共查询到20条相似文献,搜索用时 218 毫秒
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采用20步反应机理模拟了H2/空气在内径2 mm长20 mm的圆管内的预混燃烧.H2/空气预混火焰由壁面向中心传播,呈圆锥形.随着气流向后流动,燃烧区域截面温度曲线由"U"形变为"M"形,后又变为倒"U"形,分别对应壁面加热预混气体的过程,预混燃烧火焰由近壁面向中心传播的过程和燃烧后气体对外散热过程.微燃烧器对外散热量较大,约占总输入热的10%左右,其中燃烧段散热约占5%.辐射散热在壁面散热中占主导地位,占总散热的80%~90%,外壁低辐射系数的材料有利于减少散热和增加燃烧稳定性.对微燃烧而言,燃烧器壁厚增加使燃烧器散热增加,反而不利于降低燃烧器散热.燃烧器入口处壁温与壁面导热系数、壁厚不呈单调变化趋势.在导热系数为3~20 W/(m·K)、壁厚为1 mm左右时,燃烧器入口处壁温较高,有利于稳定燃烧. 相似文献
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为了揭示90°弯曲管道结构对预混火焰传播特性的影响,以丙烷-空气预混火焰为研究对象,运用高速纹影摄像、微细热电偶以及离子探针等测试手段对火焰在90°弯曲管道内的传播过程进行了实验研究.结果表明,预混火焰结构在水平管道内发生了明显变化,由规则的球形层流火焰转变为具有轴对称结构向内凹陷的湍流火焰,并伴随火焰阵面的皱褶分层.火焰进入90°弯曲管道后,受几何形状影响,火焰阵面发生畸变,对称结构被破坏,下壁面处的火焰阵面逐渐超过上壁面处的火焰阵面.由于弯管内部多波叠加作用以及湍流的影响最终使得火焰速度呈现脉动振荡. 相似文献
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本实验对微尺度燃烧器内部使用电热丝向反应物加热,提高其温度,减少散热影响,从而实现微尺度火焰稳燃.燃烧器为石英直圆管,长82.mm,通流面积3.33.mm2.氢气/空气混合气体流量为0.12.L/min、0.2.L/min、0.4.L/min,电热功率分别为0.W、1.05.W4、.70.W.实验结果显示,提高电热功率抑制熄火;提升燃料混合气体总流量也有助于抑制熄火;但高流量同时带来吹脱问题.测量燃烧器壁面温度,结合数值模拟研究内部燃烧过程.结果显示,随电热功率上升,反应温度和OH质量分数上升,证明电加热强化反应,抑制了热熄火.比较壁面散热,电热功率0.W,流量由0.12.L/min上升到0.2.L/min时,壁面散热占总能量份额由84.8%降低至81.1%.因此,在一定范围内提升燃料流量亦可抑制热熄火. 相似文献
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甲烷/氧气在微细直管内的燃烧和散热研究 总被引:1,自引:0,他引:1
为了解微细直管燃烧器的工作特点,采用内径2 mm和1.4 mm的微细不锈钢管和陶瓷管进行氧气和甲烷气体的燃烧实验,研究了氧气和甲烷在微细管内的燃烧特点以及微细管的散热损失.研究结果表明,当量比小于1时,由于CH4没有完全被氧化,生成了大量的H2和CO气体,减小了燃烧反应的放热量.当量比等于1时,CH4被完全氧化为CO2和水蒸气,此时的放热量最大,同时管壁的散热量也最大.不锈钢管的散热量最大,是发热量的22%,陶瓷管的散热量最大达到了16%.不锈钢管的壁面发射率较大,辐射损失所占的比例较大,最大达到总散热量的70%.由于陶瓷管的导热系数比不锈钢管小,沿轴线方向的壁面温度梯度比不锈钢管大,这样不利于轴向的传热,以及火焰的稳定. 相似文献
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利用数值模拟方法,研究了不同重力下有限空间内薄燃料表面逆风传播火焰的相似性.结果表明,通道高度的变化,通过影响通道间的流场和壁面的热损失,来影响通道内燃料表面的火焰传播,因此用水平窄通道模拟微重力下大空间内的火焰传播,只能得到定性相似但定量差别较大的结果,这与他人的实验结果一致.在微重力和常重力下的窄通道中,当Grashof准数足够小时(200可以作为一个定性参考值),其中的自然对流基本可以忽略,不同重力下窄通道中的火焰传播过程基本相似. 相似文献
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为了研究结构参数对全地形车排气系统散热的影响,采用Fluent软件对排气系统进行流场分析,得到壁面的温度分布图,并通过试验验证有限元模型的准确性。以消声器内隔板孔的直径、隔板到消声器前端的距离、消声器未端圆角半径为设计变量,以排气系统壁面平均温度为优化目标,应用响应面法进行结构参数优化,得到使壁面平均温度最小的一组设计变量值。最后,对改进后的排气系统进行流场分析,并与优化前的数据对比。结果表明,通过设计变量的合理改变,重点关注的消声器处壁面温度明显下降,消声器处壁面4个测试点温度分别下降了10.3%、11.5%、8.0%、10.4%。 相似文献
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稳定传播火焰在可燃云雾中产生的压力波 总被引:1,自引:0,他引:1
火焰在可燃云雾中传播时,在火焰面前方诱导出压力波。若火焰以稳定速度在均匀的静止云雾中传播,则压力波后的流场是自相似的。本文用自适应步长的四阶Runge-Kutta法对相平面上以约化参数F=t/rμu、Z=(t/rμa)^2表示的守恒方程积分,得到了自相似流场的解;以火焰面两侧的守恒关系导出了火焰面位置、燃烧速度及对应C-J条件下的燃烧速度。对膨胀比υF为7的常用碳氢燃料,本文还给出了压力波阵面压力 相似文献
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《International Journal of Hydrogen Energy》2021,46(73):36515-36527
Numerical investigation on the premixed H2/air combustion in a micro heat-recirculation combustor inserted with/without block is conducted. Effects of block setting, heat-recirculation, and flow rate on combustion characteristics and thermal performance are depicted and analyzed. The results demonstrate that the block enhances the flame stability and preheating effect, which also reduces the heat loss via exhaust gas, while it shortens reactants residence time. The combustor setting with a transverse block gains a better thermal performance than that inserted with a longitudinal block. With the increase of transverse block height, the high-temperature zone is broadened and radiation is improved. However, the block with a height of 10 mm separates the fluid field and weakens the effects of heat recirculation, leading to a lower outer wall temperature. Furthermore, the appropriate block insertion method and height contribute to the significant improvement of heat transfer, radiant efficiency and further optimization of micro power generator. 相似文献
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Measurement of the flame temperature in a micro combustor is essentially difficult due to the size constraint. A one-dimensional (1D) flame model coupled with the heat conduction in the solid wall is employed to analyze the heat transfer occurring in a cylindrical micro combustor. The flame temperature is given explicitly by taking into account the effects of the heat loss (from the flame to the wall) in the reaction zone and heat recirculation through the solid wall. With the data obtained from the simulation results of the 1D adiabatic freely propagating CH4–air laminar flames, the flame temperature in a cylindrical micro combustor can be solved iteratively. In order to validate the 1D model, the two-dimensional (2D) numerical simulations of premixed combustion of the CH4–air mixtures are carried out in a 0.5 mm radius cylindrical micro combustor. The comparisons of the flame temperature and heat recirculation between the 1D model and 2D numerical simulation indicate that despite the simplifications and assumptions made in the present study, the 1D theoretical model is able to predict the flame temperature to a reasonable accuracy. 相似文献
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The effect of gravity on the thermal performance of inclined micro heat pipes with axial conduction in the solid wall is reported. A one-dimensional, steady-state model is developed from first principles in which the continuity, momentum, and energy equations of the liquid and vapour phases, together with the Young–Laplace equation, are solved numerically to yield the heat and fluid flow characteristics of an inclined micro heat pipe which is operated optimally at a certain operating temperature. The analysis covers both the favourable and adverse effects of gravity on the performance of a micro heat pipe. The effects of gravity, through the angle of inclination, on the heat transport capacity, the optimal charge level of the working fluid, the liquid volume fraction distribution, the circulation strength of working fluid and the solid wall temperature distribution are analysed, to provide a better insight for the design of inclined micro heat pipes. 相似文献
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Xiao Yang Bo Yu Wenming Yang Huaichun Zhou 《International Journal of Hydrogen Energy》2021,46(51):25893-25907
In the field of micro and mesoscale combustion, the feature of flame-wall thermal coupling is of great significance because of its small scale nature. Thus, this work provides a comprehensive heat transfer analysis in cylindrical combustors from the perspective of numerical simulation. The combustor has a fixed length-to-diameter aspect ratio of 10, and the channel diameter is scaling up from 1 mm to 11 mm to explore the influence of chamber dimension on heat transfer and flame structure. The distribution of convective and radiative heat flux on inner surface, contribution of thermal radiation are given. Moreover, the role of radiation in flame structure is analyzed, and the convective and radiative heat losses are quantitatively analyzed. We find that radiative heat flux is smaller compared to convective heat flux, and the proportion of radiative heat flux becomes larger with an increasing diameter. Thermal radiation does not change the flame structure when the diameter is less than 3 mm. When the diameter is greater than 5 mm, thermal radiation changes the location of flame front. The heat loss becomes larger at a smaller diameter, and heat loss ratio can reach approximately 73.6% in the combustor with diameter of 1 mm. 相似文献
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Shuhn-Shyurng Hou 《International Journal of Heat and Mass Transfer》2003,46(2):311-322
The influences of flow stretch, preferential diffusion, internal heat transfer and external heat loss on the extinction of dilute spray flames propagating in a stagnation-point flow are analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. The internal heat transfer, associated with the liquid fuel loading and the initial droplet size of the spray, provides heat loss for rich sprays but heat gain for lean sprays. The flow stretch respectively weakens and intensifies the burning intensity of the lean methanol-spray flame (Le>1) and rich methanol-spray flame (Le<1). Results show that the Le>1 flame can be extinguished with or without external heat loss. Flame extinction characterized by a C-shaped curve is dominated by the external heat loss or the flow stretch. For the Le<1 flame without external heat loss, no extinction occurs under the influence of flow stretch. However, the Le<1 flame with completely prevaporized fuel sprays enduring a small amount of flow stretch can be extinguished by the external heat loss and this behavior is characterized by a C-shaped curve. Note that the W-shaped extinction curve is mainly governed by the internal heat loss. Flame extinction characterized by a W-shaped curve occurs when the Le<1 spray flame with external heat loss endures a positive stretch and experiences a partially prevaporized spray with sufficiently large liquid fuel loading and droplet size. 相似文献
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Kwon Hyoung Choi Han Bee Na Dae Hoon Lee Sejin Kwon 《Nanoscale and Microscale Thermophysical Engineering》2013,17(1):71-89
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. 相似文献
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The excess enthalpy flames and their dynamics below the flammability limit are studied by considering two flames that propagate in opposite directions in parallel channels. The model enables the coupling between the external heat loss, convection preheating, diffusion transport and finite rate chemistry. Analytical expressions for the flame temperature, separation distance, and extinction limit are obtained. The results show that flame extinction can be caused by the external heat loss without heat conduction of inner wall in the streamwise direction. The heat recirculation across the separating wall dramatically increases the flame speed and extends the flammability limit. It is shown that the maximum and minimum flame speeds corresponding respectively to the fast and slow flame modes exist at all separation distances between the two flames. It is found that the flame can adjust its separation distance to adapt to the variation of heat loss, heat recirculation and fuel concentration. There exists a maximum flame separation distance beyond which sub-limit flame does not exist. The results also showed that heat recirculation significantly extends the flammability limit. Furthermore, at low fuel concentrations, the flame can be stabilized in a narrow range of separation distance. The present study not only generalized the previous analyses of the heat recirculation flames but also provided a model for the study and control of sub-limit flames in micro power devices and reactors. 相似文献
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对一种新型的平板式微热管一零切角曲面微热管进行了实验研究。以热阻为基础,研究不同倾角、工质、充液比下微热管的热性能。为便于分析,将热管总热阻分解为4个部分:加热热阻、蒸发段热阻、冷凝段热阻和热沉热阻。通过实验得出如下结论:微热管总热阻的主要变化因素是冷凝段热阻和蒸发段热阻;与相应的无工质平板式换热器相比,实验件主要热阻变为热沉热阻.蒸发段和冷凝段热阻所占比例较低。根据不同的充液比和倾角。微热管传热极限分别由局部干烧和核态沸腾向膜态沸腾转化引起。实验表明。这种新型的微热管具有良好的应用前景,但是对于其机理还需要更深入的研究。 相似文献
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A tubular platinum reactor with a perforated annular array enables fuel/air mixtures to exchange sides, thus sustaining flames and preventing heat loss. Consequently, the combustion efficiency and operational range can be enhanced. A hydrogen/air mixture was introduced into inner and outer chambers at different equivalence ratios and flow velocities to chemically and physically investigate the interplay between the chambers. The benefits of hydrogen include a high gravimetric heating value, flame speed, and diffusion capacity and short chemical reaction time. The coexistence of heterogeneous (surface) and homogeneous (gas) reactions in the micro TPV reactor was examined and elucidated in terms of aerodynamics, mass and heat transfer, and chemical reactivity. Furthermore, a TPV reactor with TPV cell arrays was assembled, and the corresponding radiant efficiency of the emitter and the overall efficiency of the proposed micro TPV system were determined in this study. 相似文献