Mohammad Faghri

EFFECTS OF COMPRESSIBILITY AND RAREFACTION ON GASEOUS FLOWS IN MICROCHANNELS

A two-dimensional flow and heat transfer model is used to study gas compressibility and rarefaction in microchannels assuming a slip flow regime. The compressible forms of momentum and energy equations are solved with slip velocity and temperature jump boundary conditions in a parallel plate channel for both uniform wall temperature and uniform wall heat flux boundary conditions. The numerical methodology is based on the control volume finite difference scheme. To verify the model, the mass flow rate was compared with the experimental results of helium through a microchannel. Also, the normalized friction coefficient was compared with the experiments for nitrogen and helium flow in a microchannel. Finally, the axial pressure distribution was compared with the experimental results for nitrogen flow in a microchannel. The computations were performed for a wide range of knin, Re, dimensionless distance from the entrance, and for the wall parameters q∗ and T∗, to study the effects of rarefaction and compressi...

Finite-Volume Solutions for Laminar Flow and Heat Transfer in a Corrugated Duct

A finite volume methodology was developed to predict fully developed heat transfer coefficients, friction factors, and streamlines for flow in a corrugated duct. The basis of the method is an algebraic coordinate transformation which maps the complex fluid domain onto a rectangle. The method can be adopted for other convection-diffusion problems in which two boundaries of the flow domain do not lie along the coordinate lines. Representative results were found for laminar flow uniform wall temperature, and for a range of Reynolds number, Prandtl number, corrugation angle, and dimensionless interwall spacing. As seen from the streamlines, the flow patterns are highly complex including large recirculation zones. The pressure drops and friction factor results are higher than the corresponding values for a straight duct. Finally, the performance of the corrugated duct was compared with the straight duct under three different constraints - fixed pumping power, fixed pressure drop, and fixed mass flow rate. There are small differences in the heat transfer rate ratios under these constraints.

Transport Phenomena in Fuel Cells

Fuel cells are expected to play a significant role in the next generation of energy systems and road vehicles for transportation. However, substantial progress is required in reducing manufacturing costs and improving performance. This book aims to contribute to the understanding of the transport processes in solid oxide fuel cells (SOFC), proton exchange membrane fuel cells (PEMFC) and direct methanol fuel cells (DMFC), which are of current interest. A wide range of topics is covered, featuring contributions from prominent scientists and engineers in the field.

Experimental Investigation of Gas Flow in Microchannels

We present an experimental investigation of laminar gas flow through microchannels. The independent variables: relative surface roughness, Knudsen number and Mach number were systematically varied to determine their influence on the friction factor The microchannels were etched into silicon wafers, capped with glass, and have hydraulic diameters between 5 and 96 μm. The pressure was measured at seven locations along the channel length to determine local values of Knudsen number, Mach number and friction factor. All measurements were made in the laminar flow regime with Reynolds numbers ranging from 0.1 to 1000

Heat transfer with laminar pulsating flow in a pipe

Abstract The problem of heat transfer from a cylindrical pipe is formulated for a case where the flow inside the pepe consists of a periodic motion imposed on a fully developed steady laminar flow. It is shown that the velocity pulsations induce harmonic oscillations in temperature thus breaking the temperature field into a steady mean part and a harmonic part. The interaction between the velocity and temperature oscillations introduces an extra term in the energy equation which reflects the effect of pulsations in producing higher heat transfer rates.

Effect of Surface Roughness on Nitrogen Flow in a Microchannel Using the Direct Simulation Monte Carlo method

The paper presents direct simulation Monte Carlo (DSMC) results for nitrogen flow in a microchannel with surface roughness modeled by an array of rectangular modules placed on one side of a parallel-plate channel. The effects of relative surface roughness, roughness distribution, and gas rarefaction on flow are investigated and the results are presented in the form of the product of friction factor and Reynolds number. It was found that the effect of surface roughness is more pronounced at low Knudsen numbers. At high Knudsen numbers, rarefaction reduces the interaction between the gas molecules and the channel walls and results in a lower friction factor. The roughness distribution represented by the ratio of the roughness height to spacing of the modules has a significant effect on the flow and friction factor. Finally, the locally fully developed (LFD) flow model can be used to predict gas flow in a microchannel with low values of relative surface roughness.

Heat transfer and pressure drop characteristics in a corrugated duct with rounded corners

Abstract Heat transfer and pressure drop responses of a corrugated duct with rounded corners were determined numerically. The duct boundaries were approximated by a cosine function. Computations were carried out for a Prandtl number of 0.7, in the Reynolds number range from 100 to 1000, for three assigned corrugation angles, and for four values of aspect ratios. Rounding of the corners resulted in a decrease of friction factor and Nusselt number. Heat transfer performance of a duct with rounded corners was compared to a straight duct and to a duct with sharp corners under three different constraints: fixed pumping power, fixed pressure drop, and fixed mass flow. The heat transfer rates decreased or increased depending on the specific conditions.

Transport Phenomena in Fires

Controlled fires are beneficial for the generation of heat and power while uncontrolled fires, like fire incidents and wildfires, are detrimental and can cause enormous material damage and human suffering. Transport phenomena such as buoyant flow, momentum, convective heat and mass transfer as well as chemical reactions between combustible species and oxygen from the surrounding air play important turbulent mixing are important to the mechanism of flame heat transfer that govern fire release rates. The mechanisms of ignition, flame spread, steady burning flame extinction and smoke transport all need to be considered in fire modelling. In addition, temperature-dependent properties are important factors for consideration. For uncontrolled fires, the evolution in time is of great concern. This edited book presents the state of the art of modelling and numerical simulation of the important transport phenomena in fires. It describes how computational procedures can be used in analysis and design of fire protection and fire safety. Computational fluid dynamics, turbulence modelling, combustion, soot formation, thermal radiation modelling are demonstrated and applied to pool fires, flame spread, wildfires, fires in buildings and other examples. All of the chapters follow a unified outline and presentation to aid accessibility and the book provides invaluable information for both graduate researchers and R&D engineers in industry and consultancy. (Less)

FINITE-DIFFERENCE SOLUTIONS OF CONVECTION-DIFFUSION PROBLEMS IN IRREGULAR DOMAINS, USING A NONORTHOGONAL COORDINATE TRANSFORMATION

A solution methodology has been developed for convection-diffusion problems in which one boundary of the solution domain does not lie along a coordinate line. A nonorthogonal, algebraic coordinate transformation is used which yields a rectangular solution domain. This transformation avoids the task of numerically generating boundary-fitted coordinates. The discretized conservation equations are derived on a control-volume basis. These equations contain pseudodiffusion terms that result from the nonorthogonal nature of the transformation. The entire discretization procedure is documented in detail. Although it is not an essential feature of the method, the discretized equations and their solutions are tied in with the well-documented practices of the Patankar solution scheme for orthogonal systems. Application of the methodology is illustrated by two numerical examples.

Computer modeling of aerosol filtration by fibrous filters

An inline array of parallel circular cylinders, placed transverse to the flow, is used as a model for fibrous filters. The flow field within the array is obtained by solving the full Navier—Stokes equations with the assumption of periodic, fully developed flow. A control volume differencing scheme is used for this purpose. The flow field can be computed for both the viscous and laminar flow regimes. Predictions of pressure drop and particle collection due to interception and diffusion have been obtained using the inline array model, for packing densities varying from 0.029 to 0.136. The collection efficiency for deposition due to interception is directly calculated from the computed flow field. The deposition or particles due to diffusion is studied by numerically solving a separate transport equation for particle concentration, without making any boundary layer approximations. The results have been compared with the data from previous studies, both theoretical and experimental. It is shown that the resul...