Study of Non-equilibrium Process and Turbulence Generation in the Diffusive Combustion

本研究旨在阐明扩散燃烧中的非平衡过程和湍流的产生。本文的研究内容概括如下：1.采用数值模拟的方法，对密度和压力梯度之间的相互作用引起的流体运动进行了数值模拟。当涡度产生比粘性耗散更强时，火焰锋面发生形变和拉伸。当压力梯度或密度梯度较大时，涡量的产生速度较快，促进了火焰锋面的变形，加速了热的释放。基于线性化稳定性理论，研究了射流扩散火焰的流动不稳定性。对具有密度变化的二维平行流动的扰动方程进行了数值求解。结果表明，射流边界处热层的存在使流动更加稳定，提高了湍流既不增长也不衰减的临界波数和临界雷诺数。临界波数的增加主要是由于流动与密度梯度的相互作用，而粘性耗散的局部变化提高了临界雷诺数，从而导致更宽的稳定极限。应用粒子图像测速仪(PIV)测量了甲烷喷流火焰和冷甲烷喷流中的速度矢量。通过脉动速度和涡量的分布，阐明了湍流的特征。结果表明，火焰锋面内部产生的涡流将燃烧产物卷吸到燃料流中，而湍流涡流的消散是通过局部放热的分层作用实现的。

This study aims at the clarification of the non-equilibrium process and turbulence generation in the diffusive combustion. The contents of the present study are summarized as follows :1. Numerical simulation were performed to predict fluid motions induced by interactions between density and pressure gradients due to the presence of a vortex string situated in a deviated position from the jet flame axis. Deformation and stretching of the flame front take place once the vorticity production becomes stronger than the dissipation due to viscosity. When either the pressure gradient or the density gradient is high, the vorticity production proceeds at a high rate, which promotes the deformation of flame front and accelerates the heat release.2. The flow instability of a jet diffusion flame was investigated based on a linearized stability theory. The equation of disturbance was solved numerically for a two-dimensional parallel flow with density variation. Results show that the existence of a hot layr at the jet boundary makes the flow more stable, raising the critical wave number and critical Reynolds number at which turbulence neither grows nor declines. The increase of the critical wave number is due mainly to the interaction between the flow and density gradient, and the local change of the viscous dissipation raises the critical Reynolds number thereby leading to a wider stability limit.3. Particle image velocimetry (PIV) for measuring velocity vectors was applied to the flow in a methane jet flame and a cold methane jet. Characteristics of turbulent eddies were clarified from the distribution of fluctuating velocity and vorticity. Results show that eddies generated inside flame front entrain the combustion products into the fuel stream, at the same time the dissipation of turbulence eddies takes place by the laminarization due to local heat-release.