Investigation Of Stable Flow Boiling Heat Transfer And Flow Boiling Instability In Microchannels

With the rapid progress of MEMS technology, many microdevices such as micromotors, microreactors, micropumps, microchannels heat sinks, etc. have been built. The investagation of fluid flow and heat transfer characteristics in these micro-devices has attracted considerable attation in recent years. In this thesis, experimental and theoretical studies have been performed to investigate the characteristics of flow boiling heat transfer of water in microchannels.Two kinds of microchannel heat sinks were fabricated by using MEMS technology in the test section of experimental setup. The first kind was silicon-based microchannel heat sinks with three different inlet/outlet configurations and the second was glass-based microchannel heat sinks with two different inlet/outlet configurations and heating conditions. An experimental platform for investigation of single/two-phase flow boiling heat transfer characteristics in microchannels was built and the main contents in this thesis are as follows:1. Flow boiling instability in silicon-based parallel microchannels. (1) It is found that the exit vapor quality can be used to classify the stable and unstable flow boiling regimes in microchanels. A series of experiments are carried out to study flow boiling instability in microchannels with different inlet/outlet configurations based on the parameter of exit vapor quality. (2) The configuration between the inlet/outlet plenums in microchannels greatly affects the amplitude of flow boiling instabiliy. Flow boiling in microchannels with horizontal inlet/outlet configurations is found to be more stable than those with horizontal inlet/outlet configuration. In microchannels with inlet restriction configuration, stable flow boiling with no reversed flow of vapor bubble expansion can be achieved. This configuration is recommended for high-heat-flux microchannel applications to avoid large temperature fluctuations and early burnout. (3) The local boiling heat transfer coefficient peaks in the bubbly flow regimes at low vapor quality in microchannels whereas the local boiling heat transfer peaks at high vapor quality in macrochannels.2. Heat transfer characterisitics in a single microchannel with negligible axial conduction. (1) The good agreement between the numerical predictions and experimental data of wall temperature distribution and the local Nusselt number confirms that classical Navier-Stokes and energy equations are still valid to model convection in microchannels having a hydraulic diameter as small as 155μm. (2) Stable and unstable flow boiling still exist with long period of temperature and pressure oscillations in a single microchannel. And the occurrence of stable and unstable flow boiling regimes can be determined uniquely by the exit vapor quality. (3) Considering the effects of cross-section shape of microchannels, a new four-zone flow boiling model is proposed to interpret the saturated flow boiling heat transfer coefficient in rectangular microchannels. This model predicted the trend of decreasing heat transfer coefficient with increasing vapor quality in the saturated boiling region, and a good agreement between experimental data and the predictions with mean absolute error 16.2% was found.3. Microbubble emission boiling (MEB) in microchannels. (1) The occurrence of MEB in microchannels can remove a large amount of heat flux as high as 14.41 MW/m2 with only a moderate rise in wall temperature at mass flux of 883.8 kg/m2s and inlet water temperature of 20 ?C. Therefore, this heat transfer mode is very promising for next generation of chip cooling technology for microelectronic devices. (2) MEB is strongly affected by the degree of inlet water subcooling. As the inlet temperature increases to 80 ?C, there is no MEB in microchannels. Instead, bubbles continue to grow and elongate without collapsing, and reversed flow is observed. (3) A comparision of MEB and nucleate boiling is investigated in terms of bubble behavior, source of nucleate sites, and bubble occupation time. Less life time of bubbles (less than 0.5 ms) and more nucleate sites in MEB are the main reason why MEB can remove a large amount of heat flux.