Research On Convective Condensation Heat Transfer Characteristics In Horizontal Enhanced Tubes

In-tube condensation is widely used in a variety of heat transfer scenarios due to its higher efficiency and the determination of the condensation heat transfer coefficient is important for the design,development and evaluation of systems and equipment.The coefficient of condensation is influenced by a variety of parameters such as mass flow rate,vapour quality,tube diameter,saturation temperature,refrigerant properties,heat flux,etc.The complexity of the influencing factors has led to the study of enhanced intube condensing heat transfer being one of the hot issues in the field of heat transfer at home and abroad.This paper presents an experimental study of the convective condensation heat transfer characteristics in three horizontal heat transfer tubes,including smooth tubes,using the refrigerant R410 A as the working mass.The local condensation flow patterns,heat transfer coefficients and frictional resistance penalty of R410 A in different types of enhanced tubes are investigated in relation to various parameters,and the performance of the two types of enhanced tubes is evaluated and compared.The local outlet flow patterns for condensation under different operating conditions were also determined based on the visualisation experimental results and compared with existing condensation flow patterns.In addition,a comparison of empirical correlation equations for prediction of heat transfer coefficients based on experimental data and a correction fit are also carried out.The experimental results show that the in-tube condensation heat transfer coefficient and frictional pressure drop of the refrigerant decrease with decreasing vapor quality and increase with increasing mass flow rate.The average heat transfer coefficients of Cu-HX and Cu-1EHT tubes are on average 106% and 88% higher than those of smooth tubes,respectively;the average frictional pressure drops of Cu-HX and Cu-1EHT tubes are 41% and 104% higher than those of smooth tubes,respectively.The enhancement mechanism of the dimple tube(Cu-1EHT)is mainly due to the secondary structure of the bulge and petal pattern,which breaks the wall boundary layer and increases the turbulence in the tube,causing fluid separation and secondary flow,while the enhancement mechanism of the internally threaded tube(Cu-HX),in addition to the above,also serves to redistribute the condensate film to reduce the resistance to condensation heat transfer in the tube.Under experimental conditions,a total of four condensate outlet flow patterns were observed inside the test tube: stratified wavy flow(SW),intermittent flow(I),semi-annular flow(SA)and annular flow(A).Both the Cu-1EHT and Cu-HX tubes showed an earlier condensation flow pattern transition and an enlarged annular flow region compared to the smooth tube.The experimental results were compared with the flow diagram models of Hajal et al.and Cavallini et al.and the theoretical flow map were reclassified based on the actual flow patterns.In addition,empirical correlations of heat transfer coefficients commonly found in the literature were compared with experimental results for condensing heat transfer,and the prediction models of Tang et al.and Cavallini et al.were compared with experimental results for smooth tubes within ±15% error.Based on 58 experimental data points for ripple tubes with different test conditions and geometrical parameters,a prediction model for the condensation heat transfer coefficient applicable to dimple tubes was proposed,and the prediction results of the newly fitted heat transfer correlation were within ±8% error,showing good agreement with the experimental data.