Turbulent Heat and Mass Transfer, and Combustion Control of a Non-Premixed Turbulent Flame Associated with Counter Gradient Diffusion Phenomena

研究结果总结如下：（1）“弯曲矩形管道中形成的非预混湍流火焰的传热特性”：实验研究了弯曲矩形管道（180°弯曲）中形成的非预混湍流火焰的传热特性。采用LDV和细丝热电偶相结合的技术测量了速度场和温度场的关键湍流量，如雷诺应力分量和湍流热通量。这些测量提供了发生反梯度传热的异常现象的直接证据，这可以归因于弯曲管道的径向方向上的强压力梯度的存在。实验还表明，这种“反梯度”扩散的起始区与强“梯度”扩散区相邻。对湍流热通量产生项的定量评价表明，压力梯度对湍流热通量的梯度有促进作用 关于我们 在弯曲管道火焰的内壁（低压）侧引起NT扩散，在外壁（高压）侧引起逆梯度扩散。用于可视化密度场的纹影摄影显示了火焰高压侧和低压侧之间燃烧气体包的完全不同的行为。造成反梯度扩散的基本机制可以用火焰高压侧高温（低密度）气体团的独特运动来解释，高温流体运动倾向于优先被施加在流场上的压力梯度阻尼。反梯度扩散现象的出现当然会导致大多数传统湍流模型所依赖的“梯度扩散假设”的崩溃。在这样一个领域中，传热和传质过程之间的类比，这几乎总是在正常的湍流被动标量输运，可以消失。(2)“弯曲通道中非预混火焰的湍流统计”：实验研究了弯曲矩形通道中非预混湍流火焰的统计特性。两种类型的火焰，一个是在通道的中心（火焰1）和其他在内壁附近（火焰2）形成的使用的速度和温度的同时测量技术进行了测量。在这两种火焰中，反梯度扩散（CGD）出现在火焰的外壁侧的传热。火焰2显示出比火焰1更明显的CGD，并且其形状非常细长。与火焰1不同，火焰2在火焰的内壁侧不与强的“梯度扩散”热传递相关联，并且这可以加强火焰2的CGD。湍流热通量的统计分析揭示了CGD的内部结构，其中向外壁侧移动的高温流体包裹被强烈减速（火焰1），或者几乎没有扩散地返回（火焰2）。这是强压力梯度下湍流火焰的基本特征。少

The research results are summarized as follows:(1) "Heat transfer characteristics of a non-premised turbulent flame formed in a curved rectangular duct": Heat transfer characteristics of a non-premixed turbulent flame formed in a curved rectangular duct (180° bend) were investigated experimentally. Key turbulence quantities of velocity and thermal fields such as Reynolds stress components and turbulent heat fluxes were measured using a combined LDV and fine-wire thermocouple technique. These measurements provided direct evidence of the occurrence of the anomalous phenomenon of counter-gradient heat transfer, which can be ascribed to the presence of a strong pressure-gradient in the radial direction of the curved duct. The experiment also revealed that the Onset region of this "counter-gradient" diffusion was adjacent to the strong "gradient" diffusion region. The quantitative appraisal of the production terms for the turbulent heat flux showed that the pressure gradient promoted gradie … More nt diffusion on the inner-wall (low-pressure) side of the curved-duct flame and caused counter-gradient diffusion on the outer-wall (high-pressure) side. The schlieren photography for visualizing the density field showed a totally different behavior of the burned gas parcels between the high- and low-pressure sides of the flame. The essential mechanism causing the counter-gradient diffusion can be explained by the unique motion of the high-temperature (low-density) gas parcel on the high-pressure side of the flame.High-temperature fluid motions tend to be preferentially damped by the pressure gradient imposed on the flow field. The occurrence of the counter-gradient diffusion phenomenon will of course lead to the collapse of the "gradient-diffusion hypothesis," on which most conventional turbulence models rely. In such a field, the analogy between heat and mass transfer processes, which holds almost always in normal turbulent passive-scalar transport, can disappear.(2) "Turbulence statistics of a non-premixed flame formed in a curved channel": Statistical characteristics of a non-premixed turbulent flame formed in a curved rectangular channel were investigated experimentally. Two types of flame one is formed in the center of the channel (Flame 1) and the other in the vicinity of the inner-wall (Flame 2) were measured using a simultaneous measurement technique of velocity and temperature. In both flames, counter-gradient diffusion (CGD) emerged in heat transfer at the outer-wall side of the flames. Flame 2 showed more distinct CGD than Flame 1, and its shape was very elongated. Unlike Flame 1, Flame 2 was not associated with strong "gradient-diffusion" heat transfer at the inner-wall side of the flame, and this may strengthen the CGD of Flame 2. Statistical analysis of the turbulent heat-flux revealed the internal structure of the CGD, where high-temperature fluid parcels moving toward the outer-wall side are strongly decelerated (Flame 1), or return with little diffusion (Flame 2). These are the essential features characterizing the turbulent flames under the strong pressure-gradient. Less