Heat transfer characteristics of alternating discrete flow in micro-tubes

In this paper, the heat transfer performance of the alternating discrete two phase flow in micro-tubes has been numerically investigated. Two immiscible fluids are injected into a micro-tube alternately, generating a plug flow pattern with two completely separated phases. 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 (L1/D) is kept at about 7. Characteristic of the average Nusselt number is found to be almost independent 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.