Simulation On Heat Transfer Performances Of Hollow Tube Electric Heaters With Dual-side Longitudinal Flow

The dual-sided flow hollow tube electric heater is a novel solution for heating the fluid by using the heat transfer tube itself as a heating element for the improvement of the conventional U-tube electric heater.The traditional electric heater can only heat the fluid outside the heating core tubes,while the hollow tube electric heater fully utilizes the heating space inside and outside of the tubes,increasing the heat exchange area of the fluid.The outer tube side of the original design of the hollow tube electric heater is supported by full circular plates,and only the annular gap between the housing shell and the supporting plate flows,the stagnant zones are formed between the supporting plates.In this paper,an improved scheme for opening holes on the supporting plate was proposed and investigated.The holes are evenly distributed on the supporting plate,so that the flow on the shell side becomes longitudinal flow,eliminating the flow stagnant zones.In this paper,the physical and mathematical models of the longitudinal dual-sided flow hollow tube electric heaters with plate blocked scheme and perforated plate schemes of countercurrent,parallel and split were established.The model grid was drawn by ANSYS ICEM,numerical simulation was performed by software FLUENT,and the visualization and flow analysis were carried out by using the visualization post-processing analysis software TECPLOT.The main performance parameters such as shell side pressure drop,shell side shell side heat transfer coefficient,comprehensive index,heating tube wall surface average temperature and overall pressure field and velocity field distribution of the electric heaters were analyzed.By comparing the calculation results of the four schemes,the best solution for flow and heat transfer performance is obtained.The numerical simulation results show that the two perforated schemes of parallel and split can reduce the temperature difference between the fluid on both sides of the fixed tube sheet,reduce the thermal stress of the fixed tube sheet,and prolong the service life of the equipment.Under the same conditions,the electric heater of the split scheme has the highest comprehensive index h_o·?p_o^-1 and h_o·?p_o^-1/3and the lowest shell side pressure drop,and the average shell side heat transfer coefficient and comprehensive index h_o·?p_o^-1 and h_o·?p_o^-1/3 were 33.4%and 136.2%and 61.5%higher than the plate blocked scheme,respectively,and the average pressure drop and wall temperature were 43.6%and 21K lower than the blocked scheme,respectively.In addition,it can reduce the thermal stress on the sides of the fixed tube sheet.Secondly,based on the performance studies of four schemes of plate blocked,countercurrent,parallel and split,the effect of changing the diameter of electric heating tube on the performance of electric heater was studied.The outer diameters of the electric heating tubes are 24mm,25mm and 26mm respectively.12 electric heater schemes were designed and numerical simulations were carried out on them.The results show that with the increase of the diameter of the electric heating tubes,the heat transfer coefficient and pressure drop of the shell side of the four-electric heater scheme decrease;while the shell side comprehensive indexes h_o??P_o-¹ and h_o??P_o-^1/3 increase with the rise of the tube diameter.Therefore,appropriately increasing the diameter of the electric heating tube with constant tube pitch can effectively improve the flow and heat transfer performance of the electric heaters.By observing the velocity,pressure,temperature fields and other naphagrams of 12 electric heater schemes with variable diameters,it can be seen that with the increase of the diameter of the electric heating tube,the flow velocity of the fluid in the electric heating tubes has dropped significantly,while it rises at the shell side to a certain extent,effectively increasing the disturbance on the shell side,thereby enhancing the heat transfer at the shell side.At the same time,the shell side pressure drop reduces slightly due to reduced difference between the flow area at the space and at the holes.Thus the shell side flow and heat transfer performance are optimized to some extent.Although the performance of the shell side temperature field is not obvious,according to the calculation results,the temperature of the outer wall surface of the electric heating tubes decreases with the increase of the tube diameter,and the maximum temperature also decreases,which has a certain improvement in the service life of electric heaters.Thirdly,the influence of the hole diameter on the supporting plates of the perforated schemes and the outer diameter of the supporting plates of the blocked scheme on the flow and heat transfer performances of the electric heater is further studied.Nine variable hole diameter schemes with hole diameter of 14mm,15mm,and 16mm for the perforated schemes are designed,and three outer diameters of 119mm,120mm,and 121mm of the supporting plates were provided.To change the hole diameter for the perforated schemes or the outer diameter of the supporting plate for the blocked scheme means to alter the flow section area of each supporting plate.The numerical simulation results show that with the increase of the hole diameter on the supporting plate for the perforated schemes or the decrease of the outer diameter of the supporting plate for the blocked scheme,a decrease in the heat transfer coefficient and a drastic decline in the pressure drop will be resulted in on the shell side,which eventually leads to an upward trend for the comprehensive indexes h_o??P_o-¹ and h_o??P_o-1/3.Therefore,it can be known that appropriately increasing the hole diameter of the supporting plate or reducing the outer diameter of the supporting plate can improve the comprehensive flow and heat transfer performance of the electric heaters.By comparing the variable tube diameter schemes with the variable hole diameter schemes,the effect of the variable hole diameter schemes on the shell side pressure drop is greater but that on the shell side heat transfer coefficient is weaker than those of the variable tube diameter schemes,.Although the impacts of the two variable parameters on the comprehensive indexes h_o??P_o-¹ and h_o??P_o-^1/3 show upward trends,the variable tube diameter scheme performs better than those of the changing section area schemes.