自激励冲击射流流动与传热的机理研究

The self -excited precessing jet and self-sustained oscillation caused by Coanda effect could be formed in some kind of configuration even when the initial conditions are steady and the boundary conditions are quite symmetry. This is a nonlinear phenomena caused by internal inherently instability of the flow field. The researches to Coanda effect and self-sustained oscillation in the flow field of self-excited precessing jet are going to be carried out by experimental studies and numerical simulations in this project. The basic conditions that nonlinear phenomena appeared need to be found out and how the oscillation formed and developed need to be studied. The influence of such oscillation to the heat transfer also need to be explored. The hot-wire anemometer is used to test the instantaneous flow field. The PDA and PIV are used to watch the change of the whole flow field. The heat transfer characteristics will be tested and analyzed in combine with the flow field distributions. Numerical simulation is a good method to study the oscillation and nonlinear characteristics. However, the accuracy of the algorithm and the reliability of simulation model need to be tested by experimental results to guarantee the execution of the simulations. The heat transfer and flow characteristics of self -excited impinging jet beyond the experimental conditions could be studied by validated numerical model. The appearance and development of the bifurcation and oscillation in self-excited impinging jet and its influence to heat transfer characristics could also be numerically simulated in detail with different inlet Re number. This will help to seek the physical mechanism of nonlinear phenomena in the flow field and heat transfer of self-excited impinging jet.

初始及边界条件完全均匀对称的射流，在一定结构下由于柯恩达效应会发生流体的自维持振荡，形成自激励旋进流动，这是一种由系统内部因素引起的非线性现象。本项目通过实验与数值模拟相结合来研究自激励冲击射流中的柯恩达效应和自维持振荡，着重研究非线性特征出现的条件，形成和发展的过程及其对流动和传热的影响。实验采用热线风速仪对流场进行瞬时速度测量，用PDA和PIV对流场变化进行观测，并结合射流冲击传热特性进行相关分析；数值模拟是研究非线性问题的有效方法，但算法精度、数值模型的可靠性是研究工作进行的重要保证。经过实验验证的数值模型可以研究多种参数下的自激励冲击射流流动与传热的特点，尤其是入口雷诺数从小到大的变化过程中也就是流动从层流向湍流发展过程中分岔、振荡的产生、发展及其对流动和传热的影响，探索射流流动与传热中非线性现象的物理机理。

在一定的几何和结构参数下产生的自激励冲击射流，由于射流剪切层的变化会促使不稳定现象增长，数值模拟显示在进口雷诺数小于100的情况下就会出现流动的静态分岔现象，并随着进口雷诺数的增加，依次出现动态分岔、自维持振荡以及无序冲击状态。这种由射流自激励形成的自维持振荡是流体动力和剪切层不稳定形成的非线性现象，其振荡频率是由结构的几何和流动参数决定的，在本课题的研究范围内，喷嘴管内流动达到充分发展更易发生自激励现象，喷嘴膨胀部分的限制壁面对自激励的形成起到一定的反射作用，因此膨胀比大于10以后自激励作用将减弱，增加低频的主动激励有助于自激励流动的维持。自激励冲击射流的湍流强度显著增强，但指示自激励冲击射流振荡强度的St数变化不大，数值也较小，其冲击换热效果不如自由射流。高频的主动激励在信号变化率大、线形阵列射流带相差以及被冲击表面的粗糙程度提高等情况下可以强化换热，但实际影响自激励冲击射流换热机理的因素较多，从几何结构到流动状态都会有不同程度的影响。主动激励的非稳态管内流动在层流流动状态下很难强化换热，但是从层流到湍流的转换时容易激发强化换热，频率对管内流动的换热影响不如振幅的影响大。