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)“弯曲矩形管道内非预混湍流火焰的换热特性”：对弯曲矩形管道(180°弯管)中非预混湍流火焰的换热特性进行了实验研究。采用激光测速和细丝热电偶相结合的方法，测量了雷诺应力分量和湍流热流密度等速度场和热场的关键湍流量。这些测量提供了发生逆梯度换热异常现象的直接证据，这可归因于弯曲管径向存在强烈的压力梯度。实验还发现，这种“逆梯度”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，气压梯度促进了梯度…在弯管火焰的内壁(低压)侧，NT扩散较多，而在外壁(高压)侧，则引起逆梯度扩散。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的气团的完全不同的行为。高温(低密度)气团在火焰高压侧的独特运动可以解释引起逆梯度扩散的基本机理，高温流体的运动倾向于优先被施加在流场上的压力梯度所抑制。逆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“梯度扩散假说”的崩溃。在这样的场中，一般湍流被动标量传输中几乎总是存在的传热和传质过程之间的相似性可以消失。(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。采用速度和温度同时测量技术，测量了通道中心形成的火焰(火焰1)和内壁附近形成的火焰(火焰2)。在两种火焰中，火焰外壁的换热都出现了逆梯度扩散。火焰2比火焰1显示出更明显的CGD，其形状非常细长。与火焰1不同的是，火焰2与火焰内壁的强“梯度扩散”热传递无关，这可能会增强火焰2的CGD。对湍流热通量的统计分析揭示了CGD的内部结构，其中向外壁移动的高温流体被强烈减速(火焰1)，或返回时扩散很小(火焰2)。这些都是强压力梯度下湍流火焰的基本特征。较少

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