Flow boiling and condensation with refrigerant HFE-7100 for cooling high heat flux devices

Abstract

A continuous demand on effective miniature cooling systems for high heat flux applications encourages researchers to propose and investigate different cooling techniques. Two-phase flow in small-scale pumped loop systems is considered a promising solution for these applications. However, two-phase characteristics in micro passages are still to be resolved. Accordingly, the objectives of this study can be summarized as follows: (i) clarify the complex fundamental aspects of flow boiling and condensation in multi-microchannels, (ii) investigate the effect of heat flux, mass flux, vapour quality, channel aspect ratio and surface material on the boiling flow patterns, pressure drop and heat transfer coefficient, (iii) examine the influence of refrigerant mass flux, vapour quality and coolant side conditions on the condensation flow patterns and heat transfer coefficient, (iv) evaluate the existing flow pattern maps, pressure drop and heat transfer correlations available in the literature and (v) propose a new design of pumped loop system for cooling electronic components and compare its performance with other cooling techniques. An experimental investigation was conducted during this study to achieve the research objectives. Three heat sinks made of copper having different channel aspect ratios ( = 0.5, 1 and 2) but the same channel hydraulic diameter (0.46 mm), base area (500 mm2) and nearly the same surface roughness (0.3 μm) were manufactured. Another heat sink made of aluminium having channel aspect ratio of 1 was fabricated. A counter-current condenser having channel hydraulic diameter of 0.57 mm, cooled by water and made of copper, was also manufactured. HFE-7100 was chosen as a working fluid since it is suitable for electronics cooling that require surface temperature below 85 °C, such as microprocessors, IGBTs and fuel cell stacks, and considered a dielectric and eco-friendly refrigerant. Flow boiling experiments were conducted under a system pressure of 1 bar (which corresponds to a saturation temperature of 59.6 °C), inlet sub-cooling of 5 K, mass flux range of 50–250 kg/m2s, base heat flux range of 21.7–531.2 kW/m2 and exit vapour quality up to one. Flow condensation experiments were performed at a system pressure of 1 bar, inlet vapour quality near superheat, refrigerant mass flux range of 48–126 kg/m2s, inlet water temperature range of 20–40 °C and water flow rate range of 0.5–1.1 L/min. All the two-phase flow calculations corresponding to flow visualization were conducted locally and along the channels. A Phantom digital high-speed camera mounted on a microscope was used to capture the flow patterns inside the test section. The flow boiling visualization showed that bubbly, slug, churn and annular flow were captured along the channels and with increasing heat flux. Confined bubble was also seen for a short period. The size of bubbles became larger and the slug ends had bullet shapes with decreasing channel aspect ratio, while no clear effect on the annular flow was found. The effect of surface material on the flow patterns was insignificant. The present flow patterns data points were not predicted well by the existing maps. The flow boiling pressure drop was found to increase with increasing mass flux and heat flux (exit vapour quality). When the channel aspect ratio decreased, the flow boiling pressure drop increased. Aluminium surface provided higher flow boiling pressure drop compared to copper. The two-phase pressure drop correlation proposed for horizontal rectangular microchannel by Keepaiboon et al. (2016) provided a reasonable agreement with the present data with a mean absolute error ≤ 20%. The flow boiling heat transfer results indicated that the local and average heat transfer coefficient increased with heat flux. Higher local heat transfer coefficient was reached at boiling incipience and then decreased with local vapour quality. In contrast, for a given heat flux, insignificant effect of mass flux was found. A noticeable increase in the heat transfer coefficient was found with increasing the aspect ratio. At low heat fluxes, the material effect was insignificant. However, at moderate and high heat fluxes, aluminium surface showed higher heat transfer coefficient than copper. The two-phase flow heat transfer correlation by Shah (2017) showed the best prediction of all present data with a mean absolute error of 15–22%. The condensation flow visualization indicated that annular, slug and bubbly flow were visualized along the channels. Neck region was also captured as a transition from annular to slug flow. There was no existing flow patterns map that can predict all the data points of the present study. The local condensation heat transfer coefficient increased with increasing refrigerant mass flux and local vapour quality. The effect of coolant side conditions was negligible. The correlation by Cavallini et al. (2006a), for conventional channels, predicted the data with a mean absolute error of 11%. The mini/microchannels heat transfer correlations by Kim and Mudawar (2013c) and Shah (2016) predicted the experimental data with a mean absolute error of 15–16%. The proposed pumped loop system was more efficient, i.e. higher cooling load and lower pumping power, than other cooling techniques. In this system, lower pumping power was required due to the low mass flow (high latent heat of vaporization). Maximum cooling load was reached up to 265 W using microchannel evaporator having channel aspect ratio of 0.5. The working surface temperature was controlled near 80 °C. Maximum cooling load at the condenser was found to be 520.2 W at outlet sub-cooling of 3.5 K, coolant flow rate of 0.5 L/min and inlet coolant temperature of 30 °C. A microchannel evaporator having large surface enhancement, i.e. deeper channels and large number of channels, made of aluminium could be a suitable design for cooling electronic components that require high cooling loads.