VISUALIZATION-AIDED EXPERIMENTAL STUDY OF THERMAL AND HYDRAULIC CHARACTERISTICS OF SUBCOOLED FLOW BOILING OF R-134A IN A MICROCHANNEL EVAPORATOR OF A PUMPED TWO-PHASE LOOP

Conventional liquid cooling systems based on relatively inefficient forced convection are often not capable of handling high heat dissipation rates, especially for high-power electronics. A pumped two-phase loop (P2PL), which is an active two-phase cooling system, uses two-phase heat transfer (boiling and condensation) which is efficient and capable of handling high heat fluxes and is an order of magnitude higher than the forced convection of liquid cooling. In this study, an experimental investigation was performed using P2PL with R-134a for analyzing subcooled flow boiling in a microchannel evaporator. The ranges of mass flux are from 47.9 to 143.8 kg/m(2)-s, heat flux from 0 to 34 W/cm(2), and quality from subcooled to critical heat flux. Flow visualization using a sight glass tube at the outlet of the evaporator was employed to investigate the influence of heat flux and mass flow rate on the two-phase flow patterns. Five two-phase flow regimes - nucleate boiling, bubbly flow, slug flow, stratified flow, annular flow, and misty flow - were identified based on the flow visualization which coincides perfectly with the slope changes of the boiling curve. The results of this study show that for a fixed wall temperature, in the convective boiling dominant region heat flux dissipation increases with mass flux, while it remains independent of mass flux in the nucleate boiling dominant region. A peak heat transfer coefficient was observed at a high exit vapor quality which corresponds to the annular flow regime with thin-film evaporation. The transient responses were used to characterize the single-phase and two-phase flows along with the critical heat flux (CHF) by investigating the dryout propagation in the microchannel. Further, the evaporator pressure drop increased linearly with heat flux in the two-phase regimes with the point of minimum pressure drop occurring at a heat flux beyond the Onset of Nucleate Boiling (ONB).