Fluid Flow and Heat Transfer of Mixed Convection and Its Application to Thin Film Formation by CVD

实验研究了均匀向下强迫对流条件下，向上水平加热板上混合对流的流动与换热。实验是在空气中进行的，试验板宽为50 mm和100 mm，加热方式为均匀热流密度。雷诺数和修正瑞利数分别为：Rew=37-2000和Raw^*=6.2×104-5.3x×107。结果表明，当热流密度较小时，板材中心到板边的流动呈现层流楔形流动，随着热流密度的增加，板材中心的流动方向由边缘转向中心，当热流密度足够大时，板材中心的流动以浮力羽流的形式从板材中心分离出来。还测量了板材的局部换热系数。结果表明，定速强迫流下的系数随热流密度的增加而单调增大。用无量纲参数(grw^*/d/num Rew^2.5)可以很好地预测板块的总体Nusselt数Nun，其中grw^*代表Grashof数。此外，该参数很好地划分了强迫对流、混合对流和自然对流的区域，混合对流区域被确定为：0.3&lt；(grw^*/num Rew^&lt；2.5&gt；)&lt；15

Fluid flow and heat transfer of mixed convection over upward-facing horizontal heated plates placed in a uniform downward flow of forced convection were investigated experimentally. The experiments were carried out with air and with the test plates of 50 and 100mm wide, which were heated with uniform heat flux. The Reynolds and modified Rayleigh numbers ranged as: Rew =37-2000 and Raw^*=6.2x10^4-5.3x×10^7.The flow fields over heated plates were visualized with smoke. The results showed that the laminar wedge flow from the center to the edges of the plate appears when the heat fluxes are small, while that the flow turns its direction from the edges to the center with increasing the heat flux, and, finally, the flow detaches from the center of the plate as a buoyant plume when the heat fluxes are high enough. The local heat transfer coefficients from the plates were also measured. The results showed that the coefficients under the fixed velocity of the forced flow are increased monotonously with the heat fluxes. It was also found that the overall Nusselt numbers of the plates, Nun, were predicted well with the non-dimensional parameter, (GrW^*/d/Num Rew^2.5), where Grw^* stands for the Grashof numbers. Moreover, the parameter well classify the regions of the forced, mixed and natural convection, and the mixed convection region is determined as : 0.3< (Grw^*/Num Rew^<2.5>) <15