Parametric investigations of premixed methane-air combustion in two-section porous media by numerical simulation

Motivated by detailed designs of industrial porous burners published in patents, the combustion of methane-air mixtures in a two-section porous burner has been studied numerically. The software FLUENT is used to solve a two-dimensional transient mathematical model of the combustion. In order to reveal the reality of the combustion in porous media, the user defined function (UDF) is used to extend the ability of FLUENT and enable two-dimensional distributions of temperature and velocity to be obtained. Some operating or property parameters, which mainly affect the functions and quality of the industrial burner design, such as the inlet velocity of the reactants, the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, have been investigated. The results show that the contours of temperature and velocity change considerably at the interface of the porous media and near the wall, the gas temperature at the low inlet velocity limit is higher than that for the high velocity limit, the thermal conductivity in the upstream section has more influence on the temperature than that in the downstream section and finally, the temperature profiles of both the gas and the porous skeleton vary considerably with changes of the radiative extinction coefficient of the large-pore porous media.     

A numerical study on heat transfer enhancement and design of a heat exchanger with porous media in continuous hydrothermal flow synthesis system

The aim of this study is to use a new configuration of porous media in a heat exchanger in continuous hydrothermal flow synthesis (CHFS) system to enhance the heat transfer and minimize the required length of the heat exchanger.For this purpose,numerous numerical simulations are performed to investigate performance of the system with porons media.First,the numerical simulation for the heat exchanger in CHFS system is validated by experimental data.Then,porous media is added to the system and six different thicknesses for the porous media are examined to obtain the optimum thickness,based on the minimum required length of the heat exchanger.Finally,by changing the flow rate and inlet temperature of the product as well as the cooling water flow rate,the minimum required length of the heat exchanger with porous media for various inlet conditions is assessed.The investigations indicate that using porous media with the proper thickness in the heat exchanger increases the cooling rate of the product by almost 40％and reduces the required length of the heat exchanger by approximately 35％.The results also illustrate that the most proper thickness of the porous media is approximately equal to 90％ of the product tube's thickness.Results of this study lead to design a porous heat exchanger in CHFS system for various inlet conditions.     

Kinetic simulation of methane hydrate formation and dissociation in porous media

The vast amount of hydrocarbon gas deposited in the earth's crust as gas hydrates has significant implications for future energy supply and global climate. A 3-D simulator for methane hydrate formation and dissociation in porous media is developed for designing and interpreting laboratory and field hydrate experiments. Four components (hydrate, methane, water and salt) and five phases (hydrate, gas, aqueous-phase, ice and salt precipitate) are considered in the simulator. The intrinsic kinetics of hydrate formation or dissociation is considered using the Kim-Bishnoi model. Water freezing and ice melting are tracked with primary variable switch method (PVSM) by assuming equilibrium phase transition. Mass transport, including two-phase flow and molecular diffusions, and heat transfer involved in formation or dissociation of hydrates are included in the governing equations, which are discretized with finite volume difference method and are solved in a fully implicit manner. The developed simulator is used here to study the formation and the dissociation of hydrates in laboratory-scale core samples. In hydrate formation from the system of gas and ice (G+I) and in hydrate dissociation systems where ice appears, the equilibrium between aqueous-phase and ice (A-I) is found to have a “blocking” effect on heat transfer when salt is absent from the system. Increase of initial temperature (at constant outlet pressure), introduction of salt component into the system, decrease of outlet pressure, and increase of boundary heat transfer coefficient can lead to faster hydrate dissociation.     

Optimization of thermal insulation performance of porous geopolymers under the guidance of thermal conductivity calculation

Porous geopolymers are energy saving, environment-friendly and simple in preparation. However, the thermal conductivity (TC) of present porous geopolymers as well as other inorganic thermal insulation materials can be hardly below 0.050 W m^-1 K^-1 in the markets. In order to further decrease the TC of porous geopolymers, it is vital to understand the heat transfer mechanism of this type of materials. In this work, we made a comparison among the main heat transfer models for porous two-component system reported so far including series model, parallel model, geometric mean model, Maxwell-Eucken equation, Hashin spherical structure model and novel effective medium theory (NEMT) based on the data reported in references and obtained by our group for porous geopolymers, and found that NEMT could describe the heat transfer mechanism of porous geopolymers better. Then we systematically studied the relationship among the effective TC (k_e), porosity (ε), TC of solid skeleton (k_s) and TC of the uniform medium reflecting the heat conduction of the solid skeleton to air (k_m) based on the calculations with NEMT. It was found that it is essential to increase ε and decrease k_s and k_m for reducing the TC of porous geopolymers. Under the guidance of above calculations, we successfully designed and obtained some porous geopolymers with TC as low as 0.040 W m^-1 K^-1. This paper offers not only several porous geopolymers with low TC, but also an idea to design novel thermal insulation materials.     

多孔介质方腔内置芯片热流耦合的LBM数值模拟

基于格子Boltzmann方法的渗流多孔介质耦合双分布模型,对表征体元（REV）尺度下含电子芯片的多孔介质自然对流进行数值模拟研究,主要研究不同物性参数对多孔介质自然对流的影响以及单电子芯片尺寸、多芯片布局等因素对电子芯片表面散热性能的影响。得出了如下研究结果：对于恒温单芯片的多孔介质自然对流,在达西数Da=10-2时存在临界芯片尺寸。在临界芯片尺寸条件下,流场扰动较更小的芯片尺寸更强,传热性能却几乎不变。不同瑞利数Ra条件下临界芯片尺寸不同,Ra越大,临界芯片尺寸越大,在Ra=103时临界芯片尺寸为0.203125倍方腔边长,Ra=104时临界芯片尺寸为0.25倍方腔边长,Ra=105时临界芯片尺寸为0.390625倍方腔边长。当多孔介质渗透率降低时,即Da=10-4时,不存在临界芯片尺寸,且芯片表面和冷壁处的平均Nusselt数均随Ra的增大而增大。对于恒温多芯片的多孔介质自然对流,在多孔介质渗透率较大(Da=10-2)的情况下芯片横排布置可取得最佳换热效果,在渗透率较小(Da=10-4)时芯片布局宜采用对角分布。     

A numerical study on structure of premixed methane-air microflames for micropower generation

Structure of premixed methane-air microflames at normal and elevated temperatures and atmospheric pressure generated on a microtube was computationally studied, in order to understand the fundamental behavior of the microflames for micropower generation. Based on an earlier experimental investigation of the stability limits of the premixed microflames, the distributions of temperature, fuel and radicals for single microflames near the stability limits and in the stable region were predicted using a two-dimensional CFD simulation with a reduced kinetic mechanism and a detailed transport modeling. The predicted structure of microflames along the stability limits due to heat losses showed substantial fuel leaks between the microflame base and the microtube rim. This observation provided the burning mechanism that a microflame can be generated and sustained only if the injected mixture contains a certain concentration of fuel beyond a certain distance from the tube exit that can avoid quenching. The experimentally observed extended stability limits due to elevated temperature were readily explained by the predicted structure showing intensified burning and reduced heat losses. Finally, the predicted microflame structure showed that the predicted microflame length based on the maximum mass fraction of OH radical well represents the observed visible microflame length.     

Stability of lean combustion in mini-scale porous media combustor with heat recuperation

In miniaturization of burners, it is very difficult to organize stable self-sustained combustion. A mini-scale porous media combustor with heat recuperation was set up to study the stability of lean combustion and its emission. The diameter of the porous media was only 20 mm and the burner was about 140 mm in length. Experimental results showed that when the mass flow rate of the premixed gas was 0.163 g/s, the extinction limit was extended to Φ = 0.40 in the methane combustion and Φ = 0.39 in the propane combustion. For most cases, the emission of CO was lower than 100 ppm in both methane and propane combustion. The maximal concentration of NO_x was 63 ppm in the methane combustion. The ultra-lean combustion was also predicted by a numerical simulation with a 2D two-temperature model. The heat recuperation efficiency η, as high as 40%, made the ultra-lean combustion extremely stable. Although the maximal flame temperature in the porous media reached above 2000 K, the exhausts temperature was lower than 900 K.     

Experimental and numerical investigation on performance of a porous medium burner with reciprocating flow

Experimental and two-dimensional numerical investigations on the performance of an inert porous media burner with reciprocating flow are presented. Attention was focused on the combustion temperature and pressure loss in the burner, which was, respectively, packed with 4PPC (Pores Per Centimeter) ceramic foams or alumina pellets with various sizes. Results show that material and structures of porous media have significant influence on the burner performance, and that ceramic foam with high porosity is suitable for using in the combustion region whereas alumina pellets should be placed in the heat exchange zone. In addition, the highly two-dimensional characteristics of the porous media burner are validated by the numerical model, which include temperature distributions, species profile and flame structure. Numerical results were validated against experiment data.     

A research into thermal field in fluid-saturated porous media

Until now, the theory, methodology of investigations, and interpretation of thermometry data have been most completely developed for single-phase (oil, water, or gas) flows in formations. However, multiphase (oil+gas, oil+water, and oil+water+gas) flows in formations are more common in practice. This is primarily typical for fields featuring a high value of gas factor and saturation pressure, as well as for cases of formation tests at low values of bottom-hole pressure. Analysis of actual thermograms under these conditions has shown that the earlier-developed techniques for the cases of single-phase flows in the formation and the well cannot be applied here.This paper presents research data on the influence of the adiabatic and Joule-Thomson effects and the heat of fluid degassing on temperature field in porous medium.     

Experimental investigation of porous media combustion in a planar micro-combustor

The micro-combustor (emitter) is a key component of the micro-thermophotovoltaic (TPV) system. In order to improve the system efficiency, higher wall temperature and uniform distribution along the combustor wall is desirable. Porous media combustion of premixed H₂-air in a planar micro-combustor with the channel width of 1 mm is experimentally studied. The wall temperature is measured by using an infrared thermometer under the flow conditions of Ф = 0.6-1.0 and U₀ = 2-3 m/s. The effects of flow conditions and position of the porous media on both the wall temperature distribution and the emitter efficiency are investigated. The experimental results indicate that the wall temperature increases with increasing mixture velocity, and higher emitter efficiency is achieved for mixtures with Ф ≈ 0.8. In addition, the flames are found to be effectively anchored by the inserted porous media, despite the change of flow conditions. The emitter efficiency was noted to be significantly influenced by the position of the porous media, for which the mixture preheating (by the combustor wall) is believed to be a main reason.     

Inverse solution for a biochemical heat source in a porous medium in the presence of natural convection

The inverse problem approach by conjugate gradient with adjoint equations is adapted to the context of a non-isothermal bioprocess, controlled by internal heat generation from microbial oxidation, as could be found in a composting reactor for instance, to determine the heat source from internal temperature measurements. The volumetric heat source is assumed proportional to the rate of consumption of a substrate by a biomass, as described by a Monod model. Computations are performed for Rayleigh numbers equal to 0.25 and 25, for a representative biochemical reaction under typical boundary conditions, for constant and temperature-dependent model parameters. The influence of noisy input data is also considered. It is found that good solutions can be obtained when heat release and diffusion occur over very different time scales. The variation of the model parameters with temperature must be taken into account, but single sensor solutions are possible at relatively small Rayleigh numbers when convection is present.     

Re-evaluation of the Nusselt number for determining the interfacial heat and mass transfer coefficients in a flow-through monolithic catalytic converter

The two-equation porous medium model has been widely employed for modeling the flow-through monolithic catalytic converter. In this model, the interfacial heat and mass transfer coefficients have been usually obtained using the asymptotic Nusselt and Sherwood numbers with some suitable assumptions. However, previously it seemed that there existed some misunderstanding in adopting these Nusselt and Sherwood numbers. Up to now, the Nusselt number based on the fluid bulk mean temperature has been used for determining the interfacial heat and mass transfer coefficients. However, the mass and energy balance formulations in the two-equation model indicate that the Nusselt number should be evaluated based on the fluid mean temperature instead of the fluid bulk mean temperature. Therefore, in this study, to correctly model the heat and mass transfer coefficients, the Nusselt number based on the fluid mean temperature was newly obtained for the square and circular cross-sections under two different thermal boundary conditions (i.e., constant heat flux and constant temperature at the wall). In order to do that, the present study employed the numerical as well as analytical method.     

水泥回转窑中煤粉燃烧的热态数值模拟

NC型四风道煤粉燃烧器建立了燃烧器一回转窑物理模型,利用流体分析软件FLUENT对窑内燃烧情况进行了数值模拟,分析了窑内煤粉燃烧温度场及组分浓度场的分布规律,并与实际情况进行对比,计算结果符合实际燃烧规律,说明该数值分析结果可以为同类燃烧器的合理设计与优化运行提供参考。     

Heat transfer in metal foams and designed porous media

We present the characterization of heat transfer in commercial metal foam filled tubular reactors in comparison to a designed laser sintered device. The investigations are performed at empty tube Reynolds numbers ranging from 600 to 7600. Volumetric heat transfer performances up to 4.5 MW/(m³ K) were estimated by means of a simple calorimetric measurement setup, which is 3 orders of magnitude higher compared to conventional batch reactors. The heat transfer was found to increase with the ligament diameter ascribed to the enhanced turbulent kinetic energy induced. The fixed wall connection of the fully sintered device, realized by the applied manufacturing method, leads to 30% improvement of the heat transfer compared to no connection. Selective laser sintering was found to be an efficient tool for the design of continuous heat exchanger reactors covering a wide range of applications by simply adapting the geometry.     

Filtration combustion of hydrogen-air, propane-air, and methane-air mixtures in inert porous media

The filtration combustion characteristics of hydrogen-air, propane-air, and methane-air mixtures in inert porous media have been studied experimentally. It is shown that the dependences of the combustion wave velocity on the fuel-air equivalence ratio are V-shaped. For hydrogen-air mixtures, the velocity minimum is shifted to the rich region, and for propane-air and methane-air mixtures, it is shifted to the lean region. For lean hydrogen-air and rich propane-air mixtures, the measured maximum temperatures in the combustion wave are found to be reduced relative to those calculated theoretically. For methane-air mixtures, a reduction in the measured temperatures is observed over the entire range of the mixture composition. The results are interpreted within the framework of the hypothesis of selective diffusion of gas mixture components. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 8–20, July–August, 2006.     

Modeling of mass transfers in a porous green compact with two-component binder during thermal debinding

Mass transfers and phase changes of two-component binder in a porous green compact during thermal debinding process are modeled. The evaporation of low molecular weight (LMW) component and volatile fragments, the thermal degradation of high molecular weight (HMW) component, the capillary driven and pressure driven liquid phase transports, the binary diffusion in solutions, the convection and diffusion of gas phases, and the heat transfer in a porous medium are captured in the model. The model is validated with experimental data. The simulated results show that mass transfers during the early stage of thermal debinding are mainly due to capillary driven and pressure driven liquid transports. During the final stage of thermal debinding, both convective liquid and gas transports are important in binder removal. The developed model provides physical understanding of binder removal mechanisms that are essential for process optimization.     

Thermal conductivity in porous silicon nanowire arrays

The nanoscale features in silicon nanowires (SiNWs) can suppress phonon propagation and strongly reduce their thermal conductivities compared to the bulk value. This work measures the thermal conductivity along the axial direction of SiNW arrays with varying nanowire diameters, doping concentrations, surface roughness, and internal porosities using nanosecond transient thermoreflectance. For SiNWs with diameters larger than the phonon mean free path, porosity substantially reduces the thermal conductivity, yielding thermal conductivities as low as 1 W/m/K in highly porous SiNWs. However, when the SiNW diameter is below the phonon mean free path, both the internal porosity and the diameter significantly contribute to phonon scattering and lead to reduced thermal conductivity of the SiNWs.