Abstract
In this study, the enhancement of heat transfer capacity of a two-phase natural circulation loop with divergent microchannels is explored. The previous research in our labortory shows that the divergent microchannels can significantly stablize the two-phase in the microchannel. We use two different design, chip1 and chip 2, of microchannel as the evaporator to conduct the natural circulation loop experiments using 95% ethanol. The dimension of the microchannel chip is 10mm×10mm. There are 18 parallel microchannels in chip 1 with width diverging from 250μm to 350μm, with uniform depth of 200μm. This kind of design cannot increase the heat removal area effectively, the experimental results show that highest heat removal capacity is only about 70W. We therefore, re-design the microchannel evaporator as width diverging from 150μm to 250μm with depth of 350μm, and the amount of microchannels is increased to 30 for chip 2. The results reveal that the flow velocity and the mass flow rate of the loop are 2.33 times higher than the previous one, so the heat transfer capacity are advanced effectively with the highest heat removal capability of 110W. By observing the flow patterns in the evaporator and riser with a high speed video camera, we can reveal the flashing instability phenomenon same as observed in the normal scaled natural circulation loop. The instability of the loop can be suppressed while the input power is increased, after the loop reach the dryout condition, the loop will become unstable again. The experimental results shows the highest wall heat flux with different microchannel chip is 529kW/m^2 (chip 1)and 481 kW/m^2(chip 2). To make further improvement of heat transfer capacity, we change the working fluid as ethanol-water mixtures as ethanol mole fraction of 0.1. The results show that the two-phase flow in the loop is quite unstable. Moreover, when the input power is 90W, the counter flow appears in the riser. The ethanol-water mixtures may not be suitable for the natural circulation loop at low pressure.