HEAT TRANSFER DURING PHASE SEPARATION OF PARTIALLY MISCIBLE LIQUID-LIQUID SYSTEMS IN MINI-CHANNELS

This work presents the results of experimental research, which examined the possibility of using liquid-liquid phase separation for enhancing the cooling rates compared to those obtained in single-phase laminar flow in rectangular mini-channels (0.8-1mm deep) with a uniform heat flux from the channel bottom. The tested cooling liquid was a partially miscible mixture of trimethylamine (TEA) and water, with a Lower Critical Solution Temperature (LCST) of around 18°C. This mixture undergoes phase separation at the channel bottom upon being heated above the LCST. The tested TEA-water mixture showed up to a maximum of 100% cooling enhancement compared to that obtained in single-phase flow of the mixture at the same mass flow rate, up to 30% cooling enhancement compared to water, and up to 100% enhancement compared to the standard coolant of 50% ethylene glycol-water mixture. Similar results were obtained in two types of single channels (with a slightly different geometry) test sections and in a setup of six parallel channels. This implies that the heat transfer augmentation is independent of the heat sink geometry and that scaling effects are insignificant in the examined scales. Experiments were also performed at three different channel inclination angles, resulting in bottom, side, and top heating of the fluid in the channel. The results show that gravity does not affect the heat transfer during phase separation. Through the visualization of the flow in the channel, it was found that there is a clear connection between the area of the channel bottom on which the phase separation takes place and the obtained heat transfer enhancement. It can be deduced that the disengagement of the small droplets from the heated wall and the resulting local mixing have a direct connection to the higher cooling rates obtained. The present study shows the advantages of employing phase-separation of LCST mixtures for heat transfer enhancement, suggesting that the application of such mixtures can be a viable technology for the thermal management of miniaturized equipment.