Thermally induced liquid-liquid phase separation and its heat transfer at micro-scle

Abstract

Piranha Pin Fin (PPF) structure was applied to TEA-water phase separating flow to examine the heat transfer performance on advanced micro structures. It has been demonstrated that using such combination of heat transfer techniques, heat flux up to 600 W/cm2 could be dissipated at a mass flux of 600 kg/m2s. PPFs of different geometrical configurations were tested, and it was found that the PPF design with less number of fins or smaller fin size yielded the best performance boost over the case where water was used as coolant.

The increasing power density of miniaturized equipment, high degree of system integration and development of advanced electronic device cast an urgent need for advanced cooling technologies to ensure safe operation and to pursue superior performance. Micro channel based heat sinks stand out as a feasible cooling solution due to its superior heat transfer performance brought by scale effect. For single-phase and two-phase micro channel heat sinks, the systems suffer the issues of high pressure drop, flow instability and/or critical heat flux. In this thesis, we present a novel heat transfer enhancement method at micro-scale utilizing liquid-liquid phase separation of the triethylamine (TEA)-water solution. The phase separation is induced by changing the fluid’s temperature.

The mixture at its critical composition was first tested in a plain micro channel with a hydraulic diameter of 667 µm. The test aimed at providing experimental evidence that such a phase change system can potentially boost the thermal transport. Our experimental result demonstrated that the phase separating flow heat transfer is up to 2.5 times greater than that of the single-phase flow of the same mixture at the same inlet conditions. A unified heat transfer characteristic was discovered which quantifies the thermal transport behavior at different geometrical conditions. The flow pattern and domain morphology are shown to be mist flow, elongated droplet flow and annular-like flow. The pressure drop was reduced after the homogenous solution gets separated. A single-phase CFD model showed that both the latent heat effect and the flow mixing effect are responsible for the better heat transfer performance.

The mixture’s initial composition has a significant effect on the resulting heat transfer behavior, as shown in our experiment. The heat transfer coefficient of the 15% TEA mass fraction mixture results a linear increase with heat flux. 50% TEA mass fraction mixture exhibits an increasing-decreasing heat transfer coefficient with heat flux. It is postulated that the preferable wetting of the TEA-rich phase over the water-rich phase accounts for the shape of the heat transfer coefficient curves.