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Heat Transfer, Fluid Flows, and Thermal Systems

2009;():1-5. doi:10.1115/IMECE2009-10183.

The nanocomposites were prepared by adding the multi-walled carbon nanotubes (CNTs) to the melting palmitic acid to improve the thermal transfer property. Mechanochemical reaction was used to treat CNTs for dispersing into organic matrix easily. The treated CNTs were introduced hydoxide radical functional groups on the surfaces during ball milling with the potassium hydroxide. And they were successfully dispersed into the palmitic acid matrix without any surfactant. The resulted nanocomposites were stable and homogeneous. The thermal conductivities and latent heat capacities of these nanocomposites were measured using a transient hot wire apparatus and a differential scanning calorimetric instrument. Nanotube composites, containing a small amount of CNTs, have substantially higher thermal conductivities than the base palmitic acid matrix, with the enhancement increasing with the mass fraction of CNTs in both liquid state and solid state. While the latent heat capacities turned out a reverse with the CNT loadings.

Commentary by Dr. Valentin Fuster
2009;():7-13. doi:10.1115/IMECE2009-10206.

Precise process temperature control of 0.001°C under circumstances of noise-temperature change of 0.1°C is required in semiconductor manufacturing process. We studied optimum control method to minimize temperature change at an object position in a 2-dimensional vertical plate with a varying noise-heat-generation and a control-heater. We numerically calculated 2-dimensional unsteady thermal conduction in the plate with feedback control, feed-forward control, and model predictive control of the control-heat-generation. The temperature change at the object position can be decreased 1/80 times smaller than that without control-heat-generation using the feedback control with two monitoring temperatures. The temperature change at the object position can be decreased 1/1000 times (0.002°C) using the model predictive control of 5 s interval with step response pattern as a dynamic predictive model. We found that the accuracy of the dynamic predictive model is very important for precise temperature control. Experiment was performed for the model predictive control with a network model as the dynamic predictive model, and the experimental result agreed with the calculation result.

Commentary by Dr. Valentin Fuster
2009;():15-21. doi:10.1115/IMECE2009-10796.

Several modifications to physical vapor deposition (PVD) models are proposed to address the deficiencies in current theoretical studies. Simple calculations show that the flow regime of PVD fabrications will most likely vary from a continuum flow to a rarefied flow in the vacuum chamber as the vapor cloud expands toward the substrate. The flow regime for an evaporated ideal gas is calculated and then an improved equation of state is constructed and analyzed that more accurately describes vaporized metals. The result, combined with experimental observations, suggests PVD fabrication is best represented by a multi-regime flow. Then, a CFD analysis is summarized that further validates the multi-regime analysis hypothesis. Finally, a methodology for constructing and implementing the results of a theoretical multi-regime PVD model is presented.

Commentary by Dr. Valentin Fuster
2009;():23-32. doi:10.1115/IMECE2009-12122.

In industrial forming processes such as extrusion or injection molding, polymeric materials experience severe thermomechanical conditions: high pressure, high deformation rates, very fast cooling kinetics and important temperature gradients. In semi-crystalline thermoplastics, such as polypropylene, these phenomena have a major influence on the crystallization occurring during cooling, which determines the final microstructure. Predicting the solidified part properties by numerical simulation requires the implementation of a crystallization kinetics model including both the thermally and flow induced effects. In this work, a numerical model simulating polymer crystallization under non-isothermal flows is developed. The model is based on the assumption that the polymer melt elasticity, quantified by the first normal stress difference, is the driving force of flow-induced extra nucleation. Two sets of Schneider equations are used to describe the growth of thermally and flow induced nuclei. The model is then coupled with the momentum equations and the energy equation. As an application, a simple shear flow configuration between two plates (Couette flow) is simulated. The relative influence of the mechanical and thermal phenomena on the crystallization development as well as the final morphology distribution is finally analyzed as a function of the shearing intensity, in terms of nucleation density and crystallite mean sizes.

Commentary by Dr. Valentin Fuster
2009;():33-43. doi:10.1115/IMECE2009-12303.

This paper describes the relation between the Secondary Dendrite Arm Spacing (SDAS), Area of Mushy Zone with the Continuous Casting variables in low carbon steels during the solidification process in the mold zone. A Finite Element analysis of the heat flow equation, coupled with the solute distribution model and the dendrite growth model, enables the determination of the Secondary Dendrite Arm Spacing (SDAS). The CONBCAST.FOR program is developed in this work to analyze effects of process variables on the Secondary Dendrite Arm Spacing (SDAS), Area of Mushy Zone and Volume of the Bleed. Effort is also made to find the optimum casting parameters. A new concept is introduced in this work to analyze the relation between the Area of Mushy Zone and Secondary Dendrite Arm Spacing (SDAS) with the Volume of the Bleed. Quantitative work is performed by collecting the square shaped billets at two different process conditions and determined the Secondary Dendrite Arm Spacing and Volume of Bleed to analyze the relationship between SDAS, Area of Mushy zone and Volume of Bleed.

Commentary by Dr. Valentin Fuster
2009;():45-50. doi:10.1115/IMECE2009-12593.

Quality of laminates produced by Seeman Composite Resin Infusion Molding Process (SCRIMP) is studied by comparing their Fiber Volume fraction and void content. SCRIMP is a variant of Vacuum Assisted Resin Transfer Molding (VARTM). Manufacturing process parameters are then identified and varied to study the impact on mechanical properties of laminated composites. Modification to SCRIMP is carried out by infusing the resin under additional pressure. Optimal process parameters for this modified SCRIMP process are suggested to yield laminates that are repeatable and consistent in quality. Void content is reduced in the composite laminates by altering the vacuum pressure level. Thickness gradient commonly found in SCRIMP processed laminates is eliminated by allowing longer de-bulking time. Final laminate quality is measured using ASTM standardized mechanical testing.

Commentary by Dr. Valentin Fuster
2009;():51-60. doi:10.1115/IMECE2009-13185.

Predicting the changes in the temperature, displacement and stress fields during Laser Powder Deposition (LPD) is of particular importance. To create a FEM model of LPD, it is convenient to consider first Laser Glazing (LG) since it does not involve the addition of material. Once an adequate approach has been identified to model the thermal aspects associated with the effect of the laser, the complexity of adding material can be included. In this paper coupled temperature-displacement FEM models of LG and LPD developed using the commercial FEM code ABAQUS/Standard are presented. In the case of LG, a model based on the sequentially coupled thermo-mechanical theory was used to predict the temperature distribution, deformations and stresses in a rectangular plate on which the laser moved along a straight path. The results for the temperature distribution were validated using Rosenthal’s solution and experiments performed using the same material and processing parameters. For LPD, the model was developed using fully coupled thermo-mechanical theory and it was limited to thin-wall builds deposited on a plate with dimensions comparable to the wall thickness. To add material, new elements were sequentially introduced in the mesh. Qualitatively, the results obtained with the model were promising.

Commentary by Dr. Valentin Fuster
2009;():61-65. doi:10.1115/IMECE2009-10204.

The development of ripples and erosion on the material surfaces in a centrifugal slurry pump was investigated in laboratory tests using a sand-water slurry pot tester. The erosion of the primary material (KmTBCr26) used in the centrifugal slurry pumps was very serious. The ripple formation was influenced by the flow conditions, the impact angle of the solid particles and the particle size. Ripple formation was also observed in laboratory tests with structural steel (#40) and brittle ceramics (Al2 O3 , ZTA, Si3 N4 ). The ripple profile on the structural steel surface was similar to that on the high chrome cast iron (Cr26) used in the slurry pumps. With 90° impact angles, ripples also formed on the eroded surfaces of materials tested in the sand-water slurry pot. The ripple wavelength on the ceramic surface, which was influenced by the mechanical properties and material grain size, was less than that on the metallic surfaces.

Topics: Erosion , Slurries
Commentary by Dr. Valentin Fuster
2009;():67-74. doi:10.1115/IMECE2009-10379.

Unsteady jet mixing of Non-Newtonian fluids was investigated in order to develop a mixing correlation for treatment of stored radioactive waste prior to disposal. The radioactive waste was simulated by using carbopol mixtures, which possess both Newtonian and Non-Newtonian fluid rheological characteristics. A particle image velocimetry (PIV) technique with high spatial and temporal resolution was used to measure jet axial velocity, vector field velocity, and mixing properties of the carbopol mixtures. The relationship between the decaying jet axial velocity, tank geometry, fluid rheology and initial jet velocity were determined. A mathematical correlation was developed to estimate jet velocity in submerged jet-agitated tanks using the Buckingham Pi theorem and Dimensionless Numbers that influence the jet velocity and agitation in the tank.

Commentary by Dr. Valentin Fuster
2009;():75-79. doi:10.1115/IMECE2009-10421.

Turbulent drag reduction by culture solutions of dry malted rice was investigated in a 2.00mm-inner-diameter pipe flow of length 50 diameters at Reynolds numbers from 500 to 8000. The drag reducing abilities of the solutions were tested by comparing drag reduction effectiveness at different concentrations and culture times in water. Comparisons between polysaccharide biopolymer solutions and culture solutions of dry malted rice revealed that the test solutions exhibited Type B drag reduction, which were roughly parallel to, but displaced upwards from, the Newtonian Prandtl-Kármán law. The maximum drag reduction ration was about 30% at a Reynolds number of 8,000. It is shown also that the onset point of drag reduction phenomena was Ref = 200.

Topics: Drag reduction
Commentary by Dr. Valentin Fuster
2009;():81-90. doi:10.1115/IMECE2009-10473.

Micro-channels are presently used extensively in applications related to biomedicine and others. These applications commonly involve flow patterns apt for separating particles in suspension, or fluid layers or fluid portions. In order to find a desired optimum performance it is necessary, among other factors, to determine the pressure gradients that would generate the required flow patterns. Presently the driving pressure gradients are produced by means of mechanical devices, such as syringe pumps, which have obvious limitations in the case where the flow time-pattern is complex. High frequency flows with varying amplitude or swift changes in acceleration imply overcoming big inertia forces in mechanisms. In this paper it is presented a novel and alternative methods for providing complex pressure pulses for the case were the working fluid changes properties when affected by magnetic fields. It is shown that an appropriate arrangement of parallel tubes, subject to different magnetic fields, can induce a wide variety of pressure pulses in selected stations of the system. These stations can be used for connecting the working channel. An analytical model is presented together with several applications.

Commentary by Dr. Valentin Fuster
2009;():91-96. doi:10.1115/IMECE2009-10583.

In this study, rheological properties of five honey samples (Acacia, Citrus Blossom, Fynbos, Bluegum and Raw) were measured. Steady shear and dynamic rheological tests confirmed almost Newtonian behavior for all samples examined over the temperature range of 0–60° C. The water contents of honeys were between 15% and 23%. Fitting of the data in the temperature experiment, showed that primarily expected Arrhenius model is not the best model. Excluding the Raw honey that was possible to be fitted to the said model, the other honey samples were best fitted to a Hyperbolic Tangent model. For the fitted Raw honey however, the activation energy was evaluated to be 95783 J/kg mole. While some rather exceptional viscoelastic properties were observed for the samples, the thixotropic effects were virtually nonexistent. These findings can provide insight into the microstructural, physiological and sensory changes. Also, the study completes a series of studies on rheology of honeys conducted worldwide.

Topics: Rheology
Commentary by Dr. Valentin Fuster
2009;():97-102. doi:10.1115/IMECE2009-10585.

In this study, the rheological properties of different samples of olive oils, from the same producer were obtained in a wide range of temperature. At constant temperatures, the shear rate was also varied to obtain heating effects. It was found that all the samples reach a minimum viscosity in the temperature range of 120°C–150°C before thickening to higher viscosities. The viscosity remained almost unchanged in high shear rates regardless of temperature, indicating no shear thinning effects. No thixotropic effects were observed for the olive oils. These findings can provide insight into the microstructural, physiological and sensory changes at frying (high) temperatures.

Commentary by Dr. Valentin Fuster
2009;():103-110. doi:10.1115/IMECE2009-10981.

We have investigated phase transitions and internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis, by means of the Monte Carlo method. In the present study, we have considered a quasi-2D system in order to clarify the influences of volumetric fraction of particles and magnetic field strength on particle aggregations and phase transitions. Internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation and orientational pair correlation functions. The main results obtained here are summarised as follows. When the influence of magnetic interactions between particles is of the same order of the magnetic field strength, the particles form column-like clusters, and the internal structure of a suspension shows solid-like structures. For the case of strong applied magnetic field, the internal structure is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under circumstances of a strong applied magnetic field, and the internal structure exhibits solid-like ones.

Commentary by Dr. Valentin Fuster
2009;():111-119. doi:10.1115/IMECE2009-10982.

We have investigated aggregate structures and rheological properties of a colloidal dispersion composed of ferromagnetic spherocylinder particles with a magnetic moment along the particle axis direction, by means of Brownian dynamics simulations. In concrete, we have attempted to clarify the influences of the flow field, magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the internal structures of clusters, we have concentrated our attention on the radial distribution and orientational distribution functions. The present results are compared with those of the theoretical analysis for dilute dispersions and also non-dilute dispersions; the results for the latter were obtained by means of the mean-field approximation, which magnetic particle-particle interactions can be taken into account. Some important results are summarized as follows. For the case of the magnetic field strength and magnetic interactions between particles are more dominant than the viscous forces due to a simple shear flow, chain-like like clusters are formed along the magnetic field direction, although they are slightly tilted to the flow direction. When magnetic particle-particle interactions become over a certain value, such cluster formation leads to a significant increase in the viscosity of the dispersion.

Commentary by Dr. Valentin Fuster
2009;():121-127. doi:10.1115/IMECE2009-11044.

In many biomedical and industrial technological applications it is necessary to drive fluids that exhibit plastic or elasto-plastic behaviour. Particularly for cases in which fast results are demanded, or only small fluid samples are available, micro-channels are used. These normally have cross-section geometries determined by the manufacturing procedures and may be of square, rectangular, triangular and other shapes. In order to obtain the expected results, such as protein separation, particle concentrations, and the like, the rate of flow and time-dependent pattern of it must be carefully determined. In this paper it is presented an analytically rigorous method for calculating the friction coefficient in channels of arbitrary cross-section contours for plastic flow that may be conveniently described by the Bingham model. Results are presented for a variety of shapes and for different values of the yield stress.

Commentary by Dr. Valentin Fuster
2009;():129-132. doi:10.1115/IMECE2009-12144.

In this paper, a column is put in a vibrated bed as an intruder to study the force of bed by changing intruder’s mass and vibration strength. Our result indicates that the granular material is completely different with conventional fluid, which the force on intruder does not satisfy with Archimedes Law. The force is strongly influenced by its position in bed, as well as its size, shape and vibration strength. The deeper the intruder immerges, the bigger the force is. Because of the different forces, the interesting phenomena granular separation, that the intruder can not only rise to the top, but also sink to the bottom, and even keep in the middle of the bed, are present.

Topics: Force , Vibration
Commentary by Dr. Valentin Fuster
2009;():133-139. doi:10.1115/IMECE2009-12316.

Optimization of heat transfer and flow in a helical duct of rectangular cross-section used to cool the stators of electric machines is studied using computational fluid dynamics (CFD) techniques. Realizable turbulent model is used with water and oil as working fluids for two different designs (Spiral (S) and Reverse Annulus (RA) designs) and total of five configurations of the helical duct at small pitch size of 0.00254 m. The Reverse Annulus (RA) is a new design proposed by the author. Due to the curvature of the ducts, as fluid flows through curved tubes, a centrifugal force is generated. A secondary flow induced by the centrifugal force has significant ability to enhance the heat transfer rate. Results showed that the spiral design provides better heat transfer in terms of lower surface temperatures at the expense of higher pressure drop for the same flow rate of fluid.

Commentary by Dr. Valentin Fuster
2009;():141-145. doi:10.1115/IMECE2009-12748.

Steady two-dimensional natural convection in rectangular cavities has been investigated numerically. The conservation equations of mass, momentum and energy under the assumption of a Newtonian Boussinesq fluid have been solved using the finite volume technique embedded in the Fluent code for a Newtonian (water) and three non Newtonian carbopol fluids. The highly accurate Quick differential scheme was used for discretization. The computations were performed for one Rayleigh number, based on cavity height, of 105 and a Prandtl number of 10 and 700, 6,000 and 1.2×104 for the Newtonian and the three non-Newtonian fluids respectively. In all of the numerical experiments, the channel is heated from below and cooled from the top with insulated side-walls and the inclination angle is varied. The simulations have been carried out for one aspect ratio of 6. Comparison between the Newtonian and the non-Newtonian cases is conducted based on the behaviour of the average Nusselt number with angle of inclination. Both Newtonian and non-Newtonian fluids exhibit similar behavior with a sudden drop around an angle of 50° associated with flow mode transition from multi-cell to single-cell mode.

Commentary by Dr. Valentin Fuster
2009;():147-158. doi:10.1115/IMECE2009-13167.

It is an important problem in the polymer extrusion of complex profiles to balance the flow at the die exit. In this paper, we employ simulated annealing-kriging meta-algorithm to optimize the geometric parameters of a die channel to obtain a uniform exit velocity distribution. Design variables for our optimization problem involve the suitable geometric parameters for the die design, which are the thickness of the large channel and the length of the narrow channel. Die balance is based on the deviation of the velocity with respect to the average velocity at the die exit. So the cost function for the optimization problem involves the minimization of this deviation. For the design of numerical experiments, we use Latin Hypercube Sampling (LHS) to construct the kriging model. Then, based on the LHS points, the numerical solutions are performed using Polyflow, a commercial software based on the finite element method and is specifically designed to simulate the flow and heat transfer of non-newtonian, viscoelastic fluids. In our simulations, a HDPE (high density polyethylene) is used as extrusion material. Having obtained numerical simulations for N = 60 LHS points in two-dimensional parameter space (t and L), the optimization of these parameters is carried out by Simulated Annealing (SA) method in conjunction with kriging model. We show that kriging model employed in SA algorithm can be used to optimize the die geometry.

Commentary by Dr. Valentin Fuster
2009;():159-166. doi:10.1115/IMECE2009-13268.

The laminar squeeze flow of couple-stress fluids between a flat circular disc and an axisymmetric curved disc of arbitrary shape is taken into consideration. Based on the modified lubrication theory and the microcontinuum theory, the problem is solved analytically. The combined effects of fluid inertia forces, curvature and non-Newtonian couple stresses on the squeeze film behaviour are investigated through the variations in the Reynolds number, curvature and the couple stress parameters. Each of these effects and their combinations show a significant enhancement in the squeeze film behaviour and these are studied through their effects on the squeeze film pressure and the load carrying capacity of the fluid film as a function of time. It is understood that the operating life of the squeeze film discs are lengthened, especially for larger couple stress parameters, larger Reynolds numbers and concave nature of the curved discs.

Commentary by Dr. Valentin Fuster
2009;():167-175. doi:10.1115/IMECE2009-10575.

A series of wind tunnel tests was conducted on the vibration and scattering behavior of full-sized model of roof tiles, which were used widely for roofings of Japanese wooden dwellings. This study has investigated the nature and source of the vibrating and scattering behavior of roof tiles with the aim of providing a better insight to the mechanism. The roof tiles were set up on the pitched roof in the downstream of the flow from the wind tunnel. The vibrations for the roof tiles were measured by the Laser Doppler Vibrometry and the accelerometer, and the practical natural frequencies of the roof tiles were analyzed by the impulse force hammer test method. The motions of the vibration and scattering were observed by the high-speed video camera. Based on the consideration on the results of the measurements, there is a basic mechanism which can lead to flow-induced vibrations of the roof tiles. This mechanism is similar to that of the so-called fluttering instability, which appears as the self-excited oscillation in the natural mode of the structure at the certain critical flow speed. The values of the frequencies for the oscillating relate to the values of natural frequencies of the vibration.

Topics: Roofs , Wind
Commentary by Dr. Valentin Fuster
2009;():177-184. doi:10.1115/IMECE2009-10952.

In the teeming process of molten steel from a ladle, a bathtub-type vortex may be formed in the ladle. The formation of such vortex will drag the slag from the top of the molten steel into the tundish which affects the cleanness of steel. In previous works water was considered to model the molten steel but we found that water is not a suitable fluid for simulation, so a dimensional analysis was applied to model the proper fluids for molten steel and slag with a composition in a real ladle. We deduced that freon or mercury can be relevant fluid models for simulating molten steel. In addition, the effects of Re, Fr and Bo number were studied and Fr and Re numbers were found to be dominant pi-numbers and the effect of interfacial tension and surface tension were ignored because of large Bo number. The selection of fluid models for slag were based on viscosity and due to fact that adding some materials like Cao-Ca F2 for modifying slag decreases the viscosity of slag from 6.5 pa-s lower than 2 pa-s. The scale models was filled with freon and slag fluids were engine oil, fuel oil, water, DRAFSH46, DRAFSH100 and glycerol at the top of the freon. Furthermore, the cases were rotated at three different axis rotations. Results show that kinematic viscosity is responsible for slag entrainment into the drain rather than DENSITY for special Bo numbers. A Computational Fluid Dynamics (CFD) modeling was also conducted to investigate the vortex formation under imposed conditions.

Commentary by Dr. Valentin Fuster
2009;():185-193. doi:10.1115/IMECE2009-11224.

The experimental visualization of the flow patterns developed in a channel formed by parallel separated cross-corrugated plates is presented in this work. The flow visualization was carried out by seeding reflective micro-particles in water. The cross-corrugated plates were characterized by corrugations with sinusoidal profile, 0.083 m wavelength and 0.075 m amplitude, placed at ±45° relative to the main flow direction. While the wavelength-amplitude aspect ratio was kept fixed, both the uniform spacing between plates and Reynolds number were varied. The essential feature of the flow is the secondary swirling motion developed by the furrow flows because of the crossing among streams. Three flow regimes were found: steady, unsteady and chaotic mixing. At some critical Reynolds numbers, depending upon the separation between plates, the flow becomes unsteady and chaotic mixing appears first in the outlet of the channel. Chaotic mixing moves closer to the inlet of the channel as the Reynolds number is increased. The results show that the onset of chaotic mixing occurs at larger Reynolds numbers as the spacing is increased. The flow pattern of this channel configuration is compared to that reported for the chevron arrangement.

Commentary by Dr. Valentin Fuster
2009;():195-205. doi:10.1115/IMECE2009-11228.

Optimal performance of air filters and heat exchangers requires uniform inlet flow, but flow separation produces nonuniformity. The backward-facing step flow has a separation resembling those found in industrial flows. Flow resistance of the devices is a parameter which alters upstream pressure gradients, thereby affecting separation and device performance. Air filters often are modeled as porous media using an extended Darcy Law. The present work applied Computational Fluid Dynamics (CFD) to examine the changes in the step flow resulting from the resistance of a downstream air filter. Computations were performed for a backward-facing step with a 2:1 expansion ratio for a case without a filter (reattachment at ∼6 step heights) and for filters located 4.25 and 6.75 step heights downstream. FLUENT commercial CFD software was used and results were compared to many no-filter case results in the literature and our own experimental studies for the step with downstream filters. The simulations were performed for Reynolds numbers based on approach channel mean velocity and hydraulic diameter of 2000, 3750, 6550 and 10000. The different turbulence models available in FLUENT were evaluated and the Realizable k-ε model was used for the final computations. Grid independence studies were conducted. The effects of different values of the filter modeling permeability, inertial constant and thickness also were investigated for Re = 10000 with the filter at 4.25 step heights. It was found that the computational results did not compare well to no-filter cases or the experiments with filters at the lower Reynolds numbers. It is believed that the turbulence models were unsuitable for these flows at transitional Reynolds numbers. Good agreement for no-filter results and for the experiments with filters was observed for Re = 10,000. The CFD model seems to capture the physics of the separation better at the higher Reynolds numbers. The CFD velocity profiles at Re = 10,000 with the filters agree with those of the experiments. When the filter is placed at 4.25 step heights, the flow reattaches upstream of the filter with a reduction in recirculation area. When the filter is at 6.75 step heights, the separated flow tends to reattach and the opposite side tends to separate. At Re = 10,000 and the filter at 4.25 step heights, the variations of porous medium permeability, inertial constant and the filter thickness have negligible effects on the recirculation region over the parameter ranges considered.

Commentary by Dr. Valentin Fuster
2009;():207-213. doi:10.1115/IMECE2009-11269.

A fan is an important part for air circulation in household appliances and automobiles. In this research an attempt has been made to extract the flow information using the Computational Fluid Dynamics (CFD) technique. The numerical results were found for a case with a stationary fan inside the duct and the data obtained were in good agreement with the experiment. The evolution of velocity profiles at various axial locations for different flow conditions were also studied in this research. The numerical method was then extended to the cases with a rotating fan. A proof-of-concept run was also successfully carried out to showe the relationship between air flow rate in the duct and the corresponding pressure rise.

Topics: Turbulence , Ducts
Commentary by Dr. Valentin Fuster
2009;():215-222. doi:10.1115/IMECE2009-11673.

The use of numerical approach to support design and product development on industrial applications is nowadays quite widespread. Many industrial applications involve turbulent fluid flows, whose modeling still represents the bottleneck for a wider usage of numerical methods. Indeed, the application of the CFD approach to industrial problem has a number of high demanding requirements since it must deal with relatively short computational time, high geometrical complexity and high Reynolds numbers. These industrial constrains nowadays may be partially faced through RANS approach even if poor capability in predicting accurately the fluid dynamics of complex flows still represents their well known weakness. The aim of this work is to provide a model able to improve the prediction of the flow field in complex flows of industrial interest, with special attention to the presence of strong curvature. Therefore, in order to obtain an increase in the accuracy of the results compared with traditional k-ε models, the implementation of a two-equation Non Linear Eddy Viscosity Model (NLEVM) is proposed. The quadratic formulation of this model has already been validated by experimental and DNS data from literature. In the present work the cubic formulation of this model is applied to a strong-curve-geometry of industrial interest. The data are obtained through an experimental facility developed by the CFDLab of the Department of Energy at Politecnico di Milano. The measures are taken in cooperation with the Combustion and Optical Diagnostic Laboratory research group. The comparison between experimental and numerical data is carried out downstream a strong curvature by looking at mean axial velocity profiles and reattachment point prediction.

Commentary by Dr. Valentin Fuster
2009;():223-232. doi:10.1115/IMECE2009-11692.

Small orifices are widely used in different industries including gas appliances. Although characteristics of orifices such as their coefficient of discharge have been subject of interest for the past several decades, most of the previous studies focus on relatively high Reynolds number flow through relatively large diameter orifices. Moreover, the majority of previous work has focused on incompressible flows. This study focuses on the flow of different compressible gaseous fluids inside small orifices ranging from 1.3 mm to 2.1 mm hydraulic diameters for flow Re numbers of ∼8000 to ∼26000. Large-Eddy Simulation for turbulent flow is employed to solve the second-order discrete equations for compressible and incompressible flows in gas appliance orifices to predict the flow characteristics for relatively low-Re compressible flows in orifices widely used in gas appliance industry. The impacts of fluid material, the orifice hydraulic diameter, and the orifice profile on the characteristics of orifice are studied.

Commentary by Dr. Valentin Fuster
2009;():233-240. doi:10.1115/IMECE2009-11696.

The performance of cross flow hollow fiber ultrafiltration (UF) membrane with molecular weight cut off (MWCO) 100 kDaltons was studied in order to effectively remove suspended solids in wastewater. Experiments were carried out to investigate the influence of the several factors such as cross flow velocity, transmembrane pressure (TMP), water temperature, and concentration of suspended solids on the membrane performance. Several cleaning methods were applied to remove the fouling. The experimental results showed that increasing TMP, temperature and cross flow velocity all resulted in increasing permeate flux. It is observed that high TMP aggravated the fouling while high cross flow velocity alleviated the fouling. High concentrations of suspended solids led to the reduction of permeate flux. It is also found that both combination of chemical, back- and forward-washing as well as soaking cleaning methods effectively removed fouling and achieved high flux recovery. The suspended solids were effectively removed by our UF system, and the water quality is significantly improved after ultrafiltration.

Commentary by Dr. Valentin Fuster
2009;():241-249. doi:10.1115/IMECE2009-11708.

Ultrasound based on-line cleaning for hollow fiber (HF) membrane filtration of synthetic wastewater was studied. An ultrasonic transducer was submerged into a filtration system in order to get an efficient cleaning of HF membranes in fouling conditions. An ultrafiltration (UF) HF membrane with the pore size at 10,000 NMWC is employed to purify waste water. The focus of this study is on the effects of temperature, ultrasonic frequency, ultrasonic power intensity and caviation micro-bubbles as well as the transmembrane pressure (TMP) performance. Experimental evidence reveals that the permeate flux increased with the application of ultrasound after fouling by sullage solution for one hour. The micro-bubble size measured by laser PDA system shows a decreased tendency with the increase of ultrasonic frequencies, and larger micro-bubbles have greater contribution to the increase of permeate flux. Results futher shows that the permeate flux measured with lower ultrasonic frequency or higher power intensity maintained higher value in general as feeding sullage water and maintain a higher risk to extend membrane pore size. In addition, the rise of the temperature around filtration system has less impact on permeate flow rate in online ultrasound system when the temperature of feed solution maintained constant.

Commentary by Dr. Valentin Fuster
2009;():251-258. doi:10.1115/IMECE2009-12456.

Curved diffusers are an integral component of the gas turbine engines of high-speed aircraft. These facilitate effective operation of the combustor by reducing the total pressure loss. The performance characteristics of these diffusers depend on their geometry and the inlet conditions. In the present investigation the distribution of axial velocity, transverse velocity, mean velocity, static and total pressures are experimentally studied on a curved diffuser of 30° angle of turn with an area ratio of 1.27. The centreline length was chosen as three times of inlet diameter. The experimental results then were numerically validated with the help of Fluent, the commercial CFD software. The measurements of axial velocity, transverse velocity, mean velocity, static pressure and total pressure distribution were taken at Reynolds number 1.9 × 105 based on inlet diameter and mass average inlet velocity. The mean velocity and all the three components of mean velocity were measured with the help of a pre-calibrated five-hole pressure probe. The velocity distribution shows that the flow is symmetrical and uniform at the inlet and exit sections and high velocity cores are accumulated at the top concave surface due to the combined effect of velocity diffusion and centrifugal action. It also indicates the possible development of secondary motions between the concave and convex walls of the test diffuser. The mass average static pressure recovery and total pressure loss within the curved diffuser increases continuously from inlet to exit and they attained maximum values of 35% and 14% respectively. A comparison between the experimental and predicated results shows a good qualitative agreement between the two. Standard k-ε model in Fluent solver was chosen for validation. It has been observed that coefficient of pressure recovery Cpr for the computational investigation was obtained as 38% compared to the experimental investigation which was 35% and the coefficient of pressure loss is obtained as 13% in computation investigation compared to the 14% in experimental study, which indicates a very good qualitative matching.

Commentary by Dr. Valentin Fuster
2009;():259-268. doi:10.1115/IMECE2009-13303.

A numerical study was carried out to investigate steady-state and transient phase distribution, evaporation, and thermal runaway in a large-scale high-pressure trickle bed reactor operating in the low interaction regime. The thermal inertia of the catalyst particles proved to be a significant contribution to the overall energy balance. A cooling recycle stream, containing reaction products and a fresh feed, was included via a closed loop calculation. It was found that, as expected, phase distribution in the catalyst bed had a substantial impact production rate; a faulty feed distribution system can cost approximately 20% in overall steady-state product conversion. Grid resolution effects were quantified and were found to have minimal impact on macroscopic measures. Also, most results were insensitive to the extent of the modeled domain and the commercial solver version. In the event that the cooling recycle stream is lost, the external reactor shell temperature can exceed its design intent. It was found that reducing the quantity of fresh reactant feed in this situation can dramatically reduce the potential for vessel damage.

Commentary by Dr. Valentin Fuster
2009;():269-276. doi:10.1115/IMECE2009-10793.

The thrust response of a bio-inspired pulsatile vortex ring thruster is explored for variable diameter exit nozzle profiles. Our pulsatile vortex ring thruster is composed of a plunger in a submerged cavity that oscillates to intake and expel fluid through an exit nozzle creating vortex rings which generate thrust. The diameter of the exit nozzle is expanded upon fluid intake to increase volume flow. Upon fluid expulsion the nozzle diameter is reduced to increase flow velocity. This allows for an increase in total thrust. The effect is a thrusting force from a positive net flux in momentum with a zero net flux in mass. This is loosely inspired by jellyfish which utilize the same orifice for both mass intake and expulsion. The thrust is directly measured for multiple diameter profiles. A selected diameter profile is either a constant exit nozzle diameter or a sinusoidal oscillation of exit nozzle diameter that corresponds to the plunger motion. The increase in thrust is analyzed with respect to variable diameter effects and compared to thrust results of constant exit diameter nozzles.

Commentary by Dr. Valentin Fuster
2009;():277-287. doi:10.1115/IMECE2009-11208.

A numerical three-dimensional study has been carried out to analyze the hydrodynamic behavior of an incompressible periodically-reversing flow in a conical pipeline of finite length. In order to generate the oscillatory flow, a two pistons-driven pipe flow was established (the pistons were placed at the ends of the pipeline.) In the smaller piston the Womersley number was set to 16 while the axial displacement/diameter ratio was set to 1. The hydrodynamic flow behavior is considered and discussed for the accelerating and decelerating phases of the cycle. The numerical results show the generation of vortices for different penetration lengths. One of the most representative results is that in the position of 183.25°, after the pistons reach their maximum displacement, the flow pattern loses the axial symmetry that had appeared for previous phases of the cycle.

Commentary by Dr. Valentin Fuster
2009;():289-298. doi:10.1115/IMECE2009-11218.

In order to investigate the frequency and amplitude effects of the synthetic jet on the flow field, numerical simulation is carried out. Even though the final objective of this study is to understand mechanism of separation control for various objects, streamline and bluff bodies, the configuration of backward-facing step is chosen as the first step because of the simplicity. Three-dimensional Navier-Stokes equations are solved. Implicit large eddy simulation using high-order compact difference scheme is applied. The present analysis is addressed on the frequency characteristics of the synthetic jet for understanding frequency characteristics and flow-filed. Three cases are selected, No-control, F+ h = 0.2 and F+ h = 2.0, where non-dimensional frequency F+ h is normalized with the height of backward-facing step and the free stream velocity. The present computation shows that at F+ h = 2.0, separation length is 20 percent shorter than the No-control case. Strong two-dimensional vortices generated from the synthetic jet interact with the shear layer, which results in the increase of the Reynolds stress in the shear layer region. These vortices are deformed into three-dimensional structures, which make Reynolds stress stronger in the recirculation region. At F+ h = 2.0, size of the separation length is almost same as the No-control case because the mixing between the synthetic jet and the shear layer is not enhanced. Weak and short periodic vortices induced from the synthetic jet do not interacts with the shear layer very much and diffuse in the recirculation region.

Commentary by Dr. Valentin Fuster
2009;():299-308. doi:10.1115/IMECE2009-11527.

Magnetic fields are crucial in controlling flow in various physical processes of significance. One of these processes, which has significant application of a magnetic field, is continuous casting of steel, where different magnetic field configurations are used to control the turbulent steel flow in the mold to minimize defects in the cast steel. This study has been undertaken to analyze the effect of magnetic field on mean velocities and turbulence parameters in the molten metal flows through a square duct. Direct Numerical Simulations without using a sub-grid scale (SGS) model have been used to characterize the three-dimensional transient flow. The coupled Navier-Stokes-MHD equations have been solved with a three-dimensional fractional-step numerical procedure. Because liquid metals have low magnetic Reynolds number, the induced magnetic field has been neglected and the electric potential method for magnetic field-flow coupling has been implemented. Initially, laminar simulations in a square duct have been performed and results generated were compared with previous series solutions. Next, simulations of a non-MHD flow in a square duct at low Reynolds number were performed and satisfactorily compared with results of a previous DNS study. Subsequently, different levels of a magnetic field were applied to study its effect on the turbulence until the flow completely laminarized. Time-dependent and time-averaged flows have been studied through mean velocities and fluctuations, and power spectrums of instantaneous velocities.

Commentary by Dr. Valentin Fuster
2009;():309-313. doi:10.1115/IMECE2009-11578.

An immersed boundary method has been developed for Lattice Boltzmann Equations via ghost fluid approach. Image points of the ghost points inside the fluid domain are obtained by extrapolation along the boundary normal. Velocity, density and non-equilibrium value of the distribution function at ghost points are extrapolated from image point values which are calculated by interpolation from the boundary and fluid domain. The distribution function at ghost points is computed from the extrapolated non-equilibrium part and the equilibrium part which is obtained from extrapolated values of the velocities. The method is found to be second order accurate. The method is applied to concentric 2D Couette flow and 3D Taylor–Couette flow.

Commentary by Dr. Valentin Fuster
2009;():315-321. doi:10.1115/IMECE2009-11810.

The applications involving fluid flow through microchannels in industry and research have increased significantly with the evolution of microfluidic devices such as lab-on-chip systems. Most of the previous studies concerning fluid flow were done using circular microchannels. However, there is an increased usage of noncircular microchannels, especially square microchannels, in microfluidic devices. Thus there is need for experimental studies on the behavior of fluid flow in square microchannels, and the comparison of the results with the results obtained from the conventional fluid flow equations is relevant. In this study the authors are focusing on the analysis of the friction factor associated with square microchannels of rounded edges under laminar flow conditions. Microchannels with hydraulic diameters of 200, 300, 400 and 500 micrometers and length of 10 cm and 5 cm are used in the analysis. DI-water and ethylene glycol at room temperature is used as the liquid for experiments. A constant liquid flow rate is achieved in the channels using a syringe pump that can pump from 50 μl/hr to 7,500 ml/hr using a 60 ml syringe, and a high precision pressure gauge is used to measure the pressure drop across the channel. The Reynolds number of the liquid flow in all the channels is kept constant between 20 and 120 by varying the flow rate. The friction factor at each Reynolds number is calculated and the results are compared with the friction factor of conventional channels. Experiments are conducted to measure the pressure drop across the channels. The pressure drop obtained across the 5 cm channel is subtracted from the pressure drop obtained across the 10 cm channel so that the effect of entrance effect can be eliminated from the results. The fiction factor obtained from the experiments is used to calculate the Poiseuille number. The experimental values of Poiseuille number are showing a median deviation of around 9% from the conventional values for all the different channels. The uncertainty is observed to be ca.9% for all the channels at all values of Reynolds numbers. The major factor contributing towards the total uncertainty is the uncertainty in the measurement of liquid flow rate.

Commentary by Dr. Valentin Fuster
2009;():323-329. doi:10.1115/IMECE2009-11827.

The objective of the present study is to investigate low Reynolds number aerodynamics of a harmonically pitching NACA0012 airfoil. To this mean, the influence of some unsteady parameters; amplitude of oscillation, d, reduced frequency, k, and Reynolds number, Re, on the aerodynamic performance of the airfoil is investigated. Computational Fluid Dynamics (CFD) is utilized to solve Navier-Stokes equations discretized based on Finite Volume Method (FVM). The instantaneous lift coefficients are obtained and compared with analytical data from Theodorsen’s equations. The simulation results reveal that d, k, and Re are of great importance in the aerodynamic performance. They affect the maximum lift coefficients, hysteresis loops, strength and number of generated vortices within the harmonic motion, and the extent of the figure-eight phenomenon region. Thus, the optimum aerodynamic performance demands a careful selection of these parameters.

Commentary by Dr. Valentin Fuster
2009;():331-338. doi:10.1115/IMECE2009-11988.

This paper deals with the analyses of fluid flow distribution in a microfluidic device with in-line manifolds. The analysis was performed using commercially available microfluidic simulation software called CoventorWare™. The number of channels in the microfluidic device considered for this study was kept at ten due to limitations on the number of nodes and computational time. Channels with only square profile were analyzed for flow rates varying between 1 to 60 ml/min. The length of the channels was maintained at 1.5 cm for all simulations. The fluid flow distribution characteristics for different channel widths/depths (200, 100, and 75 μm) were investigated. It was observed that the flow rate decreased from the central channels to the outer channels. The flow per channel was symmetric about the geometric centre of the microdevice. The uniformity in flow was accessed using the root mean square value of flow per channel and it decreased with decrease in channel width/depth for a specific flow rate. The difference in the flow rate through the channels increased with increase in total flow rate. Similarly, the spacing between the channels was varied (300, 200, and 100 μm) for a microdevice with channel width/depth of 100 μm and its corresponding flow characteristics were studied for flow rate ranging between 1 ml/min and 60 ml/min. Finally, the length of each manifold was varied between 2500 μm and 1000 μm for understanding the effect of manifold length on flow distribution. The standard deviation of flow per channel did not show much variation with changes in spacing and manifold length. In addition each design of the manifolds was analyzed on the basis of pressure and flow rate as well as velocity profile in each of the channels.

Commentary by Dr. Valentin Fuster
2009;():339-346. doi:10.1115/IMECE2009-12368.

Fluid flow around single or multiple bluff bodies mounted on a surface has great significance in science and engineering. Understanding the characteristics of different vortices formed around wall-mounted bodies is quite necessary for different applications. Although the case of a single surface mounted cube has been studied extensively, only little attention has been paid to the flow around two or more rectangular blocks in array. Therefore, a CFD code is developed to calculate three dimensional steady state laminar fluid flow around two cuboids of arbitrary size and configuration mounted on a surface in free stream conditions. The employed numerical scheme is finite volume and SIMPLE algorithm is used to treat pressure and velocity coupling. Results are presented for two rectangular blocks of the different size mounted on a surface in various inline arrangements. Streamlines are plotted for blocks of different size ratio. Velocity and pressure distributions are also plotted in the wake region behind the obstacles. It is shown that how the behavior of flow field and vortical structures depend on the respective size and location of the larger block in comparison with the case of two inline wall mounted cubes of the same size.

Topics: Fluid dynamics
Commentary by Dr. Valentin Fuster
2009;():347-356. doi:10.1115/IMECE2009-12745.

Non-stationary signals are frequently encountered in a variety of engineering fields. The inability of conventional Fourier analysis to preserve the time dependence and describe the evolutionary spectral characteristics of non-stationary processes requires tools which allow time and frequency localization beyond customary Fourier analysis. The spectral analysis of non-stationary signals cannot describe the local transient features due to averaging over the duration of the signal [1]. The Fourier Transform (FT) and the short time Fourier transform (STFT) have been often used to measure transient phenomena. These techniques yield good information on the frequency content of the transient, but the time at which a particular disturbance in the signal occurred is lost [2, 3]. Wavelets are relatively new analysis tools that are widely being used in signal analysis. In wavelet analysis, the transients are decomposed into a series of wavelet components, each of which is a time-domain signal that covers a specific octave band of frequency. Wavelets do a very good job in detecting the time of the signal, but they give the frequency information in terms of frequency band regions or scales [4]. The main objective of this paper is to use the wavelet transform for analysis of the pressure fluctuations occurred in the bottom-outlet of Kamal-Saleh Dam. The “Kamalsaleh Dam” is located on the “Tire River” in Iran, near the Arak city. The Bottom Outlet of the dam is equipped with service gate and emergency gate. A hydraulic model test is conducted to investigate the dynamic behavior of the service gate of the outlet. The results of the calculations based on the wavelet transform is then compared with those obtained using the traditional Fast Fourier Transform.

Commentary by Dr. Valentin Fuster
2009;():357-363. doi:10.1115/IMECE2009-12809.

In spanwise rotating channel flows, the turbulent kinetic energy near the high pressure and the low pressure walls is primarily associated with longitudinal velocity fluctuations. Consequently, the primary normal Reynolds momentum flux difference is positive and the secondary normal Reynolds flux difference is negative. In the outer region on the high pressure side of the symmetry plane, the energy is redistributed with the result that the signs of both the primary and of secondary normal differences flip. This redistribution of energy by Coriolis forces occurs in a region of zero intrinsic vorticity. In this paper, the dispersion of a passive additive within the zero intrinsic vorticity region is examined by using a recently developed universal, realizable, anisotropic prestress closure for the normalized Reynolds stress. For low rotation numbers (i.e., | Ωx | ≪ ε / k), the theory shows that the transverse component of the passive additive flux is mitigated by a coupling between the shear component of the Reynolds stress and the longitudinal gradient of the mean passive additive field. At high rotation numbers (i.e., | Ωx | ≫ ε / k), the dispersion coefficient in the transverse (cross flow) direction is four times larger than the dispersion coefficient in the spanwise direction. Surprisingly, the dispersion coefficient in the longitudinal direction is relatively small. The geophysical and the engineering significance of these theoretical conclusions will be highlighted in the presentation.

Commentary by Dr. Valentin Fuster
2009;():365-370. doi:10.1115/IMECE2009-13020.

This paper reports the numerical results of flow in a concentric cylindrical vessel. Velocity plots are shown to illustrate the swirling cylinder flow inside the cylinder having free surface on top and constant rotating bottom wall. The analysis is carried out axisymmetric using Stream function and Vorticity method by writing Navier-Stokes equation and continuity equation in cylindrical coordinates in form of stream functions and azimuthal vorticity components eliminating the pressure component. Using Finite Difference Schemes, modified Navier-Stokes equation and continuity equations are solved by explicit methods. No-slip boundary condition is assumed at wall to minimize the discontinuity. Vortex formation is shown using contour plots and variation in Reynolds number and dimensions are considered as variable for the system. As the Reynolds number is increased the system undergoes vortex breakdown. Result clearly indicates the formation of another vortex near the free surface illustrating the phenomena of vortex breakdown. Effect of aspect ratio in the nature flow is also shown in the paper. Comparison of numerically achieved results and experimental results are done for validation.

Commentary by Dr. Valentin Fuster
2009;():371-381. doi:10.1115/IMECE2009-13076.

Time-Resolved Particle Image Velocimetry (TR-PIV) was used to study the vortical structures resulting from a submerged water jet impinging normally on a smooth and flat surface. A fully developed turbulent jet, exiting a long pipe, and a semi-confined flow configuration ensured properly characterized boundary conditions, which allows for straightforward assessment of turbulence models and numerical schemes. The Reynolds number based on jet mean exit velocity was 23,000. The pipe-to-plate separation was varied between 2D and 7.6D. Turbulent velocity fields are presented using Reynolds decomposition into mean and fluctuating components. Proper Orthogonal Decomposition (POD) analysis was used to identify the most energetic coherent structures of the turbulent flow field. Three velocity gradient-based vortex identification techniques, 2nd invariant Q, λ2 , and swirling strength, were found to perform equally well in identifying vortical structures along the impingement wall. The results clearly demonstrate the shortcomings of local vorticity as a vortex identifier in an impinging jet flow field.

Commentary by Dr. Valentin Fuster
2009;():383-392. doi:10.1115/IMECE2009-13082.

The effect of swirl on laminar buoyant jets with low Reynolds numbers is explored. Three dimensional direct numerical simulations are performed to solve the time-dependent, incompressible Navier-Stokes equations. We use a body fitted grid system and employ the finite volume method to discretize the governing equations. A second-order central difference scheme is employed for all spatial derivative terms. The numerical simulation is advanced in time by a fractional step method with the second-order Adams-Bashforth scheme for explicit-convection terms and the Crank-Nicholson scheme for implicit-diffusion terms. The amount of swirl and buoyancy is varied from zero to very large values and the effect on the velocity field, jet width, entrainment and vortex are examined. Comparisons with analytical and experimental models are discussed.

Commentary by Dr. Valentin Fuster
2009;():393-406. doi:10.1115/IMECE2009-13172.

The flow structure around a low aspect ratio wing at low Reynolds numbers and a fixed angle of attack of 20° is discussed using flow visualization as well as Three-Component Time-Resolved Particle Image Velocimetry (3C TR PIV). Mean quantities and statistical measurements of velocity were obtained and used to describe the average and transient characteristics of the flow field. Effects of spanwise variation from centerline to wingtip and Reynolds number variation from 1.3×104 to 6.6×104 are discussed. The role of the wing tip vortices is observed to be large in a low aspect ratio wing. The transfer of momentum via Reynolds shear stresses is shown in the leading edge region. Normalized spanwise shear stresses associated with the wing tip vortices are observed to increase with increasing Reynolds number.

Commentary by Dr. Valentin Fuster
2009;():407-416. doi:10.1115/IMECE2009-11116.

An extension of the near wall hindered diffusion theory of Brenner [1] is considered for spherical nano-particles undergoing Brownian motion under the influence of hydrodynamic drag and electrostatic interactions. Brenner’s theory is based on hydrodynamic interactions between a levitating particle and the wall and does not consider short range interactions (like electrostatic force and van der Waals force). Recent experiments by Banerjee & Kihm [2] (henceforth referred to as BK05), with nano-particles of radii ≤ 500 nm show substantial discrepancy between the experimental and theoretical values of normal reduction coefficient (ratio of near wall normal diffusivity to bulk diffusivity). However, the experimentally measured lateral reduction coefficient (ratio of tangential diffusivity of the wall to bulk diffusivity) show good agreement with theory. It is conjectured that the absence of short range interactions become critical for particle radii ≤ 500 nm in the (sub-micron) near wall region. The current work extends Brenner’s analytical theory (henceforth referred to as B61) considering various short and long range interactions. An analytical expression is derived for hindered diffusivity of a particle normal to the wall under the influence of hydrodynamic drag and electrostatic interaction with a constant surface charge density. The theory is validated with experiments of BK05 and shows a better agreement with measured values of diffusion coefficients for particles of radii 50 and 100 nm. The dependence of electrolyte concentrations on electrostatic potential energy for spheres has also been studied. Increase in ionic strength of electrolyte concentration confirms the reduction of electrostatic potential energy for different sphere sizes. Electrostatic interaction has a significant contribution in overall potential energy of the sphere in sub-micron near wall region when wall and sphere surface charge potentials are increased.

Commentary by Dr. Valentin Fuster
2009;():417-426. doi:10.1115/IMECE2009-11936.

It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or equator of the drop by subjecting the latter to a uniform electric field. In this paper, we present experimental results for the dependence of the dielectrophoretic force on the parameters of the system such as the particles’ and drop’s radii and the dielectric properties of the fluids and particles, and define a dimensionless parameter regime for which the technique can work. Specifically, we show that if the drop radius is larger than a critical value, that depends on the physical properties of the drop and ambient fluids and the particles, it is not possible to concentrate particles and thus clean the drop of the particles it carries at its surface because the drop breaks or tip-streams at an electric field intensity smaller than that needed for concentrating particles. However, since the dielectrophoretic force varies inversely with the drop radius, the effectiveness of the concentration mechanism increases with decreasing drop size, and therefore the technique is guaranteed to work provided the drop radius is sufficiently small.

Commentary by Dr. Valentin Fuster
2009;():427-431. doi:10.1115/IMECE2009-12141.

Oxygen molecules are paramagnetic while nitrogen molecules are diamagnetic. In the same gradient magnetic field, the magnetizing forces on oxygen molecules are stronger than those on nitrogen molecules, which in opposite directions. The intercepting effect on oxygen molecules by gradient magnetic field can be used for oxygen enrichment from air. The structure, which is called multi-channel cascading magnets array frame in the paper, are optimized by additional yokes. By comparison of distributions of magnetic field in multi-channel array without yokes and that with yokes, the additional yokes can eliminate the differences among different magnetic spaces in multi-channel cascading magnets’ arrays and enhances the magnetic flux densities in spaces. Joining magnets together in the length direction can make the air stay longer in the ‘magnetic sieve’ and raise the oxygen concentration of air flowing out from the optimized multi-channel cascading magnets’ arrays. The inside additional yoke can used to avoid the gradient magnetic field at the joints of the magnets and get near uniform magnetic field along length direction. The optimized multi-channel cascading magnets’ array frames can effectively promote the development of oxygen enrichment from air by “magnetic sieve”.

Commentary by Dr. Valentin Fuster
2009;():433-440. doi:10.1115/IMECE2009-12173.

A Resistive Pulse Sensor (RPS) is a device for counting and characterizing small particles by recording the electrical current change (negative pulse) during the translocation of the particle through a small pore. RPS is now widely used to characterize various micro/nano size particles, including bio-particles, proteins, and DNA. This paper presents a comprehensive multi-physical model of RPS. The model involves a coupled system of the Navier-Stokes equation for flow field, the Nernst-Planck equation for electrolyte ion concentrations, and the Poisson equation for electrical field. The model is used to simulate the quasi-steady flow of electrolyte with a fixed surface charged particle in a micro/nano-channel connecting two reservoirs. The channel and reservoir are assumed to be cylindrical and a 2-D axial-symmetry system is used. The model predicts the flow and electric fields as well as the distribution of the ion concentrations in the channel. The effects of Electrical Double Layer (EDL) on the electric current change through the channel are then investigated. Conditions for the electric current change (positive and negative pulses) are then identified.

Commentary by Dr. Valentin Fuster
2009;():441-449. doi:10.1115/IMECE2009-12219.

Electrohydrodynamic (EHD) conduction phenomenon takes advantage of the electrical Coulomb force exerted on a dielectric liquid generated by externally applied electric field. The conduction phenomenon can be applied to enhance or control mass transport and heat transfer in both terrestrial and microgravity environments with advantages of simplicity and no degradation of fluid properties for isothermal as well as non-isothermal liquids. This paper numerically studies the heat transfer augmentation of externally driven macro- and micro-scale parallel flows by means of electric conduction phenomenon. The electric conduction is generated via electrode pairs embedded against the channel wall to solely enhance the heat transfer; it is not utilized to pump the liquid. Two cases of Poiseuille and Couette parallel flows are considered where for the former, a constant external pressure gradient is applied along the channel and for the latter, the channel wall moves with a constant velocity. The electric field and electric body force distributions along with the resultant velocity fields are presented. The heat transfer enhancements are illustrated under various operating conditions for both scales.

Commentary by Dr. Valentin Fuster
2009;():451-459. doi:10.1115/IMECE2009-12533.

In this paper, the effect of the surface oscillations of a levitated droplet subject to electromagnetic and external forces, and modeled as a 3D mesh-free fluid particles system, is considered. The droplet was analyzed in a force field, which was derived from the magnetic field produced by the coil. The electromagnetic force was continuously updated with the shape and position change. To describe the fluid motion, the Navier-Stokes equations are discretized using the Moving Particle Semi-implicit (MPS) method. A numerical model based on MPS method was developed, the equations of motion were solved, the free surface of the droplet was approximated, the interface reconstructed and the oscillations frequency spectra analyzed. Two weight functions are considered and their performance compared in order to to improve the stability of MPS method.

Commentary by Dr. Valentin Fuster
2009;():461-469. doi:10.1115/IMECE2009-10110.

There has been a discussion that the thermodiffusion in porous medium is not identical to thermodiffusion in clear liquid. To validate this finding, a new thermodiffusion cell has been designed to measure thermal and molecular diffusion coefficients of binary mixture in porous medium under high pressure. A porous layer located between two liquid layers that cross two laser beams, is used. The difference of refractive index is obtained from the analysis of the interferograms recorded with a CCD camera. From the kinetics, the values of molecular and thermal diffusion coefficients through the porous medium were determined. The two binary systems dodecane (C12) and isobutylbenzene (IBB), and dodecane (C12) and tetrahydronaphthalene (THN) are used to validate the experimental setup at atmospheric pressure. Experimental results reveal an excellent agreement with benchmark values and a good agreement with theoretical data.

Commentary by Dr. Valentin Fuster
2009;():471-481. doi:10.1115/IMECE2009-10134.

The inverse heat conduction problem (IHCP) in a one-dimensional composite slab with rate-dependent pyrolysis chemical reaction and outgassing flow effects is investigated using the conjugate gradient method (CGM). The thermal properties of the composites are considered to be temperature-dependent, which makes the IHCP a nonlinear problem. The inverse problem is formulated in such a way that the front-surface heat flux is chosen as the unknown function to be recovered, and the front-surface temperature is computed as a by-product of the IHCP algorithm, which uses back-surface temperature and heat flux measurements. The proposed IHCP formulation is then applied to solve the IHCP in an organic composite slab whose front surface is subjected to high intensity periodic laser heating. It is shown that an extra temperature sensor located at an interior position is necessary since the organic composites usually possess a very low thermal conductivity. It is also found that the frequency of the periodic laser heating flux plays a dominant role in the inverse solution accuracy. In addition, the robustness of the proposed algorithm is demonstrated by its capability in handling the case of thermophysical properties with random errors.

Commentary by Dr. Valentin Fuster
2009;():483-492. doi:10.1115/IMECE2009-10241.

The Nuclear Nonproliferation Programs Design Authority is in the design stage of the Waste Solidification Building (WSB) for the treatment and solidification of the radioactive liquid waste streams generated by the Pit Disassembly and Conversion Facility (PDCF) and Mixed Oxide (MOX) Fuel Fabrication Facility (MFFF). The waste streams will be mixed with a cementitious dry mix in a 55-gallon waste container. Savannah River National Laboratory (SRNL) has been performing the testing and evaluations to support technical decisions for the WSB. Engineering Modeling & Simulation Group was requested to evaluate the thermal performance of the 55-gallon drum containing hydration heat source associated with the current baseline cement waste form. A transient axi-symmetric heat transfer model for the drum partially filled with waste form cement has been developed and heat transfer calculations performed for the baseline design configurations. For this case, 65 percent of the drum volume was assumed to be filled with the waste form, which has transient hydration heat source, as one of the baseline conditions. A series of modeling calculations has been performed using a computational heat transfer approach. The baseline modeling results show that the time to reach the maximum temperature of the 65 percent filled drum is about 32 hours when a 43°C initial cement temperature is assumed to be cooled by natural convection with 27°C external air. In addition, the results computed by the present model were compared with analytical solutions. The modeling results will be benchmarked against the prototypic test results. The verified model will be used for the evaluation of the thermal performance for the WSB drum. Detailed results and the cases considered in the calculations will be discussed here.

Commentary by Dr. Valentin Fuster
2009;():493-500. doi:10.1115/IMECE2009-10405.

A numerical model combining the ultrafast radiative heat transfer and ablation rate equation for free electron density is proposed to investigate the transient process of plasma formation in distilled water. The focused beam propagation governed by the transient equation of radiative heat transfer is solved by the transient discrete ordinates method. The temporal evolution of free electron density governed by the rate equation is solved using a forth-order Runge-Kutta method. Two laser pulses: 30 ps and 300 fs are considered. Simulation of the dynamics of plasma formation is performed. The results include the threshold laser intensity for optical breakdown, temporal evolution and spatial distribution of the free electron density as well as the maximum plasma length. To validate the model, optical breakdown thresholds for different laser pulses, the shape of plasma breakdown region and the maximum plasma length predicted by the present model are also compared with the experimental data.

Commentary by Dr. Valentin Fuster
2009;():501-505. doi:10.1115/IMECE2009-10429.

In this paper a variety of well known computer graphics algorithms (Binary Spatial Partitioning-BSP, Bounding Box-BB, and direct method of sequential search) for ray tracing are studied numerically in the context of the view factor calculations for the zonal method of radiation heat transfer analysis in complex industrial furnace geometries. The paper reports on a modified BSP algorithm which takes into account the specific types of obstructions and their arrangement in different types of metallurgical furnaces. The modified algorithm enhances the ray tracing calculations by two to three orders of magnitude. An universal algorithm to obtain an intersection with a polyhedron obstruction is developed. The method is tested for simple three dimensional and complex furnace geometries.

Commentary by Dr. Valentin Fuster
2009;():507-517. doi:10.1115/IMECE2009-10621.

Prior research has shown that the use of liquid-pistons in place of conventional solid pistons within gas compression technologies can significantly improve the efficiency of gas compression. The liquid-piston provides the prospect for a consistent and high rate of heat extraction from the compressed gas during system operation. Consequently, the input power requirements during each individual compression are lowered. To validate this concept, analytical studies of the thermal-fluids and heat transfer mechanisms during gas compression were performed. The analysis involved the development of a numerical model, using the finite-difference method, which simulated a single compression stroke and quantified the crucial parameters during compression. This model was utilized to obtain theoretical efficiency values and to recognize optimal system characteristics. The results obtained from the simulation indicated double-digit increase in efficiency with the introduction of the liquid-piston.

Commentary by Dr. Valentin Fuster
2009;():519-526. doi:10.1115/IMECE2009-10780.

In this study we show that the POD can be used as a useful tool to solve inverse design problems in thermo-fluids. In this respect, we consider a forced convection problem of air flow in a grooved channel with periodically mounted constant heat-flux heat sources. It represents a cooling problem in electronic equipments where the coolant is air. The cooling of electronic equipments with constant periodic heat sources is an important problem in the industry such that the maximum operating temperature must be kept below a value specified by the manufacturer. Geometric design in conjunction with the improved convective heat transfer characteristics is important to achieve an effective cooling. We obtain a model based on the proper orthogonal decomposition for the convection optimization problem such that for a given channel geometry and heat flux on the chip surface, we search for the minimum Reynolds number (i.e., inlet flow speed) for a specified maximum surface temperature. For a given geometry (l = 3.0 cm and h = 2.3 cm), we obtain a proper orthogonal decomposition (POD) model for the flow and heat transfer for Reynolds number in the range 1 and 230. It is shown that the POD model can accurately predict the flow and temperature field for off-design conditions and can be used effectively for inverse design problems.

Commentary by Dr. Valentin Fuster
2009;():527-532. doi:10.1115/IMECE2009-10849.

In this study the temperature increase and heat dissipation in the air gap of a cylindrical mini rotor stator system has been analyzed. A simple thermal model based on lumped parameter thermal networks has been developed. With this model the temperature dependent air properties for the fluid-rotor interaction models have been calculated. Next the complete system has also been modeled by using computational fluid dynamics (CFD) with Ansys-CFX and Ansys. The results have been compared and the capability of the thermal networks method to calculate the temperature of the air between the rotor and stator of a high speed micro rotor has been discussed.

Topics: Modeling , Rotors , Stators
Commentary by Dr. Valentin Fuster
2009;():533-542. doi:10.1115/IMECE2009-10883.

This paper examines and explains two-dimensional, steady mixed convection flow in a porous square vented cavity. The interaction between the buoyancy stemming from one or more heated elements inside a microstructure filled vented enclosure and an imposed forced flow forms the topic of this investigation. Analysis has been carried out for two different boundary conditions. Initial investigations are carried out for walls of the enclosure being isothermal. A second stage of analysis is performed keeping only the left wall isothermal and other three walls adiabatic. Natural convection takes place due to temperature difference between the isothermal wall and the fluid. Forced convection condition is imposed by providing an inlet and a square vent inside the enclosure filled with fluid saturated porous medium. The mathematical model is developed using modified Darcy flow model and energy equation. Through the adaptation of the well known finite element method, solution to this numerical problem is obtained. Governing parameters chosen are Peclet Number (Pe), Rayleigh Number (Ra), Aspect ratio (AR) and the width of the inlet as a fraction of the width (I/W) of the enclosure. For detailed analysis different value of these parameters such as five Rayleigh Numbers (1, 50, 100, 500 and 1000) and seven different Peclet Numbers (0.1, 1, 5, 10, 20, 50 and 100) are considered. Effect of inlet to cavity width ratio is examined within the range 0.1 ≤ I/W ≤ 0.5 for a particular aspect ratio. The performance of the enclosure in both cases; are determined by flow visualization and by analyzing different parameters such as Bejan Number, Nusselt Number and Entropy Generation Number. Isotherms, streamlines show substantial variation in their pattern or magnitude. Average Nusselt number and average Bejan Number increases whereas Average energy flux density decreases with increasing I/W. These fluctuations also vary for different Rayleigh or Peclet numbers. The results for both the boundary conditions are also compared to find the most effective value of I/W.

Commentary by Dr. Valentin Fuster
2009;():543-548. doi:10.1115/IMECE2009-11021.

Transient thermal analysis of electric machine under realistic operation conditions and thermal losses is studied. A symmetrical portion of the stator and rotor is modeled and all thermal losses and cooling boundary conditions are applied according to operational duty cycle. It is found that there is a temperature gradient across the stator of more than 30 °C, across the rotor of more than 70 °C and across the whole machine of more than 100 °C. These temperature gradients could cause high thermal stresses and lead to severe reduction in the machine life. It is extremely important in future designs to consider reducing the temperature gradients by optimizing the design of the electric machines through advanced cooling techniques and strategies.

Commentary by Dr. Valentin Fuster
2009;():549-552. doi:10.1115/IMECE2009-11193.

We report on a scheme to stabilize short term cryogenic temperature variations in equipment used for high frequency radio telescope receivers. A 45.9 cm3 copper helium pot is affixed to a Sumitomo/Daikin 308, GM/JT cold head. 5.7×103 cm3 of helium gas is introduced at the end of the cooling cycle and condensation until zero kPaa is reached. Short term temperature stabilization is achieved through the specific heat of the liquefied helium diminishing thermal deviations up to 46%.

Commentary by Dr. Valentin Fuster