ROLE OF BUBBLE PUMPING ON HEAT TRANSFER ENHANCEMENT OF BOILING-DRIVEN HEAT SPREADER

Abstract

The role of microporous coating on boiling heat transfer enhancement is investigated to enhance the thermal performance of the boiling-driven heat spreader, a thermal ground plane. Typical passive heat transfer devices like thermosyphon and heat pipes have been used in wick structures due to their evaporative heat transfer mechanism and working fluid circulation. However, these wick structures have fundamental disadvantages; (1) The maximum heat flux attainable in a heat spreader is determined by the capillary force and pressure loss in the structures, and the heat flux for capillary-driven heat spreaders is smaller than required for advanced thermal management in electronics about 50 W/cm2. (2) The thermal performance of a heat spreader depends on its orientation because of the low amount of the working fluid and restricted working fluid circulation pathway. The heat transfer is significantly governed by bubble pumping, in which the boiling heat transfer can be enhanced with a microporous coating inside a heat spreader. With microporous coating on metal surfaces, more nucleation cavities are making it possible to enhance boiling heat transfer, so earlier boiling incipience can be made with a relatively small wall superheat. In the present study, the main concerns related to the working principle of a boiling-driven heat spreader are investigated as follows; (1) the effect of movement of working fluid on the heat transfer characteristics of the heat spreader, (2) the effect of orientation on the bubble pumping mechanism, and (3) the effect of internal passage design on the circulation of the working fluid. So, this study proposes a new boiling-driven heat spreader with improved thermal performance and high heat dissipation of up to 200 W/cm2. Furthermore, the working principles of the boiling-driven heat spreader with a fluid circulation mechanism are confirmed by the high-speed visualization of the heat spreader and by their thermal performance testing results. In addition, a boiling-driven heat spreader is designed and fabricated using a wickless and passive-flow-controlled internal passage. The optimized bubble pumping design increases the maximum heat dissipation of the heat spreader through passive flow control. Therefore, the advanced boiling-driven heat spreader can provide valuable insights into thermal management for high-heat-flux applications in power semiconductors and laser diodes.