Heat and mass transfer and fluid flow in cryo-adsorptive hydrogen storage system

Compared to room temperature adsorption, cryo-adsorptive hydrogen storage capacity has been greatly improved, and has become the central issue of the hydrogen storage research. Accurate simulation and optimization for cryo-adsorptive hydrogen storage has important guidance and application value to the experimental research, and the finite element software Comsol Multiphysics™ and system analysis software Matlab/Simulink™ can be used to simulate the cryo-adsorptive hydrogen storage. However, the computational fluid dynamics (CFD) software Fluent™ can provide more information on the heat and mass transfer and the fluid flow than above softwares. Based on the mass, momentum and energy conservation equations, this paper uses the modified Dubinin–Astakhov (D–A) adsorption isotherm model, linear driving force (LDF) model and dynamic thermal boundary condition which are implemented by means of CFD software Fluent to simulate the hydrogen adsorption processes of charging and dormancy in the case of liquid nitrogen cooling. We study the variations of temperature and pressure during the processes of charging and dormancy. The results show that the experimental data is in good agreement with the simulation results. We also analyze the effect of variable specific heat and anisotropic thermal conductivity on the heat and mass transfer and the fluid flow in cryo-adsorptive hydrogen storage system.     

Numerical simulation of wall mass transfer rates in capillary-driven flow in microchannels

Microchannels are believed to open up the prospect of precise control of fluid flow and chemical reactions. The high surface to volume ratio of micro size channels allows efficient mass transfer rates. The capillary effect can be used to pump fluids in microchannels and the flow generated can dissolve chemicals previously deposited on the walls of the channel. The purpose of this work is to analyze the wall mass transfer rates generated by a capillary driven flow in a microchannel. The results have implications in the optimization and design of devices for biological assays. We performed simulations of the capillary-driven flow in two-dimensional rectangular and circular microchannels by solving numerically the governing momentum and mass transfer equations with a second order accuracy finite volume code. The effects of the Reynolds number, of the contact angle and of the channel geometry on the time evolution of the local and averaged wall mass transfer rates are reported and analyzed. The flow field behind the meniscus, viewed from a reference frame moving at the velocity of the meniscus, showed to have two recirculations that enhance the wall mass transfer rates close to the triple point. A correlation between the Sherwood number and the Reynolds number, the contact angle and the time is reported. The correlation can be a useful tool for design purposes of microfluidic devices with capillary driven flows in which a fast heterogeneous reaction occurs on the wall.     

Three-dimensional CFD modeling of a direct formic acid fuel cell

A three-dimensional (3D) with one straight channel computational fluid dynamics (CFD) model is developed by using the ESI-CFD software to investigate the effect of varying operating parameters on the performance of direct formic acid fuel cell (DFAFC) and formic acid crossover from the anode to the cathode side through the membrane. Formic acid concentration (4 M–10 M), temperature (313 K–353 K), anode stoichiometry (1.5–3.0), and cathode stoichiometry (2.0–3.0) are the selected operating parameters in this study. Validation results of the DFAFC are in reasonable agreement with the typical trends reported in the literature on DFAFC performance. Simulation results indicate that formic acid concentration, temperature, anode, and cathode stoichiometry influenced the DFAFC performance and the formic acid crossover. The increments of formic acid concentration or stoichiometric ratio will improve the cell performance; however, the current densities obtained are declining to the increasing temperature. The increase in temperature of the formic acid concentration is found to lead to the decrease in performance. For the formic acid crossover phenomenon, the formic acid crossover flux increases with the increments of formic acid concentration, DFAFC operating temperature, and anode and cathode stoichiometric ratios.     

Experimental and theoretical analysis of annular two-phase flow regimen in direct steam generation for a low-power system

This study aims to quantify and to model the temperature profile around an absorber tube of a parabolic trough concentrator with low fluid flow. This study was specifically developed for the solar power plant of the Engineering Institute, National University of Mexico. This work presents experimental results under saturated conditions and low pressures (1.5–3 bar) using water as the thermal and working fluid for direct steam generation (DSG). The control variable was feed flow. Solar irradiance was used as the restriction variable because all experimental tests should be developed under very specific values of this variable (for example, I > 700 W/m²). The objective of this experiment was to study the thermal behavior of a temperature gradient around the absorber tube under steady-state conditions and with low flow. Additionally, a theoretical analysis was carried out by means of the homogeneous heat conduction equation in the cylindrical coordinate system using only two dimensions (r, ). The finite-difference numerical method was used with the purpose of proposing a solution and obtaining a temperature profile. The objective of this theoretical analysis was to complement the experimental tests carried out for direct steam generation (DSG) with annular two-phase flow patterns for low powers in parabolic trough concentrators with carbon steel receivers.     

Parameter study of a porous solar-based propane steam reformer using computational fluid dynamics and response surface methodology

Solar-driven steam reforming of fossil fuels is a promising renewable method for hydrogen production that reduces emissions compared with traditional approaches such as combustion-based technologies. In the present study, a steady-state computational fluid dynamic (CFD) model is developed to investigate a porous solar propane steam reformer (PSR). P1 approximation for radiation heat transfer is coupled with the CFD model, employing User-Defined Functions (UDFs). Innovative propane steam reformers have received less attention in terms of optimization and sensitivity analysis to improve their performance and efficiency. Hence, the effects of porosity, pore diameter, inlet velocity, solar irradiation flux, inlet temperature, and foam thermal conductivity on the propane conversion, hydrogen production rate, and pressure drop are studied using response surface methodology (RSM). The inlet velocity, solar irradiation flux, and pore diameter are found to be the most influential parameters, among those mentioned, on propane conversion, hydrogen productivity, and pressure drop, respectively. Furthermore, optimization is carried out in order to minimize pressure drop and maximize hydrogen production. The reformer with the 70% propane conversion provides the lowest pressure drop maintaining the same hydrogen productivity compared with 80% and 90% propane conversions.     

Direct numerical simulation of interphase mass transfer in gas-liquid multiphase systems

This paper presents the Direct Numerical Simulation of interphase mass transfer in gas liquid multiphase system. The volume-of-fluid (VOF) method in conjunction with mass transfer model has been used. In order to study the process of interphase mass transfer two numerical simulation methods are presented. Two common mass transfer mechanisms, Diffusion through Stagnant Film (DTSF) and Equi-Molal Counter Diffusion (EMCD), are investigated. Two benchmarks, the Stefan diffusion problem and the diffusion in water and methanol Gas-Liquid system, have been used to validate the numerical methods. Afterwards two proposed numerical solution for different mass transfer mechanisms have been investigated in stratified gas liquid flows between two parallel plates. The results show by different approaches in numerical solution, the accuracy of mass transfer simulation is different.     

Review of computational heat and mass transfer modeling in polymer-electrolyte-membrane (PEM) fuel cells

Many computational fluid dynamics polymer-electrolyte-membrane fuel cell models have been presented over the last few decades. A detailed literature overview of models, ranging from one-dimensional, single-component to complete three-dimensional, large-scale setups, is presented with an emphasis on heat and mass transfer. Modeling strategies and commonly used assumptions are discussed. Solver implementations, popular numerical algorithms, and computational techniques are summarized. Additionally, model accuracy and convergence problems are highlighted while solving for these highly coupled, nonlinear systems of partial differential equations. Finally, an overview of commonly used simulation software for fuel cell modeling is given. A simple case study is presented throughout this review to support and to illustrate several discussed aspects. The paper finishes with a survey of outstanding issues and recent modeling trends.     

Numerical computation of fluid flow and heat transfer in microchannels

Three-dimensional fluid flow and heat transfer phenomena inside heated microchannels is investigated. The steady, laminar flow and heat transfer equations are solved using a finite-volume method. The numerical procedure is validated by comparing the predicted local thermal resistances with available experimental data. The friction factor is also predicted in this study. It was found that the heat input lowers the frictional losses, particularly at lower Reynolds numbers. At lower Reynolds numbers the temperature of the water increases, leading to a decrease in the viscosity and hence smaller frictional losses.     

Analysis of wind flow around a parabolic collector (1) fluid flow

Applications of parabolic collectors for solar heating and solar thermal power plant increased in the recent years. Most of the solar power plants installed with parabolic collectors are on flat terrain and they may be subjected to some environmental problems. One of problems for large parabolic collector is their stability to track the sun with respect to time very accurately. Any small off tracking as well as the collector structure stability will be affected by strong wind blowing for the regions where the wind velocity is high.In the present study, a two-dimensional numerical simulation of turbulent flow around a parabolic trough collector of the 250 kW solar power plants in Shiraz, Iran is performed taking into account the effects of variation of collector angle of attack, wind velocity and its distribution with respect to height from the ground.Computation is carried for wind velocity of 2.5, 5, 10, and 15 m/s and collector angles of 90°, 60°, 30°, 0°, −30°, −60°, and −90° with respect to wind directions. Various recirculation regions on the leeward and forward sides of the collector are observed, and both pressure field around the collector and total force on the collector are determined for each condition. The effect of absorber tube on the flow field was found negligible, while the effect of the gap between the two sections of parabola at midsection and the gap between the collector and ground were found considerable on both flow field and pressure distribution around the collector.     

Heat and mass transfer of two immiscible flows of Jeffrey fluid in a vertical channel

In this article, we examined the effect of heat and mass transfer flow of two immiscible Jeffrey fluids in a vertical channel. The highly nonlinear coupled ordinary differential equations are evaluated using regular perturbation parameters, for small values of perturbation parameter. The effect of Jeffrey's parameter on the flow and the effects of various physical parameters entering into the problem on dimensionless velocity, temperature, and concentration distribution is illustrated graphically. We observe that the Jeffrey parameter, thermal, and mass Grashof number enhance the fluid flow, while the chemical reaction parameter suppresses the fluid flow, also it is established that the Nusselt number is boosted by enhancing the thermal and mass Grashof number. We observed that the results are in very good agreement with the results obtained for a viscous fluid.     

Heat and mass transfer of magnetohydrodynamic Casson fluid flow over a wedge with thermal radiation and chemical reaction

A numerical computation to analyze the heat and mass transfer mechanism of a magnetohydrodynamic radiative Casson fluid flow over a wedge in the presence of Joule heating, viscous dissipation, and chemical reaction is carried out in this study. The flow-governing partial differential equations are transformed as ordinary differential equations by relevant similarity transformations and subsequently resolved by Runge–Kutta numerical approach with a shooting technique. The characteristics of momentum, thermal, and concentration border layers due to various influencing parameters are graphically outlined and numerically computed by MATLAB software. We present comparative solutions to construe the relative outcomes of Casson fluid versus Newtonian fluid. Computational outcomes of friction factor and Nusselt and Sherwood numbers are tabulated with suitable interpretations. An increase in skin friction values is noted due to an increment in the thermal Grashof number, whereas a decrease is observed due to the chemical reaction parameter. The Casson fluid displays a superior heat transfer mechanism than the Newtonian fluid. Obtained outcomes are in good agreement with the prevailing literature in the limiting case.     

Heat transfer and crevice flow in a hydrogen-fueled spark-ignition engine: Effect on the engine performance and NO exhaust emissions

The present work investigates the effect of heat and mass transfer on the combustion process of a hydrogen-fueled spark-ignition engine, using an in-house CFD code. The main scope is to compare the calculated local heat fluxes with the available measured ones, using three heat transfer models of increasing complexity (two existing and one developed by the authors). Moreover, the effect of mass transfer through the crevice regions is also investigated using a phenomenological crevice model. The calculated results (cylinder pressure traces, local heat fluxes and NO exhaust emissions) are compared with the corresponding measured data, at various operating conditions, maintaining constant engine speed and altering the compression ratio and the equivalence ratio. It is revealed, that the proposed heat transfer model is more accurate than the standard wall-function formulation, while with the use of the crevice model a more reliable prediction of engine performance is achieved.     

Heat Transfer and Fluid Flow over a Single Disk in a Fluid Rotating as a Rigid Body

Laminar heat transfer problem is analyzed for a disk rotating with the angular speed ωin a co-rotating fluid (with the angular speed Ω). The fluid is swirled in accordance with a forced-vortex law, it rotates as a solid body at β= Ω/ω= const. Radial variation of the disk's surface temperature follows a power law. An exact numerical solution of the problem is obtained basing on the self-similar profiles of the local temperature of fluid, its static pressure and velocity components. Numerical computations were done at the Prandtl numbers Pr = 1(?)0.71. It is shown that with increasing βboth radial and tangential components of shear stresses decrease, and to zero value at β= 1. Nusselt number is practically constant at β= 0(?) 0.3 (and even has a point of a maximum in this region); Nu decrease noticeably for larger βvalues.     

Computational fluid dynamics (CFD) modeling of heat transfer in a polymeric membrane using finite volume method

The efficiency, robustness and reliability of recent numerical methods for finding solutions to flow problems have given rise to the implementation of computational fluid dynamics (CFD) as a broadly used analysis method for engineering problems like membrane separation system. The CFD modeling in this study observes steady and unsteady (transient) heat flux and temperature profiles in a polymeric (cellulose acetate) membrane. This study is novel due to the implementation of user defined scalar (UDS) diffusion equation by using user-defined functions (UDFs) infinite volume method (FVM). Some details of the FVM used by the solver are carefully discussed when implementing terms in the governing equation and boundary conditions (BC). The contours of temperature due to high-temperature gradient are reported for steady and unsteady problems.     

Numerical study of a flat-tube high power density solid oxide fuel cell: Part I. Heat/mass transfer and fluid flow

The flat-tube high power density (HPD) solid oxide fuel cell (SOFC) is a new design developed by Siemens Westinghouse, based on their formerly developed tubular type SOFC. It has increased power density, but still maintains the beneficial feature of secure sealing of a tubular SOFC. In this paper, a three-dimensional numerical model to simulate the steady state heat/mass transfer and fluid flow of a flat-tube HPD-SOFC is developed. In the numerical computation, governing equations for continuity, momentum, mass, and energy conservation are solved simultaneously. The highly coupled temperature, concentration and flow fields of the air stream and the fuel stream inside and outside the different chambers of a flat-tube HPD-SOFC are investigated. The variation of the temperature, concentration and flow fields with the current output is studied. The heat/mass transfer and fluid flow modeling and results will be used to simulate the overall performance of a flat-tube HPD-SOFC, and to help optimize the design and operation of a SOFC stack in practical applications.     

MHD slip flow and heat transfer of UCM fluid with the effect of suction/injection due to stretching sheet: OHAM solution

In this study, the optimal homotopy analysis (OHAM) technique has been examined to solve the laminar magnetohydrodynamic flow (MHD flow) on the upper-convected Maxwell fluid on an isothermal porous stretch surface. A study on the effects of parameters like the relaxation time, suction/injection velocity, as well as the magnetic number on velocity over a sheet was conducted and these results are compared to the corresponding previously available results. It was observed that the thickness of the boundary layer is lowered by enhancing $s$, $\beta$, and $M$ values. Opposing this, it was observed that large $\beta$ values increase the $f″\left(0\right)$ magnituIIde. It is found that OHAM is an efficient method capable of giving a greater degree of accuracy in numerical values of flow parameters even after fewer approximations.     

Heat transfer and fluid flow of nanofluids in laminar radial flow cooling systems

Nanofluids are considered as interesting alternatives to conventional coolants. It is well known that traditional fluids have limited heat transfer capabilities when compared to common metals. It is therefore quite conceivable that a small amount of extremely fine metallic particles placed in suspension in traditional fluids will considerably increase their heat transfer performances. A numerical investigation into the heat transfer enhancement capabilities of coolants with suspended metallic nanoparticles inside a radial, laminar flow cooling configuration is presented. Temperature dependant nanofluid properties are evaluated from experimental data available in recent literature. Results indicate that considerable heat transfer increases are possible with the use of relatively small volume fractions of nanoparticles. Generally, however, these are accompanied by considerable increases in wall shear-stress. Results also show that predictions obtained with temperature variable nanofluid properties yield greater heat transfer capabilities and lower wall shear stresses when compared to predictions using constant properties.     

Reduction of NO emissions in a turbojet combustor by direct water/steam injection: Numerical and experimental assessment

Numerical and experimental investigations are conducted to assess the benefits and drawbacks of both water (mist) and steam direct injection within the combustion chamber of a 200 N static thrust turbojet. For this purpose, a three-dimensional CFD model of the combustion process is implemented where pollutant emissions are calculated; in parallel, a test campaign on the turbojet at sea level static conditions is carried out. In both cases the refrigerant flow is injected directly into the combustor, outside the liner. The aim of the investigations is to evaluate the impact of increasing water and steam flows (ranging from 0% to 200% of the fuel mass flow) onto the emissions levels (NO and CO) of the engine.     

Theoretical analysis of fluid flow and heat transfer in stoichiometric combustion in a naturally-ventilated control-volume

Using the conservation equations for mass, momentum and energy, a theoretical analysis of buoyancy driven flow and heat transfer for a ventilated control-volume, with an internal heat-source, has been made. The special case of stoichiometric combustion in a naturally-ventilated brick walled room, with a single rectangular opening, has been used to demonstrate the numerical calculation procedure for the prediction of the histories of the fire temperature, gas flow rate, fuel burn rate, fire power and boundary-wall temperature. The analysis may be extended for more complex space geometries and wall structures; a typical case being a railway carriage with a composite wall.     

Heat transfer in the MHD flow of a power law fluid over a non-isothermal stretching sheet

The article examines the hydromagnetic laminar boundary layer flow and heat transfer in a power law fluid over a stretching surface. The flow is influenced by linear stretching of the sheet. Also the energy equation with temperature-dependent thermal conductivity, thermal radiation, work done by stress, viscous dissipation and internal heat generation is considered. The governing partial differential equations along with the boundary conditions are first cast into a dimensionless form and then the equations are solved by Keller–Box method. The effects of various physical parameters on the flow and heat transfer characteristics are presented graphically and discussed.