Study On Flow Boiling Heat Transfer Characteristics In Microfin-Smooth Circular Tube

As an efficient heat transfer method,the flow boiling heat transfer in the tube was widely used in refrigeration,air conditioning and industrial heat dissipation.In order to obtain more efficient heat transfer capability,the flow boiling heat transfer enhancement method in tube were studied by scholars.The commonly used methods include the application of tube inserts（twisted tape,screw tape,screw coil,etc.）and the optimization of heat transfer tube surface（using microfin structure,corrugated surface,etc.）.Although these tubes can optimize heat transfer,they also increase the flow resistance.Therefore,it is necessary to improve the surface structure of heat transfer enhancement tubes to achieve the optimal comprehensive performance.In this paper,a piecewise combination of smooth tube and microfin tube is proposed to form a smooth-microfin-combined tube,which aims to improve the nucleation ability of the tube and reduce the increase of flow resistance.Based on the experimental platform for flow boiling heat transfer of different tube types,the flow boiling heat transfer and pressure drop characteristics in horizontal smooth tube,microfin tube,fin-smooth tube and smooth-fin tube were investigated and compared.The main conclusions of this paper are as follows :（1）The single-phase and two-phase verification experiments of the smooth tube show that the heating efficiency of the test section is about 87.86 %.The single-phase heat transfer coefficient is in good agreement with the prediction results of Gnielinski correlation,and the deviation is within ±20 %.The boiling heat transfer coefficient is in good agreement with the prediction results of Yoshida correlation,and its MAE is about 8.88 %.The experimental system shows good reliability.（2）The heat transfer area increase ratio of the microfin tube is about 1.57,and the enhancement factor（EF）is between 1.40 and 2.24.The area increase ratios of the combined tubes（fin-smooth and smooth-fin）were 1.29,but the EF of fore smooth-fin tube and fin-smooth tube are 1.09-1.49 and 1.43-1.78,respectively.In addition to the increase of heat transfer area,the thread distribution form also plays a key role in strengthening heat transfer.（3）At low heat flux,the heat transfer coefficient of the microfin tube is significantly affected by the mass flux,and the convective boiling dominates the heat transfer.With the increase of heat flux,nucleate boiling gradually dominates the heat transfer.When the mass flux is low,the heat transfer coefficient of the fin-smooth tube changes significantly with the mass flux,and the dominant heat transfer mechanism in the tube is convective boiling.In the smooth-fin tube,nucleate boiling dominates the mechanism.In addition,the friction pressure drop per unit length in the four test tubes increases with the increase of mass flux and heat flux.（4）Under different heat flux,the EF of the microfin tube and the smooth-fin tube gradually decrease with the increase of mass flux,while the fin-smooth tube shows an increasing trend.This rule is also reflected in the change of EF with heat flux.Under the condition of high heat flux and high mass flux,the EF of the fin-smooth tube is higher than that of the microfin tube and the smooth-fin tube.In addition,the penalty factor（PF）of the three heat transfer enhancement tubes are between 0.86 and 1.20,the three values are close,but the microfin tube is slightly larger than the combined tube.（5）At low mass flux,the merit factor（MF）of the microfin tube are the highest,while at high mass flux,the MF of the fin-smooth tube is the highest.With the increase of mass flux,the MF of the fin-smooth tube will gradually exceed that of the microfin tube and the smooth-fin tube,and the mass flux at the exceeding point will gradually decrease with the increase of heat flux.At the same time,no matter under the comprehensive performance evaluation with the same pressure drop or the same pump power as the standard,the three heat transfer enhancement tubes can produce positive heat transfer enhancement effect,that is,the increase of pump power and unit pressure drop is effectively inhibited while the heat transfer is improved.