Begell House Inc.
Computational Thermal Sciences: An International Journal
CTS
1940-2503
1
2
2009
TWO-PHASE FLOW AND MASS TRANSFER WITHIN THE DIFFUSION LAYER OF A POLYMER ELECTROLYTE MEMBRANE FUEL CELL
105-120
Steven B.
Beale
Institute of Energy and Climate Research,
IEK-3 Forschungszentrum Jülich GmbH 52425
Jülich, Germany
D. H.
Schwarz
National Research Council, Montreal Road, Ottawa, Ontario, K1A 0R6 Canada
M. R.
Malin
Concentration Heat and Momentum Ltd, Bakery House, 40 High Street, Wimbledon Village London, SW19 5AU Great Britain
Dudley Brian
Spalding
Concentration, Heat, and Momentum (CHAM), Limited, Bakery House, 40 High Street, Wimbledon Village, London SW19 5AU, England
The membrane of a polymer electrolyte membrane fuel cell must be hydrated with liquid water at all times in order to function effectively. At high current densities, liquid water in the pores of the diffusion layer inhibits oxygen transport to the cathode. The present paper shows the results of an analysis of two-phase flow and mass transfer in the diffusion layer of a fuel cell. A computational fluid dynamics code is adapted to perform calculations assuming Darcy 's law applies, with the rate of oxygen diffusion governed by Fick 's law. Both relative permeability and capillary pressure are strongly dependent on saturation. A modified version of the interphase slip algorithm is used to perform flow-field calculations. The two phases are each assigned a different pressure. Phase continuity is solved for liquid-phase saturation, from whence capillary pressure, relative permeability, and oxygen exchange coefficients are obtained. Results of numerical calculations are compared to an analytical solution with excellent agreement. Detailed calculations for a typical present-day fuel cells are presented. The results are correlated in terms of gas mass transfer driving force as a function of blowing parameter.
ADAPTIVE FEM MODEL FOR UNSTEADY TURBULENT CONVECTIVE FLOW OVER A BACKWARD-FACING STEP
121-135
Xiuling
Wang
Purdue University Northwest
David
Carrington
Los Alamos National Laboratory, T-3 Fluid Dynamics and Solid Mechanics
Darrell W.
Pepper
NCACM, Department of Mechanical Engineering, University of Nevada Las Vegas, Las Vegas, NV 89154, USA
An adaptive finite element algorithm for solving turbulent convective flow over a backward-facing step using a two-equation low−Reynolds number model has been developed. The mesh is dynamically controlled using an L2 norm error estimator. Petrov-Galerkin weighting is used for the advection terms. Complex features within the flow including boundary layer development and reattachments points are resolved using high-density localized mesh refinement. Simulation results are obtained for a Reynolds number equal to 28,000, with the channel's expansion ratio equal to 1.25 and the Prandtl number set to 0.71. A constant uniform heat flux of 270 W/m2 is specified along the wall downstream from the step; all the other walls are set to adiabatic conditions. Simulation results for mean velocity profiles, mean temperature profiles, turbulence kinetic energy, and friction coefficient distributions are obtained. Results are compared with both numerical and experimental data in the literature. Good agreement is observed.
A MULTISCALE FULL-SPECTRUM κ-DISTRIBUTION METHOD FOR RADIATIVE TRANSFER IN NONHOMOGENEOUS GAS-SOOT MIXTURES WITH WALL EMISSION
137-158
Gopalendu
Pal
Mechanical and Aerospace Engineering Department, Florida, Institute of Technology, Melbourne, Florida 32901; Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
The multiscale full-spectrum κ-distribution (MSFSK) method has become a promising method for radiative heat transfer in inhomogeneous media. The scope of the MSFSK method has been limited to radiation calculations involving inhomogeneous gas mixtures only. The objective of this paper is to develop a MSFSK model that is accurate for inhomogeneous mixtures of both gases and nongray particles, such as soot. Soot is added as one more scale in addition to the gas scales. The overlap parameters between scales are calculated by mixing molecular gases with soot particles at the narrowband level. This MSFSK method is also capable of black/gray wall emission. Because wall emission is continuous over the spectrum, similar to radiation from soot, wall emission is treated within the soot scale. Sample calculations are performed for a one-dimensional medium with step changes in species concentration and temperature and also for a two-dimensional axisymmetric medium involving combustion of methane. The MSFSK method is observed to accurately predict heat transfer from inhomogeneous gas−soot mixture with/without wall emission yielding close to line-by-line accuracy with several orders of magnitude less computational cost.
3D NUMERICAL SIMULATION OF THE EFFECT OF DROPLET INITIAL CONDITIONS ON THE EVAPORATION PROCESS
159-187
Madjid
Birouk
Department of Mechanical and Manufacturing Engineering, University of Manitoba, Winnipeg, MB, R3T 5V6 Canada
M. M. Abou
Al-Sood
Department of Mechanical and Manufacturing Engineering, University of Manitoba, Manitoba, Canada
A three-dimensional numerical model is developed to simulate the effect of a droplet's initial conditions on the vaporization process in a turbulent convective environment at ambient pressure and temperature higher than the standard conditions. A hydrocarbon (n-heptane) droplet with two different initial diameters, 0.1 mm and 1.5 mm, and initial temperatures, 253 K and 320 K, is examined. The droplet is exposed to turbulent stream of nitrogen with a mean velocity of 2 m/s, and turbulence intensity ranging between 0 and 60%. The ambient pressure and temperature range is between 0.5 MPa and 4 MPa and 324 K and 1350 K, respectively. The numerical model solves the complete set of time-dependent conservation equations of mass, momentum, energy, and species concentration in both the gas phase and liquid phase. The turbulence terms in the conservation momentum (RANS) equations of the gas phase are modeled by using the shear stress transport model. Variable thermophysical properties, gas and liquid phase transients, and radiation are all accounted for. Moreover, the effect of high pressure such as nonideal gas behavior, solubility of ambient gas into the droplet, and pressure dependence of gas- and liquid-phase thermophysical properties are also considered.
EFFICIENT CALCULATION OF RADIATION HEAT TRANSFER IN ANISOTROPICALLY SCATTERING MEDIA USING THE QL METHOD
189-206
Pedram
Hassanzadeh
University of California, Berkeley; Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Canada N2L 3G1
George D.
Raithby
Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Canada N2L 3G1
E. H.
Chui
CANMET Energy Technology Centre Natural Resources Canada, Ottawa, Canada K1 A 1M1
In general-purpose computational fluid dynamics codes, radiation heat transfer is often computed using the P1 model, which can be inaccurate, or the finite volume or discrete ordinates method, which can be computationally very expensive. The present research objective was to develop a solution method, called the QL method, which permits both the accuracy and cost to be controlled. The method has been previously shown to work well for radiation-only problems involving emission/absorption and isotropic scattering in a gray medium. This paper demonstrates the method on problems involving anisotropic scattering. It is shown that any scattering phase function can be handled with the QL method, and that accurate results are obtained at reasonable cost.
HIGHER-ORDER SPHERICAL HARMONICS TO MODEL RADIATION IN DIRECT NUMERICAL SIMULATION OF TURBULENT REACTING FLOWS
207-230
Kshitij V.
Deshmukh
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
Daniel C.
Haworth
Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
The exact treatment of the radiative transfer equation (RTE) is difficult even for idealized situations and simple boundary conditions. A number of higher-order approximations, such as the moment method, discrete ordinates method and spherical harmonics method, provide efficient solution methods. A statistical method, such as the photon Monte Carlo method, solves the RTE by simulating radiative processes such as emission, absorption, and scattering. Although accurate, it requires large computational resources and the solution suffers from statistical noise. The third-order spherical harmonics method (P3 approximation) used here decomposes the RTE into a set of 16 first-order partial differential equations. Successive elimination of spherical harmonic tensors reduces this set to six coupled second-order partial differential equations with general boundary conditions, allowing for variable properties and arbitrary three-dimensional geometries. The tedious algebra required to assemble the final form is offset by greater accuracy because it is a spectral method as opposed to the finite difference/finite volume approach of the discrete ordinates method. The radiative solution is coupled with a direct numerical solution (DNS) of turbulent reacting flows to isolate and quantify turbulence−radiation interactions. These interactions arise due to nonlinear coupling between the fluctuations of temperature, species concentrations, and radiative intensity. Radiation properties employed here correspond to a nonscattering fictitious gray gas with a Planck-mean absorption coefficient, which mimics that of typical hydrocarbon-air combustion products. Individual contributions of emission and absorption TRI have been isolated and quantified. The temperature self-correlation, the absorption coefficient-Planck function correlation, and the absorption coefficient-intensity correlation have been examined for small to large values of the optical thickness. Contributions from temperature self-correlation and absorption coefficient−Planck function correlation have been found to be significant for all the three optical thicknesses while absorption coefficient−intensity correlation is significant for optically thick cases, weak for optically intermediate cases, and negligible for optically thin cases.