Experimental investigation on the effects of swirling flow on flow boiling heat transfer and instability in a minichannel heat sink

Abstract

Flow boiling in micro/mini-channel heat sinks provides an effective thermal management solution for high-heat-flux cooling applications. However, the plain micro/mini-channel heat sink has some shortcomings, such as severe boiling instability and insufficiency for more stringent heat dissipation requirements. To overcome these shortcomings, this work designs a minichannel heat sink (MCHS) with twisted tape inducing swirling flow. The cross-section of minichannel is square, as well as the width, height and length of minichannel are 2 mm, 2 mm and 150 mm, respectively. The twisted tape length (Ltt = 50, 100 and 150 mm) and twist ratio (R = 3, 4 and 5) are selected as designed parameters to generate different swirling flow intensities. The flow boiling heat transfer and instability in such MCHS are experimentally examined under two mass fluxes (120.7 and 210.5 kg/(m2·s)), two inlet temperatures (70 and 80°C) and a range of heat flux (42.3–263.0 kW/m2). Results show that the swirling flow can mitigate the rapid growth for bubbles and suppress the coalescence of small bubbles effectively, as well as cause the bubbles to generate helical motion behavior. Especially, the micro-corners between the twisted tape and minichannel wall provide more effective nucleation sites to produce more small bubbles. Thus, the swirling flow induced by twisted tape can not only trigger onset of nucleate boiling (ONB) at low wall superheat and improve heat transfer performance, but also suppress the flow boiling instability. Smaller twist ratio or longer length of twisted tape results in stronger swirling flow, thus bringing greater heat transfer enhancement effect. Also, longer twisted tape presents better stable flow boiling, but the twist ratio has an insignificant effect on boiling instability. Compared to plain MCHS, the MCHS having twisted tape with Ltt = 150 mm and R = 3 achieves a 34.4% decrease in wall superheat of ONB and a maximum of 80.9% increase in heat transfer coefficient. Furthermore, this MCHS reduces the relative standard deviations of inlet pressure by 44.4% at heat flux of 100.5 kW/m2 and mass flux of 120.7 kg/(m2·s). These findings suggest that using twisted tape insert to induce swirling flow can improve overall performance for minichannel heat sinks effectively.