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传热的影响。会议注意了下列三个项目：(1)喷嘴的几何构型；(2)自激振荡的频率；(3)涡旋结构的模式。射流的相互作用也进行了研究，并发展了与传热系数和流速剖面相关的测量技术。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传热靶壁的温度，并利用CCD相机拍摄了传热靶壁的彩色图像。采用神经网络方法将彩色图像转化为壁面温度分布。在射流斜向进入主流的情况下，在激光照射的截面上可以清晰地看到一对大尺度的反向旋转涡，并在类似的边界条件下进行了数值预测。通过对传统垂直冲击射流与斜向射流的比较，揭示了斜向射流相对于垂直冲击射流的优越性。对于自激振荡射流，在较小的撞击距离条件下，滞止区努塞尔数分布模式发生了剧烈变化。只需要喷嘴几何尺度的微小变化就能产生可听振荡，而不需要给射流带来额外的能量。由此可见，激发态振荡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Strouhal数在0.3左右。

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.