Numerical Investigation Of Flow And Heat Transfer Characteristics Inside Helically Twisted Lobed Tubes

Compact heat exchangers got widely used recently and,due to this,they are in the close attention of many researchers.The ways to further increase the efficiency of compact heat exchangers without significantly increasing the cost of their design have been actively developed,and one of these methods is the usage of twisted n-lobed tubes.However,studies on 3 and 4 lobed tubes are usually limited to the case of water as a working medium.At the same time,there is no detailed analysis of the influence of the structural parameters of the 4-lobed tube on flow characteristics and heat transfer.Therefore,this work presents a comprehensive analysis of the influence of the structural characteristics of the 4-lobed tube on the flow characteristics and heat transfer.A comparison between 3 and 4 lobed tubes was also made.For 3 mediums,namely supercritical CO2,CH4 and water,the influence of operation conditions on flow characteristics and heat transfer characteristics was investigated.Firstly,using the ANSUYS FLUENT software package,a three-dimensional model of the considered cross section geometries was developed using the following settings: steady state,SST k-? turbulence model.Next,the influence of structural parameters on pressure drop,heat transfer coefficient and TPF was analyzed.The results showed that the higher the L/D ratio of the tube and the twist angle ?,the higher the thermal conductivity and pressure drop.However,analyzing the value of TPF(heat transfer efficiency compared with the case of a conventional round tube),it was concluded that the maximum TPF is achieved with: for 4-lobed tube: L/D = 0.05,? = ? = 1.285o;for 3-lobed tube: L/D = 0.05,? = 2.57o.After that,a comparison of these crosssection shapes was carried out,as a result of which the 4-lobed tube was selected for further investigation.Secondly,an analysis of the effect of operation conditions on flow and heat transfer characteristics for supercritical CO2,CH4 and water was made.As a result,it was revealed that for all three of the given working mediums,TPF decreases with an increase in the Reynolds number.For supercritical CO2 before the critical point with increasing inlet temperature,a lower temperature and heat flux show a higher TPF,and after the critical point a higher temperature and heat flux shows a higher TPF.This is due to the nonlinear nature of S-CO2 characteristics.With an increase in in temperature for supercritical CH4,both the pressure drop and the heat transfer.Based on the results of numerical simulation,correlation equations were developed for all three working mediums.These equations are of practical value because they allow one to obtain the main parameters of the flow and heat transfer for the cases under consideration.Verification of these equations showed their applicability for predicting the flow and heat transfer parameters obtained in the study.