Experimental investigation and analysis of two-phase flow instability of flow boiling in a mini-channel heat sink

Abstract

High heat flux and good axial temperature uniformity of channel flow boiling demonstrate that it is ideally suited for next-generation cooling systems. Such systems, however, are hampered by undesirable flow fluctuations, making channel flow boiling ineffective due to the rapid bubble growth within the channels. Furthermore, especially in the mini-channel heat sink, the interactions between multiple parallel channels and inlet/outlet plenum induce complex hydraulic and thermal oscillations. Therefore, lots of consistent and detailed experimental data are still needed to design a reliable flow boiling system. However, only limited studies have examined the effects of channel interaction on flow boiling instability. Thus, to provide a better understanding of the flow boiling instabilities, a mini-channel heat sink was tested with near-saturated inlet conditions using FC-72 as the working fluid. The mini-channel heat sink consisted of a 150 mm long and 70.4 mm wide base area, having 22 identical rectangular channels measuring 1.6 × 4.8 mm2. The flow instabilities were analyzed by observing the vapor backflow and flow fluctuation in mini-channels with variations in the mass velocity, pressure, and heat fluxes. The analysis is based on the detailed flow visualization in the parallel channels and inlet plenum. It was found that the rapid bubble generation in individual channels promotes vapor backflow and hydraulic oscillations. Further observation of the high-speed camera images shows that the vapor crossing over between parallel channels through the inlet plenum led to the synchronization of vapor oscillations, which is responsible for the parallel channel flow instability. This visualization study revealed six dominant flow patterns showing multi-channel flow instability. Based on the classification of dominant flow patterns, a better understanding of the operating conditions for stable flow boiling was obtained.