含内热源可燃多孔介质热量传递及影响因素分析

本文模拟了自然对流及恒壁温边界条件下含内热源可燃填充床内最高温度随时间的变化;计算了侧面和端面取不同换热系数时填充床内的温度分布;分析了对流换热系数、堆积状态等对床内最高温度的影响。计算结果表明:各边界取不同换热系数时对最高温度的影响不大;在环境温度低于383．15 K,或壁面温度低于381．15 K的条件下,多孔床不会发生自燃。对流换热系数较大、堆积密度较小时多孔床也不易自燃。     

渐变型多孔介质中燃气燃烧特性试验研究

试验研究预混燃气在渐变型多孔介质(GVPM)中的燃烧特性,包括在渐变型多孔介质中的温度场、火焰移动、CO和NOx生成、燃烧稳定性及多孔介质孔径结构对燃烧特性的影响规律。研究结果与几种均匀型多孔介质(HPM)中的燃烧特性进行比较,发现渐变型多孔介质中的燃烧有如下优点:均匀的温度分布、较低污染物排放、高燃烧速率、高稳定性、宽燃烧极限和较大的负荷调节范围。     

混合气体燃料(含不可燃成分)燃气热力性质的计算及讨论

燃气轮机可燃用含不可燃成分的混合气体燃料,如炼厂气等,其成分有的是变化的。为此,本文从完全燃烧方程出发,直接用燃料气各成分的容积百分比,在[1]和[2]的基础上,提出了燃气热力性质、燃料系数、燃空比和用燃料系数计算燃气成分的应用计算方法,编制了计算机程序。最后以炼厂气为例作了计算和讨论。     

超声速预混可燃气流的点火与燃烧

在激波风洞一激波管组合设备上开展了碳氢燃料超声速预混可燃气流的点火与燃烧实验研究。实验结果表明:利用激波对燃料进行预热,并以高温燃气作为引导火焰,可以有效缩短汽油空气超声速可燃混气的点火延迟时间,使之缩短到 0.2 ms以下。利用纹影照片对超声速燃烧流场结构作出了分析;研究了超声速预混可燃气流的温度以及当量比对超声速燃烧流场结构、点火与火焰传播特性的影响。     

堆积床内甲烷/空气预混燃烧的理论分析

在多孔介质内组织预混燃烧,燃气与多孔介质有强烈的换热作用,燃烧过程伴随着化学反应和热输运的强烈耦合。本文以惰性氧化铝球堆积床内的甲烷/空气预混燃烧为例,提出解析模型,对燃烧过程进行理论分析,给出温度分布的解析解,发现了超绝热火焰温度燃烧现象。     

多孔介质预混燃烧中辐射属性影响的敏感性分析

建立了惰性多孔介质中预混合燃烧的数学模型,采用辐射传递的有限体积法求解固相能量方程中的辐射源项,研究多孔介质热辐射在燃烧系统中的作用,考察辐射属性(吸收系数和散射反照率)对轴向温度场和辐射热流量影响的敏感性。研究表明,辐射属性参数波动对预测结果影响明显,固体热辐射在多孔介质预混燃烧中的影响不可忽略。     

多孔介质中甲烷预混气火焰传播特性实验研究

本文对甲烷预混气在多孔介质中的火焰传播特性进行了实验研究,在开口竖直管中充填多孔介质,通过改变预混气氧含量使火焰在不同多孔介质中传播并测量火焰传播速度。预混气中氧含量最高达到29％。实验结果表明:多孔介质中甲烷可燃预混气火焰传播速度大于其层流火焰传播速度,可达到5倍以上(当量比的甲烷-空气预混气);多孔介质当量孔直径越大,或预混气层流火焰速度越高,则预混气火焰传播速度越高;多孔介质中可燃混气的火焰传播界限变小,当量孔直径大的多孔介质其界限值较大。实验结果与Babkin提出的多孔介质中的火焰传播机理相符。     

超低热值多孔介质燃烧器功率范围试验

本文对积木式结构多孔介质燃烧器稳定燃烧超低热值燃气的功率范围进行了试验研究。用甲烷和氮气配制了试验燃气的热值为1.4~3.0 MJ/m~3,在理论当量比条件下组织了预混燃烧。通过改变甲烷的流量,测试了不同燃烧强度工况下燃烧室壁面的温度分布,进而判定了各工况条件下燃烧室内是否能保持稳定燃烧。结果表明,本文所设计的燃烧器对于超低热值气体有着良好的适应性,对于热值高于1.4 MJ/m~3的气体都有一个稳定燃烧的功率区间,且稳定燃烧的功率范围随着气体热值的增大而增大。在预热温度为1200 K的前提下,燃气热值为1.4 MJ/m~3时,稳定燃烧的燃烧强度范围为30.57~107.01 kW/m~2,而当气体热值为3.0 MJ/m~3时,燃烧极限的近似范围是107.01~229.30 kW/m~2。     

考虑弥散效应的多孔介质中超绝热燃烧的数值模拟

研究多孔介质内往复流动下的超绝热燃烧。一维模型包括气体输运、多孔介质固体的辐射、导热和气固两相间的对流换热。通过数值计算研究超绝热燃烧的形成、以及弥散效应、当量比和多孔介质材料本身对超绝热燃烧特性的影响。计算结果的有效性通过实验进行了验证并取得了相同的趋势。结果表明,组分弥散效应对气体温度分布和反应热影响很小;同一工况下,不考虑气体混合物的热弥散效应,会导致过高的气体温度计算值。同时,计算结果表明小孔径的多孔介质更有利于贫可燃极限的扩展,对30 ppi的多孔介质燃烧器,得到了当量比为0．092的可燃极限。     

甲烷-空气最小点火能量预测理论模型

 最小点火能是可燃气体危险性辨识的重要参数之一。为从理论上得到混合气体的最小点火能,建立了可燃气体火花点火的物理模型,给出了通过数值模拟得到的可燃气体最小点火能量的预测方法,采用该方法得到了甲烷-空气混合气火花点火的临界温度及最小点火能量。结果表明：甲烷-空气混合气的最小点火能量与浓度呈U型关系,浓度为8.5%的预混气的最小点火能量计算值为0.39 mJ,与实验值0.4 mJ吻合较好。     

高温空气燃烧炉内湍流混合特性的数值研究

应用自行研发的三维流动、燃烧、传热和污染物NO_x湍流生成的数值模拟程序,对高温空气燃烧实验模型炉进行了湍流扩散燃烧混合特性的数值模拟.数值预报了燃烧室内气体燃料和空气的混合物分数及其湍流脉动的三维分布.数值研究结果表明:在一定的几何条件和气体动力学条件下,高温空气燃烧的湍流混合在更广泛的区域内以较小梯度的进行;混合物分数的脉动主要分布在燃烧区,这表明高温空气燃烧的火焰厚度更大,具有燃烧释热更趋均匀的特性.数值模拟结果与相关的实验结果有相同的规律.     

The influence of thermal annealing on oxygen uptake and combustion rates of a bituminous coal char

The effect of thermal annealing on the combustion reactivity of a bituminous coal char has been investigated with a focus on the role of the formation of surface oxides by oxygen chemisorption. The combined use of thermogravimetric analysis and of analysis of the off-gas during isothermal combustion of char samples enabled the determination of the rate and extent of oxygen uptake along burn-off. Combustion was carried out at temperatures between 350 and 510 °C. Char samples were prepared by controlled isothermal heat treatment of coal for different times (in the range between 1 s and 30 min) at different temperatures (in the range 900–2000 °C). Results indicate that oxygen uptake is extensive along burn off of chars prepared under mild heat treatment conditions. The maximum oxygen uptake is barely affected by the combustion temperature within the range of combustion conditions investigated. The severity of heat treatment has a pronounced effect on char combustion rate as well as on the extent and rate at which surface oxides are built up by oxygen chemisorption. Chars prepared under severe heat treatment conditions show negligible oxygen uptake and strongly reduced combustion rates. Altogether it appears that a close correlation can be established between the extent and the accessibility of active sites on the carbon surface and the combustion rate. Despite the investigation has been carried out at temperatures well below those of practical interest, results provide useful insight into the relationship existing between thermal annealing, formation of surface oxide and combustion reactivity which is relevant to the proper formulation of detailed kinetic models of char combustion.     

交变热流下燃烧室外对流传热特性数值研究

采用数值计算对水对流换热边界下天然气燃烧及外部对流传热特性进行了研究。研究了相同燃烧功率不同水入口流速下燃烧及传热特性,得到各个区域交界面温度分布及燃烧室内辐射换热与对流换热占总换热量比例。对天然气高温空气燃烧技术在工业锅炉等设备上的应用具有很好的指导意义。     

Physical aspects of polymer combustion and the inhibition mechanism

The characteristics of polymer combustion were studied. The contributions from the conductive, convective, and radiation components of the total heat flow from the flame to the polymer surface were determined. The influence of inhibitors on the rate of chemical reactions in the preignition zone and on the rate of heat and mass exchange between the flame and the burning surface was estimated. It was shown that a change in the heat balance and heat and mass exchange accompanied by changing the optical properties of the flame and burning surface has a pronounced effect on the combustion rate at the flame edge. The formation of a protective coke layer reduces heat flow to the surface of a nonreacted polymer, leading to a decrease in the rate of evolution of the volatile combustible polymer-destruction products into the gas phase. As a result, the flame temperature decreases and it is extinguished.     

甲烷微尺度催化燃烧的数值模拟

本文联合使用计算流体力学软件FLUENT和可以计算表面反应的化学反应动力学软件DETCHEM对有逆流换热的微尺度燃烧器进行了数值计算。计算中忽略空间反应。燃料-空气混合物的当量比为0.4,反应器壁面采用等温边界条件。计算结果表明,采用催化燃烧可以实现微尺度下通常情况下无法实现的甲烷稳定燃烧。通过适当设置催化表面,可以实现燃料低温、高效转变。甲烷的总转变率受流动状态、反应温度和催化表面的大小等因素的影响。     

Interpreting the influence of fuel spray impact on mixture preparation for HCCI combustion with port-fuel injection

This paper addresses the influence of fuel spray impact on fuel/air mixture for combustion in port-fuel injection engines. The experiments include time resolved measurements of surface temperature synchronized with PDA measurements of droplet dynamics at impact and were conducted to quantify the effects of interactions between successive injections on the mixture preparation for combustion in homogeneous charge compression ignition (HCCI) engines. Analysis shows that, during engine warm up, the heat transfer over the entire valve surface occurs within the vaporization-nucleate-boiling regime and the local instantaneous surface temperature correlates with the dynamics of droplets impacting at the same point. A functional relation is found for the heat transfer coefficient, which also describes other experiments reported in the literature. Similarity does not hold after the engine warms up because heat transfer and droplet vaporization at the surface are dominated by multiple interactions between droplets arisen from diverse heat transfer regimes. However, results evidence the existence of a critical surface temperature which sets a transition between overall heat transfer regimes dominated by local nucleate boiling at lower temperatures and by local intermittent transition regimes at higher temperatures. The heat transfer within the overall nucleate boiling regime is shown to be due to a thin film boiling mechanism leading to breakdown of the liquid-film at a nearly constant surface temperature, regardless of injection frequency or any other spray conditions. While at low frequencies this regime is not limited neither by the delivery of liquid to the surface, nor by the removal of vapour from the surface, at higher frequencies it is triggered by enhanced vaporization induced by piercing and mixing the liquid film. The results further evidence the important role of spray impingement for mixture preparation as required for HCCI.     

Obstacle influence on the flow structure and mass transfer in a boundary layer with ethanol combustion on horizontal surface

Experiments with ethanol combustion on horizontal surfaces revealed the most general properties of a boundary layer with chemical and phase transformations. The list of flow features includes development of large-scale structures and manifestation of volumetric forces, which impact the flow stability and heat and mass transfer. It was demonstrated that the range of velocities ensuring flame existence is wider for flow past a rib than for flow past a backward-facing step. The nature of mass transfer in a reactive flow past an obstacle is transient and remains of that kind until the flame blow-off. For a flow above a horizontal wall at Reynolds numbers Re < 5·10⁴, the intensity of mass transfer is twice higher than for combustion below the wall. When the combustion occurs below the wall, the surface temperature gradients are higher.     

External combustion of high-speed multicomponent hydrocarbon-air flow under conditions of low-temperature plasma

The stabilization of external combustion (at the plate surface) of high-speed multicomponent (air, alcohol, and propane) flows is realized experimentally. It is shown that heat fluxes during alcohol combustion rise by a factor of about 7 and during propane combustion by a factor of 15, in comparison with the heat flux from a discharge in a high-speed air flow. The electron concentration measured at a distance of 10 cm down-stream from the electrode ends is approximately 10⁹ cm^?3 in the case of the discharge in an air flow, whereas during alcohol combustion it attains 2 × 10¹¹ cm^?3 and during propane combustion it is 3 × 10¹¹ cm^?3. The flame temperature in the area of the discharge existence varies from 2000 up to 2500 K and outside the discharge at the distance of z = 20 cm from electrodes is 1800 K, gradually decreasing downstream. It is shown that the combined discharge in the subsonic flow allows for complete combustion of liquid and gaseous hydrocarbons. The completeness of combustion in supersonic flows attains 95%, depending on the flow velocity.     

Experimental and numerical investigation of temperature fluctuations in the near-wall region of an optical reciprocating engine

Accurate prediction of in-cylinder heat transfer processes within internal combustion engines (ICEs) requires a comprehensive understanding of the boundary layer effects in the near-wall region (NWR). This study investigates near-wall temperature fluctuations of an optical reciprocating engine using a combined approach of planar laser-induced fluorescence (PLIF) thermometry and numerical conjugate heat transfer modeling. Single-line excitation of toluene and subsequent one-color emission detection is employed for PLIF thermometry, while large-eddy simulations (LES) using commercial CFD software (CONVERGE v2.4.18) is utilized for modeling. The PLIF signal is calibrated to predicted in-cylinder temperatures from a GT-POWER simulation, and precision uncertainty of temperature is found to be ±1.5 K within the calibration region. Near-wall temperature fluctuations are determined about the multi-cycle mean, and the development of thermal stratification is captured in the NWR under motored and fired conditions during the compression stroke. Regions of the largest cycle-to-cycle temperature fluctuations are identified closer to the in-cylinder head surface indicating the unsteadiness of the thermal boundary layer. Analysis includes an assessment of cyclic variability of near-wall temperature fluctuation, and the effects of compression on temperature fluctuations. Additionally, thermal stratification is found to be similar under motored and fired conditions before ignition timing. Lastly, spatial correlation analysis of temperature fluctuations is performed in the wall-normal direction, and it reveals higher correlations under fired conditions. Spatial correlations experience an initial drop outside of the buffer layer in the NWR, and the location of the drop is well captured in the simulations. Analysis of fluctuating temperatures needs to be extended to fluctuations about the spatial average temperature which directly affects the spatial thermal gradients relevant to engine heat transfer.