Experimental Study On Two-phase Flow Heat Transfer In The Annular Area Outside The Horizontal Enhanced Tubes

Aiming at the blank in the study of forced convection heat transfer outside the enhanced heat exchange tube,this paper conducted experiments on the two-phase flow heat transfer in the outer annular area of three double-sided enhanced heat exchange tubes(E1,E2,E3)with different fin-shaped array structures.The outer diameter of the tested enhanced tube is 15.88mm,and the refrigerant R410A is used to explore the heat transfer mechanism and pressure drop characteristics of the ring side under various working conditions.For one thing,the flow heat transfer characteristics of the ring side are revealed,and for another it provides help for the development and design of the heat exchanger.Through the control of variables,the heat transfer situation and pressure drop of different fin-shaped array structures on the ring side were obtained at different mass flow rate,vapor quality and saturation temperature.The effect of fin-shaped array structures on fluid disturbance and boundary layer damage greatly enhances the heat transfer of the enhanced tubes;in addition,it can not only promote the formation of condensate to enhance the heat transfer,but also accelerate the discharge of condensate to reduce the thermal resistance,strengthen the nucleation boiling,so that the two-phase heat transfer performance of enhanced fin structures several times that of the smooth tube;and the bubble blockage deteriorates the heat transfer in the flow boiling condition.The increase in mass flux,average vapor quality,and the change in physical properties caused by the decrease in saturation temperature are all conducive to heat transfer enhancement.The enhancement ratio ER of heat transfer coefficient per unit area was defined by introducing the heat transfer coefficient strengthening ratio and the reinforcement area ratio to evaluate the enhancement effect of the ring side.The highest values of single-phase,condensation and flow boiling conditions were 1.82,2.5 and 1.83 respectively.In terms of pressure drop,the proportion of acceleration pressure drop under different working conditions is extremely small,friction pressure drop is the main part,and the ring side friction pressure drop between the 3 enhanced structures is not very distinguishable.In general,the frictional pressure drop is positively correlated with the average vapor quality and mass flux,which is attributed to the enhancement of the frictional shear interaction between the gas-liquid two-phase interface,the liquid phase and the copper wall.In addition,the single-phase heat transfer comprehensive performance evaluation coefficient,PEC,was used to evaluate the comprehensive performance,and the PEC values of the enhanced tubes were all much higher than 1,and the maximum value reached 2.66.For the two-phase working conditions,pressure drop was introduced in reference to the frictional phase expression form in single-phase PEC on the basis of ER,and the two-phase comprehensive performance evaluation coefficient PEC_e on the ring side was defined.The maximum value for the condensing condition was 2.41,and the maximum value for the flow boiling condition was 1.773.The heat transfer data under each working condition were analyzed using empirical correlations.The mean relative error and mean absolute error of the 3 prediction models in single phase are all within±10%.In the condensing and flow boiling conditions,the smooth tube prediction models were also developed to the enhanced tube.The new models can predict more than 92%of the condensation data points in the range of deviation from-30%to+22%,and more than 93%of the flow boiling data points within the range of±20%deviation.