Heat transfer enhancement in microchannels using magnetic two-phase liquid-liquid plug flow

Overheating of microelectronics has become a significant issue due to their continued miniaturisation and increased heat flux that needs to be dissipated. High powered electronic devices require very effective cooling to prevent overheating. Microchannel heat sinks utilising two-phase flow are capable of very high heat transfer rates and represent a possible means of cooling such devices and, if contained within the chip, removing heat within the volume. Increasing the cooling efficiency of heat sinks used to cool these devices is critical if there is to be continued miniaturisation and increased heat output, resulting in increased heat flux that needs to be dissipated. The uptake of improved cooling technologies allowing for greater product reliability and performance has been critical to the expansion and advancement of the electronics industry. An example of this has been the adoption and widespread use of heat pipe technology in laptops and smartphones. As demand for increased performance and reliability increases, it will be vital to provide a solution to dissipate the ever-increasing heat loads of consumer electronic devices. This research work aims to address the gaps in knowledge, making electronic devices more reliable, compact, and energy efficient. Very effective cooling can be achieved using microchannel heat sinks, however, the inherent laminar flow within the microchannels limits heat transfer. To address the limitations associated with laminar flow, ferrofluids (magnetic nanofluids) were used to improve the heat conductance and to externally control mixing of the fluid. Ferrofluids are surfactant coated magnetic nanoparticles suspended in a dispersion medium. Water-based ferrofluids were used in this work as they are less toxic. To closely control the physical properties of the ferrofluid, dopamine coated magnetic nanoparticles were fabricated in the laboratory. More details can be found in chapter 3. Two-phase liquid-liquid flow can be utilised to improve heat transfer by disrupting the laminar thermal boundary layer. In this research, an aqueous sample (deionized water or water-based ferrofluids) was used as the dispersed phase, and oil (silicone oil or mineral oil) as the continuous phase. Internal recirculation within the liquid plugs generates internal vortices which improves overall heat transfer rate of the flow. Also, vortex formation also occurs at the interface of the immiscible fluids, directly influencing mixing of the flow and resulting in improved heat transfer rates. Additionally, due to the magnetic nature of ferrofluids, an external magnetic field can be applied to manipulate the magnetic plugs of fluid, disrupting the laminar thermal boundary layer of the two-phase flow and provide enhanced cooling. It was found that the addition of an external magnetic field for single-phase ferrofluid flow reduced the rate of heat transfer. This was attributed to the magnetic nanoparticles being attracted towards the wall of the channel in the region of highest magnetic flux. Over time, the magnetic nanoparticles came out of solution and remained pinned against the wall, which reduced the effective thermal conductivity of the ferrofluid flow. This is further explained in chapters 4 and 5. However, for two-phase flow, the magnetic nanoparticles remain within the liquid plug, retaining the effective thermal conductivity of the ferrofluid. Moreover, due to the internal recirculation of the liquid plugs, heat transfer is enhanced. With the addition of external magnetic fields, the alignment of the magnetic nanoparticles in accordance to the magnetic field lines results in the disruption of internal recirculation in the liquid plugs. This disruption was attributed to the enhanced heat transfer rates recorded for the flow. Currently, there is a lack of fundamental understanding of such two-phase liquid-liquid plug flow. Particularly, there is very little knowledge of the ferrofluid properties required to optimise the heat transfer enhancement in these two-phase flows. Furthermore, the potential use of external magnetic fields to enhance mixing within the flow has not been explored. It was found that two-phase ferrofluidic plug flow under the influence of external magnetic fields improves heat transfer rates by over 400% compared to single-phase flow using de-ionized water (DIW). The Thermal Performance Factor ("TPF= "  "Nu" /"f" ^"1/3"  ) is used to measure the effectiveness of the heat transfer process which considers pressure drop of the system as well as the heat transfer rate. Two-phase ferrofluidic plug flow under the influence of external magnetic fields has shown to have the best TPF, indicating about twice TPF compared to single-phase DIW flow. There is good correlation between theory and experiments throughout the thesis