Heat transfer characteristics of gas-liquid two-phase flow in microtubes
Lim, Yau Shiang
Date of Issue2014
School of Mechanical and Aerospace Engineering
It is well-known that microelectronic devices would generate high heat fluxes. Given the same heat flux generated, smaller size means more concentration of heat at one particular area. Hence, efficient thermal management is required for these devices to ensure the effective operations of these devices. Heat dissipation via microchannels has been considered as an effective method for removal of high heat fluxes due to its high surface to volume ratio. Despite the extensive research in fluid flow in microchannels over the past decade, the reported results among different researchers are still at variance. The present work is to elucidate some of the unresolved issues on one and two phase flow heat transfer characteristics in micro-sized tubes with the inner diameter less than 1 mm. Experiments have been carried out to examine the single and two-phase flow characteristics in micro-sized tubes with inner diameters at 100, 300, 500 and 900 µm respectively. Generally, pressure drop measurements are in good agreement with Poiseuille flow theory. Transition flow is found to start slightly earlier at Reynolds number below 2000 as compared to the onset of transition flow at Reynolds number of 2300 in macroscale tubes. 300 and 500 µm tubes are employed for the flow visualization investigations of adiabatic two-phase flow characteristics. Two-phase flow patterns, two-phase frictional pressure losses and void fraction are discussed and the results are compared with the available literature. Two-phase pressure losses for the bubbly and slug flows are found to be underestimated by the Lockhart-Martinelli correlation. The 300 µm tube is further utilized for the investigation of the heat transfer characteristics under constant heat flux at wall. The constant heat flux is supplied by joule heating via applying electric current through a thin uniform indium tin oxide coating. To ensure steady flow patterns, the two-phase flow is achieved by injecting nitrogen gas co-axially through a centrally positioned tube to the continuous liquid phase flow. Measurements of wall temperature along the heating zone, various flow patterns and pressure drop are recorded simultaneously. Thermal performance is found to depend on bubble size, void fraction, Reynolds number and flow patterns. The two-phase Nusselt number (〖Nu〗_(G,L)) for bubbly flow is found to increase by 176% comparing to the single-phase flow, while the corresponding pressure drop increases by less than 27%. To extend our understanding of the heat transfer characteristics in microtubes, numerical simulations for the heat transfer characteristics in axisymetric air-water two-phase flow have been carried out. Comparison between the numerical and experimental flow patterns shows that the difference is within 10%. Subsequent simulation results show that the Nusselt number enhancement can be as high as 200% while the two-phase frictional pressure loss is about 20% increment for the bubbly flow comparing to that of liquid flow alone. The numerical results also show that the heat transfer performance varies with the bubble size, frictional pressure drop and Reynolds number. Analysis of the velocity and temperature profiles near a bubble shows that the bubble obstructs the path of the liquid flow, forcing the redistribution of the axial and radial velocities around the bubble. This redistribution enhances the thermal mixing and is found to be the main reason that enhances the heat transfer performance. In addition, a new approach to enhance the heat transfer characteristics in microtube has been proposed. The main idea of this approach is to inject two immiscible fluids alternately into a microtube/microchannel, resulting in completely separated slugs for the two phases. Characteristics of both heat and mass transfer are investigated numerically. The overall Nusselt number could be increased by sixfold over the single-phase fully developed flow when the ratio of primary phase to the tube inner diameter (L_1⁄D) is kept at about 7. Characteristic of the average Nusselt number is found to be almost independent to the properties of the secondary phase. The friction factor is much dependent on the viscosity of the secondary phase. Secondary phase with higher viscosity would induce higher pressure loss and large pressure fluctuation is observed at regions close to the interfaces. The flow field that is far from the interfaces could be approximated as the single-phase fully developed flow. Weber number of the primary phase and Capillary number of the secondary phase should be kept below certain critical values in order to maintain the discrete flow patterns. Frictional loss is a more important consideration than the heat capacity when choosing a particular secondary phase. Secondary phase with lower viscosity is preferred due to the overall heat transfer enhancement will become less beneficial if the pressure loss induced by the secondary phase becomes significant. Liquid-gas discrete flow is found to have better heat transfer performance than the liquid-liquid discrete flow.