Massoud Kaviany

Effective diffusivity and water-saturation distribution in single- and two-layer PEMFC diffusion medium

Abstract The formation–distribution of condensed water in diffusion medium of proton exchange membrane fuel cells, and its tendency to reduce the local effective mass diffusivity and to influence cell performance, are studied. First the local effective mass diffusivity of a fibrous diffusion medium is determined as a function of the local porosity and local water saturation, using the network model for species diffusion. Then using this along with the hydrodynamics of capillary, two-phase flow in hydrophobic porous media, the water formation rate (hydrogen–oxygen reaction), and condensation kinetics, the one-dimensional distribution of water saturation is determined and roles of fiber diameter, porosity, and capillary pressure on cell performance are explored. The results point to a two-layer medium (similar to the added conventional microlayer) which is then analyzed for optimum performance.

Principles of heat transfer

A system is any part of the universe that can be studied. It may be an atom or a living body, the earth or the solar system. All its surrounding matter constitutes the environment of the system.

Pool-boiling CHF enhancement by modulated porous-layer coating: Theory and experiment

Abstract Modulated (periodically non-uniform thickness) porous-layer coatings, as an example of capillary artery-evaporator systems, are experimentally shown to enhance the pool-boiling critical heat flux nearly three times over that of a plain surface. The modulation separates the liquid and vapor phases, thus reducing the liquid–vapor counterflow resistance adjacent to the surface. Theories are suggested for two independent mechanisms capable of causing the liquid choking that leads to the critical heat flux. The Zuber hydrodynamic theory is modified to account for the effect of the coating modulation-wavelength on the development of the stable vapor layer above the coated surface, which effectively chokes the liquid down-flow towards the surface (above the coating). The second liquid-choking limit occurs within the porous-layer coating when the viscous drag surpasses the available capillary pumping. The lower of these liquid-choking limits, for a given set of geometrical and thermophysical parameters, is theorized to predict the observed critical heat flux. The predicted wetted-surface regime and the two limits are compared with the experimental results and good agreement is found. The theories are then used to discuss the optimization of the enhancement and suggest that completely separated liquid and vapor flow paths can result in substantial further enhancement.

Slip and no-slip temperature boundary conditions at the interface of porous, plain media: Convection

Near the interface of porous plain media, convective heat transfer may be noticeably affected by the nonuniformity of the phase distributions. The boundary effects are modeled by using interfacial slip or no-slip temperature boundary conditions. The latter uses a variable transverse total diffusivity giving a continuous variation of the temperature near and across the interface. The former uses a constant transverse total diffusivity which requires a temperature slip cross the interface (in order to obtain accurate heat flux calculations). In this study these boundary conditions are examined by the direct simulation of the momentum and energy equations for a model porous medium made of two-dimensional periodic arrange- ments of cylinders. The slip coefficient is found to depend on the bulk Peclet number Pe,. the ratio of solid to fluid conductivity k,/k,, and the gap size /I. For the no-slip boundary condition, the magnitude and the distribution of D, (~>)/a, also depend on Pe,, k,/k,, and h. For a solid bounding surface, and when k,/kr > I. the effective transverse conductivity k,,/k, dominates over the hydrodynamic dispersion, and therefore, the accurate description of the variation of k,, (r)/ki becomes critical. For a fluid bounding medium, the results show that D, (J) is nonuniform on both sides of the interface. The nonuniformity of DL(y) in the fluid medium is due to the local two dimensionality of the flow. The total diffusivity tensor D in the bulk of a two-dimensional periodic structure is also examined. The effects of the Reynolds number, Prandtl number, particle shape, particle arrangement. and flow direction, on the bulk value of D are examined. It is found that for oblique flows, the ensemble-averaged longitudinal total diffusivity D, /ccr, over the tilt angle, approaches a Pe, relation instead of a Pe: relation expected for periodic structures. THE SIMULTANEOUS presence of pore-level temperature and velocity gradients and the application of the local volume averaging technique results in the inclusion of the pore-level convection contribution as an enhanced diffusion (or dispersion). This enhanced diffusion is characterized by the total thermal diffusivity tensor given by

Principles of convective heat transfer

Convective heat transfer is the result of fluid flowing between objects of different temperatures. Thus it may be the objective of a process (as in refrigeration) or it may be an incidental aspect of other processes. This monograph reviews recent contributions to the principles of convective heat transfer for single- and multi-phase systems. It summarizes the role of the fundamental mechanism, discusses the governing differential equations, describes approximation schemes and phenomenological models, and examines their solutions and applications. After a review of the basic physics and thermodynamics, the book divides the subject into three parts. Part 1 deals with single-medium transfers, specifically with intraphase transfers in single-phase flows and with intramedium transfers in two-phase flows. Part 2 deals with fluid-solid transfer processes, as well as liquid-liquid transfer processes. Part 3 considers three media, addressing both liquid-solid-solid and gas-liquid-solid systems.

Modelling radiative heat transfer in packed beds

Abstract A comprehensive approach for modelling dependent radiative heat transfer in beds of large (geometric range) spherical particles is presented. Such a system of large spheres lies in the dependent range even for large porosities. We show that the dependent properties for a bed of opaque spheres can be obtained from their independent properties by scaling the optical thickness while leaving the albedo and the phase function unchanged. The scaling factor is found to depend mainly on the porosity and is almost independent of the emissivity. We show that such a simple scaling for non-opaque particles is not feasible. The transparent and semi-transparent particles are treated by allowing for the displacement across an optical thickness (because of transmission through a particle) while solving the equation of radiative transfer. When combined with the scaling approach, this results in a powerful method of solution called the dependence included discrete ordinates method (DIDOM). The results obtained from the DIDOM give good agreement with the results obtained from the Monte Carlo method.

Nonequilibrium in the transport of heat and reactants in combustion in porous media

Abstract Combustion in inert, catalytic and combustible porous media occurs under the influence of a large range of geometric length scales, thermophysical and thermochemical properties, and flow, heat and mass transfer conditions. As a result, a large range of phenomenological length and time scales control the extent of departure from local thermal and chemical nonequilibrium. The use of intraphase and interphase nonequilibria have allowed for the design of new combustion processes and systems, such as, catalytic reactors and converters, porous radiant burners, direct energy and gas conversion devices and systems, chemical sensors, and material synthesis processes. Improvement of current and design of yet newer and more innovative systems requires further investigations into the gas-phase and surface chemistry, solid-state and condensed-phase physics, transport in disordered structures, and mathematical and numerical methods. Here we summarize the processes leading to thermal and chemical nonequilibrium, their role in the combustion in porous media, their innovative uses and effects on applications, the current modeling of these processes and the modeling techniques that may allow for further improvements and developments.

Phonon Transport in Molecular Dynamics Simulations: Formulation and Thermal Conductivity Prediction

Publisher Summary This chapter discusses the phonon transport in molecular dynamics (MD) simulations. The chapter presents a formulation for studying the thermal transport in dielectric materials using MD simulations. The simulations allow for analysis in both the real and phonon spaces. The natural inclusion of anharmonic effects through the form of the interatomic potential presents a significant advantage over harmonic theories. The chapter describes, examines, and compares two major approaches for predicting thermal conductivity from MD simulations (the Green–Kubo method [GK] and direct methods). Each has advantages and disadvantages, and the method chosen strongly depends on the problem of interest. Generally, the GK method is superior for bulk phase simulations, while the direct method is best for finite structures. In terms of applying MD simulations to real systems, current computational resources cannot accurately model anything close to a micron in size on an atom-by-atom level. The upscaling of MD results to larger length scale models is a promising and exciting avenue. Upscaling has been applied in a different context to phonon transport across material interfaces by Schelling and Phillpot.

Adiabatic Reverse Combustion in a Packed Bed

Downward propagation of a combustion front in a packed bed of wood particles, with air supply from below, is examined theoretically and experimentally. Using a single-step reaction, with a kinetic model of char oxidation as the dominant mechanism, and assuming local thermal equilibrium, but allowing a local chemical nonequilibrium between the solid and gas phases, the front speed u F, the adiabatic temperature T r, and the extent of solid consumption ( p)7/( p )s are determined as functions of the entering air pole-velocity (u)n s. Both oxygen-limited and fuel-limited regimes, with the boundary marked by the stoichiometric burning, are examined. In the oxygen-limited regime, T,., u F, ( p),S/( p),s, and the thickness of the front 6F all increase with (u),, g. In the fuel-limited regime, the reverse occurs and the extinction at high (u),, g is predicted. In the oxygen-limited regime, where the bed is not yet fluidized, the experimental results are in good agreement with the predictions. NOMENCLATURE a a pre-exponential factor (s- 1 ) Asg solid-gas surface area (m- 1 ) Cp specific heat capacity (J/kg-K) d diameter (m) D total thermal diffusivity (m2/s) D m mass diffusivity (mZ//s) D d axial thermal dispersion coefficient (m2//s) A E a activation energy for reaction (J//mole) F radiation exchange factor Ai c heat of combustion (J/kg) k thermal conductivity (W/m-K) K kinetic rate (s- 1 ) h d diffusion-controlled reaction rate (kg//m3-s) haE kinetically controlled reaction rate (kg//m3-s) h reaction rate (kg/m3-s) Leg gas Lewis number Pe d P6clet number Rg gas constant (J/mole-K) Sc Schmidt number (Shd)Sg Sherwood number t time (s) T temperature (K) u velocity (m/s) V representative elementary volume (m 3)

Thermoelectric performance of films in the bismuth-tellurium and antimony-tellurium systems

Coevaporated bismuth-tellurium and antimony-tellurium films were fabricated under various deposition conditions (controlled evaporation rates of individual species, substrate temperature, and substrate material), and their thermoelectric (TE) properties (Seebeck coefficient, electrical resistivity, and carrier concentration) were measured in search of optimal TE performance. The tellurium atomic concentration was varied from 48% to 74%, the substrate temperature ranged from 130 to 300 °C, and glass, mica, magnesium oxide, and sapphire substrates were used. The chemical composition and crystal structure of the films were recorded (using microprobe and x-ray diffractometer, respectively), analyzed, and compared with available standard Bi2Te3 and Sb2Te3 single-crystal samples. High-performance TE films had tellurium atomic concentration around 60% and were deposited at a substrate temperature between 260 and 270 °C.