IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Experimental studies on critical heat flux (CHF) have been conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length at different reduced pressures ranging from 0.24 to 0.99 with R-134a as the working fluid. The onset of CHF was determined by the sudden rise in the wall temperature of the electrically heated tube. Experiments were performed over a wide range of parameters: mass flux values from 200 to 2000 kg/m2 s, pressure from 10 to 39.7 bars and heat flux from 2 to 80 kW/m2 and exit quality from 0.17 to 0.94. The results show considerably lower critical heat flux at high pressures. Well known CHF prediction methods, such as the look-up table and correlations of earlier workers show poor agreement at high pressures. A new correlation has been proposed to estimate the CHF in uniformly heated vertical tubes up to the critical pressure and over a wide range of parameters. © 2005 Elsevier Ltd. All rights reserved.

Department Of Chemical Engineering

GENERAL

International Journal of Heat and Mass Transfer

Studies on critical heat flux in flow boiling at near critical pressures

The interdigitated flow configuration is unique in the sense that the reactants are forced through the porous electrodes in this configuration, whereas this occurs by diffusion in the conventional types of flow fields. Previous studies have shown that this reduces the possibility of flooding of the cathode electrode resulting in improved efficiency of the fuel cell, however, at the cost of a higher-pressure drop. In the present study, computational fluid dynamics (CFD)-based simulations have been used to study the effect of important geometric parameters such as the channel, the header and the electrode dimensions. The results show that for low permeabilities the flow is uniform across the porous electrode; however, for permeabilities in the range of 10-11 to 10-10 m2, the flow may become non-uniform. Under these conditions, calculations in four-cell and eight-cell parallel interdigitated configuration show that significant flow maldistribution among the parallel channels is likely to occur. © 2005.

Journal of Power Sources

Flow maldistribution in interdigitated channels used in PEM fuel cells

Single U- and Z-type parallel-channel configurations for gas distributor plates in planar fuel cells reduce the pressure drop but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. In this paper, previous analytical solutions obtained for single U- and Z-type flow configurations are extended to multiple U- and multiple Z-type flow configurations of interest to fuel cell applications. Algorithms to calculate flow distribution and pressure drop in multiple U- and Z-type flow configurations are developed. The results are validated by comparison with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. It is found that there is a significant improvement in the flow distribution in some configurations without paying for extra pressure drop. The possibility of unmatched distribution on the cathode and the anodes sides is also highlighted. Careful design of the flow configuration is therefore necessary for optimum performance. © 2005 Elsevier B.V. All rights reserved.

Pressure drop and flow distribution in multiple parallel-channel configurations used in proton-exchange membrane fuel cell stacks

The critical heat flux (CHF) condition is an important parameter in the design and operation of nuclear reactor fuel rod assemblies. It has traditionally been evaluated using look-up tables or empirical correlations. In the present paper, a set of relations, in the form of a one-dimensional three-field model, is presented which enables a mechanistic prediction of high-quality CHF known as dry out. This also allows a more fundamental treatment of post-dry out heat transfer. The ensemble of relations is validated by comparing the predictions with data obtained from the TPTF test series of JAERI and the THTF test series of ORNL. The rod bundles have been modelled on a sub-channel basis. The effect of spacer grids on the flow and heat transfer has been studied by including their geometrical characteristics only. Good predictions of the dry out location and the post-dry out variation of the rod temperature are obtained over the range of parameters investigated in these tests, namely, in the pressure range of 30–130&nbsp;bar, mass flux range of 50–800&nbsp;kg/m2&nbsp;s and inlet condition ranging from sub-cooled to an inlet quality of 0.89.

World University root node

Calculation of dry out and post-dry out heat transfer in rod bundles using a three field model

Rotating wheel air classifiers are often used in many process industry applications. The internal geometry of these equipment is quite complicated and has not been investigated in detail. In the present study, the flow field inside a rotating wheel air classifier has been calculated using CFD techniques, taking full account of the internal geometrical features. The predicted overall flow rate and the flow pattern are in good agreement with measurements and flow visualization studies. The calculations show that the induced flow depends strongly on geometric parameters such as the location of the inlet and outlet ports and the type of shutters used. Trajectory calculations of single particles show that the particle motion is influenced principally by centrifugal force, air drag, and wall-rebound characteristics. The wall rebound is possibly one of the means of how large particles enter the fines stream, leading to low efficiency at high speeds or large particles. Experiments of the classification using angular and radial shutter vanes show distinct range of operability of each type. These results have been interpreted coherently in the light of the flow pattern and particle trajectory calculations. © 2005 American Institute of Chemical Engineers.

AIChE Journal

Experimental and numerical study of a rotating wheel air classifier

Parallel-channel configurations for gas-distributor plates of planar fuel cells reduce the pressure drop, but give rise to the problem of severe flow maldistribution wherein some of the channels may be starved of the reactants. This study presents an analysis of the flow distribution through parallel-channel configurations. One-dimensional models based on mass and momentum balance equations in the inlet and exhaust gas headers are developed for Z- and U-type parallel-channel configurations. The resulting coupled ordinary differential equations are solved analytically to obtain closed-form solutions for the flow distribution in the individual channels and for the pressure drop over the entire distributor plate. The models have been validated by comparing the results with those obtained from three-dimensional computational fluid dynamics (CFD) simulations. Application of the models to typical fuel-cell distributor plates shows that severe maldistribution of flow may arise in certain cases and that this can be avoided by careful choice of the dimensions of the headers and the channels. © 2004 Elsevier B.V. All rights reserved.

Flow distribution and pressure drop in parallel-channel configurations of planar fuel cells

Maintaining proper water balance between the production of water due to reaction and its removal by evaporation is very important for the successful operation of a Polymer Electrolyte Membrane (PEM) fuel cell. Imbalance between the two processes can result in either flooding of the electrodes/ gas channels or the dehydration of the membrane. The water management issue is especially critical for ambient temperature operation of the fuel cell. Several experimental and theoretical studies relevant to water management have been carried out to investigate means of reducing the flooding of electrodes/channels or the dehydration of membrane. Bernardi [9] and Wang et al. [11] have developed theoretical models for the prediction of when flooding/dehydration may take place. In the present study, an improved model is developed which combines the advantages of these two models. The Bernardi [9] model is extended to include mass transfer resistances. Following Wang et al. [11], the Stefan-Maxwell description of multicomponent diffusion is replaced by Fickian diffusion. In addition, water vapour diffusion to both anode and cathode sides is included in the model. The overall model is in the form of a closed-form expression for the critical or threshold or balance current density at which the water production rate and the water vapour evacuation rate are exactly balanced. The model shows that the balance current density is a function of operating conditions, properties of electrode, flow and geometric parameters in the gas channels. It has been validated by comparing the predictions with the experimental data of Tuber et al. [5] and Eckl et al.

Proceedings of the 3rd International Conference on Fuel Cell Science, Engineering, and Technology, 2005

An Improved Model to Predict Flooding/Dehydration in PEM Fuel Cells

Mass transfer from solids, which has important applications in a number of chemical and pharmaceutical industries, has been studied experimentally and semiempirically under turbulent flow conditions, and correlations are available in the literature to calculate the mass-transfer coefficients from pellets, rotating cylinders and disks etc. However, mass transfer under laminar flow has not been sufficiently addressed. One of the difficulties here is the strong Reynolds number dependence of the flow pattern, for example, due to the onset of Taylor vortices for the case of a rotating cylinder. This problem is circumvented by using a computational fluid dynamics (CFD)-based solution of the governing equations for the case of a cylinder rotating inside a stationary cylindrical outer vessel filled with liquid. The parameters cover a range of Reynolds number (based on the cylinder diameter, and the tangential speed of the cylinder), Schmidt number and the ratio of the outer to inner cylinder diameters. The results confirm that the circumferential velocity profile is a strong function of the Reynolds number and varies from a nearly Couette-type flow at very low Reynolds numbers to a boundary layer-like profile at high Reynolds numbers. The onset of Taylor vortices has a strong effect on the flow field and the mass-transfer mode. The calculations show that the Sherwood number has a linear dependence on the Reynolds number in the Couette-flow regime, and roughly square-root dependence after the onset of Taylor vortices. Correlations have been proposed to calculate the Sherwood number taking account of these effects. © 2005 American Institute of Chemical Engineers.

Effect of Taylor Vortices on mass transfer from a rotating cylinder

On axial coherence of interfacial waves in countercurrent flow

An isothermal, quasi-two-dimensional model is presented, which enables the prediction of when two-phase flow and flooding may occur in a typical polymer electrolyte membrane (PEM) fuel cell stack. The model solves, at a given section of the membrane-electrode assembly (MEA), for the local current density and water vapor concentration in the anode and the cathode electrodes and in the gas channels by solving the Stefan-Maxwell equations for multi-component diffusion in the MEA. Two-phase flow, which would eventually lead to flooding, is expected to occur when the relative humidity approaches 100% on the anode or the cathode side. The model involves a system of non-linear algebraic equations, which has been solved using the optimization toolbox in MATLAB. The experimental results of Tuber et al., who reported on the visual observations of two-phase flow in a PEM fuel cell, are used to validate the model. This is an abstract of a paper presented at the 7th World Congress of Chemical Engineering (Glasgow, Scotland 7/10-14/2005).

7th World Congress of Chemical Engineering, GLASGOW2005, incorporating the 5th European Congress of Chemical Engineering

A model for the prediction of onset of flooding in PEM fuel cells

Pipelines and ducting systems used in process industry applications are characterized by large sizes, sharp bends and successive bends connected by short straight sections. Systematic studies of such flow situations have not been reported in the literature and the design of such ducting systems is based on rules of thumb and scaled-model testing. Taking advantage of the potential size-insensitivity of CFD-based calculations, a large number of calculations of typical flow situations has been carried out in the present study to address such issues as the optimum location of guides vanes, the influence of an upstream bend on the loss coefficient and the scaling laws for proper extrapolation of laboratory results to industrial scale applications. It is shown that the ideal radial location of a vane for a square duct is given by (RiRo )0.5 and that the influence of an upstream bend on the loss coefficient is beneficial in most cases. Thus, the calculation of bend losses treating each bend as being isolated is likely to be conservative. The velocity profile is uniformly improved by the use of vanes. The loss coefficient is not very sensitive to the fluid scaling, but the effect of wall roughness has to be considered carefully in scaled-down conditions. © 2004 Institution of Chemical Engineers.

Chemical Engineering Research and Design

Pressure losses and flow maldistribution in ducts with sharp bends

The pressure losses in the flow distributor plate of the fuel cell depend on the Reynolds number and geometric parameters of the small flow channels. Very little information has been published on the loss coefficients for laminar flow. This study reports a numerical simulation of laminar flow though single sharp and curved bends, 180° bends and serpentine channels of typical fuel cell configurations. The effect of the geometric parameters and Reynolds number on the flow pattern and the pressure loss characteristics is investigated. A three-regime correlation is developed for the excess bend loss coefficient as a function of Reynolds number, aspect ratios, curvature ratios and spacer lengths between the channels. These have been applied to calculate the pressure drop in typical proton-exchange membrane fuel cell configurations to bring out the interplay among the important geometric parameters. © 2004 Elsevier B.V. All rights reserved.

Pressure losses in laminar flow through serpentine channels in fuel cell stacks

With the increasing use of computational fluid dynamics (CFD)-based simulations in the assessment of thermal hydraulic behaviour of nuclear reactors, there is a need to benchmark the predictions of CFD codes. The case of a frictionless U-tube manometer [Ransom, V., 1992. Oscillating manometer. In: Hewitt, G.F., Delhaye J.M., Zuber N. (Eds.), Multiphase Science and Technology, vol. 6. Hemisphere, New York, USA, p. 591] has been previously used as a test case to benchmark the dispersive and dissipative characteristics of a numerical prediction. In the present paper, this case is extended to cover two new aspects that are often found in nuclear reactor systems: variable density and dissipation. Through a theoretical model, it is shown that the dynamic behaviour of the new system is more complicated than that of a constant-density frictionless U-tube manometer. Experiments of the test case have been carried out to substantiate the essential features of the response. CFD simulations of the system show both dissipative and dispersive errors compared to the theoretical result. The results can therefore serve as a useful benchmark for computer codes used to study the thermal hydraulics of nuclear reactor systems, especially in evaluating the role of numerical damping in systems in which physical damping exists inherently. © 2003 Elsevier B.V. All rights reserved.

Nuclear Engineering and Design

The case of an oscillating manometer with variable density and dissipation: Experimental and numerical study

Sedimentation tanks are used in the process industry to separate the solid particles from the slurry to get the clarified liquid. A detailed study of the hydrodynamics of sedimentation tanks is presented here using an Eulerian-Lagrangian approach to study the motion of solids in the tank. The model, in its present form, is applicable only to nonflocculent discrete (Type I) settling. It is shown that a typical particle-eddy interaction can be characterized by a lower cut-off size below which the particles would be entrained by the eddy; and an upper cut-off size above which the particle would continuously settle through the sedimentation tank in spite of the recirculation. The effect of inlet configuration on the flow field as well as on the settling characteristics has been investigated. The simulations show that both the upper and the lower cut-off sizes for a sedimentation tank are considerably reduced by providing a tulip type of inlet with a conical deflector as compared to a straight inlet.

Journal of Environmental Engineering

Computational study of particle-eddy interaction in sedimentation tanks

A finite volume method-based CFD model has been developed to simulate steady, turbulent, two-dimensional annular gas-liquid flow in a duct. The gas flow is treated as being equivalent to flow through a rough-walled duct. The effect of the liquid film on the gas phase is included in the form of modified wall functions which incorporate the well-known triangular relationship (Annular Two-Phase Flow, Pergamon Press, Oxford, 1970) that exists among wall shear stress, film flow rate and film thickness in annular flow. The presence of droplets is accounted for by solving an additional scalar transport equation for the mass fraction of the droplets. Entrainment and deposition of droplets are included as source term and boundary condition, respectively, in the mass fraction equation. It is shown that the resulting model, while retaining simplicity of formulation, gives good predictions of the literature data of annular flow parameters under equilibrium and non-equilibrium conditions. © 2004 Elsevier Ltd. All rights reserved.

Chemical Engineering Science

A multidimensional model for annular gas-liquid flow

A set of closure relations, in the context of a one-dimensional three-fluid model, is presented for the prediction of dryout and post-dryout heat transfer at high pressure (P/Pcr&gt;0.3) conditions. It is shown that the traditional models based on low pressure data for interfacial friction, droplet size and the transition criteria for onset of annular flow cannot be readily extended to high pressure situations. The proposed new relations are validated by comparing with literature data in the pressure range of 30–200 bar, mass flux range of 500–3000 kg&nbsp;m−2&nbsp;s−1 and tube inner diameters in the range of 10–25 mm. Good agreement is obtained for the dry out quality and the tube wall temperature in the post-dry out region except for cases of low mass flux at high pressures. The predictions show that at high pressure, high mass flux conditions, annular flow may prevail for low gas phase volume fraction, the droplet and the liquid film volume fractions being an order of magnitude higher than those encountered in typical air–water experiments.

Prediction of dryout and post-dryout heat transfer at high pressures using a one-dimensional three-fluid model

Helical ribbon impellers are widely used in chemical and process industries for the mixing of pseudoplastic fluids of high viscosity. The design of such impellers is based on an assumed linear relation between shear rate and the rotation speed of the impeller. A number of computational fluid dynamics (CFD) simulations of the flow field have been carried to verify this hypothesis. It is shown that while the shear rate varies greatly within the mixing vessel, there does exist a linear relationship between the impeller speed and the local shear rate near the tip of the impeller. The proportionality constant K s associated with this linear relation is found to be dependent on the geometric parameters of the system, but is largely independent of the flow behavior index. Based on these results, a new correlation, applicable to both Newtonian and power-law fluids for power consumption, is proposed.

VIT Unitversity

Mixing of Pseudoplastic Fluids Using Helical Ribbon Impellers

In 1957 Metzner and Otto proposed a simplified way of calculating the power consumption in impeller-driven systems for power-law type of fluids. Assuming a proportionality between the shear rate and rotational speed of the impeller, they reported that the proportionality constant was independent of the geometric and fluid properties of the mixing system. This was later contested by a number of researchers, and different correlations have been given to evaluate the proportionality constant. An in-depth investigation of the Metzner-Otto concept is discussed with the help of computational fluid dynamics (CFD) techniques to calculate the flow field created by an anchor impeller. Fundamentally, both of the assumptions are broadly valid for typical operating parameters used in anchor-driven impeller systems. A new correlation, based on the Metzner-Otto concept and making use of the present CFD simulations, was developed to calculate power consumption in anchor-driven mixing systems.

Mixing of power-law fluids using anchors: Metzner-Otto concept revisited

CFD-based computations of the flow field, power consumption and mixing time are presented for a mechanically stirred eight-blade paddle impeller in an unbaffled vessel over a range of Reynolds numbers covering laminar, transitional and turbulent flow regimes. The flow field calculations were performed using the sliding mesh technique to account for the motion of the impeller, and mixing time studies were done using a simulated tracer injection experiment. The effect of grid density and the choice of the turbulence model were investigated. The results are compared with flow field data from Dong et al.1, and power and mixing time correlations from the literature and show satisfactory agreement. It is shown that the product of mixing time and rotational speed remains constant for paddle impellers for laminar flow and that the use of a low Reynolds number turbulence model is necessary for good prediction of mixing time in the transitional flow.

CFD study of power and mixing time for paddle mixing in unbaffled vessels

Systematic experiments were conducted to study the postflooding situation in air - water countercurrent flow in a tube of 0.025 m internal diameter with smooth inlet and outlet conditions for the two phases. The downflow rate of water and the pressure gradient were measured over a range of flow rates for test-section lengths of 0.6, 1.2 and 2.0 m. Additional experiments were conducted with increasing and decreasing water flow rates at a constant air flow rate. The length of the test section or the way the flooding point was approached did not significantly affect the onset of flooding. The data show that, as the gas flow rate is increased, the pressure gradient can fall significantly in the postflooding situation due to depleting the downflow rate of liquid. Dimensionless correlations are proposed to calculate the downflow rate and the pressure gradient given for overall flow parameters.