Combustion of liquid fuel inside inert porous media: an analytical approach

This paper presents a numerical analysis of combustion of liquid fuel droplets suspended in air inside an inert porous media. A one-dimensional heat transfer model has been developed assuming complete vaporization of oil droplets prior to their entry into the flame. The effects of absorption coefficient, emissivity of medium, flame position on radiative energy output efficiency and optimum oil droplet size at the entry, defined as the maximum size for complete vaporization before entering the combustion zone, have been presented. The inert porous medium with low absorption coefficient will produce high downstream radiative output with large oil droplet sizes.     

A universal model of opposed flow combustion of solid fuel over an inert porous medium

In this study, a universal model is developed to examine the behavior of combustion wave observed in porous solid matters (e.g., smoldering, self-propagating high-temperature synthesis (SHS), diesel particulate filter (DPF) regeneration process). Analytical expressions of the combustion characters of solid combustible (e.g., diesel particulate matters trapped in a DPF) deposited over an inert porous medium are obtained employing large activation energy asymptotic taking into account the sensible transport processes; namely, heat transfer between the porous medium and gas phases, radiation heat transfer from the porous medium, heat loss from the porous medium to the environment, mass transfer of oxygen from the gas stream to the surface of solid fuel and the effective diffusion in modeling the species diffusion. Then it has been validated that the present model is applicable and adaptable for predicting the characteristics of smoldering combustion and thus SHS process. As a result, the features of combustion wave of the present phenomena would be useful to other processes. From practical point of view and for deep understanding of the behavior of combustion wave of these processes, we investigate the effects of various physical parameters over a wide range of conditions. We observe that the moving speed of the reaction front increases with the increase of porosity of the porous medium, mass transfer coefficient and initial fuel mass fraction; while it decreases owing to the increase of heat transfer rate from the porous medium to the gas, heat loss to the environment and radiative heat transfer. Furthermore, the results reveal that extinction tends to occur due to lower porosity of the porous medium, higher radiative heat transfer from the porous medium, higher heat transfer rate from the porous medium to the gas and higher heat losses from the porous medium to the environment. Even the observed near-extinction behavior in reaction front speed versus heat loss diagram is found to be similar what we got in gaseous premixed flame propagating through the porous medium. An extinction limit diagram has been presented as a function of radiation-conduction parameter and the gas flow velocity. In addition to, the impact of radiation and the combined effect of the inclusion of Knudsen diffusion and tortuosity are demonstrated in terms of the spatial temperature and species profiles to examine how these two parameters modify the reaction front structure. Furthermore, the governing equations have been solved numerically and it is observed that asymptotic analysis gives a good agreement with the numerical solution.     

Modeling of trickle flow liquid fuel combustion in inert porous medium

Present work is a numerical analysis of fuel oil combustion inside an inert porous medium where fuel oil flows through the porous medium under gravity wetting its solid wall with concurrent movement of liquid fuel and air under steady state conditions. A one-dimensional heat transfer model has been developed under steady state conditions using a single step global reaction mechanism. The effects of optical thickness, emissivity of medium, flame position and reaction enthalpy flux on radiation energy output efficiency as well as the temperature, position and thickness of vaporization zone have been presented using kerosene as fuel. Low values of optical thickness and emissivity of porous medium will ensure efficient combustion, maximize downstream radiative output with minimum upstream radiative loss.     

Combustion and heat transfer in model two-dimensional porous burners

A two-dimensional model of two simple porous burner geometries is developed to analyze the influence of multidimensionality on flames within pore scale structures. The first geometry simulates a honeycomb burner, in which a ceramic is penetrated by many small, straight, nonconnecting passages. The second geometry consists of many small parallel plates aligned with the flow direction. The Monte Carlo method is employed to calculate the viewfactors for radiation heat exchange in the second geometry. This model compares well with experiments on burning rates, operating ranges, and radiation output. Heat losses from the burner are found to reduce the burning rate. The flame is shown to be highly two-dimensional, and limitations of one-dimensional models are discussed. The effects of the material properties on the peak burning rate in these model porous media are examined. Variations in the flame on length scales smaller than the pore size are also present and are discussed and quantified.     

Characterization of heat transfer between phases inside a porous medium as applied to vegetal set representations

Convective heat transfer between vegetal sets and the surrounding air in the context of forest fires has not yet been fully investigated and understood in existing studies. This process may have a great influence on many environmental problems such as forest fires. This study is devoted to the computational heat transfer characterization of tree structures. These structures were generated by Iterated Function Systems (IFS) and the fluid flow was computed using balance equations (mass, momentum, heat, etc.). The heat transfer was then characterized using the macroscopic Stanton number on several tree structures. The main objective of this study was to demonstrate that the macroscopic Stanton number only depends on the macroscopic Reynolds number using a power law. In addition, the power law exponent was found to be quasi-constant for all the configurations tested in this work and it tends to be universal.     

Radiation heat transfer analysis of the monolith type solid oxide fuel cell

In this study, a modeling framework for heat and mass transport is established for a unit monolith type SOFC, with emphasis on quantifying the radiation heat transfer effects. The Schuster–Schwartzchild two-flux approximation is used for treating thermal radiation transport in the optically thin yttria-stabilized-zirconia (YSZ) electrolyte, and the Rosseland radiative thermal conductivity is used to account for radiation effects in the optically thick Ni–YSZ and LSM electrodes. The thermal radiation heat transfer is coupled to the overall energy conservation equations through the divergence of the local radiative flux. Commercially available FLUENT™ CFD software was used as a platform for the global thermal-fluid modeling of the SOFC and the radiation models were implemented through the user-defined functions. Results from sample calculations show significant changes in the operating temperatures and parameters of the SOFC with the inclusion of radiation effects.     

Numerical study of transient heat transfer in semitransparent porous medium

Transient solutions were obtained for heat transfer through a semitransparent porous medium placed in a flow passage and submitted to incident radiation. The one-dimensional physical model takes into account, conduction, convection and radiation simultaneously. The porous medium is assumed to be homogenous continuum, which absorbs, emits and scatters thermal radiation. A fully implicit time-marching algorithm was used to solve the nonlinear coupled energy equations for gas and porous medium numerically. The present study utilizes the differential–discrete–ordinate (DDO) method to account for the radiation contribution. The effects of the Reynolds number, optical depth, anisotropic scattering, conduction–radiation parameter and scattering albedo on temperatures and fluxes profiles are investigated.     

Local analysis of heat transfer inside corrugated channel

Experiments are performed to study effects of hydrodynamic conditions on the enhancement of heat transfer for single phase flow. These experiments have been conducted for a wide range of Reynolds numbers, (0 < Re < 7500) in order to obtain the different regimes from steady laminar to turbulent. A two-dimensional corrugated test section which has been instrumented with thermocouples can be heated by electrical cartridges. The local temperature measurements are used to evaluate the local and global heat transfer coefficient of the wavy heat exchanger. As expected, the heat transfer is always higher than those in rectangular channel; it is essentially due to the mixing induced by the recirculation in the wake of the corrugations.     

Charge,mass and heat transfer interactions in solid oxide fuel cells operated with different fuel gases—A sensitivity analysis

The interaction between charge, heat and mass transfer occurring in SOFCs is investigated applying a finite-volume-based SOFC model. The strong interactions are the consequence of the high degree of integration of different processes (chemical/electrochemical reactions, diffusion, heat and mass transfer) within SOFCs. The understanding of these interactions is a key for the future development and application of SOFCs. The investigation was conducted by means of a sensitivity analysis for two different fuel gases, where one gas features a considerable amount of methane inducing steam reforming reactions as additional disturbance factor in the energy and mass balance system of SOFCs. In order to isolate the impact of the varied model parameters and the according changes in the interactions of charge, mass and heat transfer from side effects, the sensitivity analysis was conducted at constant fuel utilization. It was found that the impact of different fuel gases on the operational conditions of SOFCs dominates geometrical and material-induced phenomena. The power output was most affected by the fuel, followed by the values for the activation polarization activation energy that reflects the employed electrode catalysts activity.     

A review of investigations on liquid fuel combustion in porous inert media

Utilization of a porous medium for combustion of liquid fuels is proved to be a promising approach for future applications. The porous medium burner for liquid fuels is more advantageous than the conventional open spray flame burner for several reasons; these include enhanced evaporation of droplet spray owing to regenerative combustion characteristics, low emission of pollutants, high combustion intensity with moderate turn-down ratio and compactness. This article provides a comprehensive picture of the global scenario of research and developments in combustion of liquid fuels within a porous medium that enable a researcher to determine the direction of further investigation. Accordingly, a glossary of the important terminology, the modeling approach, advances in numerical and experimental works and applications are included. The papers published in standard journals are reviewed and summarized with relevant comments and suggestions for future work.     

Moving boundary flow and heat transfer in an anisotropic porous medium

The axisymmetrical flow of a fluid injected through a circular opening into an anisotropic porous medium confined between two isothermal surfaces is studied. The governing equations are solved numerically according to the quasi-static approach, using a Landau type transformation to immobilize the interface in the new coordinate system. It is found that the anisotropy of the medium, as well as the inlet pressure, may significantly influence the shape and propagation speed of the moving front.     

管内填充多孔介质强化换热的数值研究

管内填充多孔介质强化换热的基本原理是构造热边界层,增大壁面附近流体的温度梯度,并且流动阻力增幅不大。本文运用数值模拟的方法,模拟填充多孔介质管内的流场和温度场,探讨填充比例φ、渗透率Da以及空隙率ε对管内对流换热的影响规律。研究表明,提高填充比例φ和减小渗透率Da都能明显提高换热效果,但也增加了管内流动阻力。空隙率ε对强化换热作用不大,但高空隙率可以明显降低管内流动阻力,在实际中应选用空隙率较大的多孔介质。     

Multimode heat transfer in cyclic flow reversal combustion in a porous medium

Experimental and numerical studies of combustion and multimode heat transfer in a porous medium, with and without a cyclic flow reversal of a mixture through a porous medium, were performed. Parametric studies were done in order to understand combustion characteristics such as maximum flame temperature and radiative heat flux using a one‐ dimensional conduction, convection, radiation and premixed flame model. The porous medium was assumed to emit and absorb radiant energy, while scattering is ignored. Non‐local thermodynamic equilibrium between the solid an d gas is taken into account by introducing separate energy equations for the gas and the solid phase. As a prelimina ry study, the combustion regime was described by a one‐step global mechanism with an internal heat source uniformly dist ributed along the reaction zone. The effects of the flame position, cyclic flow reversal, period of the cyclic flow rever sal, the optical thickness and the flow velocity on the burner performance were clarified by a rigorous radiation analysis. Th e model was validated by comparing the theoretical results with the experiments. It was shown that, for maximizing the fl ame temperature and the net radiative heat flux feedback, the flame should be stabilized near the centre of the po rous medium with a cyclic flow reversal, the period of which should be as small as possible. A high optical thickness prod uced a high flame temperature and a high net radiative feedback. Also, a high flow velocity at low period of the cyclic f low reversal of mixture yielded a high value of both the flame temperature and the net radiative feedback. Thermal structure predictions in terms of the gas‐phase and the solid‐phase temperature distributions along the axis of the combustor show good agreement with the experimental ones. Copyright © 1999 John Wiley & Sons, Ltd.     

Field synergy principle analysis on convective heat transfer in porous medium with uniform heat generation for thermally developing flow

The field synergy analysis for forced convective heat transfer in porous medium between two parallel plates is augmented from the earlier study on developed flow to the present thermally developing flow. The temperature variations along the axial and transverse directions for both adiabatic and isoflux boundary conditions are studied. Green’s function solution integrated with the extended weighted residuals method is adopted in this study to first obtain the temperature profile. The variation of intersection angle in the context of field synergy analysis is discussed in relations to the Nusselt number, boundary conditions and the shape factor.     

Transient heat transfer in air saturated porous media in a short cylinder

The rate of heat transfer occurring in a fluid saturated porous media contained in a short cylinder has been experimentally studied. The experiments considered the situations of heating and cooling the porous bed from an initially uniform temperature to the imposed boundary temperature to steady-state. A modified Rayleigh and a modified absolute Nusselt number were used to correlate the experimental results. Comparison was made to a finite difference simulation.     

Transient natural convective heat transfer in porous medium with solidification of binary mixture

In this paper an enthalpy porosity method associated with finite control volume scheme and SIMPLE iteration was employed to solve Navier–Stokes equation coupled with energy equation through Ergun equation and Boussinesq approximation for studying the effect of two-dimensional transient natural convective heat transfer from a closed region of porous medium with the different porosity on solidification in carbon–iron system. As shown in the results, it is fund that the thickness of solidification layer is increased with time due to thermal coupled flow induced by natural convection; and the wall temperature is faster changed in porous medium with larger porosity, which corresponds to slow the growth of the solidification layer in binary system.     

Second-law analysis of flow and heat transfer inside a microannulus

The purpose of this work is to investigate the entropy generation in a microannulus flow. Fully developed laminar flow is considered with uniform heat flux at the walls. The viscous dissipation effect, the velocity slip and the temperature jump at the wall are taken into consideration. The velocity and temperature profiles are obtained analytically and used to compute the entropy generation rate. The effects of Kn, Br, Br/Ω and r? on velocity, temperature profiles, entropy generation rate and average entropy generation are discussed. The present analytical results for the case with and without the viscous dissipation effect are compared with those available in the literature and an excellent agreement is observed. Entropy generation is shown to decrease with an increase in Kn while increasing Br, Br/Ω and r? results in increasing entropy generation.     

Turbulent heat and mass transfer through air curtains devices for the confinement of heat inside tunnels

The influence of flow instabilities on the efficiency of a twin-jet air curtain formed by a cold stream parallel to a hot stream was studied. Two cases were considered: the Reynolds number and mean flow velocity of both streams were set to 1000 and 3 m/s, respectively. The results obtained from 3D LES simulations were compared with experimental data and earlier LES numerical results using the FDS code. Energy spectra exhibit decay laws with − 5/3 and − 3 slopes. Instabilities with a characteristic Strouhal number of about 0.4 were detected. Kelvin-Helmoltz type instabilities play a more significant role than the Görtler type instabilities present at the wall for applications of plane impinging jets to ambience separation.     

Natural convection heat transfer from a horizontal cylinder embedded in a porous medium

This paper presents the results of an experimental investigation of heat transfer by natural convection from a horizontal cylinder embedded in porous media consisting of randomly packed glass spheres saturated by either water or silicone oil. It is shown that the overall range of the Rayleigh number, Ra, can be divided into two subregions, called ‘low’ and ‘high’, in each of which the Nusselt number, Nu, behaves differently. It is demonstrated that the low Ra region corresponds to Darey flow and the high to Forchheimer flow. Correlation equations for Nu for the Darcy regime are presented that account for viscous dissipation, and others for the Forchheimer regime that involve the first and second Forchheimer coefficients. The variation of properties with temperature and the wall effect on porosity (and consequently on heat transfer) are considered. The paper includes information concerning the resistance to flow in porous media that was obtained in conjunction with the heat transfer study.