The heat transfer and flow field characteristics of a periodical turbulent jet impinging on a flat plate are experimental studied and the results as well as the mechanism of heat transfer enhancement are discussed. The periodical jets are produced by a special mass flow rate controller. The results of two typical periodical jets (i. e. , sinusoidal and rectangular jets) are presented and the signal frequency is performed as a parameter in the experiments. The experimental results show that the heat transfer performances of two periodical jets with same average flow rate are obviously different. The rectangular jet, which has a step change in signal function itself, is much more effective for heat transfer improvement than the sinusoidal impinging jet. A higher frequency is promising in heat transfer for both signals. Particle Image Velocimetry (PIV) is used to measure the phase-locked flow field of the periodically impinging jet. For the higher frequency case of a rectangular impinging jet which has a step change in signal itself, the plate surface is exposed to both a strong impinging jet and a strong vortex during a periodical cycle and thereby it improves the convection process. In the flow field of a sinusoidal impinging jet, the phase-averaged velocity is lower than the case in a rectangular jet. The entrainment to the ambient fluid and the impingement as well as the vortex spreading are not so strong as that of a rectangular jet, thus it leads to only a slight heat transfer augmentation even at a higher frequency.%实验研究了周期性变化的湍流射流冲击平板时的传热和流场特性,对实验现象和射流冲击强化传热机理进行了探索和分析.利用一个特殊的质量流量控制装置产生周期性的冲击射流,采用典型的正弦和矩形变化的射流,在不同频率下进行实验.研究表明,两种周期性射流冲击传热性能有很大差异,矩形冲击射流的传热性能优于正弦冲击射流,提高频率有助于射流冲击传热的强化.用粒子图像测速(PIV)技术对流场进行锁相实验测量,研究表明,在较高频率下,阶跃变化的矩形射流冲击时整个变化周期内平板表面处于强烈的冲击和涡流不断的产生和传播状态,从而能有效地强化对流换热过程.平缓连续变化的正弦射流的锁相平均速度场低于矩形射流,卷吸和冲击作用、涡流的形成和传播过程均没有矩形射流冲击下那么强烈,从而导致换热强化效果相对较弱.
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