Abstract
In order to reduce the large space of 1L condensing water tank in our previous microchannel natural circulation loop (NCL), this study employs thermoelectric cooler incorporating with cooling copper channels instead as the condensing section in this NCL to develop the cooling methodology for the electronics, such as Central Processing Unit (CPU). According to the real size of current CPU, the base area of microchannel evaporator in the present NCL is modified from 10.5x10.5mm2 to 31x31 mm2. Our previous studies had recognized that the divergent microchannels can significantly stabilize the two-phase microchannel NCL. In addition to increase the wall-to-base area ratio, the divergent design is only applied to the front section of all the 78 parallel microchannels with uniform depth of 300 μm. Accordingly, the width of each microchannel is diverging from 150 μm at the inlet to 300 μm at the location of 16mm from the inlet, and then with an uniform cross-section with a width of 300μm until the outlet. Thus, it will result in a wall-to-base area ratio of 2.1. The 99.8% ethanol is adopted as the working fluid in the NCL. Its boiling temperature is about 78.4 ℃ at 1 atm. With a filling ratio of 100%, the experimental results show that thermoelectric cooler together with cooling copper channels may increase loop flow resistance and reduce heat removal capability. The temperature at the evaporator outlet is higher than the saturated temperature at 1 atm. It implies that the fluid pressure inside the loop is possibly higher than 1 atm. The higher pressure may be caused by the confined space for bubble growth after the boiling inception and thus affect the heat removal capability of NCL. Therefore, this study also investigates different filling ratios on the performance of the present NCL. The experimental results reveal that the filling ratio has a significant effect on the two-phase flow characteristics of this NCL. The optimum filling ratio is supposed to be about 90%. In order to further improve heat transfer capacity of the NCL, this study enlarges the diameter of the riser and downcomer from 4mm to 8mm. However, the results show the counter-current flow appears in the riser due to the thicker liquid film deposited in the riser and the larger flow resistance existing in the cooling cooper channels. This will reduce the heat removal capability of this NCL. Based on the above results, the thermoelectric cooler incorporating with cooling copper channel may not be suitable for this microchannel NCL. It needs a further improvement to increase the heat removal capability and meets the cooling requirement of CPU.