Study On Flow Boiling Instability In Rectangular Microchannel Based On Bubble Dynamics

As a more efficient cooling method, phase-change heat transfer technique in microchannels is widely applied in industry while the flow instability phenomenon is general in microchannel cooling for electronic cooling application. Research on this issue has attracted more and more attentions of domestic and foreign scholars. In this paper,an experimental platform was constructed to study two phase flow boiling phenomenon in microchannels.The test section with 14 parallel rectangle microchannels was machined by spark and every channel has a hydraulic diameter less than 200μm. The flow instability in parallel microchannel is studied using deionezed water as working fluid based on the visual observation and the synchronous parameter measurement technology. Then,the flow instability is theoretically analyzed accroding to the parameters such as temperature/pressure and the results of visualization in order to enrich the understanding of parallel multimirochannels flow instability.In this paper, by studying the drop-flow characteristic curve, the influence of working parameters to static flow instability starting point(OFI point) is analyzed in the parallel micro channels. The main results are as below: 1)In the condition of a certain inlet undercooling, an increase in heat flux can make the OFI point which is the lowest point of pressure drop versus mass flow characteristic curve move toward the high mass flow rate.In the condition of a certain heat flux, an increase in inlet undercooling can make OFI point which is the lowest point of pressure drop versus mass flow characteristic curve move toward to the high mass flow rate. 2)In the condition of a certain heat flux and temperature, the fluctuation amplitude of pressure drop increases gradually and the system enters the non-steady boiling state from steady boiling state with the decreasing of mass flow rate. Meanwhile, combined with visualization techniques, flow patterns and parameter fluctuations in the condition of steady and unsteady boiling mode are analyzed respectively. It can be observed that in steady boiling stage small bubbles generally could not continue to grow up and were rushed downstream under the effect of a large inertial force of flowing liquid. The slug flow which appeared could not stop the liquid entering the channel and only caused small amplitude fluctuations of import and export pressure. In the non-steady boiling stage, due to the higher wall superheat and small inertial force of flowing liquid, the size of the bubbles rapidly grow to the size of channels. The bubbles were squeezed by wall and the phenomenon of steamcore backflow appeared. Such periodic alternating flow of vapor-liquid caused big and cyclical amplitude fluctuations of import and export pressure. Thus, the conclusion is that in parallel microchannels, liquid / vapor- liquid two-phase alternating flow are caused by steamcore backflow and liquid backflush.This paper presents an analytical model that it is developed to investigate the periodic pressure fluctuations by the analysis of periodic bubble growth in a rectangular microchannel. When boiling occurs in a rectangular microchannel, the growing bubble is assumed to be composed of three stages, i.e. free growth, partially confined growth and fully confined growth.When the length of fully confined bubble expands to some extent, the tail interface of bubble will reverse resulting in dramatically pressure increase. The variation characteristics of the tail interfacial velocity of bubble 、the length of fully confined bubble and entrance pressure fluctuation with time is got by solving the conservation equations of the momentum of the liquid column coupled with the equations of the force balance at the bubble interface and takes the upstream compressibility into account. The predictions of model are found to be favorably in coincidence with the experimental data in the literature.