Experimental investigations of techniques to enhance the cooling rate of hot fluids by utilizing Gallium as a heat sink material
Heat transfer enhancement is crucial for many industrial applications. Liquid water has been known as the commonly used coolant for such applications. When water captures heat from the to-be-cooled heat generation source, it loses its cooling deriving force; hence, it needs to be cooled before it can be reused. This research proposes novel techniques to cool hot water in batch-wise operation, at cooling rates faster than those attainable by conventional means. In these techniques, the water is allowed to lose its heat in a direct-contact manner to a high thermal conductivity sink material, which is taken in this research to be gallium. When gallium is used as a heat sink material, while being in its solid state, it acts as a phase change material (PCM) due to its relatively low melting temperature. In other words, when the solid gallium captures heat from the hot source, it melts to a liquid phase that can experience some kind of superheating upon further dumping of heat into it. Consequently, this may cause appreciable diminishing of the temperature difference driving heat transfer from the source. To overcome such possible liquid gallium superheating issues, the present research proposes for the first time in the literature a new technique integrate chunks of additional un-encapsulated PCM within the solid gallium; (up to 10% volume fraction in this work) that has lower melting temperature than that of the gallium.
Also, this research proposes new techniques to enhance the rate of heat transfer from the hot water to the gallium either by vibrating the heat sink (gallium and water) under a range of amplitudes (0.3, 0.5 and 0.7 mm) and frequencies (20, 35 and 50 Hz) or discretizing the hot water into small bubbles travelling through a gallium bath with height of 13 or 18 mm. For the water bubbling technique, three supporting empirical models from the literature were tailored to the experimental conditions of the present study to predict the experimentally investigated the bubbling process and water/gallium heat exchange scenarios.
The results have shown that when the PCM loading within gallium increases, higher water cooling rates can be achieved; with the less important role played by the way the PCM is structured in the gallium. Also, the vibration experiments revealed that in the range of the studied parameters, the increase in the vibration parameters leads to higher heat removals; with the more domineering effect of the amplitude over frequency. Finally, the bubbling technique had resulted in a promising performance when the liquid gallium bath temperature was maintained at low-temperature levels. The predicted results obtained by the three empirical models used for the bubbling process were in very good agreement with experimental results.
In conclusion, this work provides a better understanding of heat transfer enhancement applications and proposes in new concepts and unconventional techniques for effective cooling of hot fluids (e.g. hot water) that surpass the heat exchange concepts implemented in existing traditional heat exchangers.