Studies on heat transfer in microchannel

Abstract

Due to rapid evolution in a wide range of technologies in twentieth century, heat dissipation requirement has increased very rapidly especially from compact systems. There is an urgent need for high-performance heat sinks to ensure the integrity and long life of these petite systems. Use of forced convection cooling has been limited by the requirement of the excessively high flow velocity and associated noise and vibration problems. Microchannel heat sink seems to be most reliable cooling technology due to its superior command over heat carrying capability. Detail literature review of the microchannel heat sink addressing various aspects such as flow visualization, flow regime map, bubble dynamics, pressure drop and heat transfer characteristics, different instabilities and critical heat flux (CHF) has been carried out. Formation of the first bubble at nucleation site is an inception of flow boiling in microchannel. Present study is emphasis in understanding bubble dynamics in microchannel and its theoretical modeling. A new energy balance bubble growth model has been proposed to predict the bubble growth behavior at nucleation site in microchannel. It is assumed that heat supplied at nucleation site is divided between the liquid phase and the vapor phase as per instantaneous void fraction value. The energy consumed by the vapor phase is utilized in bubble growth and overcoming resistive effects; surface tension, inertia, drag, gravity and change in momentum due to evaporation. Developed model shows good agreement with available experimental results. In addition, the bubble waiting time phenomenon for flow boiling is also addressed using proposed model. Waiting time predicted by the model is also close to that obtained from experimental data. Further, bubble growth model is extended to address bubble dynamics during bubble growth at nucleation site for microchannel in terms of non-dimensional energy ratio numbers and their variation from bubble inception until departure. New non dimensional energy ratio is also proposed, which helps in differentiating inertia controlled and thermal diffusion controlled regionduring bubble growth at nucleation site. This new non dimensional energy ratio (E1) is the ratio of the energy required for bubble growth to the energy required to overcome the surface tension effect. Moreover, effort has been made to develop new CHF model for microchannel combining non dimensional analysis and energy based bubble growth model. Two separate CHF correlations for refrigerants and water have been developed following a semi-empirical approach. Both CHF correlations show good agreement with experimental CHF data than previously proposed CHF correlations. Eventually, heat transfer and pressure drop behavior of single phase flow in open type microchannel are measured experimentally. An experimental test rig is design and fabricated to carryout experimental investigation. Performance of the two configurations of the microchannel heat sink is tested: (1) microchannels with fins, (2) microchannels without fins. It is found that fins in the microchannel intensified heat transfer performance of open type microchannel heat sink by 15%. Whereas, penalty in pressure drop increased by around 18%. Overall thermal performance of the extended open type microchannel is found to be above unity for all operating condition.