Sub-atmospheric boiling heat transfer and thermal performance of two-phase loop thermosyphon

This experimental study investigates the thermal performances of a two phase loop thermosyphon (TPLT) by examining the boiling heat transfer and instabilities, the thermodynamic cycles and the constituent and overall thermal resistances with the aid of boiling flow structures collected from visualization tests. With water as the working fluid, the phase-change pressures in this TPLT at the temperature range typical for electronic cooling applications are sub-atmospheric. Followed by increasing the boiling heat flux at sub-atmospheric pressures, the transition of boiling structures from intermittent Taylor bubble to continuous bubbly flows reverses the transition route at positive pressures and triggers two different types of boiling instabilities. A set of selected results illustrates the interdependent impacts of boiling heater power (Q) and condenser thermal resistance (R _th,con) on each thermal property investigated. Three sets of heat transfer correlations for determining the boiling heat transfer coefficients over pool-boiling, intermittent and vapor regions of the evaporator along with two sets of empirical correlations that permit the evaluation of individual and interdependent Q and R _th,con effects on overall thermal resistances and evaporator pressures of the tested TPLT are generated. The applicable conditions for these empirical correlations considerably extend the lower end of pressure range available in the open literature, which add design capabilities for two-phase heat transfer devices operating at sub-atmospheric pressures.