Flow Boiling of HFE-7200 in Multi-Microchannel Heat Sinks for High-Heat Flux Applications

Abstract

The thermal management of high power-density electronics due to miniaturisation is a key bottleneck in the continued enhancement of processing power and device functionality in compact packing assemblies. With air cooling generally limited to heat fluxes below 1 MW/m2 and single-phase liquid cooling exhibiting high pressure drop and temperature gradients across the heat sink, flow boiling in microchannels, offering high heat transfer rates with enhanced temperature uniformity on low coolant charge, is an emerging solution to manage heat fluxes in modern electronics projected to reach 10 MW/m2 with hotspots of 6 to 8 times the background heat flux. However, poor understanding of fundamental issues in flow boiling and lack of design tools to optimise two-phase cooling have hindered the implementation of the technique in industry. Additionally, whilst heat fluxes up to 7 MW/m2 have been demonstrated using microchannel flow boiling, high coolant subcooling is typically employed, which may not be attainable in integrated two-phase cooling systems for compact electronics packages. Accordingly, the objectives of the present experimental study is to (i) clarify and understand the fundamental aspects and basic mechanisms of flow boiling in multi-microchannels, (ii) investigate the effect of operating parameters, including heat flux, mass flux, system pressure and degree of subcooling, on microchannel two-phase flow patterns, flow instabilities, heat transfer and pressure drop, (iii) evaluate existing flow regimes maps, heat transfer and pressure drop correlations in literature, (iv) explore the use of a surface coating to enhance heat transfer rates in a parallel microchannel heat sink and (v) propose a conceptual design of a small-scale, integrated two-phase pumped cooling system for multiple heat sources within a compact electronics enclosure, including estimations of the power consumption of the system based on experimental data. Flow boiling experiments were conducted in a plain and coated copper microchannel heat sink with a square footprint area of 400 mm2 and 44 parallel channels of nominal width, height and length of 360 μm, 700 μm and 20 mm respectively. The hydraulic diameter of the plain channels was 475 μm. A dielectric working fluid, HFE-7200, with a boiling point of 76 °C at 1 atm was employed, as it is suitable for the cooling of electronics (typical junction temperatures of +85 °C to +125 °C) and is able to reject waste heat at low pressures from the condenser to high ambient environments of up to +70 °C. The experimental range investigated was between inlet pressures of 1 – 2 bar, mass fluxes of 200 – 400 kg/m2 s, subcooling degree of 5 – 20 K for exit vapour qualities of up to 1, corresponding to wall and base heat fluxes 24.8 – 234.3 kW/m2 and 93.7 – 896.3 kW/m2 respectively. High-speed flow visualisation was conducted along the microchannels at each condition to monitor flow pattern developments in the heat sink. Flow patterns developed from bubbly to slug, churn and annular flow with increasing heat flux along the channel. Nucleating bubbles were observed on the channel side walls and were periodically suppressed with liquid film thinning in the slug and annular flow regime. Similar but earlier flow pattern evolutions were observed in the coated channels due to higher bubble generation frequency on the coated surface. Increasing mass flux and decreasing the degree of inlet subcooling generally accelerated flow regime transitions in the microchannels. Increasing system pressure delayed flow pattern development in the plain channels while the opposite was observed in the coated channels. Saturation pressure, degree of inlet subcooling and the surface coating were found to have an effect on flow boiling instabilities in the microchannel heat sink. The flow pattern transition boundaries were compared with predictions in literature. Both the local and averaged flow boiling heat transfer coefficients increased with increasing heat flux. A peak in local heat transfer coefficient was generally observed near the onset of boiling and depreciated with increasing vapour quality corresponding to nucleate boiling suppression. Heat transfer enhancement of up to 50 % was achieved using the surface coating. Nevertheless, in some instances, excessive bubble nucleation activity in the coated channels led to prolonged wall dryout and the premature occurrence of critical heat flux. Increasing saturation pressure enhanced two-phase heat transfer in the microchannels while varying the degree of subcooling had only a marginal effect on average flow boiling heat transfer. The mass flux effect was only notable at high heat fluxes where annular flow was dominant, corresponding to the increasing contribution of the convective boiling mechanism to two-phase heat transfer. The heat transfer results were compared with correlations reported in literature. Two-phase pressure drop increased with increasing heat flux, exit vapour quality and mass flux in the channels. Flow boiling pressure drop was generally higher in the coated channels, where the penalty may be as much as 83 %. There appears to be an interdependence between the magnitude of pressure drop, wall dryout and two-phase flow oscillations in the microchannels. Increasing system pressure lowered channel pressure drop due to significant changes in the thermophysical properties of the fluid while increasing subcooling degree reduced pressure drop due to delayed flow pattern development in the channels. The pressure drop results were compared with past correlations with varying success. All correlations assessed did not capture the effect of inlet subcooling on flow boiling pressure drop. A maximum cooling capacity of 800 kW/m2 was demonstrated with uniform heat sink wall temperatures below typical allowable junction limits. The surface coating enhanced temperature uniformity and further lowered wall temperatures on the heat sink without significant compromise in the pump power consumption of the system. The power consumption of the heat sink is only a small fraction of the power consumption of the system due to the complex experimental circuit. Based on experimental data, the proposed design of a small-scale pumped two-phase cooling loop for a multiple heat source high heat flux electronics package was projected to be less than 10 % of the device power input. A prototype of an integrated cooling loop should be investigated, with a focus on flow boiling behaviour when subject to dynamic and non-uniform heat dissipation profiles.