Experimental Study On Convective Condensation Heat Transfer Mechanism Of Horizontal Dimple Tubes And Helix Microfin Tubes

In this paper,the single-phase flow heat transfer and pressure drop of refrigerant R410A in smooth tubes,dimple tubes and helix microfin tubes were analyzed and compared.At the same time,the flow condensation research was carried out,and then the influences of three surface structures on condensation heat transfer and frictional pressure drop under different mass flow rates,vapor quality and saturation temperatures,as well as different tube diameters and tube materials were used were explored.The experimental data were also analyzed in depth by comparing with the flow pattern inside the tube.The working conditions involved in the test process include:the saturation temperature is set at 35–45℃;the refrigerant vapor quality ranges from 0.1to 0.9;the mass flow rate ranges from 70 to 450kg m^-2 s^-1.Results indicated that EHT tube produced the highest single-phase flow heat transfer coefficient and pressure drop,followed by HX tube,and smooth tube had the smallest single-phase flow heat transfer coefficient and pressure drop.At the same time,the PEC factor of EHT tubes gradually increased in the range of 1.59-2.52 with the increase of mass flow rate,while that of HX tubes slightly decreased in the range of0.58-0.86 with the increase of mass flow rate.The condensation heat transfer coefficient（HTC）and frictional pressure drop decreased with an increase of the saturation temperature.Additionally there was an increase of the HTC with increasing mass flux and vapor quality;this was the result of shear force enhancement.At very low flow rates,the HTC of the enhanced tubes decreased with increasing mass flux.The helix microfin tube provided the maximum heat transfer surface area and the best drainage capacity.However,liquid droplets driven by surface tension made it easier for the fluid to reach the upper region of the tube,which reduced the heat transfer area between the gas and the tube wall.However,for the dimple structure of dimple tube,the turbulence generated by this type of reinforced surface was much stronger than the disturbance generated by helix microfin tube.The fluid flow characteristics can be reflected in the flow pattern,although the flow patterns at the outlets of the three tubes were consistent under the same conditions,the distribution of the liquid in the enhanced tube was more uneven and the liquid level was higher than that in the smooth tube.For the test conditions considered here the HTC enhancement ratio was in the range from 1.15 to 2.05 for the 1EHT tube and for the HX tube it ranged from 1.18 to 1.69,while the performance factor that taking into account the pressure drop is 1.02-1.57 and 1.23-1.78 respectively.Heat transfer performance of the smooth tube was determined to be slightly affected by the thermal conductivity of the tube;however,for enhanced tubes there was a more dramatic increase in the HTC for an increase of thermal conductivity.Additionally,better heat transfer performance was demonstrated for smaller diameter tubes.The specific sensitivity factor is introduced to evaluate the heat transfer enhancement effect of enhanced tubes.Through the analysis of specific sensitivity factor,it can be concluded that reducing saturation temperature is not a suitable method for enhancing heat transfer of enhanced tubes.However,using tube made of higher thermal conductivity and reducing the tube diameter are effective measures to improve the efficiency of enhanced tube.The results of pressure drop showed that the frictional pressure drop of dimple tube which palyed the most important role increased by 30%-40%,while that of helix microfin tube only increased by 5-20%.The frictional pressure drop increased with the increase of mass flow rate;the acceleration pressure drop and frictional pressure drop increased with the increase of vapor quality.Smaller tube diameter often led to larger frictional pressure drop;compared with different types of heat exchange tubes,three-dimensional enhanced tubes produced the highest frictional pressure drop,followed by the helix micro-fin tube and the smooth tube.Comprehensive evaluation of heat transfer performance of the enhanced tubes showed that the helix microfin tubes have good performance under most working conditions especially at low flow rates,and the three-dimensional dimple tubes were more suitable for high mass flow.