Heater size and gravity level effects on nucleate pool boiling heat transfer under controlled wall heat flux condition

With the ability of dissipating large amount of latent heat in nearly isothermal process, pool boiling heat transfer is an efficient solution for cooling problem of different sized electronic devices in different gravities. For the practical heat dissipation of electronic device usually occurs at controlled wall heat flux condition, it is essential to reveal pool boiling heat transfer characteristics at different sizes and gravities under controlled wall heat flux condition. In this paper, boiling heat transfer mechanisms on two sized smooth silicon chips (chip S 1 x 1 and chip S 2 x 2) in ground gravity and microgravity were studied. It was found that the gravity scaling parameters developed under controlled wall temperature condition cannot be applicable to the controlled wall heat flux condition. Under controlled wall heat flux condition, the microgravity boiling heat transfer performance on chip S 1 x 1 is in surface tension dominated boiling (SDB) regime in the whole heat flux range. While for chip S 2 x 2, it is in SDB regime when heat flux is smaller than 6.44 W.cm(-2), but enters into buoyancy dominated boiling (BDB) regime when heat flux is larger than 6.44 W.cm(-2). The critical point of the two boiling mechanisms occurs when the threshold gravitational acceleration equals to the experimental microgravity. In BDB regime, the power law coefficient reflecting the gravity effect on nucleate boiling heat transfer performance (m(BDB)) for controlled wall temperature condition is larger than that for controlled wall heat flux condition, indicating a greater impact of gravity level under controlled wall temperature condition. Moreover, m(BDB) for controlled wall temperature condition is affected only by non-dimensional wall temperature T*. While for controlled wall heat flux condition, m(BDB) increases with increasing T* but decreases with increase in heater size, and heater size has a greater in-fluence on m(BDB). Based on the available data, the gravity scaling parameters for controlled wall heat flux boiling were developed, which can accurately predict the microgravity boiling heat transfer performance on two sized chips.