Experimental Study On Condensation Heat Transfer Mechanism In Composite Enhanced Tubes And Internal Thread Tubes

In this paper,the heat transfer of condensation flow in internally threaded tubes with the single structure,diamond pattern composite enhanced tubes and strip pattern composite enhanced tubes with the composite structure were investigated experimentally.The effects of the three enhanced tubes on heat transfer coefficient,friction pressure drop and flow pattern in the condensation were investigated.The condensation heat transfer performance in the enhanced tubes and smooth tubes was investigated at various working conditions such as different mass flow rates,refrigerant vapor quality and saturation temperature.The flow patterns in enhanced tubes and smooth tubes were investigated at different vapor quality and mass flow rates.The strengthening effect of the enhanced structure on the condensation heat transfer and the promotion effect on the flow pattern transition are analyzed.On the one hand,this study reveals the heat transfer effect of the new structure and enriches the condensation heat transfer database.And provides references and design ideas for the design of enhanced structures,which is beneficial to the miniaturization and efficiency of condensing heat exchangers on the other hand.The results of the investigation show that the condensation heat transfer coefficient of the internal threaded tube is about 1.60～1.74 times that of the smooth tube,the heat transfer coefficient of the diamond pattern composite enhanced tube is about 1.17～1.54 times that of the smooth tube,and the heat transfer coefficient of the strip pattern composite enhanced tube is about 1.1～1.28 times that of the smooth tube.The threaded structure of the internal threaded tube changes the refrigerant flow direction and promotes fluid mixing,while the internal threaded tube has the largest heat transfer area.The crossed lines and bulging tiny structure of the diamond pattern composite enhanced tube intensify the fluid disturbance,triggering the secondary flow and promoting heat exchange.The stripe structure of the striped composite enhanced tube facilitates the export of condensate,and the patchy surface enhancement structure causes the destruction and thinning of the thermal boundary layer,reducing the heat transfer thermal resistance.Frictional pressure drops increase by 18～38% for internally threaded tubes,17～50% for diamond pattern composite enhanced tubes,and 7～17% for strip pattern composite enhanced tubes compared to smooth tubes.The enhanced structure leads to local fluid extrusion,generating greater shear and friction,while the resulting fluid disturbance and the superposition of secondary flow will also generate more frictional losses.Generally speaking,as the refrigerant mass flow rate increases,the heat transfer coefficient increases.The diamond pattern composite enhanced tube has excellent heat transfer at high mass flow rate,and the internal threaded tube has high heat transfer performance.Before reaching the dryness state,the heat transfer coefficient shows an increasing trend as the vapor quality increases.Condensation heat transfer coefficient decreases with the increase of the saturation temperature.By using the turbulence enhancement factor C and the area expansion ratio K,the Cavallini correlation is modified.The resulting correlation deviated from the experimental results within ±20%.The stratified wavy,intermittent and annular flows were observed in the heat exchanger tubes with increasing mass flow rate and vapor quality by visual observation experiments.The flow pattern maps of the smooth tube are in good agreement with the flow pattern predictions of Hajal et al.The enhanced structure promotes the early transformation of the flow pattern,where the promotion effect is more significant for the diamond pattern composite enhanced tube.