Influence of smooth heater size on critical heat flux and heat transfer coefficient of saturated pool boiling heat transfer

In order to study the influence of smooth heater size on the performance of pool boiling heat transfer, saturated boiling experiments of FC-72 were conducted on eight sized smooth silicon chips (the test surfaces are denoted as S5 similar to S30, and their dimensionless dimensions are in the range of L-h/L-c = 7 similar to 42). The bubble dynamics during boiling were observed with a high-speed camera. The results indicated that in natural convection regime, the heat transfer coefficient decreases with increasing heater size at a given heat flux. Besides, the larger surfaces enter into the nucleate boiling regime earlier. In a fully developed nucleate boiling regime, S5 surface displays the optimal heat transfer performance. The reality is that no vapor columns are produced even in the high heat flux range due to its small bubble departure diameter and high bubble departure frequency. For S8 similar to S15 surfaces, the wall temperatures increase continuously with the increase of heat flux, while S20 similar to S30 surfaces present a trend of first keeping constant and then increasing. The active nucleation site densities on S15 similar to S30 surfaces follow the law of N-a = 0.45q(2) in the whole nucleate boiling regime. By contrast, for S5 similar to S12 surfaces, the nucleation site densities abide by this law only in a fully developed nucleate boiling regime. In addition, the departure diameter and departure frequency of bubbles increase with the increase of heat flux for all surfaces. It is observed that with the increase of smooth heater size, CHF presents a changing trend of first increasing (in range of L-h <= lambda(D), lambda(D) is the most dangerous Taylor wavelength), then decreasing (in range of lambda(D) < L-h <= 3 lambda(D)) and finally leveling off (in range of L-h > 3 lambda(D)). Accordingly, the CHF value maximizes at the heater size L-h = lambda(D). In the segment of decreased CHF, the decline rate of CHF in range of lambda(D) < L-h <= 2 lambda(D) is larger than that in range of 2 lambda(D) < L-h <= 3 lambda(D). Meanwhile, the influencing mechanisms of heater size on CHF were explained by consideration into bubble patterns and behavior.