Investigation Of Heat Transfer And Flow Characteristics Of A Triple Concentric Pipe Minichannel Heat Exchanger

With the development and progress of industrial technology,modern industry has higher and higher heat transfer requirements.Especially in the face of the projects that need to meet high-intensity cooling requirements,there are always new design challenges that need to be addressed to realize more efficient heat transfer,which aims to achieve energy conservation and consumption reduction.Micro/mini channel heat exchangers have the potential to develop into a new generation of heat transfer technology due to their compact structure,high mass and heat transfer efficiency,and low cost.The concentric pipe heat exchangers are widely used in the different chemical industries because of their simple structure and convenient manufacture.In recent years,the triple concentric pipe heat exchanger,as a special concentric pipe heat exchanger,has gained wide attention for its higher heat transfer efficiency than the double concentric pipe heat exchanger.Therefore,this study combines the minichannel heat transfer technology and the triple concentric pipe heat exchanger and proposes a triple concentric pipe minichannel heat exchanger（TCPMHE）.A comprehensive study of heat transfer and flow characteristics of TCPMHE was implemented in this paper,aiming to provide some guidance for the further industrial application of the triple concentric pipe minichannel heat exchanger,and then realize the purpose of providing a new means for energy saving and emission reduction.Firstly,experiments were conducted to examine the influence of different parameters on heat transfer and flow performance of TCPMHE,and the parameters included the inner annulus size,the volume flow rate and inlet temperature of hot water in the inner annulus.The results were compared comprehensively with the double concentric pipe minichannel heat exchanger（DCPMHE）.Results showed that the friction factors（f）of DCPMHE and TCPMHE were almost equal.Although the Nusselt number（Nu）values of DCPMHE were 21.8%-50.7%higher than those of TCPMHE,the efficiency（ε）values of TCPMHE were much higher than those of DCPMHE,and the enhancement factor（η）ranged from 1.45 to 1.65.Through the dimensionless analysis,the correlations between Nu,f andεfor the TCPMHE and different experimental parameters were established respectively;The maximum relative deviations between the predicted values and the experimental values were no more than±8%,±4%and±11,and the average relative deviations were 3.6%,0.8%and 4.3%,respectively,which can provide a design basis for the industrial application of TCPMHE.Subsequently,using the passive heat transfer enhancement method,the helical coil was selected as the insert of TCPMHE,and the influences of volume flow rate and inlet temperature of hot water in the inner annulus on the heat transfer and flow performance of TCPMHE with different specifications of wire diameter and pitch of helical coil were investigated,and the results were compared comprehensively with the TCPMHE without helical coil insert.Results showed that the Nu values of TCPMHE with the helical coil insert were increased up to 1.3 times,whereas f was increased up to 5.8 times.Meanwhile,the performance evaluation criterion（PEC）was used to evaluate the comprehensive thermal performance of TCPMHE with different helical coil inserts,and the PEC value ranged from 1.02 to 1.72,which showed that the insertion of the helical coil improved the thermohydraulic performance of the TCPMHE.The backpropagation neural network（BPNN）was used to build an artificial neural network（ANN）model.The model enabled accurate prediction of Nu,f andεof the inner annular side of TCPMHE with insert for different operating and structural parameters.For the test set,the maximum relative deviations of Nu,f andεbetween the predicted values using the ANN model and experimental values the were no more than±10%,±10%and±14%,and the average relative deviations were 3.4%,2.8%and 2.5%,respectively,indicating that the BPNN model could be used to optimize the design of TCPMHE with the helical coil insert.Finally,the helical coil was selected as the insert of TCPMHE,and Si O₂micro-nanofluid with different concentrations was used as the hot fluid working medium in the inner annulus of TCPMHE.The heat transfer enhancement and flow performance of the TCPMHE were investigated using the compound enhanced heat transfer method.Results showed that using the compound enhanced method,Nu was increased up to 1.7 times,while f was increased up to 4.6 times andεwas increased up to 53.1%.Besides,PEC was used to evaluate the comprehensive thermal performance of TCPMHE under different conditions.Compared with the TCPMHE without helical coil insert and with deionized water as the fluid working medium,the PEC_c-no values of TCPMHE using the compound enhanced method were all greater than 1,and the PEC_c-no values ranged from 1.15 to 1.60,indicating that the comprehensive thermal performance of TCPMHE was significantly improved by the compound enhanced heat transfer method.Finally,the dimensionless correlations for Nu,f andεof TCPMHE using micro-nanofluid strengthening and compound enhanced methods were carried out,respectively,The maximum relative deviations between predicted and experimental values were no more than±7%,±12%,±8%and±11%,±8%,±9%,and the average relative deviations were 2.5%,6.0%,2.9%and3.9%,2.7%,2.4%,respectively,which provides a method for optimal the design of the novel heat exchanger.