微燃烧装置流量控制实验研究

基于燃烧的微/小能源系统具有传统电池不可比拟的能量密度,正成为微能源/动力系统的研究热点。文章分析讨论了做液体乙醇微尺度扩散火焰实验时,用微量注射泵供给微量液体乙醇燃料不连续而造成微尺度火焰脉动的问题。对微量注射泵进行了简单的改进,改进后,燃料供给相对连续,微火焰的脉动很小,为进行微燃烧的实验研究提供了更好的实验条件。     

小尺度乙醇扩散火焰及管壁温度场的实验研究

用不同的小尺度陶瓷圆管作为燃烧器,对液体乙醇在空气中的扩散火焰燃烧进行了实验研究.分析了火焰燃烧的三种不同的特性以及燃烧区域,考察了影响火焰高度与宽度的相关因素,并分析了燃烧时管壁的温度场.结果表明:稳定燃烧时火焰的高度与宽度均随流量的增大而线性增加,并随着内径的减小而减小.随着内径的减小,淬熄流量减小,振荡流量减小,稳定燃烧范围减小.同一流量下,管壁的温度以管口为基点呈指数规律递减.     

稀释合成气微混合燃料喷射燃烧火焰特性研究

研究了耦合CO2稀释和微混合燃料喷射燃烧的火焰特性.结果表明:耦合CO2稀释和微混合燃料喷射燃烧是一种降低合成气扩散火焰NOx生成量的有效途径;实验范围内排放的NOx质量浓度一般低于2mg/m3,排放的CO质量浓度低于10mg/m3,排放的CO质量浓度随火焰热功率的增大而降低;燃烧器出口温度、壁面温度和喷嘴出口温度等均随火焰热功率的增大而升高.     

电场作用下的液体乙醇微尺度层流扩散燃烧

采用实验和数值模拟方法研究了电场作用下液体乙醇层流扩散微燃烧的火焰结构、火焰无量纲高度和宽度以及Re之间的关系.结果表明,实验拍摄火焰结构图像与数值模拟高温区结构相似,正向电场火焰结构尺寸较反向电场高;H/d与Re呈线性关系,正向电场的H/d随Re增加幅度较反向电场快;W/d随Re先是线性增加,后期变化幅度较小;火焰中...     

氧质量分数对高温空气下火焰氮氧化物生成影响

揭示高温空气燃烧过程中的火焰结构和氮氧化物生成机理.以对向流扩散火焰为对象,利用基于详细基元反应动力学模型的燃烧数值解析方法研究了氧质量分数对高温空气(温度为1 300 K)/甲烷扩散火焰火焰结构和氮氧化物生成的影响.结果表明,随着氧质量分数的逐渐减小,火焰结构和NO的生成机理发生显著变化,扩散火焰的NO生成主要由热力...     

预热温度对高温空气/甲烷扩散火焰中NO生成的影响

NO生成机理的基础研究对利用高温空气燃烧技术非常重要.本文以对向流扩散火焰为对象,利用基于详细基元反应动力学模型的燃烧数值解析方法研究了预热温度对高温空气(630～1 800 K)/甲烷扩散火焰中氮氧化物生成的影响.结果表明,随着预热温度的逐渐升高,NO的生成机理发生显著变化,扩散火焰中的NO生成主要由快速型机理控制变...     

稀释燃烧的火焰稳定性

采用实验研究的方法探讨了反应物预热温度与稀释率两个因素对稀释燃烧火焰稳定性的影响.实验以氮气稀释的甲烷-空气对冲扩散火焰为研究对象,确定了不同反应物预热温度与氧化剂稀释率(氧气体积分数)时火焰的熄火极限,结果表明,增大反应物预热温度拓宽了火焰稳定燃烧区域,而增加氧化剂稀释率(降低氧气体积分数)会降低稀释火焰的稳定性,二者对火焰稳定性的影响作用相反.为了进一步分析反应物预热温度与稀释率对火焰稳定性的影响程度,引入了估算的Damkohler数,分析表明,在实验研究范围内,反应物预热温度对火焰稳定性的影响比稀释率的影响显著,是火焰稳定性的主要影响因素.     

氢气预混层流燃烧球形膨胀火焰的拉伸研究

采用高速纹影系统和定容燃烧弹对氢气预混层流燃烧球形膨胀火焰的拉伸进行了研究.分别改变燃空当量比(0.3～4.0)、初始温度(283～400,K)、初始压力(0.05～0.30,MPa)的条件下对比分析拉伸的变化规律,以及拉伸对火焰速度、燃烧速度的影响规律,还解释了拉伸对不等扩散不稳定和流体力学不稳定的影响机理.研究结果表明,随着半径的增大,拉伸率成幂函数关系逐渐减小,其指数和系数值随燃空当量比、温度、压力的变化有明显的变化规律;火焰拉伸的存在使得火焰速度和燃烧速度发生改变,同时它还是火焰不等扩散不稳定和流体力学不稳定发生的前提条件.     

喷管内传递特性对微尺度扩散火焰的影响

对均匀空气流中微尺度甲烷扩散燃烧进行了数值模拟,重点考察微喷管内的流动和传热传质对微尺度燃烧特性的影响.结果表明,在低流速下,内径为0.3 mm的微喷管内进气速度为1.0 m/s时燃料与空气的混合已经发生,混合气被管外的热量预热,同时火焰的热损失增加.在喷管直径一定时,减小燃料喷出速度,传热传质现象对微尺度甲烷扩散火焰特性的影响增强;当进气速度为0.5 m/s时,甲烷在微喷管内开始燃烧,放出热量.在进行微尺度解析计算时,必须包含一定的喷管区域.     

微尺度甲烷扩散火焰及其熄灭特性

用不同直径微/小尺度圆管对甲烷在空气中的扩散燃烧进行实验研究,分析了微/小尺度火焰的结构,考察了影响火焰高度的相关因素,详细探讨了火焰的熄灭极限特点,并拟合了淬熄速度与喷管出口直径d之间的经验关系式.结果表明,火焰的高度与喷管出口速度呈线性关系,随d减小而减小;H/d(火焰高度/喷口直径)与出口处Re值成正比,与d无关;随尺度d的减少,下限(淬熄速度)增大,火焰的稳燃区间变小,稳定燃烧条件苛刻.     

高温高压条件下乙醇-空气-稀释气预混合气层流燃烧特性研究

利用定容燃烧弹和高速纹影摄像手段研究了不同初始压力、初始温度、气体稀释度和燃空当量比下乙醇-空气-稀释气预混层流燃烧特性的基础特征参数,如绝热火焰温度、层流燃烧速度、层流燃烧质量流量、层流燃烧火焰厚度和已燃气体Markstein长度。研究结果表明:在给定初始压力、初始温度和气体稀释度的情况下,绝热火焰温度、质量燃烧流量和层流燃烧速度的最大值均出现在当量比1.0～1.1,层流火焰厚度在当量比1.1处取得最小值;已燃气体Markstein长度随当量比的增加呈下降趋势;在给定当量比条件下,绝热火焰温度随初始压力、初始温度的增加而增加,随氮气稀释度的增加而降低;层流燃烧速度随初始压力和氮气稀释度增加而降低,随初始温度增加而增加;层流质量燃烧流量随初始压力和初始温度的增加而增加;随氮气稀释度增加而减小;层流火焰厚度和已燃气体Markstein长度随初始压力和初始温度的增加而减小,随氮气稀释度的增加而增加。     

Explosion bomb measurements of ethanol-air laminar gaseous flame characteristics at pressures up to 1.4 MPa

The principal burning characteristics of a laminar flame comprise the fuel vapour pressure, the laminar burning velocity, ignition delay times, Markstein numbers for strain rate and curvature, the stretch rates for the onset of flame instabilities and of flame extinction for different mixtures. With the exception of ignition delay times, measurements of these are reported and discussed for ethanol-air mixtures. The measurements were in a spherical explosion bomb, with central ignition, in the regime of a developed stable, flame between that of an under or over-driven ignition and that of an unstable flame. Pressures ranged from 0.1 to 1.4 MPa, temperatures from 300 to 393 K, and equivalence ratios were between 0.7 and 1.5. It was important to ensure the relatively large volume of ethanol in rich mixtures at high pressures was fully evaporated. The maximum pressure for the measurements was the highest compatible with the maximum safe working pressure of the bomb. Many of the flames soon became unstable, due to Darrieus-Landau and thermo-diffusive instabilities. This effect increased with pressure and the flame wrinkling arising from the instabilities enhanced the flame speed. Both the critical Peclet number and the, more rational, associated critical Karlovitz stretch factor were evaluated at the onset of the instability. With increasing pressure, the onset of flame instability occurred earlier. The measured values of burning velocity are expressed in terms of their variations with temperature and pressure, and these are compared with those obtained by other researchers. Some comparisons are made with the corresponding properties for iso-octane-air mixtures.     

Experimental determination of laminar burning velocities and Markstein lengths for 75% hydrous-ethanol, hydrogen and air gaseous mixtures

Hydrogen-rich mixtures generated by the on-board reforming of biomass-derived hydrous-ethanol can be used as a potential alternative fuel (i.e., reformed ethanol fuel, RE fuel). In this paper, outwardly propagating spherical flames were employed to observe the laminar flame characteristics of the gaseous mixtures composed of simulated RE fuel (mixture of 75% hydrous-ethanol and hydrogen) and air in a constant-volume combustion vessel at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 80%, and an equivalence ratio from 0.6 to 1.6. The results show that the unstretched flame propagation speeds and burning velocities increase with increasing hydrogen fraction, especially when the fraction is above 40%. When the hydrogen fraction is less than 40%, the Markstein length and flame instability decrease and increase with the equivalence ratio, respectively, while the reverse holds when the hydrogen fraction is greater than 40%. At an equivalence ratio below 1.4, the Markstein length decreases with increasing hydrogen fraction, indicating a positive correlation between the flame instability and hydrogen fraction. At an equivalence ratio above 1.4, a negative relationship is observed. Finally, it is concluded that a hydrogen fraction of approximately 40% in simulated RE fuel is feasible for spark ignition engines by comparing the laminar burning characteristics of ethanol-air mixtures.     

Single-phase pressure drop and heat transfer characteristics of turbulent liquid nitrogen flow in micro-tubes

Experiments have been conducted to investigate the single-phase pressure drop and heat transfer characteristics of liquid nitrogen in four micro-tubes with the diameters of 1.931, 1.042, 0.834 and 0.531 mm. The friction factors are compared with the conventional correlations over a Reynolds number range of 10,000–90,000. The effect of the variable thermal properties of liquid nitrogen, i.e., viscosity and thermal conductivity, on the flow and local heat transfer in the micro-tubes is clarified. The average Nusselt numbers are determined and compared with the correlations for the conventional channels and micro-channels, respectively. It is found that large roughness of the micro-tube causes high friction factor, and the modified Colebrook correlation can well predict the experimental friction factors by using the measured surface roughness. With the increase of liquid nitrogen temperature, the pressure drop decreases as a result of the lower viscosity. Opposite to water, the local heat transfer coefficient of liquid nitrogen flow in the micro-tube drops by 12.5% along the tube. The experimental data show that the average Nusselt numbers for the micro-tubes are higher than those predicted by the correlations for the conventional channels. Taking into account the effect of surface roughness of the micro-tubes on the heat transfer, the modified Gnielinski correlation enables to predict the experimental Nusselt numbers with a mean absolute error (MAE) of 6.4%.     

Experimental study on the small‐scale diffusion flame of ethanol and the wall temperature field

An experimental investigation was carried out on the diffusion flames of liquid ethanol burning in air. Ceramic tubes with different inner diameters were used as burners. Three different flame structures at different flow rates were identified and different combustion regions were divided based on the experimental results. Some relevant factors which may affect the flame height and width were discussed. The outer surface wall temperature field of the tubes was measured in the combustion process. The results showed that both flame height and flame width all increased proportionally with an increasing flow rate in the steady flame region. Both flame height and flame width decreased with a decreasing tube inner diameter at the same flow rate. By decreasing the tube inner diameter, the quenching flow rate was decreased, the flow rate according to the produced periodic explosive flame decreased, and the flow rate range of the steady flames decreased. The outer wall temperature field presented an exponential distribution, and the wall temperature reached the greatest value at the outlet of the ceramic tube. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20282     

Flame spread over aviation kerosene with an obstacle in liquid phase

The phenomena of flame spread over aviation kerosene with an obstacle in liquid phase are investigated experimentally through surface temperature measurement by using infrared camera,schlieren images of subsurface flow in front of and behind obstacle and residence time of flame obtained from video recording.Experimental results reveal that obstacle has no effect on gas phase controlled flame spread.But for liquid phase controlled flame spread,flame can be stopped by an obstacle with its top edge flush with oil surface,and the residence time decreases with the increase of initial temperature of fuel.That conduction and radiation only play a subsidiary role in flame spread over liquid fuel was proved by schlieren images and surface temperature profiles.     

基于激光诊断的生物柴油碳烟生成特性研究

制作了1个以液体为燃料可以产生层流扩散火焰的燃烧器,应用双色激光诱导炽光法(LII)技术来测量火焰中的绝对碳烟体积分数.基于在2个波长上获取的火焰中某1点的LII信号,获得被激光加热的碳烟粒子温度,同时得到此点的碳烟浓度,通过映射得到火焰的二维碳烟浓度分布.采用激光诱导荧光法可获得碳烟前驱物多环芳香烃在火焰中的二维分布,将激光诱导荧光和激光诱导炽光相结合,在柴油和生物柴油混合燃料的层流扩散火焰上进行测试.研究结果表明,随着生物柴油掺混比例的增加,碳烟和多环芳香烃的最大浓度都随之降低,浓区分布面积也进一步缩小.     

乙醇-氢气-空气混合物层流燃烧特性研究

氢气是一种高效的添加剂,可以改善生物质燃料的层流燃烧特性。为研究氢气对乙醇-空气预混层流火焰燃烧特性的影响,利用定容燃烧弹结合高速纹影摄像技术,系统研究了初始温度为400 K,初始压力为0.1 MPa和0.4 MPa,氢气含量为0%、10%、30%、50%、70%和90%,当量比为0.7 ~ 1.4时的氢气-乙醇-空气混合燃料的层流燃烧速度（LBV）、火焰厚度和马克斯坦长度等参数,并采用辐射校正公式使LBV更加精准。通过数值仿真构建预混火焰模型,与实验结果进行对比。结果表明,氢气比例的增加可以提高混合燃料的层流燃烧速度。当氢气比例小于50%时,LBV随氢气比例的增加线性增长。而当氢气比例大于50%,LBV随氢气的增加呈指数增长。初始压力的上升虽然降低了LBV,但提高了LBV的增长率。此外,随着氢气比例和初始压力的增加,火焰厚度减小,马克斯坦长度降低,火焰的不稳定性增强。     

浸没难挥发性可燃液体的多孔介质表面火焰传播特性

实验研究了浸没难挥发性可燃液体的多孔介质表面火焰的传播特性.丁醇和石英砂分别作为浸入的难挥发性可燃液体和多孔介质.研究结果表明:液面高度和介质粒径对于火焰传播过程中火焰的结构、火焰传播速度以及砂层的温度分布都有着显著的影响;火焰在传播过程中,火焰前锋前面的砂表面预热区内存在一气化可燃物析出区域(约为5.6,mm),其对火焰的传播起着重要作用.