Experimental Study On Condensation Heat Transfer Inside Protrusion Tubes

Heat exchangers are widely used in various industries such as refrigeration,aviation,and chemical industry.Improving the heat exchange efficiency of heat exchangers is of great significance for improving energy efficiency and alleviating the energy crisis.The application of enhanced heat transfer technology reduces the heat exchange area,reduces the volume and weight of the heat exchanger,reduce power loss and improve the heat exchange efficiency of the heat exchanger.This article mainly studies the single-phase and condensation heat transfer characteristics of refrigerant flowing inside protrusion tubes,observes the local condensation flow pattern of refrigerant at the outlet of test tubes,and analyzes the effects of mass flow rate,vapor quality and saturation temperature on the condensation heat transfer coefficient and frictional pressure drop of refrigerant.The single-phase heat transfer coefficient and pressure drop of the refrigerant R134 a flowing inside protrusion tubes are higher than that of the smooth tube;the heat transfer coefficient of refrigerant R134 a flowing inside the copper tube is higher than that of the stainless steel tube,but the pressure drop is smaller than that of the stainless steel tube.Both the Dittus-Boeler correlation and the Gnielinisk correlation can well predict the Nu number of single-phase heat transfer in smooth tubes.Protrusion tubes can improve single-phase heat transfer performance.The enhanced performance coefficient of protrusion tubes is larger than 1.At low mass flow rates,the condensation flow pattern of the refrigerant in the copper smooth tube are mainly stratified flow,stratified-wave flow,and semi-annular flow.The condensation flow pattern of the refrigerant in the copper and stainless steel protrusion tube are mainly stratified flow,stratified-wave flow,semi-annular flow and annular flow.With the increase of the vapor quality and the mass flow rate,the condensation liquid film will wave more obviously,and the refrigerant liquid film will become thinner.El Hajal et al.flow pattern map and Cavallini et al.flow pattern map can predict the condensation flow patterns in the smooth tube.Whether it is the copper tube or the stainless steel tube,the condensation heat transfer coefficient and pressure drop of the refrigerant flowing inside protrusion tubes are larger than that of the smooth tube.Because the surface structure of the dimpled protrusions and petal protrusions increases the heat transfer area of the protrusion tubes.The dimpled protrusions also cause a swirling flow of the refrigerant liquid film,which disturbs the refrigerant liquid film.The petal and dimpled protrusions will continuously destroy the temperature boundary layer,and will form shunt and mix of the refrigerant liquid film near the wall of the test tube,resulting in secondary disturbances.The petal and dimple protrusion enhance the turbulence of the refrigerant liquid film.The condensation heat transfer coefficient and frictional pressure drop of the refrigerant inside tubes will increase with the increase of the vapor quality and the decrease of the saturation temperature.At low mass flow rates,the mass flow rate has effect on condensation heat transfer coefficient slightly.