An Experimental Study on New Technology of Dynamic Cooling with Jet Impingement

为了改善射流冲击冷却装置的性能，研究了大尺度纵向涡和射流产生的自激振荡对冲击换热的影响。重点研究了以下三个方面：(1)喷嘴的几何形状；(2)自激振荡的频率；(3)旋涡结构的模式。此外，还对射流间的相互作用进行了研究，并发展了与换热系数和流速分布相关的测量技术。LDV和PIV系统分别用于测量局部速度波动和横截面速度矢量分布。利用热致变色液晶片测量了换热靶壁的温度，并用CCD摄像机拍摄了其彩色图像。采用神经网络方法将彩色图像转换为墙体温度分布。在射流倾斜进入主流的情况下，在激光片照射的截面上可以清晰地看到一对大尺度的对旋涡，这是在类似边界条件下的数值预测。将常规垂直冲击射流与现有的斜向冲击射流进行了比较，得出了斜向冲击射流优于垂直冲击射流的结论。在自激振荡射流的情况下，在相对较小的撞击距离条件下，滞止区内的努塞尔数分布模式发生了很大的变化。只需要对喷嘴的几何尺寸进行微小的改变，就可以产生可听得见的振荡，而不会给射流带来任何额外的能量。因此，激振对射流冲击换热有很大的影响。与受振荡影响最大的情况相对应，基于振荡频率的斯特劳哈尔数约为0.3。

Effectiveness of large-scale longitudinal vortices and self-excited oscillation generated by a jet on impingement heat transfer were investigated in order to improve the performance of jet impingement cooling devices. Attention was paid to the following three items; (1) geometrical configurations of jet nozzle, (2) frequencies of the self-excited oscillation, and (3) patterns of vortical structures. The interaction of jets were also investigated together with the development of measuring techniques related to the heat transfer coefficients and flow velocity profiles. LDV and PIV systems were used to measure local velocity fluctuations and cross-sectional velocity vector distributions, respectively. The temperatures of the heat transfer target wall were measured using thermochromic liquid crystal sheets, the color images of which were taken with a CCD camera. Neural network method was applied to transform the color images into the wall temperature distributions. In the case of the jet obliquely discharged into a main flow, a pair of large-scale counter-rotating vortices was clearly visualized in the cross-section illuminated by a laser sheet, which was numerically predicted under similar boundary conditions. Comparison between the conventional vertical impinging jet and the present oblique jet reveals the superiority of the oblique jet to the vertical one. In the case of the self-excited oscillation jet, the patterns of Nusselt number distributions in the stagnation region drastically change under the conditions of relatively small impingement distance. Only small change of the geometric scales of the jet nozzle was required to generate the audible oscillation without any extra energy to the jet. Thus, the excited oscillation was found to have a great influence on the jet impingement heat transfer. Corresponding to the cases most influenced by the oscillation, Strouhal number based on the oscillation frequency was around 0.3.