Boiling at sub-atmospheric conditions with enhanced structures

Liquid cooling with phase change is a very attractive option for thermal management of electronics because it achieves very high heat transfer coefficients compared to single phase liquid cooling. Phase change liquid cooling can be implemented in a thermosyphon loop, where heat is transferred from the heated surface to the evaporator and rejected to the ambient from the condenser. Optimized design of the evaporator requires a fundamental understanding of boiling in the evaporator. Past studies with dielectric working fluids have shown the importance of using boiling enhancement structures in lowering incipience overshoot, increasing heat flux and reducing evaporator volume. Water possesses superior thermal properties than dielectric liquids, but there is a lack in the understanding of boiling of water from enhancement structures. Since in silicon devices, a maximum surface temperature of 85/spl deg/C is typically allowed, boiling of water at sub-atmospheric pressures for lowering the saturation temperature and thus aiding in early initiation of nucleate boiling is required. The present study aims to provide a detailed understanding of the effects of boiling enhancement structure and sub-atmospheric pressures on the boiling of water and investigate their effectiveness in electronics cooling applications. The effects of system pressures and enhancement structure geometry on the boiling heat transfer are investigated. Experiments were performed at three different pressures, 9.7, 15 and 21 kPa using a stacked enhancement structure with four different geometries (1, 2, 4 and 6 layers). The results are compared with sub-atmospheric pressure boiling from a plain surface.