Study On The Enhancement Of Moist Air Condensation Heat Transfer Outside Horizontal Three-Dimensional Finned Tube

China’s industrial energy consumption is mainly dominated by fossil fuels,and the efficient and deep recovery of a large amount of gaseous waste heat resources generated during its utilization will effectively boost the realization of China’s ‘dual carbon’strategic goal.In the process of deep recovery of gaseous waste heat resources,condensation heat transfer of moist air in the presence of a large amount of non-condensable gas is one of the key problems.This problem also exists in the air conditioning,seawater desalination,nuclear passive cooling system,and other fields.Therefore,the condensation heat transfer characteristics of moist air and its enhancement have always been the research focus in the field of engineering thermophysics,and also has important academic value and engineering application value.Three-dimensional finned tube is an effective heat transfer element,but there is few research on the condensation heat transfer of moist air outside three-dimensional finned tube.In this paper,the condensation heat transfer characteristics of moist air outside the horizontal three-dimensional finned tube are studied by means of experiment and numerical simulation.Firstly,the condensation of moist air on a vertical plate surface of three-dimensional tube was set as research object,and the latent and sensible heat transfer model was built,and its changing law and influence on the total heat transfer process were revealed by numerical calculation.Secondly,the enhanced heat transfer performance of three-dimensional finned tube under different moist air parameters was studied experimentally,and the influence of fin structural parameters,such as fin height,width,circumferential spacing and axial spacing on heat transfer performance was studied,and the optimized fin structural parameters were obtained.Then,the motion characteristics of condensate on three-dimensional finned tube with different surface wettability and its influence on heat transfer performance were analyzed.Finally,through numerical simulation,the change law of moist air condensation heat transfer outside the single tube and tube bundle were analyzed,and the flow and heat transfer performance of tube bundle under different arrangement and different moist air parameters were obtained.The main conclusions are as follows:(1)A theoretical model of condensation heat transfer of moist air on a vertical flat surface was established.The influencing mechanism and changing law of single-phase convective heat transfer and condensation heat transfer of moist air condensation heat transfer were elucidated.The condensation heat transfer characteristic under different moist air temperature,velocity,steam mole fraction,wall temperature was obtained.The results show that,the single-phase convective heat transfer coefficient basically remains unchanged with the increase of the relative humidity of moist air,but condensation coefficient increased obviously,which improves the overall heat transfer performance.Therefore,the phase-change heat transfer is the main influential factor;Under the condition of 95% relative humidity,333 K moist air temperature and 303 K wall temperature,the single-phase convective heat transfer coefficient and phase-change heat transfer coefficient have the same trend with moist air velocity(0.1-16 m/s),and the ratio between them remains constant about 0.17.(2)The condensation heat transfer characteristics of moist air on horizontal three-dimensional finned tube were experimentally studied,and the influence of moist air parameters and three-dimensional fin structure on heat transfer performance was revealed.The results show that the condensation heat transfer coefficient of three-dimensional finned tube can reach 1.7 times that of smooth tube when the steam mole fraction is 0.05,the moist air temperature is 373 K and flow rate is 2.4 m/s.It is found that fin height has the greatest influence on heat transfer coefficient and contributes 75.7% for it.The contribution rate of axial fin spacing increased with the increase of relative humidity.The condensation heat transfer coefficient of three-dimensional finned tube increase at first and then decrease when the relative humidity is 0.6 and axial fin spacing increase from 1.5 mm to 2.5.The heat transfer performance is the best when the axial fin spacing is 2.0 mm.The effect of single-phase and phase-change convective heat transfer on three-dimensional finned tube were considered at the same time,and the empirical correlation formula was fitted for the prediction of Nu number.The deviation between the predicted results and experimental results was less than ±15%.(3)Experiments were carried out to study the convective heat transfer characteristics of moist air outside three-dimensional finned tube with different wettability.The results show that although surface hydrophobicity and super-hydrophobicity induced dropwise condensation,the retention time of condensate on the surface of the three-dimensional finned tube increased significantly,which restricted the further improvement of the condensation heat transfer of three-dimensional finned tube.The heat transfer coefficient of the hydrophilic three-dimensional finned-tube is the highest,and can be 1.94 times that of the hydrophilic smooth tube.(4)A novel heat transfer enhancement geometric model of three-dimensional finned tube with different circumferential fin height was proposed.The Eulerian Wall Film model and Diffusion Balance Model were adopted to calculate the condensation heat transfer characteristic of moist air outside three-dimensional finned tube.The distribtion of condensation rate on three-dimensional finned tube with different fin height was obtained,and the ratio of single-phase and phase-change heat transfer with different moist air parameters was obtained.The results show that when the moist air velocity is 2.4 m/s,the three-dimensional fins have larger condensation heat transfer rate compared with base tube surface.In addition,in all alien finned tubes,the greater fin height increment ratio,the stronger the heat transfer ability can be achieved than finned tube with uniform fin height.When the steam mole fraction increases from 0.05 to 0.15,three-dimensional finned tube with 1.5 fin height increase ratio can achieve 36%to 152% higher heat flux compared with finned tube with uniform fin height.(5)The condensation heat transfer characteristics outside horizontal three-dimensional finned tube bundle under different tube arrangement and moist air parameters were studied by numerical simulation.The results show that compared with the in-line tube bundle,the increment of heat flux between staggered three-dimensional finned tube bundle and smooth tube was higher.Besides,when steam mole fraction(0.1～0.15)and moist air velocity(1～10 m/s)increased,the convective heat transfer coefficient increment ratio between three-dimensional finned tube bundleand smooth tube bundle becomes lower.Based on the calculation results of staggered three-dimensional finned tube and smooth tube bundle,empirical correlations were fitted to predict the flow and heat transfer performance,and the deviation of Nu number between the predicted results and numerical results was less than ±10%,and the deviation of flow fraction factor f was less than ±20%.