperature approaches the melting temperature of sand.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A013. doi:10.1115/GT2014-25687.

Many gas turbine engines operate in harsh environments where the engines ingest solid particles. Ingested particles accelerate the deterioration of engine components and reduce the engine’s service life. Understanding particle impacts on materials used in gas turbines at representative engine conditions leads to improved designs for turbomachinery operating in particle-laden environments. Coefficient of Restitution (COR) is a measure of particle/wall interaction and is used to study erosion and deposition. In the current study, the effect of temperature (independent of velocity) on COR was investigated. Arizona Road Dust (ARD) of 20–40/μm size was injected into a flow field to measure the effects of temperature and velocity on particle rebound. Target coupon materials used were 304 stainless steel and Hastelloy X. Tests were performed at three different temperatures, 300 K (ambient), 873 K, and 1073 K while the velocity of the flow field was held constant at 28 m/s. The impingement angle of the bulk sand on the coupon was varied from 30 ° to 80 ° for each temperature tested. The COR was found to decrease substantially from the ambient case to the 873 K and 1073 K cases. This decrease is believed to be due to the changes in the surface of both materials due to oxide layer formation which occurs as the target material is heated. The Hastelloy X material exhibits a larger decrease in COR than the stainless steel 304 material. The results are also compared to previously published literature.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A014. doi:10.1115/GT2014-25691.

The effect of circumferential inflow swirl on the instability of the shear layer formed between the core and bypass flows discharged from an axisymmetric twelve-lobed mixer is studied through a combined experimental and computational investigation. A series of unsteady Navier-Stokes simulations are performed with 0 and 31 degrees of circumferential swirl specified in the core stream of the lobed mixer. Comparison of the axial- and swirling-inflow cases highlights the effect of swirl on the instability-driven transient flow structures that develop within and downstream of the lobed mixer. Medium- and large-scale unsteady motions are captured by the fine spatial and temporal resolution of the unsteady Navier-Stokes simulations. The simulations are validated against four-wire thermal anemometry measurements in a scaled lobed-mixer wind-tunnel model with turbulent, swirling inflow conditions. The simulation results illustrate that while the axial-inflow case develops layers of streamwise vorticity uniformly along the lobe walls, the core flow in the swirling-inflow case separates from the suction side of the lobe wall near the lobe trough. Roll-up and axial stretching of the separated flow produces Λ-shaped vortical structures upstream of the discharge plane. The Λ-shaped structures interact with the shear layers discharged from the lobe trailing edge and accelerate the breakdown of the shear layer in the swirling-inflow case relative to the axial-inflow case. The extent of this interaction is shown to strongly affect the streamwise mixing rate of the flow downstream of the discharge plane.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A015. doi:10.1115/GT2014-25705.

One of the most troublesome problems in the development of a component-based engine model is the compressor modeling because of the strong dependence of its performance on rotational speed and the treatment of compressor characteristic curves plays a significant role in modeling and simulation of gas turbine for the analysis of its off-design performance and thus it is crucial to describe compressor map exactly.

Usually part of rotational speed characteristic curves of compressor including on-design operating point are known during actual modeling and simulation, and reasonable interpolation and extrapolation have to be done in order to make good use of flow characteristics and efficiency characteristics under more constant rotational speed lines during off-design performance simulation.

Due to that the accuracy of traditional approaches for component characteristic treatment is not satisfactory, and the performance of interpolation and extrapolation of artificial neural network is poor, a linear multiple regression method, i.e., partial least-squares regression method was proposed in this paper.

Partial least-squares regression modeling method was used to reproduce the compressor maps. Different polynomial functions of various powers were used to obtain respectively expressions of compressor maps. Fitting accuracy and performance of interpolation and extrapolation of this method were analyzed, and the simulating experimental results show that partial least-squares regression method can ensure good fitting accuracy and good performance of interpolation and extrapolation for compressor thermodynamic modeling with both maximum RMS errors less than 0.5%.

It can be expected that the application of partial least-squares regression modeling has a certain reference value to improve the solution accuracy for thermodynamic model of industrial gas turbine.

Topics: Compressors , Modeling
Commentary by Dr. Valentin Fuster
2014;():V01AT01A016. doi:10.1115/GT2014-25726.

An earlier combined vibration and temperature analysis technique for rotating machinery fault diagnosis produced good results. This was, however, applied to a rotating machinery experimental rig operating at a constant speed. In the current study, using a similar approach, vibration data and temperature measurements obtained at different steady state rotating speeds for different fault conditions namely; crack, misalignment and rub, are fused together in a single analysis step. The objective in this case is to develop a computationally efficient fault diagnosis technique that can exploit data measured at different rotating speeds. The result obtained is an easily interpreted diagrammatical representation that shows clear discrimination between the different fault conditions tested and the healthy condition, thus offering the potential for practical application on variable speed machinery such as aero-engines.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A017. doi:10.1115/GT2014-25733.

An analytical study was undertaken using the performance model of a two spool direct drive high BPR 300kN thrust turbofan engine, to investigate the effects of advanced configurations on overall engine performance. These include variable bypass nozzle, variable cooling air flow and more electric technique.

For variable bypass nozzle, analysis on performance of outer fan at different conditions indicates that different operating points cannot meet optimal performance at the same time if the bypass nozzle area kept a constant. By changing bypass nozzle throat area at different states, outer fan operating point moves to the location where airflow and efficiency are more appropriate, and have enough margin away from surge line. As a result, the range of variable area of bypass nozzle throat is determined which ensures engine having a low SFC and adequate stability.

For variable cooling airflow, configuration of turbine cooling air flow extraction and methodology for obtaining change of cooling airflow are investigated. Then, base on temperature analysis of turbine vane and blade and resistance of cooling airflow, reduction of cooling airflow is determined. Finally, using performance model which considering effect of cooling air flow on work and efficiency of turbine, variable cooling airflow effect on overall performance is analyzed.

For more electric technique, the main characteristic is to use power off-take instead of overboard air extraction. Power off-take and air extraction effect on overall performance of high bypass turbofan engine is compared. Investigation demonstrates that power offtake will have less SFC.

Topics: Engines , Turbofans
Commentary by Dr. Valentin Fuster
2014;():V01AT01A018. doi:10.1115/GT2014-25765.

This paper presents a numerical study aimed at characterizing the aerodynamics of an advanced propeller distinguished by its high rotational speed, blade sweep and airfoil sections. Many of the difficulties encountered when applying CFD to an open rotor (a propeller) arise due to removal of the casing existing in a conventional aero-engine turbomachinery. For this purpose the propeller computational domain needed to be well parameterized to keep sufficient outer domains distances where the appropriate boundary conditions are imposed. The mesh of a certain resolution was extended radially, five times the tip radii, to fully capture the stream-tube and minimize the effect of free-stream boundary conditions. Comparisons of obtained flow field results with some available experimental data shows in general similar quantitative results and trends. The estimated propulsive efficiency is shown to be strongly dependent upon the cruise flight Mach number, advance ratio and pitch angle. The maximum propulsive efficiency reached a value of 76.2 % around flight Mach number of 0.8, twist angle of 66 deg and advance ratio of 4.1. The effect of blades number has revealed a higher propulsive efficiency for the six and eight-bladed propellers but at the expense of lower power and thrust coefficients.

Topics: Propellers
Commentary by Dr. Valentin Fuster
2014;():V01AT01A019. doi:10.1115/GT2014-25772.

Recent trends in design for civil intakes lead towards shorter diffuser sections, unorthodox installations and more loaded lips. All these features increase the risk of lip stall in flight at incidence or in cross wind and increase the level of forcing seen by the fan blades because of the interaction with non-uniform flow from the intake.

In this study we analyze the behavior of prediction tools for intake distortion. In particular we compare the performance of popular turbulence models for standard intake flows and we discuss their behavior on the grounds of their behavior for elementary flows.

We conclude our study by comparing forcing and distortion figures of merit from different models.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2014;():V01AT01A020. doi:10.1115/GT2014-25839.

In the jet engine, icing phenomena occur primarily on the fan blades, the fan exit guide vanes (FEGVs), the splitter, and the low-pressure compressor. Accreted ice disturbs the inlet flow and causes large energy losses. In addition, ice accreted on a fan rotor can be shed from the blade surface due to centrifugal force and can damage compressor components. This phenomenon, which is typical in turbomachinery, is referred to as ice shedding. Although existing icing models can simulate ice growth, these models do not have the capability to reproduce ice shedding. In the present study, we develop an icing model that takes into account both ice growth and ice shedding. Furthermore, we validated the proposed ice shedding model through the comparison of numerical results and experimental data, which includes the flow rate loss due to ice growth and the flow rate recovery due to ice shedding. The simulation results for the time at which ice shedding occurred and that were obtained using the proposed ice shedding model were in good agreement with the experimental results.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A021. doi:10.1115/GT2014-25847.

This paper discusses a fundamental study on icing phenomena in a high-temperature environment. Generally, ice accretion is a phenomenon to form ice layer on a body due to impingement of super-cooled water droplets. In recent years, it is known that ice accretion occurs in the engine core such as the low pressure compressor and the first stage of the high pressure compressor, where the temperature is about 30 degree C. The ice accretion in the engine core is called as “ice crystal accretion”. Some scenarios are given for the ice crystal accretion, but the mechanism has not been sufficiently clarified yet. Moreover, the current icing model is not available in the environment where the temperature is above the freezing point. In this paper, we develop a new icing code which is applicable to a warm environment. The new icing model consists of four iterative computations for turbulent flow, droplet/ice trajectory, thermodynamics of icing, and heat conduction within a wall. First, we validate our new icing model with a flat plate instead of a compressor stator blade as the fundamental study of ice crystal accretion. Then, we simulate ice accretion on a two-dimensional compressor stator blade in a high-temperature environment, in order to clarify the ice-crystal physics.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A022. doi:10.1115/GT2014-26008.

On possible fan concept for future high and ultra-high bypass ratio turbofan engines is the counter-rotating (CR) fan. Several studies [1][2][3][4] dealt already with the optimization of CR fans, however the mass flow and the total pressure ratio were typically given and fixed for a specified application. The results of these studies showed a benefit of the CR fan compared to the conventional single-rotating (SR) fan, which strongly depended on the engine cycle. Following this experience, it was necessary to further specify the efficiency benefits more precisely in association with fan total pressure ratio and fan inlet axial Mach number. The results are discussed in this present paper. A special emphasis was given on determining the optimal pressure ratio, for which the CR-fan expectably achieves the maximal efficiency benefit.

The idea was to perform a global optimization study without any constraints for the operating point inside of a broad (ΠFan, Max) –range, for the rotational speeds and with only a few constraints for the geometry of the blades to avoid infeasible geometries. An adequate range for the fan pressure ratio (ΠFan) and for the axial Mach number (Max) was chosen for the global optimization covering the entire range from current to potential future ultra-high bypass ratio engine applications, also taking into account a reduced nacelle diameter and thus high axial fan inflow Mach numbers.

The focus of the present study was to develop a method for the global optimization of a fan stage.

As a result of this study, the maximal achievable efficiency is shown as a function of the fan pressure ratio and the axial Mach number. Thus the efficiency differences between the CR and SR fan can be calculated through the differences between the surfaces for any given set of parameters defining a potential engine. This allows for a generalized assessment of this particular fan concept over the entire range of relevant applications.

Topics: Fans , Optimization
Commentary by Dr. Valentin Fuster
2014;():V01AT01A023. doi:10.1115/GT2014-26091.

Today many of the routes between small to medium sized airports and large hubs are operated by regional aircraft, powered by turboprop or turbofan engines. In the future the open rotor engine might provide an alternative option. The open rotor would combine the possibility of high cruise speed with high propulsive efficiency. Also, since the open rotor essentially is a turboprop with the possibility to fly fast, there is a benefit of high specific range at low cruising speeds, thus giving it a wide range cruise operation.

In this paper a regional aircraft for 70 passengers and 3000 km range is studied. The aircraft is evaluated with both a counter rotating open rotor and a turbofan engine. Aircraft design parameters such as wing area and sweep are varied together with engine parameters such as engine power and propeller disc loading.

Results show that the open rotor aircraft has a 17.0 % higher specific range at the optimal cruise Mach number compared to the turbofan aircraft. For higher speeds, at Mach 0.78, the difference is reduced to 15.0 %. The long range cruise Mach number is around Mach 0.7 for the open rotor aircraft while for the turbofan aircraft it is slightly higher, around Mach 0.72.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A024. doi:10.1115/GT2014-26142.

Boundary Layer Ingesting (BLI) turbofan engines could offer reduced fuel burn compared with podded engines, but the fan stage must be designed to run continuously with severe inlet distortion. This paper aims to explain the fluid dynamics and loss sources in BLI fans running at a cruise condition. High-resolution experimental measurements and full-annulus unsteady CFD have been performed on a low-speed fan rig running with a representative BLI inlet velocity profile. A three-dimensional flow redistribution is observed, leading to an attenuation of the axial velocity non-uniformity upstream of the rotor and to non-uniform swirl and radial angle distributions at rotor inlet. The distorted flow field is shown to create circumferential and radial variations in diffusion factor with a corresponding loss variation around the annulus. Additional loss is generated by an unsteady separation of the casing boundary layer, caused by a localised peak in loading at the rotor tip. Non-uniform swirl and radial angles at rotor exit lead to increased loss in the stator due to the variations in profile loss and corner separation size. An additional CFD calculation of a transonic fan running with the same inlet profile is used to show that BLI leads to wide variations in rotor shock structure, strength and position and hence to loss generation through shock-boundary layer interaction, but otherwise contained the same flow features as the low-speed case. For both fan geometries, BLI was found to reduce the stage efficiency by around 1–2% relative to operation with uniform inlet flow.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A025. doi:10.1115/GT2014-26184.

The present investigation aims at performing a probabilistic analysis of the secondary air system of a three-stage low-pressure turbine rotor in a jet engine. Geometrical engine to engine variations due to the tolerance of the different parts as well as the variation of engine performance parameters are taken into account to analyse the impact on the aerodynamic behaviour of the secondary air system. Three main functions of the secondary air system have been investigated at one engine condition — take-off. At first the variation of the turbine rotor cooling flow consumption was studied. Secondly the axial bearing loads were considered and finally the system was analysed with regard to its robustness towards disc space hot gas ingestion.

To determine the uncertainty in the accomplishment of these tasks and to identify the major variation drivers, a Latin Hypercube sampling method coupled with the correlation coefficient analysis was applied to the 1D flow model. The incapability of the correlation coefficient analysis to deal with functional relationships of not monotonic behaviour or strong interaction effects was compensated by additionally applying in such cases an Elementary Effect analysis to determine the influential variables. As the 1D flow model cannot consider thermal and centrifugal growth effects, a simple mathematical model was deduced from the physical dependencies enhancing the 1D flow model to approximately capture the impact of these effects on the labyrinth seals.

Results showed that the cooling mass flow and axial bearing load are both normally distributed while their uncertainties are mainly induced by the uncertainties of the state variable of the primary air system. The investigated chamber temperature ratio to analyse the hot gas ingestion showed a not normally distributed histogram and a strong influence of interaction terms. Therefore the results of the correlation coefficient analysis were complemented with the results of an Elementary Effect analysis.

Topics: Pressure , Turbines
Commentary by Dr. Valentin Fuster
2014;():V01AT01A026. doi:10.1115/GT2014-26208.

This paper presents results of investigations on the interaction between a targeted oil jet and a rotating shaft in an aero engine typical bearing chamber. Measurements were performed at atmospheric temperature and pressure in order to study the influence of the operating conditions, nozzle diameters and impingement angles on the efficiency of such an oil supply system. The flow phenomena of the jet-shaft interaction were visualised.

A qualitative analysis of the jet-shaft interaction revealed massive droplet generation due to the jet break-up in the air crossflow and its impact on the shaft. The latter could be reduced with shallower impingement angles. Measurements showed that the oil inflow rate, the shaft speed and the nozzle diameter have a strong influence on the collected oil quantity, which is expressed as catch efficiency, i.e. the ratio of collected and supplied oil. The impingement angle was also identified to have a strong influence on the catch efficiency. The ratio of the momentum fluxes of supplied oil and chamber air flow is proposed as a parameter to correlate the catch efficiency to the operating conditions.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A027. doi:10.1115/GT2014-26249.

This paper describes a novel ducted fan inlet flow conditioning concept that will significantly improve the performance and controllability of ducted fan systems operating at high angle of attack. High angle of attack operation of ducted fans is very common in VTOL (vertical take off and landing) UAV systems. The new concept that will significantly reduce the inlet lip separation related performance penalties in the edgewise/forward flight zone is named DOUBLE DUCTED FAN (DDF). The current concept uses a secondary stationary duct system to control inlet lip separation related momentum deficit at the inlet of the fan rotor occurring at elevated edgewise flight velocities. The DDF is self-adjusting in a wide edgewise flight velocity range and its corrective aerodynamic effect becomes more pronounced with increasing flight velocity due to its inherent design properties. Most axial flow fans are designed for an axial inlet flow with zero or minimal inlet flow distortion. The DDF concept is proven to be an effective way of dealing with inlet flow distortions occurring near the lip section of any axial flow fan system, especially at high angle of attack. In this present paper, a conventional baseline duct without any lip separation control feature is compared to two different double ducted fans named DDF CASE-A and DDF CASE-B via 3D, viscous and turbulent flow computational analysis. Both hover and edgewise flight conditions are considered. Significant relative improvements from DDF CASE-A and DDF CASE-B are in the areas of vertical force (thrust) enhancement, nose-up pitching moment control and recovery of fan through-flow mass flow rate in a wide horizontal flight range.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A028. doi:10.1115/GT2014-26292.

The Advisory Council for Aeronautics Research in Europe (ACARE) has set an ambitious array of objectives to be accomplished by 2050. It is often claimed that complying with those targets will not require evolution but, rather, revolution. If the growth in aviation has to be sustained in the future then we must come up with radical aircraft and engine configurations which can meet the demands of future aviation.

The contra-rotating fan is one such system which can play an important role in the future engine configurations, such as the hybrid engine configuration that is being investigated in the EU cofounded AHEAD project.

In order to design a CRF system, a 1-D code has been developed based on the inverse Blade Element Method (BEM) to design a contra rotating fan. The CRF design obtained from this methodology is then analyzed with a full 3D RANS simulation.

The numerical analysis revealed that the performance of the first rotor satisfies with the given design requirements in terms of both pressure ratio and isentropic efficiency, thus proving the efficacy of using the 1-D code for designing the CRF. However, the performance of the rear rotor does not reach the design demands. It was observed that there is a strong flow separation around the root and a strong normal shock in the blade passage near the tip. It was found that there is a great difference between the blade metal inlet angles and the relative flow inlet angles near the root of the rear rotor. One of the main reasons for this is the calculation of the axial velocity depending on the vortex design and the resolution of the radial equilibrium. Based on the CFD simulations, the design code could be further modified to improve the design of CRF.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A029. doi:10.1115/GT2014-26369.

As the propulsor fan pressure ratio (FPR) is decreased for improved fuel burn, reduced emissions and noise, the fan diameter grows and innovative nacelle concepts with short inlets are required to reduce their weight and drag. This paper addresses the uncharted inlet and nacelle design space for low-FPR propulsors where fan and nacelle are more closely coupled than in current turbofan engines. The paper presents an integrated fan-nacelle design framework, combining a spline-based inlet design tool with a fast and reliable body-force-based approach for the fan rotor and stator blade rows to capture the inlet-fan and fan-exhaust interactions and flow distortion at the fan face. The new capability enables parametric studies of characteristic inlet and nacelle design parameters with a short turn-around time. The interaction of the rotor with a region of high streamwise Mach number at the fan face is identified as the key mechanism limiting the design of short inlets. The local increase in Mach number is due to flow acceleration along the inlet internal surface coupled with a reduction in effective flow area. For a candidate short-inlet design with length over diameter ratio L/D = 0.19, the streamwise Mach number at the fan face near the shroud increases by up to 0.16 at cruise and by up to 0.36 at off-design conditions relative to a long-inlet propulsor with L/D = 0.5. As a consequence, the rotor locally operates close to choke resulting in fan efficiency penalties of up to 1.6 % at cruise and 3.9 % at off-design. For inlets with L/D < 0.25, the benefit from reduced nacelle drag is offset by the reduction in fan efficiency, resulting in propulsive efficiency penalties. Based on a parametric inlet study, the recommended inlet L/D is suggested to be between 0.25 and 0.4. The performance of a candidate short inlet with L/D = 0.25 was assessed using full-annulus unsteady RANS simulations at critical design and off-design operating conditions. The candidate design maintains the propulsive efficiency of the baseline case and fuel burn benefits are conjectured due to reductions in nacelle weight and drag compared to an aircraft powered by the baseline propulsor.

Topics: Pressure
Commentary by Dr. Valentin Fuster
2014;():V01AT01A030. doi:10.1115/GT2014-26442.

For the sake of investigating impact of inlet distortion on the fan stage performance, numerical simulation of the whole structure of a flush-mounted S-shaped inlet and the rear fan stage was conducted in this paper. The single fan stage with uniform air admission was researched at the same time for comparison. Considering substantial boundary layer ingesting, a scheme of suction control imposed at the first bend of the inlet was also carried out. The results show that the total pressure ratio as well as the efficiency of the fan stage decreases dramatically and the choked mass flow has a reduction about 1.20% as compare with the uniform air condition. With suction control, aerodynamic performance of the fan stage improves slightly, the choked mass flow and total pressure ratio at the maximum isentropic efficiency point increase about 0.28% and 0.25% respectively, and the stable operation range is extended. With effect of rotating rotor, the significant low energy region at bottom of the airintake exit decrease continually as it travels downstream to the rotor and covers nearly three flow passages at the front-edge rotor blade, moreover, the high-energy fluid mixes with the low-energy fluid.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A031. doi:10.1115/GT2014-26449.

Modern high bypass turbofan engines have single stage fans with a low hub-tip radius ratio. The fan map is a very important element for off-design performance simulations. Such a map consists of tables with corrected mass flow, pressure ratio and efficiency for a range of corrected spool speeds.

Applying the data read from a fan map to both the core and the bypass stream is inaccurate because the transonic flow field of the bypass stream is very different to the subsonic flow field of the core stream. A better approximation of reality is to use a hybrid map with total mass flow, bypass pressure ratio and efficiency. Constant factors are employed to derive the core stream pressure ratio and efficiency.

For more accurate simulations two maps may be employed, one for the core and another one for the bypass stream. The total mass flow of the fan is the same in these two maps while pressure ratio and efficiency are different for the two streams. The data for each point in this so-called “Split Map” are valid for a pre-defined bypass ratio.

This paper describes an alternative to the split map methodology which takes the variability of the bypass ratio into account in a different way. The hypothesis is that the overall fan performance is not affected by variations in bypass ratio. The fan performance map is completed by an additional table with core stream efficiency.

This enhanced map is used as follows. When scaling the map, the bypass ratio as well as the pressure ratio and efficiencies for the core and bypass streams are known. Assumed values for fan tip speed, hub-tip radius ratio and fan inlet Mach number yield the core stream velocity triangle. The rotor blade exit flow angle from this triangle remains the same in all other operating conditions.

The core flow velocity triangle analysis with known rotor blade exit angle yields the work done on the core stream during off-design. The pressure ratio is calculated from this work and the efficiency read from the core stream efficiency table mentioned above.

Finally, the bypass stream pressure ratio and efficiency are calculated from the overall map and the core stream data applying the actual bypass ratio.

Topics: Design , Fans
Commentary by Dr. Valentin Fuster
2014;():V01AT01A032. doi:10.1115/GT2014-26472.

Hybrid, Implicit Large Eddy Simulations (ILES) for an idealized aero engine intake in a crosswind is performed. The ILES zone is smoothly blended to a near wall Reynolds Averaged Navier-Stokes (RANS) zone. The flow has a region of high favourable pressure gradient (FPG) where the streamwise acceleration parameter (KS) is found to be greater than 3×10−6. This is sufficient to laminarize the boundary layer (BL). As a consequence, the turbulence in the boundary is severely suppressed and this interacts with a shock causing separation and distortion at the engine fan face. This is known to be undesirable for aero engines. The separated shear layer reenergizes turbulence and this promotes reattachment. The calculation in the RANS zone has been enhanced with a novel three-component RANS model and this is used in the hybrid RANS/ILES framework. Simulations also consider the modelling of roughness. The turbulent statistics and the engineering relevance of these are also discussed in this work. Broadly, encouraging agreement is found with measurements. Substantial accuracy improvements are found relative to standard RANS model simulations. The accuracy of the hybrid simulations is also contrasted with pure ILES and the critical need for the RANS layer shown for modest grids.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A033. doi:10.1115/GT2014-26481.

As a result of the new engine design trends, the likelihood of tightly-wound vortices being ingested by the engine rises. Therefore, the risk associated with the ingestion of swirl distortion becomes a major concern.

A numerical analysis of the response of a transonic fan stage to the ingestion of different distorted flow patterns is carried out using steady-state CFD. The CFD approach is generated and validated against experimental data for undistorted inlet conditions. Following the validation, a wide range of configurations with vortex flow distortions are analysed and evaluated. The change in global performance is quantified and the flow field is extensively analysed. Consequently, the parameters that have the most critical impact on the performance of the fan stage are identified.

The study identifies a close relation between the number of vortices ingested and the change in rotor performance. However, the deviation from the clean rotor performance has been found to be independent of the circumferential distance between vortices. Additionally, the effects of the radial location, polarity and vortex magnitude have been assessed. Ingested co-rotating vortices cause a significant reduction in pressure ratio and corrected mass flow. In contrast, counter-rotating vortices are associated with an increase in the pressure ratio and corrected mass flow. The change in rotor performance increases with the strength. However, a dramatical drop in pressure ratio is observed for counter-rotating vortices when the vortex strength exceeds a critical value.

Topics: Vortices
Commentary by Dr. Valentin Fuster
2014;():V01AT01A034. doi:10.1115/GT2014-26486.

The objective of the current paper is to gain an understanding of the effects of inlet swirling flow on the flow field through short annular transition diffusers and nozzles. These devices are representative of the primary driving nozzles for certain exhaust ejector systems. It is known that strongly swirling flow can degrade ejector performance due to core separation. It is believed that minor changes in driving nozzle shape can improve ejector performance significantly.

Two configurations of a diffuser/nozzle were tested experimentally and numerically under different swirl strengths. The two configurations were mounted on an annular flow wind tunnel. Two shapes of the annulus’ centre body end; square and elliptical, were used. Based on the hydraulic inlet diameter, average velocity and temperature in the annulus of the wind tunnel, the measurements were carried out at Mach range of 0.21 to 0.26 with Reynolds number of 2.3 to 2.7×105.

Ansys14 was used for the CFD simulations. The measured velocity profiles in the annulus were used as input flow conditions in the CFD investigation. The RNG k-ε turbulence model was used in the CFD simulations. The measured velocity profiles at the device exit, and measured surface pressures on the annulus, duct and nozzle walls were compared with the CFD predictions. The measured back pressure coefficient and total pressure loss through the diffuser systems were compared with the CFD predictions. A reasonable agreement between the experimental data and numerical predictions was observed.

It was found computationally that the size of the central recirculation zone behind the annulus centre body has negative effects on the diffuser performance under different swirl numbers. The square shape of the annulus’ centre body end increased the back pressure and total pressure loss coefficients over the elliptical shape. However, the flow uniformity at the duct and nozzle exits improved with the square shape over the elliptical end. These differences may have a significant effect on ejector pumping.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A035. doi:10.1115/GT2014-26556.

Rain ingestion can significantly affect the performance and operability of gas turbine aero-engines. In order to study and understand rain ingestion phenomena at engine level, a performance model is required that integrates component models capable of simulating the physics of rain ingestion. The current work provides, for the first time in the open literature, information about the set-up of a mixed-fidelity engine model suitable for rain ingestion simulation and corresponding overall engine performance results. Such a model can initially support an analysis of rain ingestion during the pre-design phase of engine development. Once components and engine models are validated and calibrated versus experimental data, they can then be used to support certification tests, the extrapolation of ground test results to altitude conditions, the evaluation of control or engine hardware improvements and eventually the investigation of in-flight events.

In the present paper, component models of various levels of fidelity are firstly described. These models account for the scoop effect at engine inlet, the fan effect and the effects of water presence in the operation and performance of the compressors and the combustor. Phenomena such as velocity slip between the liquid and gaseous phases, droplet break-up, droplet-surface interaction, droplet and film evaporation as well as compressor stages re-matching due to evaporation are included in the calculations. Water ingestion influences the operation of the components and their matching, so in order to simulate rain ingestion at engine level a suitable multi-fidelity engine model has been developed in the PROOSIS simulation platform. The engine model’s architecture is discussed and a generic high bypass turbofan is selected as a demonstration test case engine.

The analysis of rain ingestion effects on engine performance and operability is performed for the worst case scenario, with respect to the water quantity entering the engine. The results indicate that rain ingestion has a strong negative effect on high-pressure compressor surge margin, fuel consumption and combustor efficiency, while more than half of the water entering the core is expected to remain unevaporated and reach the combustor in the form of film.

Topics: Engines , Simulation
Commentary by Dr. Valentin Fuster
2014;():V01AT01A036. doi:10.1115/GT2014-26627.

Non-uniform inlet flow has come back into focus of research during the last years due to the need of increasing the operational range of airborne engines. Higher climbing rates for lower noise pollution at airports as well as boundary layer ingesting inlet designs lead to the demand of inlet distortion resistant engines and compressors, in particular. To fulfill this design task, a deep understanding of the dominant flow physics of the distortion transport through the compressor as well as the influence of the compressor on the upstream flow field is needed. This paper starts with the transport of a circumferential total pressure distortion through a compressor stage. Using numerical results, previously validated by experimental data, a phenomenological approach for the transport is presented. The most important finding is the essential role of the different propagation speeds of the static pressure distortion and the inflow velocity distortion and its decoupling. A static pressure and an inflow velocity distortion are present for all kinds of total pressure distortions caused by the upstream flow field redistribution of the compressor. This decoupling causes not only a significant circumferential increase of the distorted sector but also a strong variation of the distortion magnitude downstream of the compressor stage. All relevant phenomena are present in the phenomenological approach as well as in the numerical and the referred experimental results. Inlet distortions result in a decrease of stability margin [1],[2]. The crucial area for the stability of most modern transonic compressors is the tip region; therefore, the tip region was under particular investigation. The numerical results show that the flow field in the distorted area is shifted toward the stall line. The shock system and the tip clearance vortex behave similar to the results near stall with uniform inflow. No local stall can be observed, although the local operating points within the distorted sector travel beyond the stall line of the compressor map with uniform inflow.

Finally, a new analytical approach for the critical distortion angle is presented. The main finding is the circumferential extent has to be big enough to separate the zones of decoupled distortion quantities.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A037. doi:10.1115/GT2014-26666.

Boundary Layer Ingestion (BLI) is currently being researched as a potential method to improve efficiency and decrease emissions for the next generation of commercial aircraft. While re-energizing the boundary layer formed over the fuselage of an aircraft has many system level benefits, ingesting the low velocity boundary layer flow through a serpentine inlet into a turbofan engine adversely affects the performance of the engine. This work reports an experimental investigation of the effects of a BLI-type distortion on a turbofan engine’s performance. A modified JT15D-1 turbofan engine was used in this study. Inlet flow distortion was created by a layered wire mesh distortion screen designed to create a total pressure distortion profile at the aerodynamic interface plane (AIP) similar to NASA’s Inlet A boundary layer ingesting inlet flow profile. Results of this investigation showed a 15.5% decrease in stream thrust and a 14% increase in TSFC in the presence of BLI-type distortion. The presence of the distortion screen resulted in a 24% increase in mass-averaged entropy production along the entire fan flow path compared to the non-distorted test.

Topics: Engines , Turbofans
Commentary by Dr. Valentin Fuster
2014;():V01AT01A038. doi:10.1115/GT2014-26882.

This paper elaborates on the theoretical development of a mathematical approach, targeting the real-time simulation of aeroelastic rotor blade dynamics for the multidisciplinary design of rotorcraft. A Lagrangian approach is formulated for the rapid estimation of natural vibration characteristics of rotor blades with nonuniform structural properties. Modal characteristics obtained from classical vibration analysis methods, are utilized as assumed deformation functions. Closed form integral expressions are incorporated, describing the generalized centrifugal forces and moments acting on the blade. The treatment of three-dimensional elastic blade kinematics in the time-domain is thoroughly discussed. In order to ensure robustness and establish applicability in real-time, a novel, second-order accurate, finite-difference scheme is utilized for the temporal discretization of elastic blade motion. The developed mathematical approach is coupled with a finite-state induced flow model, an unsteady blade element aerodynamics model, and a dynamic wake distortion model. The combined aeroelastic rotor formulation is implemented in a helicopter flight mechanics code.

The aeroelastic behavior of a full-scale hingeless helicopter rotor has been investigated. Results are presented in terms of rotor blade resonant frequencies, airframe–rotor trim performance, oscillatory structural blade loads, and transient rotor response to control inputs. Extensive comparisons are carried out with wind tunnel and flight test measurements found in the open literature, as well as with non-real-time comprehensive analysis methods. It is shown that, the proposed approach exhibits good agreement with flight test data regarding trim performance and transient rotor response characteristics. Accurate estimation of structural blade loads is demonstrated, in terms of both amplitude and phase, up to the third harmonic component of oscillatory loading. It is shown that, the developed model can be utilized for real-time simulation on a modern personal computer. The proposed methodology essentially constitutes an enabling technology for the multidisciplinary design of rotorcraft, when a compromise between simulation fidelity and computational efficiency has to be sought for in the model development process.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A039. doi:10.1115/GT2014-26922.

The effects of sand and dust ingestion often limit the useful life of turbine engines operating in austere environments and efforts are needed to reduce the quantity of particulate entering the engine. Several Engine Air Particle Separation (EAPS) systems exist to accomplish this task. Inertial Particle Separators (IPS) are of particular interest because they offer significant weight savings and are more compact. This study focuses on the how small particles are affected by the dynamic fluid forces present in the IPS. Using Multi-Phase Particle Image Velocimetry (MP-PIV), 10um and 35um glass spheres were tracked through the IPS. Further, the data was also used to analyze the particles Coefficient of Restitution, (COR), where they impact the Outer Surface Geometry (OSG) of the IPS.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A040. doi:10.1115/GT2014-26991.

This paper describes the work done and strong interaction between the Technology Evaluator (TE), Green Rotorcraft (GRC) Integrated Technology Demonstrator (ITD) and Sustainable and Green Engine (SAGE) ITD of the Clean Sky Joint Technology Initiative (JTI). The GRC and SAGE ITDs are responsible for developing new helicopter airframe and engine technologies respectively, whilst the TE has the distinctive role of assessing the environmental impact of these technologies at single flight (mission), airport and Air Transport System levels (ATS). The assessments reported herein have been performed by using a GRC-developed multidisciplinary simulation framework called PhoeniX (Platform Hosting Operational and Environmental Investigations for Rotorcraft) that comprises various computational modules. These modules include a rotorcraft performance code (EUROPA), an engine performance and emissions simulation tool (GSP) and a noise prediction code (HELENA). PhoeniX can predict the performance of a helicopter along a prescribed 4D trajectory offering a complete helicopter mission analysis. In the context of the TE assessments reported herein, two helicopter classes are examined namely a Twin Engine Light (TEL) configuration for Emergency Medical Service (EMS) and Police missions and a Single Engine Light (SEL) configuration for Passenger/Transport missions. The different technologies assessed reflect three simulation points which are the ‘Baseline’ Year 2000 technology, ‘Reference’ Y2020 technology, without Clean Sky benefits, and finally the ‘Conceptual’, reflecting Y2020 technology with Clean Sky benefits. The results of this study illustrate the potential that incorporated technologies possess in terms of improving performance and gas emission metrics such as fuel burn, CO2, NOx as well as the noise footprint on the ground.

Topics: Simulation
Commentary by Dr. Valentin Fuster
2014;():V01AT01A041. doi:10.1115/GT2014-27017.

Designing propulsion system architectures to meet next generation requirements requires many tradeoffs be made. These trades are often between performance, risk, and cost. For example, the core of an engine is the most expensive and highest risk area of a propulsion system design. However, a new core design provides the greatest flexibility in meeting future performance requirements. The decision to upgrade or redesign the core must be justified by comparison with other lower risk options. Furthermore, for turboshaft applications, the choice of compressor, whether axial or centrifugal, is a major decision and trade with the choice being heavily driven by both current and projected weight and performance requirements. This problem is confounded by uncertainty in potential benefits of technologies or future performance of components. To address these issues this research proposes the use of a Bayesian belief network (BBN) to extend the more traditional robust engine design process. This is done by leveraging forward and backward inference to identify engine upgrade paths that are robust to uncertainty in requirements performance. Prior beliefs on the different scenarios and technology uncertainty can be used to quantify risk. Forward inference can be used to compare different scenarios.

The problem will be demonstrated using a two-spool turboshaft architecture modeled using the Numerical Propulsion System Simulation (NPSS) program. Upgrade options will include off the shelf, derivative engine (fixed core) with no technologies, derivative engine with new technologies, a new engine with no technologies, and a new engine with new technologies. The robust design process with a BBN will be used to identify which engine cycle and upgrade scenario is needed to meet performance requirements while minimizing cost and risk. To demonstrate how the choice of upgrade and cycle change with changes in requirements, studies are performed at different horsepower, ESFC, and power density requirements.

Topics: Engine design
Commentary by Dr. Valentin Fuster
2014;():V01AT01A042. doi:10.1115/GT2014-27047.

NASA is actively funding research into advanced, unconventional aircraft and engine architectures to achieve drastic reductions in vehicle fuel burn, noise, and emissions. One such concept is being explored by Boeing, General Electric, Virginia Tech, and Georgia Tech under the Subsonic Ultra Green Aircraft Research (SUGAR) project [1]. A major cornerstone of this research is evaluating the potential performance benefits that can be attributed to using hybrid electric propulsion. Hybrid electric propulsion in this context involves a non-Brayton power generation or storage source, such as a battery or a fuel cell, which can be used to provide additional propulsive energy to a conventional Brayton cycle powered turbofan engine. Employing additional power sources for thrust production increases the number of degrees of freedom both from a design and configuration standpoint and from an operational one. In order to assess and understand the myriad number of potential new configurations a modeling and simulation tool is needed; however, current state of the art propulsion modeling tools such as the Numerical Propulsion System Simulation (NPSS) are not natively capable of assessing novel hybrid electric configurations.

This research addresses the gap between hybrid electric propulsion and conventional cycle analysis tools by developing a suite of native NPSS elements suitable for hybrid electric engine cycle design and analysis. Elements have been developed for a fuel cell, battery, motor, generator, and electrical distribution system. Both room temperature and cryogenically cooled superconducting variants are developed. The elements are designed such that they can be seamlessly integrated into existing NPSS cycle models to assess any system configuration or architecture the designer can envision.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A043. doi:10.1115/GT2014-27137.

Increases in computing power have enabled designers to efficiently use Monte Carlo simulation to perform robust design analyses of engine systems. Unfortunately, the designer is now faced with the problem of being able to generate more data than he is capable of analyzing or understanding in any tractable manner. Traditional filtered Monte Carlo methods involve creating surrogate model representations (such as Artificial Neural Networks) of a physics based model in order to rapidly generate tens of thousands of model responses as design and technology input parameters are randomly varied within user defined distributions. The downside to this approach is that the designer is often faced with a large design space with which he must perform a significant amount of post processing to arrive at probabilities of meeting design requirements.

This research enhances the traditional filtered Monte Carlo robust design approach by regressing surrogate responses of joint confidence intervals for metric responses of interest. Fitting surrogate responses of probabilistic confidence intervals rather than the raw response data changes the problem the engineer is able to answer. Using the new approach the question can be better phrased in terms of the probability of meeting certain requirements. For example, one will be able to ask, “What engine overall pressure ratio is needed to achieve a 90% chance of reaching a 25% reduction in TSFC from the current state of the art?” The more traditional approach does not have the ability to include confidence in the process without significant post-processing.

The process will be demonstrated using a two-spool axi-centrifugal turboshaft architecture modeled using the Numerical Propulsion System Simulation (NPSS) program. The new robust design process will be used to demonstrate the probabilistic impacts of both technology and design variables on choosing a cycle that is robust to both growth in mission requirements and changes in anticipated technology levels as the design matures. More specifically, the research will present engine cycle variables including component pressure ratios and temperatures that provide for high probability of meeting notional system level requirements, such as ESFC, under different levels of technology uncertainty. Technology uncertainty will be defined in terms of available material properties and component efficiency.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A044. doi:10.1115/GT2014-27200.

This paper presents a numerical investigation of lobed mixer performance at experimentally validated low speed conditions and conditions representative of high speed engine operation. The purpose of this study was to first assess and understand how variations in bypass-to-core area ratio (AR) can affect engine performance, then isolate those effects to determine the efficacy of increasing the number of mixer lobes. The area ratio was manipulated via adjustment of the lobe crest and valley radiuses. No other geometric features were altered in any of the 5 mixers studied (12-lobe AR of 3, 2.5 and 3.5, 16-lobe AR of 3 and 18-lobe AR of 3). Results indicate that performance can be affected by area ratio. Low-speed results showed that pressure loss and thrust output were improved at lower area ratios. High speed results showed the opposite. This behavior is believed to be the result of a bypass-to-core momentum ratio difference between the two test conditions. These effects were avoided when studying the number of lobes by maintaining a constant area ratio. Results indicate that adding lobes enhanced exhaust mixing but hampered performance at low speed conditions. No appreciable performance difference was observed at high speed conditions. Fluid viscosity and associated viscous mixing losses are believed to be the parameters at fault for the reduced low-speed performance results.

Commentary by Dr. Valentin Fuster
2014;():V01AT01A045. doi:10.1115/GT2014-27287.

An architectural modeling tool has been developed to support Accessory Gearbox (AGB) design. This is a structured approach which allows the Preliminary Design team to design an AGB with high fidelity in a short time and earlier in the design process. The program can accommodate a large variety of accessories with different attributes to generate a gearbox of minimum size. The final gearbox shape includes curvature which enables a close fit to the engine carcass. By setting the size, shape and location of the gearbox and accessories earlier, the designer can begin the process of locating external configurations around the gearbox more efficiently. The tool systematically explores the design space to optimize geometry based on multiple design criteria. It successfully constructs the intermediate gear train needed within the gearbox. It provides graphical output to the user with primitive models of the gears, accessories and housing. Once located in engine coordinates, these primitives provide the reference for the next level for detailed design.

Topics: Gears
Commentary by Dr. Valentin Fuster
2014;():V01AT01A046. doi:10.1115/GT2014-27290.

Large eddy simulations are performed for hot and cold single stream jets with an acoustic Mach number of (Ma = Vj/a = 0.875). The temperature ratio (Tj/T) for the hot jet is 2.7 and for the cold jet it is 1.0. Grids with 34 million points are used. The simulation results for the flow field are in encouraging agreement with the mean velocity and Reynolds stress measurements. The Ffowcs Williams-Hawkings (FW-H) equation is used to predict the far-field noise. In this study four cylindrical FW-H surfaces around the jet at various radial distances from the centreline are used. The FW-H surfaces are closed at the downstream end with multiple endplates. These endplates are at x = 25.0D – 30.0D with Δ = 0.5D apart. It is shown that surfaces close to jet get affected with pseudo sound. To avoid pseudo sound, surfaces must be placed in the irrotational region. To account for all the acoustic signals end plates are necessary. However, a simple averaging process to cancel pseudo sound at the end plates is not sufficient.

Commentary by Dr. Valentin Fuster

Fans and Blowers

2014;():V01AT10A001. doi:10.1115/GT2014-25013.

In this paper, a novel global optimization algorithm has been developed, which is named as Particle Swarm Optimization combined with Particle Generator (PSO-PG). In PSO-PG, a particle generator was introduced to iteratively generate the initial particles for PSO. Based on a series of comparable numerical experiments, it was convinced that the calculation accuracy of the new algorithm as well as its optimization efficiency was greatly improved in comparison with those of the standard PSO. It was also observed that the optimization results obtained from PSO-PG were almost independent of some critical coefficients employed in the algorithm. Additionally, the novel optimization algorithm was adopted in the airfoil optimization. A special fitness function was designed and its elements were carefully selected for the low-velocity airfoil. To testify the accuracy of the optimization method, the comparative experiments were also carried out to illustrate the difference of the aerodynamic performance between the optimized and its initial airfoil.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A002. doi:10.1115/GT2014-25140.

Volume flow rate, specific isentropic enthalpy difference, rotor outer diameter and rotational speed of a fan can be transformed to speed number and diameter number. These two non-dimensional numbers are related together in the so-called Cordier-diagram. For axial, radial and mixed flow fans, there is a single empirical relationship between both quantities and it is well accepted that this line represents “optimum” fan designs with high efficiency. Based on velocity triangles, a relationship between flow coefficient and pressure coefficient exists. This so-called performance curve captures off-design operating points as well as the design point of a fan. Therefore, the performance curve can be transformed to the Cordier-diagram to predict the relationship between speed number and diameter number. It is shown that the Cordier-line depends mainly on velocity triangles and the common argument of high efficiency, claimed in the majority of the literature, plays only a secondary role. Nevertheless, the requirement of high efficiency influences the fan design for a certain design point. This paper focuses mainly on axial flow fans. It gives a theoretical interpretation of the influence of blade loading criteria and design limits on the Cordier-line: (1) De Haller number, (2) cascade loading parameter, (3) Lieblein diffusion factor, (4) Strscheletzky swirl number. Criterion (1) reflects the minimum velocity ratio to avoid endwall separation in a linear compressor cascade. Criterion (2) is a combination of lift coefficient and cascade solidity. It reflects the aerodynamic loading of the suction side blade boundary layer. Criteria (1) and (2) are included in criterion (3). Finally, criterion (4) indicates the risk of hub separation due to strong swirl flow. The investigation shows that the transformation of these criteria to the Cordier-diagram gives very similar results. Furthermore, it is shown that the axial fan design limits in the Cordier-diagram are represented by certain hub-to-tip radius ratios.

Topics: Design , Fans , Axial flow , Blades
Commentary by Dr. Valentin Fuster
2014;():V01AT10A003. doi:10.1115/GT2014-25281.

The meta-model based optimization is widely used in the aerodynamical design process for rotating machines, and the main industrial cost of such techniques comes from physical evaluations of answers, either by experimental or numerical means. Design of experiment (DoE) with Latin Hypercube sampling has been studied for the design of an automotive fan system for engine cooling. Surrogate models constructed with Kriging and Co-Kriging methods are estimated with the help of a reference numerical model. The objective of the present work is to assess the necessary number of sampling points for the initial DoE for this kind of meta-model method and to study the influence brought by the sample dispersion. The objective being to execute future aerodynamic optimizations at a reduced cost in term of timeframe and CPU effort. Two parameters, camber and chord length were used to investigate geometrical changes and they are completed with a physical parameter which is the flow rate. The optimization should lead to a higher level of performances with given constraints of rotational speed, torque and packaging. A criterion was defined for the initial necessary number of evaluations and the variances for different DoE design were controlled for the sake of comparison. Starting from an initial meta-model, a variance based method was used for further training with additional points. Uncertainties due to lack of information outside the domain led the model to regularly propose new points on the borders, yielding to high sample variance. A genetic-algorithm was employed to explore the final meta-model and to conduct a multi-objective optimization. Results are presented in terms of Pareto Front and are analysed with SOM to understand the relations between factors and objectives. A final optimal design was selected, and proposed to demonstrate the relevancy of the method.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A004. doi:10.1115/GT2014-25310.

In this report the diffusor geometry for a Pitot-Tube Jet pump (PTJ pump) is optimized, using 3D, steady CFD methods (software: ANSYS Fluent) as well as mesh morphing (software: Sculptor). The design point is Q = 16 m3/h. A complex optimization loop is set up, which takes geometric constraints, as well as manufacturing limits into account. The optimization is multi-objective and best practice guidelines are derived for future diffusor designs with respect to the reduction of total pressure losses and the diffusor displacement in the fluid of the rotor cavity. Both, advantages and disadvantages, are listed and evaluated.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A005. doi:10.1115/GT2014-25319.

A high-efficiency centrifugal fan with low noise emission is used to investigate the influence of the casing width on the flow performance. Common design concepts use a width ratio of casing to impeller outlet from 2.5 up to 3. This range of the width ratio results in a large casing depth, which is not the best approach with regard to the overall dimension and the material costs for large industrial machines. Furthermore, centrifugal fan designs with a width ratio above 3 are disadvantageous because of their lower overall efficiency. In addition to an optimized casing depth, the position of the impeller in the casing also influences the performance. Using the results of several CFD simulations, we show that by creating a very small gap between the impeller rear plate and the casing wall, smaller energy losses and higher–performance can be achieved.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A006. doi:10.1115/GT2014-25339.

The investigated axial flow fan investigated in our paper consisted of an advanced axial flow stage, an inlet chamber and a diffuser. The fan stage with high aerodynamic loading and the hub/tip ratio of 0.6 had the design flow and pressure coefficients of 0.60 and 0.83, respectively. The test and computed CFD aerodynamic performance of the axial flow fan and the fan’s stage were compared, with acceptable results. Subsequently, analysis of the computed 3D flow was carried out within the wide working range at the rotor blades stagger angle variation of ±20°.Consequence of the rotor blades adjustment is that the blade elements work often at the off-design working conditions with the flow separation on the blades suction and pressure sides. The flow is strictly three-dimensional.

Velocity profile distortion and swirl due to the flow separation in the stator blade row decreases the diffuser pressure recovery and efficiency. The diffuser in the axial flow fan environment achieves a significantly higher efficiency in comparison with conical diffuser furnished with ducted-flow inlet conditions due to the increased turbulent mixing.

Inlet chamber loss coefficient slightly decreased with the increasing flow rates due to the Reynolds number effect. Core flow in the inlet chamber is without occurrence of significant vortex inducing motion with the exception of the area near the tube where the fan’s shaft is located.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A007. doi:10.1115/GT2014-25403.

Acceptance tests on large fans to prove the performance (efficiency and total pressure rise) to the customer are expensive and sometimes even impossible to perform. Hence there is a need for the manufacturer to reliably predict the performance of fans from measurements on down-scaled test fans. The commonly used scale-up formulas give satisfactorily results only near the design point, where inertia losses are small in comparison to frictional losses. At part- and overload the inertia losses are dominant and the scale-up formulas used so far fail. In 2013 Pelz and Stonjek introduced a new scaling method which fullfills the demands ( [1], [2]). This method considers the influence of surface roughness and geometric variations on the performance. It consists basically of two steps: Initially, the efficiency is scaled. Efficiency scaling is derived analytically from the definition of the total efficiency. With the total derivative it can be shown that the change of friction coefficient is inversely proportional to the change of efficiency of a fan. The second step is shifting the performance characteristic to a higher value of flow coefficient. It is the task of this work to improve the scaling method which was previously introduced by Pelz and Stonjek by treating the rotor/impeller and volute/stator separately. The validation of the improved scale-up method is performed with test data from two axial fans with a diameter of 1000 mm/250mm and three centrifugal fans with 2240mm/896mm/224mm diameter. The predicted performance characteristics show a good agreement to test data.

Topics: Fans
Commentary by Dr. Valentin Fuster
2014;():V01AT10A008. doi:10.1115/GT2014-25498.

Fans operating at the edges of large-scale air-cooled steam condensers often do so under distorted inlet air flow conditions. These conditions create variations in the aerodynamic loads exerted on a fan blade during rotation which causes it to vibrate. In order to isolate the sources of the unsteady aerodynamic loads as well as their effects on blade vibration, a potential flow fluid dynamics code was written to determine the aerodynamic loads exerted on a fan blade as a function of its rotation. The lift and drag forces were exported to a finite element code approximating the fan blade as a cantilever beam. With these two sets of code the response of the blade when subjected to varying aerodynamic loads could be determined. Furthermore, the effect of changing certain parameters such as blade stiffness or damping can be investigated. It was found that the blade’s response closely resembles that which was measured at the full-scale facility and that slight changes to the blade’s stiffness can potentially reduce the vibrational amplitude but may also lead to resonance.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A009. doi:10.1115/GT2014-25564.

The optimization process of a fan based on 3D viscous CFD calculations is time consuming, especially if many variables are taken into account. With the focus on computational cost efficiency and reliable CFD-results a specific optimization algorithm for radial fans based on CFD calculations is presented. The algorithm is derived from the classical knowledge of flow phenomena occurring in radial fans. The leading edge is adjusted in reference to the stagnation point caused by the incoming flow. The trailing edge is adjusted to achieve the required pressure rise. The 5 blades of the investigated fan are constructed as 3D free surface blades; each blade is separated into 5 profile sections. The optimization process regarding the blade includes 10 independent parameters of the leading and trailing edges.

An additional potential to increase efficiency is obtained by changing the meridional shape of the impeller. To investigate the meridional shape, the blade adjustment algorithm is coupled with a response surface method using the Kriging approximation to find a highly efficient meridional shape.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A010. doi:10.1115/GT2014-25707.

The pressure fluctuations on the volute surface induced by internal unsteady flow are the important sources of fan casing vibration and noise generation. In this paper, a three-dimensional and unsteady flow of a whole impeller-volute structure have been carried out by using commercial CFX code in order to obtain the wall pressure fluctuations on the volute of a large scale centrifugal fan (especially in the vicinity of volute tongue). The two important different flow rates have been simulated, the best efficiency point (BEP) and 130 percent of the BEP (1.3 × BEP), multi-domain structure grids have been applied in all the domains of current simulations. The pressure fluctuations of setting locations on the volute have been obtained by this method. Characteristics of these fluctuations in time and frequency domains were mainly analyzed. The results showed that the amplitudes of these pressure fluctuations over the volute changed with the flow rates variation. The blade passing frequency and their second harmonic frequency were observed clearly, and an important peak presented at the blade passing frequency (BPF). The amplitude of BPF has related with the position of the volute. On the circumferential direction of the volute, the highest values appeared in the vicinity of volute tongue; on the axial position, the peak value was discovered near to impeller shrouds. All the calculation results have been compared to the experimental results showing a good agreement.

Topics: Pressure , Blades
Commentary by Dr. Valentin Fuster
2014;():V01AT10A011. doi:10.1115/GT2014-25776.

There has been renewed interest in the contra-rotating (CR) fan configuration in aviation and other applications where size and weight are important design factors. Contra-rotation recovers swirl energy compared with the single-rotor design, but this advantage is not fully harnessed due to, perhaps, the issue of noise. This study explores passive noise reduction for a small, axial-flow, contra-rotating fan with perforated trailing-edge for the upstream rotor and perforated leading-edge for the downstream rotor. The fan is designed with simple velocity triangle analyses which are checked by 3D flow computations. The aerodynamic consequence and the acoustic benefit of such perforated blading are investigated experimentally. The results show that there is a reduction of total pressure compared with the baseline CR fan at the same rotating speeds, but this is easily compensated for by slightly raising the rotating speeds. A reduction of 6∼7 dB in overall noise is achieved for the same aerodynamic output, although there is a moderate noise increase in the high frequency range of 12.5∼15.0 kHz due to blade perforations. The effect of inter-rotor separation distance is also investigated for the baseline design. A clear critical distance exists below which the increased spacing shows clear acoustic benefits.

Topics: Noise control
Commentary by Dr. Valentin Fuster
2014;():V01AT10A012. doi:10.1115/GT2014-25794.

Railway tunnel and metropolitan metro system ventilation fans are subjected to positive and negative pressure pulses. As a train travels along a tunnel it drives air down the tunnel. This creates a ‘piston effect’ that results in positive and negative pressure pulses on the ventilation fans. A pressure pulse effect transiently drives a ventilation fan to a higher pressure operating point. If the operating point is beyond the fan’s pressure developing capability, there is a risk it may stall. Tunnel ventilation fan designers classically utilise a stabilisation ring to stabilise the fan’s characteristic and thus mitigate the mechanical consequences of driving a fan into stall.

A stabilisation ring consists of an annular chamber that is incorporated into the fan casing over the fan blade’s leading edge. As a tip-limited axial fan approaches stall, boundary layer fluid centrifuges up the blade. The fan stalls at the point when flow inside the annulus reverses direction in the blade tip region. The stabilisation ring provides an annular chamber into which this fluid may flow. It incorporates a set of vanes that redirect the reverse flow into an axial direction, and then reintroduce it into the main-stream flow up-stream of the fan blade leading edge. Although effective in stabilising the fan’s characteristic, stabilisation rings typically reduce fan efficiency by three per cent, and consequently are becoming progressively less acceptable as required minimum fan efficiencies increase.

The reported research combines experimental measurements of overall fan performance with and without a fitted stabilisation ring and a numerical analysis of the flow-field within the stabilisation ring. Visualisation of the flow-field within the stabilisation ring provides an insight into the physical flow mechanisms that enable the stabilisation ring to stabilise the fan’s characteristic. A conclusion of the research is that at the fan’s peak efficiency operating point, flow through the stabilisation ring separated from the stabilisation ring vanes. Therefore, redesigning the vanes within the stabilisation ring to avoid separated flow offers the potential to eliminate this aerodynamic loss mechanism, thus reducing the efficiency loss classically associated with applying a stabilisation ring.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A013. doi:10.1115/GT2014-25809.

Noise level of AC (Air-Conditioner) outdoor unit fan system has always been receiving much concern. It involves both aerodynamic noise and vibroacoustic noise. But most previous studies have been focused on aeroacoustics, and less attention is paid to the latter.

The objective of present study is to identify the influential factors of vibroacoustic noise and quantify its contribution to the overall noise level of the fan system. A numerical approach has been developed for predicting the flow-induced fan casing vibration and noise radiation. A fluid-solid-sound unidirectional coupling technique is used to transfer the unsteady loading to the structure, and the results arising from structural vibration analysis are used as sound radiation boundary conditions. Unsteady fan flow is solved by Large Eddy Simulation (LES) method. Then, the fluid force produced by the fluctuating pressure component acting on the inner casing surfaces is obtained, and it is used as external excitation in the Finite Element Analysis (FEA) model of the casing structure. Further, harmonic response analysis is conducted and the obtained results are used to calculate sound radiation through Indirect Boundary Element Method (IBEM), while the displacement amplitude obtained in structural analysis is used as boundary condition.

Experimental tests are conducted respectively on fan aerodynamic and aeroacoustic performance, and casing vibration. The numerical approach is partially validated by the experimental data. The validated models are used to predict the vibroacoustic noise, based on which, a quantitative evaluation of its contribution to overall sound level is conducted.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A014. doi:10.1115/GT2014-25865.

Regulations require that industrial fans utilised in power generation, cement and steel applications must operate as part of a process that produces erosive particles. Over time these erosive particles erode centrifugal fan impeller blades, changing the blade profile and consequently, degrading fan performance. To replace the eroded impellers, operators must shut down the process. If one must replace an impeller between scheduled maintenance intervals, the associated costs with lost production become significant. Consequently, the industrial fan community is interested in predicting the erosion, and ultimately, a fan impeller’s in-service life when operating in an erosive environment. Industrial fan designers face challenges when attempting to predict impeller erosion. Industrial centrifugal fan impeller blades are routinely constructed from cambered plate, usually with backward or forward sweeping, with the inevitable consequence of separated flow regions. This separated flow is within a highly three dimensional flow-field making difficult an accurate prediction of the flow-field though an impeller with cambered plate blades. Assuming that one can accurately predict this three dimensional flow-field one must then go on to simulate the erosive particles’ trajectory.

This paper builds on the work of other scholars who have developed a computational approach that accurately predicts the flow-field though an impeller with cambered plate blades. The authors report an unsteady numerical analysis with the finite volume open-source code OpenFOAM. The analysis was undertaken using a moving mesh technique, based on Arbitrary Mesh Interface technology. Reynolds Averaged Navier-Stokes equations for incompressible flow were solved with a non-linear first order turbulence closure. They modelled particle transport and dispersion using a Lagrangian approach coupled with a Particle Cloud Tracking (PCT) model. Understanding the particle size effect facilitates identifying critical regions on the impeller blades most prone to erosion for each combination of particle sizes. Identifying the most critical regions thus provides a basis for modifying overall impeller and individual blade geometry in an effort to reduce susceptibility to erosion. This then increases in-service life, and consequently the time between maintenance intervals.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A015. doi:10.1115/GT2014-25916.

The paper presents a methodology for on-site investigation of short-ducted industrial axial flow fans, as an easily realizable and effective means for concerted diagnostics on fan aerodynamics and acoustics along the rotor radius. The methodology relies on the accessibility of the fan from the upstream direction only. It involves experiments such as i) measurement of inlet axial velocity profile along the radius, combined with ii) beamforming studies using the Phased Array Microphone technique. The application of the methodology has been demonstrated in a case study of a ventilating fan. The semi-empirical data processing demonstrated significant changes of aerodynamic properties along the blade span. The acoustic studies regarded a frequency range being significant from the viewpoint of human audition. The phased array data have been processed and evaluated on the basis of two in-house developed beamforming codes, based on the Delay and Sum as well as the Rotating Source Identifier (ROSI) methods. The measurements revealed that the detected noise is dominated by rotating sources of broadband noise. By means of the ROSI code, pitchwise-resolved information has been obtained on the rotor noise. By such means, noise sources such as locally thickened suction side blade boundary layers and tip leakage flow have been identified. The spanwise variation of sound pressure has been compared with cascade loss indicators used in fan analysis and preliminary design, such as the total pressure loss coefficient and the Lieblein diffusion factor. The sound pressure has been found to increase locally with the total pressure loss and diffusion along the dominant portion of blade span, in the frequency bands being the most significant from the viewpoint of audibility of the noise generated by the fan.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A016. doi:10.1115/GT2014-25927.

Large axial flow fans are used in forced draft air cooled heat exchangers (ACHEs). Previous studies have shown that adverse operating conditions cause certain sectors of the fan, or the fan as a whole to operate at very low flow rates, thereby reducing the cooling effectiveness of the ACHE. The present study is directed towards the experimental and numerical analyses of the flow in the vicinity of an axial flow fan during low flow rates. This is done to obtain the global flow structure up and downstream of the fan. A near-free-vortex fan, designed for specific application in ACHEs, is used for the investigation. Experimental fan testing was conducted in a British Standard 848, type A fan test facility, to obtain the fan characteristic. Both steady-state and time-dependent numerical simulations were performed, depending on the operating condition of the fan, using the Realizable k-ε turbulence model. Good agreement is found between the numerically and experimentally obtained fan characteristic data. Using data from the numerical simulations, the time and circumferentially averaged flow field is presented. At the design flow rate the downstream fan jet mainly moves in the axial and tangential direction, as expected for a free-vortex design criteria, with a small amount of radial flow that can be observed. As the flow rate through the fan is decreased, it is evident that the down-stream fan jet gradually shifts more diagonally outwards, and the region where reverse flow occur between the fan jet and the fan rotational axis increases. At very low flow rates the flow close to the tip reverses through the fan, producing a small recirculation zone as well as swirl at certain locations upstream of the fan.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A017. doi:10.1115/GT2014-25932.

For a competitive low pressure axial fan design low noise emission is as important as high efficiency. In this paper a new design method for low pressure fans with a small hub to tip ratio including blade sweep is introduced and discussed based on experimental investigations. Basis is an empirical axial and tangential velocity distribution at the rotor outlet combined with a distinctive sweep angle distribution along the stacking line. Several fans were designed, built and tested in order to analyze the aerodynamic as well as the aeroacoustic behavior.

For the aerodynamic performance particular attention was paid to compensate the influence of reduced pressure rise and efficiency due to increasing blade sweep. This was achieved by a method of increasing the blade chord depending on the local sweep angle which is based on single airfoil data. The tested fans without this compensation revealed a significant noise reduction effect of up to approx. 6 dB(A) for a tip sweep angle of 64° which was accompanied by an unsatisfactory effect of reduced overall aerodynamic performance.

The second group of fans did not only confirm the method of the aerodynamic compensation by a nearly unchanged pressure rise and efficiency characteristic but also revealed an increased aeroacoustic benefit of in average 9.5 dB(A) compared to the unswept version. Beside the overall characteristics the individual differences between the designs are also discussed using results of wall pressure measurements which show some significant changes of the blade tip flow structure.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A018. doi:10.1115/GT2014-26013.

In radial compressors or blowers, a low solidity circular cascade diffuser (LSD) is one of the effective devices to improve the pressure recovery at design flow rate while guaranteeing a wide operating range. The improvement is mainly attributed to the so called secondary flow effect, which reduces the flow separation on the LSD blade at small flow rates. However, it is very difficult to find out the effective shape of the blade in order to promote this secondary flow effect. In this paper, a multipoint and multi-objective optimization technique is applied to design the LSD blade of a centrifugal blower. The optimization method has been developed at the von Karman Institute for Fluid Dynamics (VKI), which makes use of an evolutionary algorithm, a metamodel as a rapid exploration tool, and a high fidelity 3D Navier-Stokes solver.

The optimization is aiming at improving the static pressure coefficient at design point and at low flow rate condition while constraining the slope of the lift coefficient curve. Seven detailed design parameters describing the shape and position of the LSD vane were introduced, e.g. the radial spacing between impeller exit and the LSD leading edge, the radial chord length and the mean camber angle distribution of the LSD blade with five control points. Moreover, a small tip clearance of the LSD blade was applied in order to activate and to stabilize the secondary flow effect at small flow rate condition. The optimized LSD blade has an extended operating range of 114 % towards smaller flow rate as compared to the baseline design without deteriorating the diffuser pressure recovery at design point. The diffuser pressure rise and operating flow range of the optimized LSD blade are experimentally verified. It is found that the optimized LSD blade shows good improvement of the blade loading in the whole operating range, while at small flow rate the flow separation on the LSD blade has been successfully suppressed by the secondary flow effect. This is fully corresponding to the CFD predictions and demonstrates the effectiveness of the optimization methodology, by limiting the experimental testing to only two geometries.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A019. doi:10.1115/GT2014-26048.

Contra-rotating fans have several advantages over single stage axial fans. If they are well designed, the exit flow field is almost irrotational. This helps to increase the aerodynamic efficiency by up to 16%, when compared to single stage fans. However, since the second stage interacts with the flow disturbances from the first stage, the associated noise generation is a disadvantage. This may be remedied by carefully tuning the design.

The optimization of a contra-rotating fan involves a large set of design parameters. These include the geometrical parameters of the fan blades, the winglets, the guide vane as well as the hub diameter. We demonstrate an evolutionary algorithm which helps to automate the optimization process. It is controlled by two objective functions: (1) aerodynamic efficiency and (2) the emitted tonal noise.

For the evaluation of the sound pressure, we implemented a new lattice-Boltzmann solver. Due to its algorithmic structure, this is ideally suited for massive parallelization. To leverage this potential, it is designed to run on general-purpose graphics processing units (GPGPUs). To further accelerate the optimization, it is supported by a meta-model based on a radial-basis function network.

We demonstrate the method for a small contra-rotating fan. Our numerical results are compared with physical tests. The new algorithmic arrangement has shown to drastically cut development costs and time.

Topics: Optimization
Commentary by Dr. Valentin Fuster
2014;():V01AT10A020. doi:10.1115/GT2014-26253.

The main focus of this work is on the geometrical modifications that can be applied to the fan wheel and the volute tongue of a radial fan to reduce the tonal noise. The experimental measurements are performed by using the in-duct method in accordance with ISO 5136. In addition to the experimental measurements, CFD (Computational Fluid Dynamics) and CAA (Computational Aeroacoustics) simulations are carried out to investigate the effects of different modifications on the noise and performance of the fan. It is shown that by modifying the blade outlet angle, the tonal noise of the fan can be reduced without affecting the performance of the fan. Moreover, it is indicated that increasing the number of blades leads to a significant reduction in the tonal noise and also an improvement in the performance. However, this trend is only valid up to a certain number of blades, and a further increment might reduce the aerodynamic performance of the fan. Besides modifying the impeller geometry, new volute tongues are designed and manufactured. It is demonstrated that the shape of the volute tongue plays an important role in the tonal noise generation of the fan. It is possible to reduce the tonal noise by using stepped tongues which produce phase-shift effects that lead to an effective local cancellation of the noise.

Topics: Noise control , Blades
Commentary by Dr. Valentin Fuster
2014;():V01AT10A021. doi:10.1115/GT2014-26386.

The increasing demand for comfort and quietness from the automotive industry transforms the acoustics performances of subsystems as a critical input for the selection of a specific design. Among this market, rotating systems noise takes a growing importance and automotive alternators are strongly impacted by this aspect. Alternators contain many different types of rotating parts such as cooling fans and claw poles and their corresponding flow-induced noise contributions and interaction mechanisms driving the noise generation have to be assessed as early as possible in the product development process.

Experimental methods have been historically used to identify and reduce the most obvious phenomena at the origin of the broadband and tonal contents of the noise. Considering the complexity of this device, it appears practically more and more difficult to understand the involved mechanisms and to identify and treat the remaining aeroacoustics sources. The use of digital solutions to simulate the corresponding flow-induced noise contributions and to provide an insight on the noise generation mechanisms represents an alternative to this experimental approach. Furthermore, numerical approach allows a broader design space exploration, where experimental testing can sometime be limited by other constraints, such as mechanical, thermal and electromagnetic aspects. Another advantage of using CAE method is to reduce the product development cycle and the number of expensive prototypes. The highly detailed geometry features and the constrained environment of alternators however represented a real challenge for computational aeroacoustics solutions.

In this paper, an unsteady and compressible computational approach based on the Lattice Boltzmann Method (LBM) is used to simultaneously predict the 3-D turbulent flow and the corresponding acoustic field of an automotive alternator. The complete rotor-stator model including all geometrical details and the truly rotating geometry is simulated. Numerical and experimental far field sound pressure levels and acoustic power comparisons are presented. Additional transient and spectral flow analysis are performed to diagnose flow-induced noise problems and to provide a better understanding of the aeroacoustics sources.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A022. doi:10.1115/GT2014-26415.

The objective of this work is to present, by means of experimental, analytical and numerical techniques that sound pressure level generated by radial-bladed centrifugal fans of electric motor cooling systems may be expressed by a logarithmical ratio of the peripheral velocity of rotor, volumetric flow and efficiency of the fan. The proposed methodology proved to be efficient and simple in the prediction of generated noise by radial-bladed centrifugal fans of TEFC motors with accuracy of ± 3 dB.

In addition, the acoustic resonance mode of the fan cavity were determined by means of numerical simulations, which its results were validated through experiments using waterfall spectrum.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A023. doi:10.1115/GT2014-26468.

The EU-funded MACCSol project is developing a new modular air-cooled condenser design for power plant applications in water scarce regions. In that scope, this work is to examine the influence ambient winds may have on an axial ventilator’s performance. An axial fan test rig was built inside a wind tunnel environment at the University of Erlangen to realize different wind velocities and angles. Total fan pressure was captured using an array of 81 Kiel probes. The validity of the test rig adaptations to the wind tunnel restrictions was shown in comparison to results from ISO 5801 standard fan test rigs. Two different fan geometries were examined in their characteristic fan curves’ reactions to wind influence at the free fan inlet. The two fans’ characteristic curves showed effects differing in magnitude, but similar in their tendency. While frontal winds tended to improve fan performance, cross winds reduced it. In reverse operating mode, the effect of wind at the fan outlet demonstrated little but positive influence on the fan curve. In order to reduce negative cross wind influence at the fan inlet, different conical and cylindrical inlet extensions were tested. Short conical shrouds performed best.

Topics: Fans , Wind
Commentary by Dr. Valentin Fuster
2014;():V01AT10A024. doi:10.1115/GT2014-26563.

Complex experimental study of the turbulent swirl flow behind the axial fan is reported in this paper. Axial fan with nine blades, designed to generate Rankine vortex, was positioned in the circular pipe entrance transparent section with profiled free bell mouth inlet. Two test rigs were built in order to study the turbulent swirl flow generated on the axial fan pressure side in the case of axially unrestricted and restricted swirl flows. One-component laser Doppler anemometry (LDA) and stereo particle image velocimetry (SPIV) were used in the first test rig in the measuring section 3.35D, measured from the test rig inlet. One of the latest measurement techniques, high speed SPIV (HSS PIV), was used for the measurements in the second test rig in the section 2.1D downstream the fan’s trailing edge. Achieved Reynolds numbers in the first test rig are Re = 182600 and 277020, while in the second Re = 186463. Turbulent velocity field non-homogeneity and anisotropy is revealed using the LDA system. Calculated turbulent statistical properties, such as moments of the second and higher orders, reveal complex mechanisms in turbulent swirl flow. It is shown for the used axial fan construction that swirl number has almost constant value for two various duty points generated by changing rotation number. Study of the instant and mean velocity fields obtained using SPIV discovers vortex core dynamics. Obtained percentage of the unique positions of the total velocity minimum are 10% for the first regime, while 11.5% for the second regime in the first test rig. HSS PIV experimental results have also shown the three-dimensionality and non-homogeneity of generated turbulent swirl flow. Experimentally determined and calculated invariant maps revealed three-component isotropic turbulence in the vortex core region.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A025. doi:10.1115/GT2014-26605.

An analytical formulation for axial fan performances at and off design point is proposed for an arbitrary work distribution. It is shown that the total pressure, total-to-static pressure and hydraulic efficiency characteristics can be described by means of hyperbola, straight lines and parabola. This formulation is applied to axial fans with free vortex work distributions, allowing a theoretical study onto the influence of the hub-to-tip ratio. Theoretical predictions are compared with numerical simulations. Good qualitative agreements are found. Conclusions onto the best practice guidelines for the design of axial fan according to a free vortex work distributions are presented.

Topics: Design , Fans
Commentary by Dr. Valentin Fuster
2014;():V01AT10A026. doi:10.1115/GT2014-26719.

The Slip phenomenon strongly influences the working conditions and performance of turbomachines. It is of interest to know accurately the parameters influencing the slip factor and its effect on the turbomachines. The present work incorporates an experimental analysis of the slip factor for different types of impellers. The main purpose is to provide an insight into the effect of different blade exit angle on the slip factor. Therefore, a single stage centrifugal blower with three types of impellers viz. backward curved, backward curved with radial tipped and forward curved was developed for experimentation. A total of 12 test locations, at an interval of 30°, were selected near the impeller outlet regions. The volute of the blower was kept the same for all types of impellers. To analyze a 3-dimensional flow near the impeller region, a five hole probe is used. Pressure measured by a 5-hole probe was recorded with the help of pressure transducers. Experimental results indicate that, the slip factor was not constant at a whole impeller width as well as the impeller outlet radial periphery. The value and nature of the curve for the slip factor was different at all angular positions, along the width of the impeller. This may be due to the effect of rotor-stator interaction. The effect of rotor-stator interaction increases as we move from the tongue to the exit of the volute. This affects the impeller outlet flow at different angular positions. From the experimental results, it was observed that, the impeller outlet blade angle also has a significant effect on the slip factor. For the present case the value of the slip factor was the highest with the backward curved impeller and the lowest with the forward curved impeller.

Topics: Impellers
Commentary by Dr. Valentin Fuster
2014;():V01AT10A027. doi:10.1115/GT2014-26858.

Aerodynamic noise prediction is a major challenge in computational aeroacoustics due to the complexity of phenomena involved such as turbulence and laminar to turbulent transition. Accurate numerical methodologies, capable to provide reliable predictions in a reasonable computational time, are of large interest for the industrial design of Heating, Ventilation and Air-Conditioning (HVAC) systems. The objective of the present research work is to benchmark different CFD/CAA simulation setup (i.e. mesh topologies, boundary conditions) for predicting the broadband noise generated by low speed axial fans to develop guidelines for reliable and computationally affordable simulation.

In previous works the authors investigated the capabilities of the Zonal LES technique coupled with the Ffowcs Williams-Hawkings acoustic analogy for the prediction of the noise generated by an unducted low speed axial fan. The results showed a good agreement with aerodynamic and aeroacoustic experimental data.

Despite the achievements obtained so far, the high physical and numerical complexity of the problem calls for further investigations. The latest developments, presented here, focus on the impact of the mesh topology and the inflow turbulence on the far field noise prediction.

Two computational meshes with different topology are investigated: an unstructured-hybrid mesh, which can be generated with fast and highly automated methods, and a structured-hybrid mesh, which allows better control of the volume mesh around the blade. Both meshes are designed to adequately resolve the boundary layer, providing LES driven values of y+, x+ and z+ on the blade surface for the operating condition considered.

Two different levels of inflow turbulence are studied, one representing an ideal turbulence-free unbounded environment, and one mimicking the experimental measurements environment.

All the aerodynamic and aeroacoustic simulation results presented are benchmarked with experimental data acquired by the authors.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A028. doi:10.1115/GT2014-26877.

The work presented in this paper is concerned with a methodology for substituting time consuming CFD investigations of the operational characteristics of axial fans by CFD-trained meta-models. For that, the fan geometry is parameterized by 25 physically interpretable quantities allowing for a huge variety of potential fan designs. The parameters are varied by Design of Experiment (DoE) and characteristic curves of approximately 10,000 fan designs are produced using the Reynolds-averaged Navier Stokes (RANS) method. Pressure rise, efficiency, and circumferentially averaged flow profiles upstream and downstream of the rotor are extracted from the RANS results and used to train the meta-models which are Artificial Neural Networks (ANN) or, more specifically, Multilayer Perceptrons (MLP). Special care is taken to mitigate extrapolation weaknesses of the MLPs which could compromise their suitability to compute the target function in optimization algorithms. With these extra efforts, it is possible to aerodynamically optimize axial fans for arbitrary design points within the range typical for axial or even mixed-flow fans according to Cordier’s diagram of turbo machinery. On top of that, designs with good efficiency are also found outside the well known Cordier range. In particular, an extension of feasible operating points towards untypically high specific fan diameters is observed. These findings are relevant for designs aiming at high total-to-static efficiency and make optimized axial fans compete with other fan types, especially with mixed-flow fans.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A029. doi:10.1115/GT2014-26899.

Structural excitation and direct production of acoustic sources in the flow lead to the sound emission of side channel blowers. Pressure fluctuations in the region of the interrupter excite the structure and result in a tonal sound emission at the blade passing frequency (BPF). Furthermore the flow phenomena produce acoustic sources that are radiated in the far field. These different mechanisms of the structural and acoustic excitation are calculated in a simplified way. In a second step each source is separately coupled into the side channel blower. The structural excitation is realized with a shaker. The acoustic sources are generated with a horn driver (speaker) and are induced into the system through the outlet of the blower. Using this setup, the contribution of each source to the tonal sound in the far field can be estimated and compared to the total measured sound pressure level (SPL) at the corresponding frequency.

Topics: Acoustics , Sound
Commentary by Dr. Valentin Fuster
2014;():V01AT10A030. doi:10.1115/GT2014-26983.

According to the direction of flow, fans can be classified in the following three types: axial, mixed flow and radial fans. If large volume flows are required with a proportionally high pressure increase, mixed flow fans can be used. Similar to axial fans, mixed flow fans provide a large flow rate and because of the radial portion in the flow deflection they increase the pressure ratio. Contrary to axial and radial fans, in technical literature there are only a few recommendations concerning the design method of mixed flow fans available.

The present study focuses on the comparative research of four different blade design methods adapted to mixed flow fans. The selected methods created a straight blade, a profiled blade, a foreward curved “Engelberg” blade and a backward curved “Pfleiderer” blade. The contour of the hub and shroud remained unchanged during the entire design process as a fixed parameter. The 4 different blade sets were manufactured and experimentally tested on a suction side chamber test stand according to DIN EN ISO 5801.

The measurement results show, for comparison, the characteristic curves relating flow rate and pressure increase and the achieved overall efficiencies. Here, the design method of “Pfleiderer” has proved its worth, both through its precise meeting of the design point as well as by achieving the highest efficiency.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A031. doi:10.1115/GT2014-27071.

Large diameter fans with low solidity are widely used in automotive application for engine cooling. Their designs with small chord length help reducing the torque on the electrical motor and provide a good aerodynamic compromise between several operating conditions, some of these being at high flow rate. Their global performances are measured according to the ISO standard DP 5801, which allows comparison of results from different facilities. However, some variations in global performances are observed when considering results from two different test rigs. On a fan selected for the purpose of this study, up to 6 % of efficiency is lost on the worst case.

As efficiency is more than ever a key factor to select a component, some experimental and numerical investigations were conducted to analyze the fan behavior on each facility. Two sets of measurement and simulation are performed and compared. Geometries considered for the domain of computation include the test rig plenum, the torquemeter, the ground and a large domain for the atmospheric conditions. The exact fan geometry with tip clearance and under-hub ribs is also considered. Numerical results show in both cases a good agreement with experiment when convergence is reached and for low flow rate when computations are switched to unsteady mode. Comparisons show that simulations are able to capture the different fan behaviors depending on the configuration and those efficiency losses previously observed are correctly predicted.

These results are further analyzed to perform some post-processing. Blade loading remains identical for both cases but disparities appear in the wake and its interaction with the surrounding. Tiny details that are often neglected during experiment and/or simulation appear to be the cause of slight variations. Position of the torquemeter and shape of the plenum are among the parameters that varies and that have cumulative effects. Efficiency being a ration of pressure and torque, variations are rather important.

Finally, these results are discussed in terms of rules for conception and a new geometry less sensible to loss of efficiency is proposed.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A032. doi:10.1115/GT2014-27112.

Dynamic loads in turbomachines may lead to severe fluid induced vibrations, in particular, if resonance conditions are matched. The major sources of such unsteadiness are rotor/stator interaction, flow separation on the blade suction side at part-load and separation due to the curvature of the shroud. In the present work, some of these phenomena are investigated using computational techniques (Computational Fluid Dynamics - CFD) as well as novel measurement methods (High-Speed Particle Image velocimetry - PIV). The measurements provide a unique database of the velocity fields in an industrial impeller at various operation conditions. They are the base for validation of the computational methods. In view of the complexity of the separated, transient and turbulent flow field, this is still an open issue. Besides that, the analysis of the transient flow fields allows the determination of the “modes” of unsteady forces und thus, to shed some light on the sources of unsteadiness in the flow.

Commentary by Dr. Valentin Fuster
2014;():V01AT10A033. doi:10.1115/GT2014-27176.

The controlled vortex design is a common criterion to distribute circulation along the blade span for high total pressure axial fan rotors when diameter and/or rotational speed limitations are imposed. In addition, industrial fans have to comply with Standards which impose high total efficiency.

Researchers involved in the field of fan blading fluid-dynamics showed that forward sweep of the profiles stacking line may give beneficial effects in controlled vortex design blading. However, the published literature is not always unanimous in quantifying these effects and still lacks of clearly outlined design criteria. The paper searches for design guidelines that increase the performance of a rotor-only tube axial fan featuring a constant swirl blade design without reduction in total efficiency. The original fan is experimentally tested and considered as base design for CFD models that were build to estimate the effect of design modifications. The study of the interaction between blade sweep angle, tip clearance and radial shift of the meridional flow across the rotor suggested a design procedure which increases fan total pressure of about 10% at design point and significantly extends the stable operation range while keeping similar values of total efficiency within the whole operation range.

Topics: Design , Rotors
Commentary by Dr. Valentin Fuster

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