Numerical studies of gas-liquid Taylor flows in vertical capillaries using CuO/water nanofluids

Abstract

Heat transfer and pressure drop characteristics of gas-liquid Taylor flows in a mini tube with 1 mm inner tube diameter were investigated numerically using a moving frame of reference method. A CuO/water nanofluid was used as the continuous phase, while nitrogen was adopted as the dispersed phase. The inlet Reynolds number ranged from 250 to 600, and the volume concentration of the CuO particles was in the range of 0% to 3%. The results show that a thicker liquid film and a relatively longer bubble are obtained for Taylor flows with nanofluids compared with those using pure water. The heat transfer process could be divided into three stages with increasing time. At the initial stage, a quick increase of the thermal boundary layer results in a dramatic decrease of the heat transfer coefficient. With increasing time, heat transfer coefficient oscillation is obtained because of the advection of cold liquid from the tube center to the heated wall. With the combined effect of thermal diffusion and recirculation in liquid slugs, the fully developed status of Taylor flow is obtained. Heat transfer coefficients increase with decreasing gas void fraction and with increasing nanoparticle concentration. The overall two-phase pressure gradients increase with increasing nanoparticle concentration and Re, but with decreasing gas void fraction. The increase in the thermal conductivity and the viscosity of nanofluids is the main reason for heat transfer enhancement and pressure drop penalty.