Numerical And Experimental Study Of The Gas-liquid Taylor Flow And Condensing Annular Flow In Mini/micro Channels

Heat transfer enhancement is an unfailing topic because of its wide industrial applications.As the technology developing,more efficient methods to enhance heat transfer are needed.Based on this,the research hotspot of heat transfer enhancement has transferred from single-phase flow in convectional channels to multi-phase flow in mini/micro channels.Though the applications of the multi-phase flow in mini/micro channels are wide,the hydraulic and thermal characteristics and the mechanism of heat transfer enhancement for the multi-phase flow have not been fully understood.In this thesis,thermal and hydraulic characteristics of the gas-liquid Taylor flow without phase change and the condensing flow in mini/micro channels and the mechanism of heat transfer enhancement for these flow patterns are studied numerically and experimentally.The gas-liquid Taylor flow without phase change in mini/micro channels is an important two-phase flow pattern and is widely adopted in micro-reactor and microelectronic device.The numerical results show that the velocity of the Taylor bubble is higher than that of the two-phase mixing velocity at the channel inlet.The dimensionless bubble velocity and the liquid film thickness increase with increasing capillary number(Ca).The internal recirculation in the liquid slug will increase the pressure drops and enhance the heat transfer.Fluid streamlines in Taylor flows corresponds to the fully-developed temperature contours.Compared with the single-phase flow,the Taylor flow can reach the fully-developed status faster with a higher Nusselt number.Three-dimensional Taylor bubbles in square-and rectangular-channels are asymmetrical at lower Ca because of the confinement effect of tube walls,while axisymmetric bubbles are obtained at higher Ca for the slighter influence of tube walls.The volume of the recirculation region decreases and the dimensionless recirculation time increases with increasing Capillary number,which means that the intensity of recirculation decrease with increasing Ca.The heat transfer enhancement and the pressure drop penalty in the Taylor flow increase with the increasing recirculation region and the decreasing recirculation time.Compared with the single-phase flow and Taylor flow,the condensing flow in mini/micro channels can take advantage of the latent heat leading to a higher heat transfer coefficient.The heat transfer and frictional characteristics of the condensing flow in mini/micro channels with circular,flattened,and microfin cross-sections have been studied numerically and experimentally in the present thesis.The results show that heat transfer coefficients and frictional pressure gradients of the condensing flow increase with increasing vapor quality,mass flux and gravitational acceleration and decrease with increasing tube diameter and saturation temperature.The liquid film thickness increases with increasing saturation temperature and tube diameter and decreases with increasing mass flux,vapor quality and gravitational acceleration,while the reverse is true for the local heat transfer coefficient.When the film Reynolds number(Re)is low,the turbulence in the liquid film is smaller,and the effect of turbulence on the effective heat conductivity in the liquid film is insignificant.With increasing film Re,the effect of the turbulence in the liquid film is higher than that of the liquid film thickness and dominant the heat transfer coefficient of the condensing flow.The mass transfer from the vapor phase to the liquid phase only exists near the liquid-vapor interface.The vapor phase in the channel will flow to the interface region where the mass transfer exists.A longitudinal vortex is formed in the vapor phase because of the combinational effect of gravity and the mass sink for the vapor phase.The vortex shrinks and even disappear when the gravity effect decreases.The heat transfer coefficient and frictional pressure gradients in the flattened tube are higher than that of the circular tubes with the same heat transfer surface areas.The heat transfer enhancement for the flattened tubes is more pronounce at higher channel aspect ratio,mass flux and vapor quality.The liquid film tends to accumulate at the small radius region rather than the bottom of the flattened tube leading to thin liquid films on the top and bottom surfaces.The heat transfer enhancement is more pronounced when the surface tension and inertia force dominate the condensing flow,while a smaller heat transfer enhancement is obtained when the gravity effect is pronounced.The swirling flow induced by the helical angle will bring the liquid film at the tube bottom to the tube top,leading to an axisymmetric liquid-vapor interface during condensation.The streamlines for the condensing flow in microfin tubes is helical because of the disturbance of the microfins.The gravity effect is suppressed and the swirling flow dominates the condensing flow in microfin tubes.Compared with smooth tubes,the area of the heat transfer surface and the annular flow regions in microfin tube is higher.Together with the thinner liquid film and the swirling effect caused by the microfins,higher heat transfer coefficient and pressure drop penalty are obtained for the microfin tubes.