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

为了提高射流冲击冷却装置的性能，研究了射流产生的大尺度纵向涡和自激振荡对冲击换热的影响。着重研究了三个问题：（1）喷嘴的几何形状;（2）自激振荡的频率;（3）涡结构的形态。射流的相互作用也进行了研究，与相关的传热系数和流速分布的测量技术的发展。分别采用LDV和PIV系统测量了局部速度脉动和截面速度矢量分布。传热目标壁的温度使用热致变色液晶片测量，其彩色图像由CCD摄像机拍摄。采用神经网络方法将彩色图像转化为壁温分布。在射流斜喷入主流的情况下，在激光片光照射的截面上可以清楚地看到一对大尺度反向旋转的涡，这是在类似的边界条件下数值预测的。通过对传统的垂直冲击射流和斜射流的比较，揭示了斜射流相对于垂直冲击射流的优越性。在自激振荡射流情况下，在相对较小的碰撞距离条件下，滞止区的努塞尔数分布模式发生了急剧变化。只需改变喷嘴的几何尺寸，就能产生可闻的振动，而不需要对射流施加任何额外的能量。因此，激励振荡被发现有很大的影响，射流冲击换热。对应于受振荡影响最大的情况，基于振荡频率的Strouhal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.