多孔介质微小燃烧器的传热特性分析

采用计算流体力学软件Fluent,对H_2/空气预混气在全填充多孔介质平板微燃烧器内的燃烧过程进行数值模拟.研究了多孔介质导热系数、壁面导热系数、当量比、孔隙率对微燃烧器回热循环的影响规律.模拟结果表明:预热区对流回热效率、多孔介质导热效率与多孔介质导热系数呈正相关趋势;壁面导热系数增大会使预热区对流回热效率下降,壁面对流回热效率上升;预热区对流回热效率、壁面对流回热效率与当量比呈负相关趋势;多孔介质孔隙率是影响回热效率的重要因素,随着孔隙率的增大,预热区对流回热效率下降,壁面对流回热效率上升.     

壁面覆盖部分多孔介质方腔自然对流流动的数值模拟

多孔介质壁面封闭腔体的自然对流在生产实际中有重要的应用。针对左侧部分多孔介质壁面方形封闭腔体,基于有限元法对封闭腔体的自然对流换热进行了数值模拟,得到了在不同Ra、孔隙率条件下腔体内空气的温度分布、速度分布。结果表明:随着Ra的增大,腔体内的流场及温度场发生了明显的变化,多孔介质壁面孔隙率的变化对腔体的流动换热的影响很小。     

多孔介质微燃烧器的稳燃范围的数值研究

应用计算流体力学软件Fluent,对氢气/空气预混气在部分填充多孔介质的微平板燃烧器中的实验现象进行了模拟,研究了多孔介质热导率、壁面热导率、多孔介质孔隙率对稳燃范围的影响.模拟结果表明:稳燃范围的大小与多孔介质热导率呈正相关趋势,较高的多孔介质热导率将会拓宽稳燃范围;随着壁面热导率的增加,稳燃范围与壁面热导率呈V型比例;多孔介质孔隙率也是影响稳燃范围的一个重要因素,在0.5~0.9的区间内,随着孔隙率的增大,稳燃范围也随之增大.     

管中心填充两层多孔介质强化传热研究

采用圆管中心层流充分发展段中心流等截面分层填充金属多孔介质以实现强化传热,建立了流动与传热数学模型,并着重分析了三种新型强化传热管的速度、温度分布及传热综合性能.结果表明,与光管相比,填充金属多孔介质后,管中心流体温度更均匀,壁面附近流体的温度梯度更大,圆管中心流体速度分布趋于平坦,壁面附近流体速度梯度增大,壁面与流体间的换热显著增强;对三种强化传热管,在两区域内填充孔隙率相同的多孔介质,可望得到较高的综合性能指标值.     

天然气在渐变型多孔介质中的预混燃烧启动特性

针对天然气在渐变型多孔介质燃烧器中的点火启动过程进行了试验研究,通过监测燃烧器壁面或气体温度在点火后的变化,得到了影响启动时间的因素及特性,对特定的燃烧器而言,启动时间与预混气体当量比、流速以及点火位置有关,在冷态下点火,随着当量比接近理论当量比,启动时间减小;混合气体流速增大,启动时间增大;点火位置从燃烧器外移到燃烧器人口时,启动时间可大大缩小,采用小流速、近理论当量比条件下点火,对多孔介质层预热,有利于火焰迅速向上游移动,然后再调整到需求当量比或流速,可以大大减小燃烧器启动时间,采用孔径变化率高的渐变型多孔介质结构,也可以达到缩短启动时间的目的。     

燃烧器材料和结构尺寸对微型热光电系统的影响

微型燃烧器的燃烧性能是影响微型热光电(MTPV)系统的重要因素之一.本文通过对多孔介质燃烧器的结构、材料、尺寸的研究,分析了燃烧器的管径/壁厚、材料等因素对燃烧器燃烧、辐射性能的影响.为微型热光电动力系统的研究开发提供了有意义的参考依据.     

多孔介质回热微燃烧器的扩散燃烧

设计了多孔介质回热微燃烧器.进行了微燃烧器的扩散燃烧特性实验研究,得到了其燃烧效率、出口尾气温度、壁面温度和热损失率随燃烧热功率和过量空气系数的变化规律.实验发现,在较宽的操作范围内,微燃烧器具有较高的燃烧效率和出口尾气温度,而且随着燃烧功率和过量空气系数的增大,微燃烧器的壁面温度和热损失率反而减小.分析表明,采用回热夹层和多孔介质相向的进气方式,使得反应气体的流动方向与散热方向相反,有效回收了热量损失,提高了微燃烧器的热效率和出口尾气温度.所设计的多孔介质回热微燃烧器对开发微燃烧透平发电系统具有重要应用价值.     

渐变型多孔介质中预混燃烧温度分布试验

进行了预混天然气在等孔隙率渐近变孔径的多孔介质中的燃烧试验，用热电偶测量了燃烧室温度分布，并与单一孔径（d=1mm）的均匀多孔介质中燃烧结果进行了比较。结果表明，渐变型多孔介质燃烧器比均匀型多孔介质燃烧器具有更多的优点：燃烧室温度分布更加均匀，燃烧更加稳定，并能更好的适应当量比和流量／功率的变化，由于孔径的变化，多孔介质中气流扰动增加，有利于火焰的稳定，当量比和流速变化范围增大。     

往复流多孔介质燃烧器的二维数值模拟与结构改进

对往复式惰性多孔介质燃烧器进行了二维数值模拟,模型的有效性通过实验数据进行验证.在燃烧器中分别填充4孔/cm泡沫陶瓷或小球,研究其内部的燃烧温度和压力损失.结果表明,由相同材料制成但结构不同的多孔介质对燃烧器内的高温区域和压力损失有显著的影响.孔隙率较大的泡沫陶瓷适合于布置在燃烧区,而孔隙率较小的小球适合于布置在热交换区域.改进燃烧器结构,即在燃烧器的中间布置泡沫陶瓷,而在两端布置小球.对于当量比为0.1的甲烷与空气混合气,得到了更为宽广的高温区域和适度的压力降.     

天然气在渐变型多孔介质中的预混燃烧

设计了孔径沿程变化的渐变型多孔介质(GVPM)燃烧器，为了解天然气在其中的预混燃烧特性，对燃烧室气体、固体温度分布和CO、NO；污染物排放进行了测量．试验研究了渐变型多孔介质中燃烧的温度场分布、火焰移动、污染物排放、稳定性及多孔介质孔径结构对燃烧特性的影响规律．将研究结果与几种均匀型多孔介质(HPM)中的燃烧进行比较，发现渐变型多孔介质中的燃烧可以有更多的优点，包括均匀温度场分布、极低污染物排放、高火焰速度、高稳定性、宽燃烧极限和有很大的负荷调节范围等．     

材料和喷口直径对多孔介质微燃烧的影响

为了进一步优化微燃烧室的设计,以最大化提高微燃烧室的能量转换效率及微热光电系统的整体工作效率,在前期工作的基础上设计了不同多孔介质材料及喷嘴/燃烧室内径比的多孔介质微燃烧室.通过实验验证,针对多孔介质微燃烧室内的氢氧预混燃烧进行了数值模拟计算,研究结果表明,多孔介质材料,喷嘴/燃烧室内径比对微燃烧室内的微尺度燃烧有重要影响,微燃烧室在多孔介质材料为SiC, 喷嘴/燃烧室内径比为0.27时燃烧效率最高,有利于提高微热光电系统的整体效率.     

Porous media combustion for micro thermophotovoltaic system applications

The micro combustor is the key component of the micro TPV power generator. To obtain high power density and performance efficiency, it is important for a micro combustor to achieve a high and uniform temperature distribution along the wall. In this paper, we compare the performance of a micro cylindrical combustor with and without employing porous media. Results indicate that packing the combustor with porous media can significantly enhance the heat transfer between the high temperature combustion products and the emitter wall. The use of porous media increases the contact area thereby increasing the temperature along the wall of the micro combustor resulting in an increase in its radiation energy. The effects of some parameters on radiation energy of the micro combustor are also highlighted.     

Numerical study on methane/air combustion characteristics in a heat-recirculating micro combustor embedded with porous media

Micro-combustor is a portable power device that can provide energy efficiently, heat recirculating is considered to be an important factor affecting the combustion process. For enhancing the heat recirculating and improving the combustion stability, we proposed a heat-recirculating micro-combustor embedded with porous media, and the numerical simulation was carried out by CFD software. In this paper, the effect of porous media materials, thickness and inlet conditions (equivalence ratio, inlet velocity) on the temperature distribution and exhaust species in the micro combustor are investigated. The results showed that compared with the micro combustor without embedded porous media (MCNPM), micro-combustor embedded with porous media (MCEPM) can improve the temperature uniformity distribution in the radial direction and strengthen the preheating capacity. However, it is found that the embedding thickness of porous media should be reasonably arranged. Setting the thickness of porous media to 15 mm, the combustor can obtain excellent comprehensive capacity of steady combustion and heat recirculating. Compared the thermal performance of Al₂O₃, SiC, and ZrO₂ porous media materials, indicating that SiC due to its strong thermal conductivity, its combustion stabilization and heat recirculating capacity are obviously better than that of Al₂O₃ and ZrO₂. With the porous media embedded in the micro combustor, the combustion has a tempering limit of more than 10 m/s, and the flame is blown out of the porous media area over 100 m/s. The reasonable equivalence ratio of CH₄/air combustion should be controlled within the range of 0.1–0.5, and “super-enthalpy combustion” can be realized.     

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow is numerically studied in order to improve the understanding of the complex heat transfer and optimum design of the combustor. The heat transfer performance of a porous media combustor strongly depends on the thermophysical properties of the porous material. In order to explore how the material properties influence reciprocating superadiabatic combustion of premixed gases in porous media (short for RSCP), a two‐dimensional mathematical model of a simplified RSCP combustor is developed based on the hypothesis of local thermal non‐equilibrium between the solid and the gas phases by solving separate energy equations for these two phases. The porous media is assumed to emit, absorb, and isotropically scatter radiation. The finite‐volume method is used for computing radiation heat transfer processes. The flow and temperature fields are calculated by solving the mass, moment, gas and solid energy, and species conservation equations with a finite difference/control volume approach. Since the mass fraction conservation equations are stiff, an operator splitting method is used to solve them. The results show that the volumetric convective heat transfer coefficient and extinction coefficient of the porous media obviously affect the temperature distributions of the combustion chamber and burning speed of the gases, but thermal conductivity does not have an obvious effect. It indicates that convective heat transfer and heat radiation are the dominating ways of heat transfer, while heat conduction is a little less important. The specific heat of the porous media also has a remarkable impact on temperature distribution of gases and heat release rate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 336–350, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20120     

预混气体在多孔介质中往复流动下超绝热燃烧的理论探讨

预混合气在多孔介质中往复流动下的超绝热燃烧技术(简称RSCP)被称为划时代的燃烧技术，文章探讨了RSCP燃烧器的工作原理，全面阐述了多孔介质和换向装置在其中的作用；从能量守恒定理出发，通过数学分析给出了超绝热火焰产生的理论依据，提出超绝热现象是多孔介质中积累的热量的热传播波与混合气燃烧时的燃烧波叠加的结果。     

预混气体在多孔介质中往复式超绝热燃烧的数值研究

根据气、固两相局部非热平衡假设,建立了RSCP系统的二维非稳态数学模型,对于固体能量方程中的辐射源项采用辐射传递的有限体积法求解,研究了当量比、换向半周期、混合气流速对温度分布、辐射热流量和放热率的影响,考察了最高温升和可燃极限与这些参数之间的关联．研究表明,燃烧室内温度呈梯形分布,高温区较宽;气体的最高温度明显高于绝热火焰温度;贫可燃极限显著扩展,对提高燃烧效率和节约能源有重要作用。     

多孔介质内往复流动下超绝热燃烧的实验研究

对RSCP的燃烧特性进行了实验研究．建成了RSCP实验台，它由泡沫陶瓷燃烧器、电磁阀控制的周期换向进排气管路系统和测量系统组成．对各种工况参数(燃料空气当量比、气体流速、循环半周期)下多孔介质内轴向温度分布进行了系统的测量．实验结果表明，较之常规的自由火焰燃烧器，RSCP具有增强火焰稳定性、拓宽燃料可燃极限等优点．对丙烷-丁烷混合气，其贫可燃极限可扩展到当量比0．065．在实验基础上，探讨了RSCP实现超绝热燃烧的机理，总结出有关工况参数对其燃烧特性影响的规律．     

On predicting two-dimensional heat transfer in a cylindrical porous media combustor

Convective heat transfer within a cylindrical inert porous media combustor is studied. A two-dimensional, two temperature mathematical model, based on fluid mechanics, energy and chemical species governing equations is used for combustion simulation. The finite volume method is used to solve the discrete model for methane combustion with air. Results are presented for unsteady velocity and temperature distributions, as well as for the displacement of the combustion zone, to assess the effects of inlet reactants velocities in the range 0.3–0.6 m/s; excess air ratios between 3 and 6 and porosities of 0.3 up to 0.6 in the convection heat transfer and combustion processes.     

渐变型多孔介质燃气装置的试验研究

在一台0.8 t/h渐变型多孔介质燃气锅炉上进行了传热特性、污染物排放特性和阻力特性试验研究,结果表明,该多孔介质燃气炉具有体积小、负荷调节比大、换热效率高、污染物排放低和气流阻力小等特点.该燃气锅炉的特点是在燃烧室中设置有多孔介质与布置在其中的冷却介质管束,同样功率的多孔介质燃气炉体积只有常规自由火焰锅炉体积的1/5,换热效率大于90%,Nox排放质量浓度低于45 mg/m3,压降小于0.001 Mpa.