A free-particles-based technique for boiling heat transfer enhancement in a wetting liquid

摘要

An easy-to-implement technique for pool boiling heat transfer enhancement is proposed and evaluated through an experimental investigation. This free-particle technique brings about nucleate boiling at a low degree of superheat by means of metal particles that are not fixed to the heated surface, but rather are free to move with respect to the surface. The effects of copper particles with sizes ranging from tens of nanometers to 9 mm on nucleate boiling heat transfer and critical heat flux (CHF) of the wetting dielectric fluid FC-72 are investigated. Visualizations of the bubble nucleation characteristics due to the free particles are presented. Experimental results show that the introduction of microscale free particles onto a superheated surface effectively facilitates bubble nucleation and thus increases the nucleate boiling heat transfer coefficients. Millimeter-sized as well as nanoscale free particles do not have a strong effect on the boiling heat transfer performance of this wetting fluid. Introduction of a large quantity of microscale free particles reduces CHF by increasing the resistance to liquid replenishment and vapor departure; however, by properly selecting particle size and quantity, an improvement in both nucleate boiling heat transfer and CHF is observed. For the case where 0.2 g of 10 μm-diameter free particles are placed on a polished copper surface, corresponding to a particle layer thickness of approximately 67 μm, the average nucleate boiling heat transfer coefficient is enhanced by 76.3% over the heat flux range of 10 to 159 kW/m~2, while CHF is increased by 10%.