Flow regimes and convective heat transfer of refrigerant flow boiling in ultra-small clearance microgaps

Abstract

Understanding two-phase convective heat transfer under extreme conditions of high heat and mass fluxes and confined geometry is of fundamental interest and practical significance. In particular, next generation electronics are becoming thermally limited in performance, as integration levels increase due to the emergence of ‗hotspots' featuring up to ten-fold increase in local heat fluxes, resulting from non-uniform power distribution. An ultra-small clearance, 10 μm microgap, was investigated to gain insight into physics of high mass flux refrigerant R134a flow boiling, and to assess its utility as a practical solution for hotspot thermal management. Two configurations -a bare microgap, and inline micro-pin fin populated microgap -were tested in terms of their ability to dissipate heat fluxes approaching 1.5 kW/cm 2 . Extreme flow conditions were investigated, including mass fluxes up to 3,000 kg/m 2 s at inlet pressures up to 1.5MPa and 2 exit vapor qualities approaching unity. Dominant flow regimes were identified and correlated to two phase heat transfer coefficients obtained using model-based data reduction for both device configurations. The results obtained were compared to predictions using correlations from literature, with the maximum heat transfer coefficient reaching 1.5 MW/m 2 K in the vapor plume regime in the case of the finned microgap.

Experimental setup for studying two-phase flows in micro-and minichannels at ultra-high heat fluxes: methodology and first experimental results

“…Working fluid leaves the heat exchanger with a desired degree of subcooling, passes through an ultrasonic flow meter (4) and enters the test section (5). Excessive heat is dumped into the atmosphere by means of secondary fluid loop -8) which is also connected to the main fluid tank (6). The test section was designed to achieve ultrahigh heat flux of up to 2.5 kW/cm 2 .…”

“…Effects of microchannel aspect ratio on heat transfer is studied for different conditions [3][4] and in all cases authors note the effect of channel geometry on the flow boiling heat transfer characteristics. Boiling under heat fluxes above 1 kW/cm 2 was studied in [5][6][7]. Heat fluxes above 1 kW/cm 2 were achieved during flow boiling on diamond substrate covered by porous copper.…”

“…Heat fluxes above 1 kW/cm 2 were achieved during flow boiling on diamond substrate covered by porous copper. In [6] flow patterns and heat transfer characteristics of boiling in microgaps under heat fluxes of up to 4 kW/cm 2 was studied for different channel heights but only in case of uniform heating from the wall. Importance of understanding of liquid-vapor interactions during boiling was demonstrated in [7].…”

Flow Boiling in Minigap in the Reversed Two-Phase Thermosiphon Loop

“…Particularly of interest are the conditions for flow visualization regarding flat, two-dimensional configurations of flow [2]. Several issues, mentioned in the recent literature, are related to the influence of size [3,4], orientation [5][6][7] and surface [8][9][10][11][12][13] structure of the minigap on heat transfer and pressure drop and also to the passive methods [8,[14][15][16].…”

Measurements of the evaporation rate upon evaporation of thin layer at different heating modes

“…Investigation of evaporation process in a two-layer system under non-isothermal conditions is important for many applications, such as microelectronics cooling [1]. The advancement in high performance heat exchangers, vapor chambers and heat pipes [2] with microscale passages stimulates also development of a new fundamental knowledge of the processes with phase change [3,4].…”