【作者】 何沛；

【导师】 周俊虎； 岑可法； 王智化；

【作者基本信息】 浙江大学 ， 工程热物理， 2010， 博士

【摘要】 本文采用直接数值模拟方法对无反应空气射流和合成气射流火焰进行了研究。具体研究内容包括三维燃烧直接数值模拟的数值方法、空气射流及合成气射流火焰。模拟求解的控制方程为无量纲化完全可压缩的Navier-Stokes方程,采用具有四阶精度的8阶中心有限差分的空间求导格式和五步四阶Runge-Kutta时间步进格式,同时添加了10阶显式中心差分过滤格式,该数值方法表现出了良好的稳定性。推导出了适用于三维燃烧情况的三维特征无反射边界条件,同时还采用了缓冲区低通过滤的缓冲区边界条件。此外确定了三维程序并行计算方法,结合所采用的显式差分方法取得了令人满意的并行加速比。首先对一典型亚音速圆形空气射流进行了直接数值模拟。计算得到了平均速度、雷诺应力、二阶和三阶速度相关及湍流能谱等湍流统计量,通过同实验数据的对比发现DNS结果同实验结果符合非常好。对涡量、速度梯度进行了计算以揭示湍流结构的演化过程,给出了射流上游段涡环形状的拟序结构在向下游运动的过程中逐渐破碎的过程的图像。计算出射流轴线平均速度衰减率系数Bu为5.5,这同其他研究者的数据非常接近。得到表征射流平均速度径向分布曲线形状的常数Ku为75.4。统计计算出了速度相关系数,发现在轴向区域x=17d-20d湍流统计量呈现出较好的自相似性,且与实验数据符合很好。最后计算了射流轴线上轴向速度和径向速度的一维湍流能谱,结果显示在湍流惯性子区能谱衰减符合k-5/3衰减律。在射流轴线上x=19d附近轴线速度和径向速度的一维能谱因为湍流的各项异性而呈现出一定差别。其次模拟了木屑合成气三维射流火焰燃烧现象,在模拟中采用合成气的真实组成,以对合成气射流火焰结构、物质分布特别是污染物NO的分布情况进行更加准确的研究,同时采用包含NO反应的多步化学反应机理来模拟反应过程。采用激光诱导荧光技术对合成气射流火焰进行了测量,得到了火焰中OH的瞬时分布图像和平均浓度的径向分布。模拟得到了燃烧场中组分质量分数、温度、混合分数、化学反应热释放率和标量耗散率等的分布图像,发现模拟得到的OH瞬时图像表现出了同PLIF测量结果相同的褶皱形状。然后统计了得到了各物理量在混合分数空间的PDF分布图和条件平均值分曲线,发现OH的最大值出现在最大热释放率的左侧,也即偏向高温的贫燃料侧,这种OH和热释放率峰值的偏离说明对于部分预混射流火焰来说,采用OH作为火焰锋面的指示量并不完全精确。而后对在射流不同位置标量耗散率和化学反应速率间的关系进行了研究,发现在射流初始段标量耗散率越大的地方化学反应越快,在射流下游这种现象逐渐消失。最后对射流火焰的着火过程进行了分析,较大的标量耗散率推动燃料和氧化剂的混合,且在混合分数空间热释放率不断增大最终全面着火现象发生。更多还原

【Abstract】 Round jet flow and syngas jet flame were studied via direct numerical simulation technique. The main research subject were the numerical techniques for there dimensional direct numerical simulation of combustion, air jet flow and syngas jet flame.The fully compressible non-dimensional Navier-Stokes conservation equations were solved here. For the spatial discretization eighth-order explicit central finite difference scheme was adopted and fourth-order five stage Runge-Kutta scheme was used for temporal marching. Besides, tenth-order explicit central finite difference scheme was used to eliminate the unwanted oscillation with high wave number. All these numerical techniques perform very well and have strong stability. Non-reflecting characteristic boundary conditions for three dimensional combustion were obtained. Exit zone where low order filtering was implemented near computational domain boundary were used. Proper parallel computational methods were used combined with the explicit difference schemes, and high speed-up was obtained.First we do a DNS for a subsonic round turbulent air jet in a coflow with laminar flow. The jet mean velocity, Reynolds stresses, second and third order velocity moments and turbulent energy spectral were obtained. These simulation results agree very well with experimental data. Vortex rings were found in the upstream of the jet, further downstream these large scale coherent structure started to break down and merge with each other. Image of this evolution process were showed by vortex magnitude and velocity gradient. The decay rate Bu of jet centerline mean velocity is found to be 5.5, which was very close tR RtKHV rHVu(?). u, wKLF(?)tVSrRILOH of the mean velocity along radial direction, was found to be 75.4. Then we calculate the second and third order velocity moments and found that in the axial region of x=17d-20d the jet statistics exhibit self-similar behavior and agree well with published experimental data in fully self-similar region. The one-dimensional energy spectra of longitudinal and lateral velocity are also presented. The inertial region where the spectra decays according to the k-5/3 is observed. At around x=19d on the jet centerline, their was some difference between autospectra of longitudinal and of lateral velocity due to anisotropy.Then we do a direct simulation of wood residue syngas jet flame in a hot coflow environment. In order to investigate the jet flame structure, species distribution and the formation of NO, wood residue syngas with real composition was considered. The reaction mechanism is a multi-step mechanism which include NO reaction. PLIF was used to measure the OH image and species mean concentration along radial direction. Image of the species mass fraction, temperature, mixture fraction, heat release rate and scalar dissipation rate were obtained. The computed OH image was compared with the OH-PLIF image and the flame front structure was very similar. Many physical variables scatter plot in the mixture fraction space were plotted and also the conditional mean value on the mixture fraction. It is found that the peak of OH curve appears on the left side of the maximum heat release rate, which means on the lean side with high temperature. The discrepancy between peak position of OH curve and heat release rate for this partial premixed jet flame indicates that it is not very precise to use OH as the flame front indicator. Then the relationship between scalar dissipation rate and chemical reaction rate was analyzed and it is found that at the upstream position of the jet high heat release rate appears in the place where the scalar dissipation rate is large. Further downstream this phenomenon disappear gradually. At the end we do analysis of the ignition process of the jet flame. Large scalar dissipation promote the mixing between fuel and oxidizer and the heat release rate grow up in the mixture space, and at last ignition take place.更多还原