微热光电系统中的微燃烧研究

描述了一种新颖的MEMS动力源概念，即微热光电(TPV)系统。该系统将使用氢气作为燃料，每立方厘米体积能够发出1～10W的电力。微燃烧室是该系统中最重要的元件之一，为了获得较高的电能输出，燃烧室壁面的温度分布要求高而且均匀。由于微燃烧室面容比大，热损失显增加；着火困难并使火焰窒熄。为了测试微燃烧室内燃烧的可行性和确定影响燃烧的有关因素，进行了实验和数值模拟。结果表明燃烧室壁面能够得到要求的高温，且温度分布均匀。     

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 and Experimental Studies of Mixing Enhancement and Flame Stabilization in a Meso-Scale TPV Combustor With a Porous-Medium Injector and a Heat-Regeneration Reverse Tube

Understanding the flow dynamics, chemical kinetics, and heat transfer mechanism within a miniature thermophotovoltaic (TPV) combustor is essential for the development of devices for combustion-based power microelectromechanical systems, which may have a much higher energy density than that of conventional batteries. In this study, methods for enhancing the intensity and uniformity of the combustion chamber wall (emitter) illumination through the design of combustion and thermal management of the combustor in a miniature TPV system are proposed, discussed, and demonstrated. The proposed miniature TPV system consists of a swirling combustor with the combustion chamber wall acting as the emitter, a heat-regeneration reverse tube, and mixing-enhancing porous-medium fuel injection, which improves the low nonuniform illumination or incomplete combustion problems associated with conventional miniature TPV systems. Experiments and numerical simulations are performed to analyze the details of the flame structure and flame stabilization mechanism inside the meso-scale combustor with and without a reverse tube. Results indicate that the proposed swirling combustor with a heat-regeneration reverse tube and porous medium can improve the intensity and uniformity of the combustion chamber (emitter) illumination and can increase the surface temperature of the chamber wall. From the systematic numerical and experimental analysis, suitable operational parameters for the meso-scale TPV combustor are suggested, which may be used as a guideline for meso-scale TPV combustor design.     

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.     

Combustion characteristics and thermal enhancement of premixed hydrogen/air in micro combustor with pin fin arrays

Targeted at improving the combustion stability and enhancing heat transfer in micro combustor, the combustion characteristics and thermal performance of micro combustor with pin fin arrays are numerically investigated by employing detail H₂/O₂ reaction mechanism. It is shown that the micro combustor with staggered pin fin arrays exhibits the highest average temperature and heat flux of external wall, while the micro combustor with in-line pin fin arrays displays the most uniform temperature distribution of external wall. When the equivalence ratio is 1.1, all micro combustors exhibit the highest mean temperature and heat flux of external wall. The micro combustor materials with high thermal conductivity can not only improve the average temperature and heat flux of external wall, but also enhance heat transfer to the upstream which can preheat the mixed gas. Therefore, the materials with high thermal conductivity, such as red copper and aluminum, can make up for the nonuniform temperature distribution of micro combustor with staggered pin fin arrays, so as to realize uniform high heat flux output of external wall.     

微型热光电系统多孔介质燃烧器性能的实验研究

为保证微型热光电动力系统能稳定、高效地工作,燃烧器壁面需有较高的温度,且分布均匀．对采用多孔介质结构的微型燃烧器进行了实验研究,分析了孔隙率、CH_4/O_2混合比等因素对燃烧器性能的影响．结果表明,采用多孔介质结构可以改善燃烧器内的燃烧传热过程；合理选择孔隙率和工况参数,可以优化燃烧器壁面温度分布,提高系统工作性能．     

采用正交各向热异性壁面材料提升微通道火焰稳定性的数值模拟研究

为深入分析不同壁面导热率对微尺度燃烧器稳燃性能的影响,使用基于OpenFOAM框架开发的低马赫数反应流求解器,并结合DRM-19化学反应机理对正交各向热异性的微型平行平板内甲烷/空气预混火焰稳定性开展了数值模拟。模拟结果表明：相比于各向热同性壁面材料,正交各向热异性材料能够显著提升壁面对通道上游附近未燃气体的预热作用,同时减少燃烧器向外界环境的热损失,可达到提高火焰吹出极限的效果。     

Combustion in micro-cylindrical combustors with and without a backward facing step

Micro-combustors are critical components for micro-power systems using hydrogen and hydrocarbon fuels as an energy source. The micro-thermophotovoltaic (TPV) power system requires an output of high and uniform temperature from the wall of the combustor. This paper presents the experimental results of three types of stainless cylindrical micro-combustor with or without a backward facing step. Hydrogen was used as the fuel. The temperatures at exit and along the wall of the combustors were measured. The results show that the backward facing step provides a simple yet effective solution to enhance the mixing of fuel mixture and prolong the residence time. In addition, the step is very useful in controlling the position of flame and widening the operational range of the flow rate and H₂/air ratio. A high and uniform temperature distribution has been achieved for micro-combustors with a backward facing step. This result is relevant to the application of micro-TPV power systems we are currently pursuing.     

微热光电系统带环形翅片燃烧室的数值模拟

最大化提高微燃烧室的燃烧效率对于微热光电系统是非常关键的。建立了微燃烧室内的流动、传热和燃烧模型并进行了数值模拟。利用试验结果验证了模型的可靠性。在此基础上,模拟了带有环形翅片燃烧室的情况,表明环形翅片能增强微燃烧室内混合气体的湍流扰动,改善燃烧状况,有效地提高燃烧效率。     

Effects of major parameters on micro-combustion for thermophotovoltaic energy conversion

For a micro-thermophotovoltaic (TPV) energy conversion device, high surface to volume ratio in the micro-combustor provides a great potential to achieve high surface radiation power output per unit energy input. This work investigated experimentally the effects of three major parameters on micro-combustion, namely hydrogen to oxygen mixing ratio, nozzle to combustor diameter ratio, and wall thickness to combustor diameter ratio. The results show that the high average wall temperature can be achieved at slightly oxygen rich mixing ratios. Nozzle to combustor diameter ratio affects both the magnitude and uniformity of wall temperature distribution. The newly designed thin wall combustor which yields a reduction of axial heat conduction loss is able to increase wall temperature more than 150 K. Optimized design of these parameters will have significant impact on the enhancement of radiation heat output in micro-TPV energy conversion.     

微型能源动力装置及微尺度燃烧研究

基于燃料燃烧的微型能源动力装置具有高能量密度特性,可提供瓦到百瓦级的能量输出,因此在过去的20年间受到广泛关注。国内外学者研制了微型的燃气轮机、内燃机、推进装置、燃烧器、热电转换装置及热光电转换系统等不同类型的能源动力装置。然而,由于微尺度条件下燃烧环境和常规尺度存在差异,材料、密封及润滑等方面的技术瓶颈,目前大部分微型能源动力装置的性能未能到达预期的目标。由于微尺度燃烧基础理论有别于传统的常规尺度燃烧理论,随着其重要性的凸显,国内外学者对其进行了广泛深入的研究,更加清晰地揭示了微尺度火焰及燃烧的基本特性。本文首先介绍了国内外微型能源动力装置及系统的研究进展,然后对微尺度条件下预混及非预混火焰的研究现状进行了总结,在本文的最后部分提出了微燃烧相关亟待解决的科学及工程问题。     

不同喷管间距甲烷微火焰阵列温度场和燃尽特性研究

为了优化微阵列火焰燃烧加热系统,在相同的燃料负荷和喷管物理条件下,构建了甲烷预混和微火焰阵列燃烧模型,并研究了不同喷管中心间距对温度场和燃尽率特性的影响规律。研究结果表明,由若干微小喷管火焰优化组成的阵列可形成温度均匀的加热场;随着喷管中心间距减小,火焰间相互影响程度增加,均温加热场的温度提高;喷管中心间距继续减小,微喷管阵列火焰开始聚并、向大火焰转变,燃烧反应区间变长、均温场处的燃尽率下降,微喷管火焰丧失微火焰特性;因此确定微火焰阵列加热场喷管中心间距这一重要参数时,需综合考虑温度均匀性、热负荷、燃尽率和污染物等因素。     

Numerical investigation on combustion characteristics of premixed hydrogen/air in a swirl micro combustor with twisted vanes

The combustion characteristics of the swirl micro combustor with twisted vanes (Swirl-MC-TV) and the conventional micro combustor (Conventional-MC) are investigated and compared under different inlet velocities (8–40 m/s), wall materials (quartz, steel, and SiC), and equivalence ratios (0.6–1.4). The results show that the larger area of recirculation zones and the stronger recirculation intensity are the key factors for Swirl-MC-TV to stable combustion. When the inlet velocity is 40 m/s, compared with the Conventional-MC, the wall heat loss of the Swirl-MC-TV is reduced by 15.9%, and the reaction heat and combustion efficiency of the Swirl-MC-TV are increased by 17.5% and 5.9%, respectively. When the wall materials of steel and SiC, combustors have a better preheating effect and higher combustion intensity. When the equivalence ratio is greater than 0.6, the wall heat loss of Swirl-MC-TV is larger but the combustion efficiency and the reaction intensity are still higher than Conventional-MC.     

A numerical study on combustion and emission characteristics of premixed hydrogen air flames

Main challenges for micro power generators that utilize combustion process for energy production are inadequate residence time, destructive radical wall interactions and intensified heat loss which are mainly rooted from size limitation of such devices. To achieve high and uniform energy output, and bring in a solution to these challenges in an environment friendly manner without any kind of fundamental modification, effect of equivalence ratio on combustion and emission behavior of premixed hydrogen/air flames is numerically investigated in this study. For this purpose, an experimentally tested micro cylindrical combustor model is constructed and premixed hydrogen/air combustion in this model is simulated by varying equivalence ratio between 0.5 and 1.2 to find an optimal equivalence ratio with respect to drawbacks of micro power generators. Combustion and turbulence models implemented in this study are Eddy Dissipation Concept and Standard k-ε models, respectively. A detailed hydrogen/air reaction mechanism which consists of 9 species and 19 steps is employed to accurately gain insight into combustion process. Simulation results show that as the equivalence ratio decreases; centerline temperature distribution gets a lower value and the place where chemical reactions take place moves downstream. The most uniform temperature distribution is achieved between 0.8 and 1.0 equivalence ratios. The highest NO_x formation is at 0.9 equivalence ratio and its mass fraction decreases sharply when the equivalence ratio reduces from 0.9 to 0.5.     

小型斯特林发动机不同外加热工况下运行的探索性实验

该文就其微型化可能产生的一系列问题,如：输出频率过高、产生转矩过小、冷热端差过小等进行了分析。提出自由活塞式斯特林发动机比热声式更具微型化的优势。并对一台长12cm、直径2．5cm的小型活塞式斯特林发动机进行了实验。实验中发动机热端温度范围64～626℃,通过测量其外壁面温度和声信号随时问的变化,得到发动机的稳定性和输出功率随外加热功率变化规律。该发动机的理论最高功率密度为102kW／m^3。     

Experimental study on micro modular combustor for micro-thermophotovoltaic system application

As one of the key components of micro modular thermophotovoltaic power generators, every micro combustor should be able to produce a high and uniform temperature distribution on the surface. In this work, three micro modular combustors with different fuel supply systems were designed and tested. The results indicated that the in-line design with only one fuel supply tube could not equally distribute H₂/air mixture to every combustor. The wall temperatures of the two central combustors were obviously higher than the two side combustors. However, both the in-line design with two fuel supply tubes and the parallel design could equally deliver the fuel/air mixture to every combustor, and an uniform temperature distribution could be obtained for every combustor. The total radiation energy and radiation efficiency of the micro modular combustors were also calculated for various flow speeds and H₂/air equivalence ratios. A radiation efficiency of 27.3% could be achieved when the H₂/air equivalence ratio was 0.8 and the flow speed was 6 m/s.     

NUMERICAL SIMULATION OF FLAME PROPAGATION NEAR EXTINCTION CONDITION IN A MICRO COMBUSTOR

In the present study, a numerical simulation of flame propagation and extinction in a micro combustor that is subject to excessive heat loss to the wall, particularly during flame propagation, is described. Heat loss to the wall was empirically modeled from measurement data on a similarly configured micro combustor. A PISO based numerical scheme was used for differencing the conservation equations. An H₂-air reaction mechanism involving 16 species and 10 reaction steps was used to approximate the combustion process. A cylindrical computation domain was used to simulate the experiments. The combustor volume has a small height to radius ratio and an axial gradient of properties can be significant. In the present study, however, axial gradients were ignored, leaving radius as the only spatial coordinate. Instead of evaluating heat transfer from the temperature gradient near the wall surface, an empirical bulk heat transfer coefficient was used to approximate heat loss to the wall. A comparison of the computation and measurements showed a good agreement in flame propagation speed and cooling process, after the flame had been quenched by an artificially imposed extinction condition.     

Frequency evaluation of a gas‐fired pulse combustor

Theoretical study and experimental investigation of a Helmholtz‐type gas‐fired pulse combustor are presented. In this study, a mathematical model was developed to evaluate the operating frequency of the pulse combustor. A variety of experiments were also performed to prove the validation of theoretical frequency model and compare the theoretical frequency with the experimental one. In the experiments, the length and diameter of the tail pipe, the volume of the combustion chamber and the gas supply pressure of the combustor were changed within certain limits. In the experiments, related to tailpipe length, there was a perfect agreement between the tailpipe lengths of 1.2 and 1.6 m. As the theoretical frequency was compared with the experimental one, the maximum deviation was 5.8 Hz (1.25% errors). If the frequency comparisons related to the volume of the combustion chamber were taken into consideration, the calculated frequency and measured ones agreed very closely. In the volumes between 1/2 V_c and V_c, maximum deviation was 5.4 Hz (7.7% errors). In this study, a tunable pulse combustor, which is used for increasing the frequency of industrial processes, was also developed. Copyright © 2005 John Wiley & Sons, Ltd.     

不同散热条件下微燃烧的数值模拟

散热是影响微尺度燃烧器燃烧稳定性的重要因素之一.本实验通过在一个长40 mm、内径2 mm、外径4 mm的石英玻璃直圆管表面施加不同的外部吹风温度,控制其表面散热.研究4、107、756℃外部风温下,微燃烧器的工作性能,其中燃料混合气体流量为0.16、0.28、0.32 L/min.实验测得燃烧器壁面温度,结合数值模拟研究内部燃烧过程.计算结果显示,提高燃料流量或外部风温可以提升反应强度、抑制熄火.如在风温107℃时,燃料气体当量配比下,当流量由0.16 L/min上升到0.32 L/min时,峰值温度由1538 K上升到1620 K;在流量0.28 L/min时,燃料气体当量配比下,当外部风温由4℃上升到756℃时,峰值温度由1592 K上升到1731K.     

大加速度场中层流扩散火焰流场的数值计算

通过对大加速度场中层流燃烧室流场的数值计算,建立了大加速度场中二维层流燃烧的数学模型,对控制方程组进行离散,采用SIMPLE算法和交错网格设计并调试程序。在调试成功的程序上对甲烷和空气在大加速度场中的扩散燃烧过程进行了数值模拟。计算结果表明,沿燃烧室轴线方向的均匀大加速度场会对扩散火焰的速度场和温度场等产生明显影响。一方面使得燃料与空气的扩散混合过程得到强化,扩散火焰的形状变短变粗,火焰面温度升高．因而能够提高其燃烧速度;另一方面,由于浮力作用驱动高温气流的流动方向与燃料射流的方向相反,将形成一种不稳定的流场结构,并同时诱发燃烧过程的不稳定。