往复式多孔介质燃烧器流动特性的试验研究

研究了往复式多孔介质燃烧器在冷态条件下气流在其中的流动特性规律。在多孔介质的类型和运行温度不变的情况下，往复式多孔介质燃烧器的压降与空截面流速的平方成正比，与多孔介质的厚度成正比；燃烧器的稳定时间基本不受多孔介质厚度和空截面流速的影响。建立了计算压降的简单数学模型，理论模型的计算结果与实验数据符合较好。     

泡沫陶瓷多孔介质内流体流动及表面传热模拟

气隙扩散蒸馏脱盐技术利用具有大比表面积的多孔介质作为蒸发器，海水在多孔介质内部流动并在表面蒸发，多孔介质起到了强化液体蒸发的作用;但由于多孔介质结构极其复杂，很难使用传统的实验技术从微观水平观测到多孔介质孔隙通道内流体的流动状态以及传热现象。针对此问题采用计算机数值模拟方法，拍摄实际碳化硅泡沫陶瓷CT图片，构建三维模型进行有限元模拟分析。结果表明，多孔介质内流体会优先通过较大孔隙通道。流体在多孔介质表面向环境空气的散热量随孔隙密度增大而增大，孔隙密度从10提高至30 PPI，散热量提高约1.43倍。进口热流体与环境空气温差越大，向环境的散热量越大，孔隙密度在30 PPI条件下，进口热流体温度从49.38增加至68.67 ℃，散热量提高近2.07倍。     

多孔介质太阳能集热组合墙的耦合传热与流动分析

针对接触型和分隔型多孔介质太阳能集热组合墙系统,分析了太阳辐射及环境温度变化时,组合墙内传热与流动变化.多孔介质太阳能集热组合墙中,多孔介质起半透明隔热体和蓄热体的作用.多孔介质集热层的孔隙率、粒径、材料热导率和多孔介质集热层在组合墙中的位置对系统的采暖效果影响较大.     

多孔介质微燃烧器的试验研究

对微尺度下的氢气/空气预混气在多孔介质中进行预热燃烧时的燃烧特性进行了试验研究,在回热燃烧器中对不同ppi(每英寸长度上的孔洞数)的多孔介质进行对比试验,分别测试了氢气/空气预混气在预热下的燃烧效率与氢气流量、过量空气系数α以及多孔介质ppi之间的关系.结果表明,在多孔陶瓷的蓄热和混流作用下,燃烧速度和燃烧效率均得到了显著的提高,稳定燃烧界限也有一定的扩大.为进一步减小微尺度条件下的燃烧热量损失,提高燃烧效率,提供了试验依据.     

氢气对超低体积分数甲烷催化燃烧的影响

搭建了催化燃烧实验台,在保证催化燃烧室入口气体温度、流速相同的情况下,通过改变气体中甲烷和氢气的体积分数,得到不同体积分数甲烷气体在加入不同体积分数氢气情况下的催化燃烧特性.结果表明:在保证催化燃烧时入口气体温度为520℃条件下,加入低体积分数的氢气可有效加快甲烷催化燃烧的反应速度,降低甲烷的起燃温度,提高甲烷转化率;加入的氢气体积分数越高,对甲烷的催化燃烧助燃效果越好;而甲烷体积分数越高,氢气对甲烷的催化燃烧效果也越显著.     

太阳能热化学反应器中甲烷水蒸气重整的参数研究

文章基于小型太阳能反应器建立了甲烷水蒸气重整的热化学反应数学模型,采用该模型计算得到了甲烷水蒸气重整制氢过程中不同工况参数（孔隙率、气体入口速度、辐照度、反应器压力）对甲烷转化率、氢气产率以及温度分布的影响规律。研究结果表明：沿反应器中心线方向,甲烷的摩尔分数不断降低,氢气的摩尔分数逐渐升高;混合气体最终的摩尔分数及多孔区域最终的温度趋于稳定;甲烷转化率及氢气产率均随着辐照度与孔隙率的增加而增加,随着压力与气体入口速度的增加而降低。     

刷式密封泄漏流动特性影响因素的研究

采用基于改进Darcian多孔介质模型的Reynolds-Av-eraged Navier-Stokes方程求解技术,用数值模拟的方法分析了在一定径向间隙条件下压比和刷丝束厚度对刷式密封泄漏流动特性的影响规律。根据发表的刷式密封泄漏量试验数据,确定了刷丝束多孔介质的渗透率系数。利用所确定的刷丝束多孔介质渗透率系数,分别计算了在一定径向间隙条件下7种压比和5种刷丝束厚度时某轴端刷式密封的泄漏量和泄漏流动形态。计算结果表明,压比和刷丝束厚度均影响刷式密封的泄漏量,在一定压比条件下,泄漏量随着刷丝束厚度的增加而减小;在一定刷丝束厚度条件下,泄漏量随着压比的增加而增加。因为刷式密封泄漏量与压比和刷丝束厚度近似成线性变化,所以压比和刷丝束厚度对刷式密封内泄漏流动形态的影响可以忽略。     

太阳能热气流发电系统的传热与流动数值分析

建立了集热棚、烟囱以及多孔蓄热层的太阳能热气流发电系统传热与流动数学模型,分析了太阳辐射对蓄热介质的蓄热特性的影响.计算结果表明,在太阳辐射为200～800W/M2的范围内,随着太阳辐射的增强,蓄热介质的蓄热比例先减小后增大;烟囱底部的最小相对压力显著减小,流动速度增大;系统内空气的温升增大,蓄热介质表面的温度也显著升高.     

非能动安全壳竖直平板降膜流动特性数值模拟

核电安全日益受到关注,非能动系统作为第三代核电系统具有很高的安全性。采用FLUENT流体体积分数(volume of fraction,VOF)模型和k-ε湍流模型对非能动安全壳冷却系统(passive containment cooling system,PCCS)三维平板降膜流动进行数值模拟。结果表明:1)在降膜过程中有波动现象,最终波动趋于平缓;2)水与空气逆流流动过程中发生轻微的液滴夹带;3)降膜流动受重力、表面张力与壁面黏滞力共同作用,液膜厚度沿横向分布均匀,沿高度方向平均液膜厚度越来越小,并且受进口水流速度与入口宽度影响,水流量一定时增加进口水流速度与入口宽度,平均液膜厚度增大,空气入口流速对水膜厚度影响相对较小。     

多孔介质-双层玻璃幕墙传热与流动特性

基于多孔介质具有吸收和贮存太阳能的特点,在双层玻璃幕墙通道内设置了多孔介质层,利用多孔介质充分收集与贮存太阳能用于建筑供暖,并采用数值模拟法研究了幕墙的传热与流动特性.结果表明,玻璃幕墙能充分利用太阳能加热新风供暖,集热效率高又节能,有推广价值.     

Simulation of methane steam reforming in a catalytic micro-reactor using a combined analytical approach and response surface methodology

In this study, a steady-state analytical model for heat and mass transfer in a 2D micro-reactor coated with a Nickel-based catalyst is developed to investigate microscale hydrogen production. Appropriate correlations for each species’ net rate of production or consumption, mass diffusivity, and the heat of reactions are developed using a detailed reaction mechanism of methane steam reforming. The energy and species conservation equations are then solved for the reactive mixture coupled with the wall energy equation. Finally, the response surface methodology (RSM) is employed to study the effects of channel height, inlet velocity and temperature, wall thickness and conductivity, and external heat flux on CH4 conversion. It is found that the inlet gas temperature, among different parameters, has the most influence on the overall performance of the microchannel hydrogen production. Also, the maximum necessary heat of reforming reaction increases by 84% and 26% if the CH₄ conversion changes from 50% to 60% and 60% to 70%, respectively. The developed analytical simulation can be a useful tool for designing experiments in micro-scale hydrogen production.     

A study on flow and heat transfer characteristics for saturated falling-film evaporation of liquid oxygen in a vertical channel

In this study, a mathematical model for the laminar falling film is presented in order to simulate the evaporation heat transfer characteristics in falling liquid oxygen films. The model takes into account the effect of the interfacial shear. The values of the film thickness, the heat transfer coefficient as well as the interfacial shear are obtained under given conditions by solving the model with an iteration method. The influences of the inlet Reynolds number, channel length and the interfacial shear on the flow and heat transfer characteristics of the falling film evaporation are analyzed in detail. Effects of key factors on the circulation ratio of the inlet fluid mass flow rate to the generated vapor mass flow rate, an important design parameter for reboilers/condensers, are particularly analyzed. In addition, the variations of the average vapor velocity and interfacial film velocity are also discussed. The analysis results could provide theoretical guidance for the simulation and design of downflow reboilers/condensers applied in air separation units.     

Numerical investigation of the impact of gas and cooling flow configurations on current and water distributions in a polymer membrane fuel cell through a pseudo-two-dimensional diphasic model

For optimal performances, proton exchange membrane fuel cells require fine water and thermal management. Accurate modelling of the physical phenomena occurring in the fuel cell is a key issue to improve fuel cell technology. Here, an analytic steady state diphasic 2D model of heat and mass transfer is presented. Through this model, the aim of this work is to study the influence of local events on the global performances of a fuel cell. A part of the complete model is a microscopic representation of the coupling between water transport and charge transfers in the electrodes. The thickness of the liquid layer around the reactive agglomerates is deduced from the saturation. The evolution of the quantity of water within the catalyst layer is monitored and its influence on the global performances of the cell is investigated. In gas diffusion layers (GDLs), liquid and vapour water transport through are computed regarding the temperature. The flow direction of cooling water modifies the current density distribution along the cell. The impact of the direction of air and hydrogen feeding channels are investigated. It can modify greatly the fuel cell mean current density and the net water transport coefficient. The counter-flow mode was preferable. Likewise, thanks to a better membrane hydration, it results in independent performances regarding the hydrogen inlet relative humidity or stoichiometry.     

Mathematical modelling and simulation of nitrite hydrogenation in a membrane microreactor

Nitrite hydrogenation using heterogeneous catalysis is an important process for purification of wastewater or potable water. The main aim of this study is to explore a new mechanistic model and simulation for a heterogeneously catalysed reaction in a microporous catalytic layer. The system studied involves a liquid solution containing certain amount of nitrite, and a membrane reactor in which the nitrite penetrates into the catalytic layer to react with hydrogen. The developed model considers coupling between equations of momentum transfer in free and porous media and convection-diffusion of nitrite. It was found that there is great agreement between measured data and modelling values. Increasing velocity was the main reason for reduction of nitrite conversion and also there was a slight increase in nitrite conversion with increasing the thickness and porosity of catalytic layer. Furthermore, it was found that the diffusion mass transfer mechanism is favourable for nitrite hydrogenation while convective mass transfer of fluid flow has negative impact on nitrite hydrogenation.     

Fixed bed membrane reactors for WGSR-based hydrogen production: Optimisation of modelling approaches and reactor performance

The production of high-purity hydrogen using the water–gas-shift reaction in both conventional fixed bed reactor and hydrogen perm-selective membrane reactor at low to medium scale is studied in this work by developing and comparing models with different complexity levels. A two-dimensional rigorous reactor model considering radial and axial variations of properties (including bed porosity), setting mass, energy and momentum differential balances, and nesting a rigorous model for mass transfer within the porous catalyst was considered as reference for comparison. Different simplifications of this model for taking into account mass-transfer effects within the catalyst pellet (Thiele modulus, evaluation of apparent kinetic constants, empirical correlation for effectiveness factors or just neglecting these effects) were tested, being observed that these effects are not negligible and that the first two approaches are accurate enough for taking into account mass transfer within catalyst pellets. Regarding to the reactor model, it was observed that one-dimensional models are not adequate, especially for the membrane reactor. Analogously, neglecting the momentum balances in the reactor (as made is most simulations reported in the literature) leads to important misspredictions in the behaviour of the membrane reactor performance. Finally, the influence of the main operation parameters (inlet temperature, pressure, space velocity, etc.) was studied using the detailed reactor model, concluding that space velocity and pressure are the most important parameters affecting reactor performance for membrane reactors.     

电站锅炉SCR系统流场的冷态试验与数值模拟的研究

采用冷态试验和数值模拟相结合对某电厂660 MW燃煤锅炉SCR脱硝系统中整流格栅、多孔板、导流板的设计方案进行了速度场和浓度场分布及压降的研究。结果表明数值模拟结果和冷态试验结果较为吻合。其中AIG上游速度分布CV值为15.4%,催化剂入口速度分布CV值为8.1%,浓度场CV值为5.7%,满足系统设计要求。催化剂入口处安装整流格栅可以优化进入催化剂层的气流方向,使烟气垂直进入催化剂层。     

Heat Transfer from Pulsating Laminar Impingement Slot Jet on a Flat Surface with Inlet Velocity: Sinusoidal and Square Wave

Heat transfer from a pulsating laminar impingement slot jet on a flat surface was investigated numerically and experimentally. Inlet velocity was considered sinusoidal velocity and square wave velocity. Experimental studies were done only for the sinusoidal velocity state. An inverse heat conduction method, conjugated gradient method with adjoint equation, was used for the experimental estimation of the local heat transfer coefficient along the target surface. Effect of the square wave velocity of the laminar impingement slot jet was studied numerically. The results show pulsations in flow change flow patterns and the thermal boundary layer thickness because of the newly forming thermal boundary layer is extremely small each time the flow is resumed. Heat transfer rate in this state enhances due to pulsating inlet velocity in comparison with steady state. Heat transfer increases with increasing pulsation amplitude. Enhancement in mean heat transfer on the target plate for sinusoidal velocity is rather than square wave velocity.     

Numerical investigation of a multichannel reactor for syngas production by methanol steam reforming at various operating conditions

A novel multichannel reactor with a bifurcation inlet manifold, a rectangular outlet manifold, and sixteen parallel minichannels with commercial CuO/ZnO/Al₂O₃ catalyst for methanol steam reforming was numerically investigated in this paper. A three-dimensional numerical model was established to study the heat and mass transfer characteristics as well as the chemical reaction rates. The numerical model adopted the triple rate kinetic model of methanol steam reforming which can accurately calculate the consumption and generation of each species in the reactor. The effects of steam to carbon molar ratio, weight hourly space velocity, operating temperature and catalyst layer thickness on the methanol steam reforming performance were evaluated and discussed. The distributions of temperature, velocity, species concentration, and reaction rates in the reactor were obtained and analyzed to explain the mechanisms of different effects. It is suggested that the operating temperature of 548 K, steam to carbon ratio of 1.3, and weight hourly space velocity of 0.67 h⁻¹ are recommended operating conditions for methanol steam reforming by the novel multichannel reactor with catalyst fully packed in the parallel minichannels.     

Modeling and simulation of methane steam reforming in a thermally coupled membrane reactor

A distributed mathematical model for thermally coupled membrane reactor that is composed of three channels is developed for methane steam reforming. Methane combustion takes place in the first channel on a Pt/δ–Al₂O₃$\text{Pt}/\delta \text{–}{\text{Al}}_2{\text{O}}_3$ catalyst layer that supplies the necessary heat for the endothermic steam reforming reaction. In the second channel, catalytic steam reforming reactions take place in the presence of Ni/MgO–Al₂O₃ catalyst. The combustion catalyst forms a thin layer next to the reactor wall to minimize the heat transfer resistance. Selective permeation of hydrogen through the palladium membrane is achieved either by co-current or counter-current flow of sweep gas through the third channel. The burner is modeled as a monolith reactor and the reformer is assumed to behave as a pseudo-homogenous reactor. The mass and energy balance equations for the thermally coupled membrane reactor form a set of 22 coupled ordinary differential equations. With the application of appropriate boundary conditions, the distributed reactor model for steady-state operation is solved as a boundary value problem. The model equations are discretized using spline collocation on finite elements. The discretized nonlinear modeling equations, along with the boundary conditions, form a system of algebraic equations that are solved using the trust region dogleg method. The performance of the reactor is numerically investigated for various key operating variables such as inlet fuel concentration, inlet steam/methane ratio, inlet reformer gas temperature and inlet reformer gas velocity. Simulations for both the co-current and the countercurrent flow modes are also performed using different sweep gas flow rates. For each case, the reactor performance is analyzed based on methane conversion and hydrogen recovery yield.     

Phase change in the cathode side of a proton exchange membrane fuel cell

A three-dimensional steady state two-phase non-isothermal model which highly couples the water and thermal management has been developed to numerically investigate the spatial distribution of the interfacial mass transfer phase-change rate in the cathode side of a proton exchange membrane fuel cell (PEMFC). A non-equilibrium evaporation-condensation phase change rate was incorporated in the model which allowed supersaturation and undersaturation take place. The most significant effects of phase-change rate on liquid saturation and temperature distributions are highlighted. A parametric study was also carried out to investigate the effects of operating conditions; namely as the channel inlet humidity, cell operating temperature, and inlet mass flow rate on the phase-change rate. It was also found that liquid phase assumption for produced water in the cathode catalyst layer (CL) changed the local distribution of phase-change rate. The maximum evaporation rate zone (above the channel near the CL) coincided with the maximum temperature zone and resulted in lowering the liquid saturation level. Furthermore, reduction of the channel inlet humidity and an increase of the operation temperature and inlet mass flow rate increased the evaporation rate and allowed for dehydration process of the gas diffusion layer (GDL) to take place faster.