ABSTRACT

The local heat transfer characteristics of circular multiple air jet impingement on semicircular concave surface were studied experimentally. For constant Reynolds number (Re = 9,000), the effect of jet to concave surface spacing (z/d = 1, 4.2, 6), ratio of curvature (D/d = 4.28, 6, 8.6), jet-to-jet spacing (s/d = 2.4, 4, 5.6), and number of jets were considered. The local heat transfer characteristics were determined by using infrared thermal camera and thin foil method. Lower curvature value ratio (D/d = 4.28) resulted higher average heat transfer coefficient due to broader spread of heat transfer distribution, whereas higher curvature ratio (D/d = 8.6) resulted in lower average heat transfer coefficient due to steeper heat transfer distribution in the stagnation area. At lower D/d, z/d, and s/d with seven jet configuration, the heat transfer coefficients were found effective as compared to other configurations due to broad heat transfer distribution and less interaction between the jets. At given D/d and s/d configuration, the stagnation Nu at θ = 0° do not coincide with jet hole center axis and this shift is more pronounced at higher z/d value. This could be due to additional push before jet impinging on concave surface. The secondary peaks were observed between two neighboring stagnation regions in Nu distribution along the center longitudinal line for lower z/d and higher s/d values. This resulted due to upwash that was obtained from jet interactions related to collisions with the wall jets. The correlations of stagnation line average Nu and overall average Nu were obtained. The coefficient of variance of local heat transfer coefficient for each configuration was determined, and correlation was proposed to analyze the degree of non-uniformity.

摘要

实验研究了圆形多重空气射流冲击半圆凹面的局部传热特性。对于常数雷诺数 ( Re  = 9,000)，射流对凹面间距的影响 ( z / d  = 1, 4.2, 6)，曲率比 ( D / d  = 4.28, 6, 8.6)，射流对射流考虑了间距 ( s / d  = 2.4、4、5.6) 和射流数量。采用红外热像仪和薄箔法测定局部传热特性。下曲率值比 ( D / d = 4.28) 由于传热分布更广泛而导致平均传热系数更高，而较高的曲率比 ( D / d  = 8.6) 由于停滞区的传热分布更陡峭而导致平均传热系数较低。在较低的D / d、z / d和s / d下，七射流配置，传热系数与其他配置相比是有效的，因为传热分布广泛且射流之间的相互作用较少。在给定的D / d和s / d配置中， θ = 0°处的停滞 Nu 与射流孔中心轴不重合，并且这种偏移在较高的z / d值时更为明显。这可能是由于射流撞击凹面之前的额外推动。对于较低的z / d和较高的s / d，在沿中心纵向线的 Nu 分布中的两个相邻停滞区域之间观察到次级峰值。这是由于从与壁射流碰撞相关的射流相互作用中获得的上洗。获得了停滞线平均Nu和总体平均Nu的相关性。确定每种配置的局部传热系数的方差系数，并提出相关性来分析不均匀程度。