Experimental and numerical study on heat transfer and pressure drop during mPCM slurry flow in microchannels.
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Microencapsulated phase change material (mPCM) slurries have been used as the working fluids for enhancement of convective heat transfer, thermal energy storage and thermal energy transport due to their high latent heat and almost constant temperature during the change of phase between liquid and solid. This project aimed at investigating experimentally and numerically heat transfer and pressure drop characteristics during mPCM slurry flow in microchannels. A test rig has been designed and built. The test section consists of six microchannels of length 500 mm, width 1 mm and height 1.5 mm, machined inside the aluminum blocks. Local surface temperature and local heat flux along the channel were determined, from temperatures measured at 98 precisely-known locations in the aluminum blocks, by the inverse solution of the two-dimensional (2-D) heat conduction (Yu et al. (2014)). Experiments were performed using pure water to validate the experimental setup and the data are also used for comparison with mPCM slurry. For mPCM slurry, experiments were conducted at mass concentrations of 5% and 10%, with Reynolds number ranging from 340 to 1800. The effects of mass concentration on local surface temperature, local heat flux, local Nusselt number, average Nusselt number, bulk temperature rise and pressure drop, were investigated. Moreover, the effect of Stefan number on heat transfer performance of mPCM slurry flow was also investigated. The average Nusselt numbers at Re = 1200 for 5% and 10% mass concentrations were 12.1% and 28.3% higher than those of pure water, respectively. For the same heat transfer rate of 400 W, the fluid temperature rise was found to be 1.03 K and 2.68 K lower at mass concentrations of 5% and 10% as compared to water. The pressure drop for mPCM slurry was found to be lower at same heat transfer rate than pure water. An empirical correlation was also developed and all experimental data can be predicted within ±15%. Moreover, numerical simulations of three-dimensional conjugated heat transfer during the melting and freezing of mPCM slurry flow in microchannels were carried out. Numerical model was validated with experimental data and found to be in a good agreement within maximum 15%. The bulk temperature rise in case of mPCM slurry was lower than that of pure water. Furthermore, the delay in thermal boundary layer development was observed for mPCM slurry. Numerical simulations of three-dimensional conjugated heat transfer of mPCM slurry in microchannel heat sink was performed. The effects of geometrical parameters including height and width of separating wall of the channel on surface temperature, bulk temperature, thermal resistance and heat transfer rate were investigated. The numerical results show that: (i) increase in width from 1.2 to 4.0 resulted in 38.5%, 48%, 41% and 51% increase in surface temperature, bulk temperature, thermal resistance and heat transfer rate, respectively, (ii) Increase in height shows more uniform surface temperature. Furthermore, the effect of circular, square and rectangular cross sectional shapes of channel on heat transfer and pressure drop was examined. It is found that the rectangular shape showed 67% lower thermal resistance but circular shape transferred 62% more heat per unit pumping power. Keywords: Microencapsulated phase change material, Slurry, Microchannel, Heat transfer, Heat sink, Inverse heat conduction, Latent heat, Thermal Energy storage, Air conditioning, Refrigeration, Electronics cooling.
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