Study On Heat Transfer,Airflow Resistance And Frosting Performance Of Perforated Finned-tube Heat Exchangers

The air source heat pump（ASHP）has been widely used in China to control thermal comfort level in an indoor space because of energy efficient,compact,inexpensive to install and environmental protection etc.However,frost always form on the surfaces of the outdoor coil,namely finned-tube heat exchanger,when an ASHP works at the conditions of low temperature and high wet in winter.The accumulated frost dramatically decreases the performance of the outdoor coil by increasing both heat exchange and air flow resistance,which adversely degrades the system,increases the energy consumption,or even results in an undesired shutdown.With the rising of serious energy crisis,there has been a growing interesting in relieving the harm of frost accumulation on the ASHP.The fin type of the outdoor coil is one of the key factors that affect the heat transfer and airflow resistance of air sides,so does the frost accumulation.In order to improve the flow and heat transfer performance of air side under dry conditions and frosting conditions and delay the frost growth,this paper suggests to perforate circular holes and elliptical holes on the surface of fin based on the theory of fluid mechanics,heat transfer and thermodynamics.The main contents of this study included are as following.Perforated fin study.Based on orthogonal experiment method,fins perforated with elliptical holes and circular holes were designed while the diameter,d,the transverse tube pitch,W₁,and longitudinal tube pitch,Y₁,were chosen as variable parameters.Besides,the performance of the heat transfer and airflow resistance for plain finned-tube heat exchanger and perforated finned-tube heat exchanger were calculated by Computational Fluid Dynamics,CFD,which was verified by experimental data obtained from the public literature.The results of range-method showed that the rankings of the most influence on the Colburn factor,j,and the friction factor,f,were d,W₁,and Y₁.Besides,circle10 fin and ellipse10 fin were designed to avoid missing of the best performance of perforated fin.Compared with the plain fin,the j values of fins perforated with biggest elliptical holes（d₁=5.6mm/d₂=4.5mm）and circular holes（d=5mm）are increased by 11.46%²3.92%,7.83%²3.13%while the f values are increased by 11.58%²7.43%,10.24%²2.95%.The calculations for the comprehensive index of j/f^1/2 and j/f^1/3 indicate that the performance of perforated fins outperform the plain fin under the conditions of the same pressure drop and pump power.And ellipse 9 fin and circle 10 fin appear to the best performance.According to the mechanism analysis,the development of temperature and velocity boundary layers are prevented effectively at the region of elliptical holes or circular holes on the fin,which enhances flow turbulence and achieve the aim of heat transfer enhancement.But at the same time,the airflow resistance gets to improve.Frost test study.Combined with the optimization results of CFD,the three finned-tube heat exchangers based on ellipse 9 fin,circle 10 fin and plain fin were custom-made,respectively.The contrastive tests were made of the three finned tube heat exchangers under frosting conditions.The results of experiments are described as follows.1)Effects of fin type.The increase of airflow resistance of ellipse 9 and circle 10 finned-tube heat exchangers are slower about 10min²0 min than the plain finned-tube heat exchangers at the conditions of inlet velocity of moist air set as 1m/s,2m/s,and 3m/s,which indicates that the perforated finned-tube heat exchangers can delay the growth rate of frost layer volume.Compared with plain finned-tube heat exchangers,the frost accumulation amount,the heat exchange amount,the COP and the equivalent coefficient of convective heat transfer of the unit installed with circle10 finned-tube heat exchangers get increase by 9.85%²6.89%,7.58%²4.83%,5.48%⁷.69%and 24.18%⁵4.40%,respectively.The increase of the parameters above of the unit installed with ellipse 9 finned-tube heat exchanger are 14.64%²8.33%,8.76%²5.99%,8.28%⁹.76%and 29.39%⁶2.66%,respectively.2)Effects of inlet velocity.The increasing of inlet velocity of moist air also contribute to delay the volume growth of frost layer,and improve the heat transfer capacity,the average equivalent heat transfer coefficient,the COP and the frost accumulation amount of the unit.However,the influence of delaying the frost growth will not increase when the air velocity reach a value.Meanwhile,a higher air velocity will result in a greater airflow resistance,so a suitable air velocity should be selected according to actual conditions.Analysis for frost growth delaying and correlation fitting.The observation of experiment reveals that the frost layer grows and distributes on the surfaces of the finned-tube heat exchanger unevenly.It is to exhibit that the frost layer grows more fastly on the surfaces of front fins than rear fins.The nonuniform distribution of the frost layer results in losing heat transfer performance of rear fins because of airflow decreasing sharply caused by blocking of frost layer on front fins.The both of perforating holes and increasing airflow velocity can enhance the disturbance to moist air on the surfaces of fins,which prompts the flow of condensation water from front fins to rear fins before freezing.Then it is to prompt the uniform distribution of the frost layer on the surfaces of front and rear fins,slowdown the frost layer growth,and prolong the defrosting period.Meanwhile,the effect of disturbance enhanced also increases frost accumulation amount.The correlations of airflow resistance and heat convection of ellipse 9 and circle 10finned-tube heat exchangers are fitted under frosting conditions utilized of the test data.The dimensionless time,Fo,was used in the correlations which can be used to calculate the performance of airflow resistance and heat convection of perforated finned-tube heat exchangers at different times under frosting condition.Frost accumulation prediction model study.In this study,the ANFIS method was developed to estimate the frost accumulation with high accuracy.For the ANFIS model,the six input parameters of j/f^1/3,wall temperature,air velocity,air temperature,relative humidity and time over an extensive range were proposed as input vectors to predict the frost accumulation.And the index of j/f^1/3 was proposed firstly to estimate quantitatively the influence of finned-tube exchanger geometry on frost accumulation.The 5005experimental data points used to develop and validate the presented model was obtained from this work and the public literature.Statistical analysis of R² and RMSE were carried out to indicate the performance of the developed models.R²=0.9985 and RMSE=0.6945for the training,R²=0.9985 and RMSE=0.6945 for the testing,respectively,by comparison with the corresponding experimental data,show that the ANFIS model are trained and tested adequately.The error of this investigation compared to the experimental data is considerably big at the beginning and decreases soon,finally it keeps within 5%.That demonstrates that the index of j/f^1/3,representing the geometry of finned-tube exchanger,such as the fin type,the fin pitch,the number of rows,etc,can be used in the ANFIS model to predict the frost accumulation,which estimate quantitatively the influence of finned-tube exchanger geometry on frost accumulation and make the ANFIS model to estimate the frost accumulation on different geometry of finned-tube exchanger geometry under different frost conditions.Compared with existing theoretical model,ANFIS model in this study shows advantages of high accuracy,convenience,wide application etc.