Chong, Jen Haw
(2018)
Modelling of subcooled flow boiling in a rectangular micro-channel heat sink.
PhD thesis, University of Nottingham.
Abstract
Attaching micro-channel heat sinks operating under flow boiling conditions on heat sources of electronic components is an efficient cooling technique which still requires further improvements of designs. When developing this system, the efficient heat transfer performance is essential, however, this development often entangles with difficulties. The difficulties arise as existing prediction approaches are underdeveloped and inadequate to perform the accurate prediction in wide ranges of operating conditions. This inadequacy persists due to incomplete discoveries of involved mechanisms that involve fluid and dynamics for the heat transfer during the flow boiling. Also, the mechanisms involved in the flow boiling process are complicated, hindering the development of more reliable approaches. By addressing this issue, this study explores and investigates the relating mechanisms.
The mechanisms of fluids during the flow boiling of subcooled liquids in micro-channel heat sinks immediately before and during the nucleation of first bubbles were explored in this study. This study then addressed the mechanisms of heat transfer enhancement of flow boiling. Later, this study repeated with different substrate materials of micro-channel heat sinks and working fluids. This study serves the purpose to better understand the involved mechanisms during the flow boiling of subcooled liquids in micro-channel heat sinks for the development of more reliable approaches to predict the heat transfer.
This study regarding the mechanisms during the flow boiling in micro-channel heat sinks implemented the numerical model associated with the Volume of Fluid (VOF) in which corresponding governing equations were solved using a computational fluid dynamics (CFD). In this model, computational domains of micro-channel heat sinks in three dimensions that include the sub-domains of solids and fluid were created to consider the conjugate heat transfer for better estimation of data. The data collected in this study were from operating parameters of heat flux, mass flux, and inlet temperature of the micro-channel at 500-3197 kW/m2, 115-389 kg/m2 s, and 23-53°C, respectively. The micro-channel heat sinks operated at the atmospheric pressure, and the corresponding substrate materials chosen were steel, silicon, aluminium and copper, and working fluids selected were water and ethanol. The numerical results agree well with the experimental data from the previous study.
The results show that although the bubble nucleation is absent, the heat transfer mechanisms in micro-channels possesses the nucleate boiling characteristic involving the transient conduction with the existence of the phase change process. The heat transfer mechanisms from the phase change process with the incomplete evaporation induce the ascending and descending flows and liquid-vapour mixture on the heating surfaces. From the results, four different modes of heat transfer mechanisms from the phase change process associated with ascending and descending flows and liquid vapour mixture become apparent. The ascending and descending flows on the heating surfaces appear with local increases of pressure gradients near to the heating surfaces facilitating the heat transfer enhancement due to phase change. On the other hand, the liquid-vapour mixture produced from the phase change process impeding the heat transfer. In overall, the heat transfer enhancement due to the phase change at the side surfaces in the micro-channel is more extensive as compared to the bottom surface for each condition tested in this study. Meanwhile, the amount of the liquid-vapour mixture accumulating on the bottom surface is more massive as compared to the side surfaces, leading to the impedance of the heat transfer. These heat transfer mechanisms also persist during flow boiling in micro-channels. The heat transfer enhancement due to phase change from the side and bottom surfaces also varies when employing different operating conditions before and during flow boiling.
This study provides better insights for researchers and designers in industries regarding the local mechanisms for the heat transfer during the flow boiling in micro-channel heat sinks. These understandings assist the researchers to develop the more reliable prediction methods to design new and better heat transfer performance of micro-channel heat sinks and avoid repeating experiments which are costly and tedious in procedures.
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