Heat transfer and pressure drop characteristics of gas-liquid Taylor flow in mini ducts of square and rectangular cross-sections

The thermal and flow characteristics of gas-liquid Taylor flow in vertical mini square and rectangular ducts were studied numerically for water and ethylene glycol as the liquid phase and nitrogen as the gas phase. The effects of fluid properties, flow parameters, and aspect ratio on the bubble shape, recirculation time, friction factor, and Nusselt number are discussed. The results indicate that the gas phase is confined by the tube wall in square and rectangular tubes leading to an asymmetrical Taylor bubble at low Capillary numbers, while an axisymmetric bubble is formed at high Capillary numbers. The liquid film thicknesses in the square and rectangular ducts are not uniform with a thicker liquid film formed at the tube corner. The recirculation region decreases and the dimensionless recirculation time increases with increasing Capillary number, which means that the intensity of recirculation decreases with increasing Capillary number. The friction factor decreases with increasing two-phase mixing velocity and aspect ratio and increases in gas void fraction, while the reverse is true for the two-phase Nusselt number. Compared with the ethylene glycol/nitrogen cases, the addition of Taylor bubble plays a more significant role on the pressure drop increase and heat transfer enhancement for the water/nitrogen cases because of the larger recirculation volume and smaller dimensionless recirculation time. Two empirical correlations are developed to predict the apparent slug Nusselt number and the film-to-slug Nusselt number for gas-liquid Taylor flow in square and rectangular ducts.