Study On Heat Transfer And Flow Characteristics Of New Type Of Pitted Tube

High viscosity fluid heat exchangers are used in various fields such as chemical,aerospace,machinery,and food.Due to the low thermal conductivity and high heat transfer resistance of high-viscosity fluids,high-viscosity fluid heat exchangers also have problems of weak heat transfer and high resistance.In order to improve the reliability of the equipment and design a more compact and efficient high-viscosity fluid heat exchanger,this paper proposes the idea of the pitted tube based on the reduction characteristics of the pits on the golf ball.The numerical simulation of the flow and heat transfer characteristics of the high viscosity fluid watershed is presented,which provides a theoretical reference for the design of high viscosity fluid heat exchangers.The main work contents and results are as follows:(1)Using the method of three-dimensional numerical simulation,the flow and heat transfer characteristics of lubricating oil traversing a single pipe are studied and analyzed.The results show that the existence of pits strengthens the energy and momentum exchange in the boundary layer,and strengthens the heat transfer of the pitted tube.The pits cause the boundary layer separation of the pitted tube to be postponed,and the flow resistance of the pitted tube also decreases.The enhanced heat transfer and drag reduction effect of the pits make the comprehensive heat transfer performance of the pitted tube superior to that of the smooth pipe.(2)Numerical simulation calculation of the pitted tube with different inlet oil speed,pit structure parameters and pit arrangement type is carried out.The results show that the comprehensive heat transfer performance of the pitted tube can be improved by reducing pit diameter d,increase pit depth h,reduce axial pit distance p and circumferential pit distance l within the scope of this study.The Nu value of the pitted tube is increased by28.6%-55.4% compared with the smooth pipe.The f value is also reduced by 0.59% to26.9% compared with the smooth pipe,and the PEC value is in the range of 1.05 to 1.59.(3)Set the Nu value,f value and PEC value as the objective function.The response surface design in Design-Expert software is used to optimize the parameters of the pitted pit structure.The optimal parameter combination obtained by the system is: Re=1144,d=2.2mm,h=1.1mm,N=12,p=10mm,corresponding to Nu=209.95,f=1.08,PEC=1.6.(4)The optimal single tube structure of the pitted tube obtained above is a calculation unit,and the numerical simulation of the pitted tube bundles with in-line arrangement and staggered arrangement is carried out.The results show that: because of the pits of pitted tube,the flow velocity between pitted tube bundles is higher than that of smooth tube bundles,and the temperature field changes are more significant than that of smooth tube bundles.The flow field and temperature field changes in the staggered tube bundles are more complicated than those in the in-line tube bundles.(5)The heat transfer and flow characteristics of in-line tube bundles and staggered tube bundles with different relative transverse pitch s1/D,relative longitudinal pitch s2/D and fluid Reynolds number Re were simulated.The results show that the Nu value of the in-line pitted tube bundles is increased by 7.6%～47.7% compared with the the in-line smooth tube bundles,and the f value is decreased by 0.66%～7.4%.The Nu value of the staggered pitted tube bundles is11.6% to 50.2% higher than that of the staggered tube bundles,and the f value is reduced by 2.4% to 10.1%;the comprehensive heat transfer performance of the different tube spacing and arrangement type of the pitted tube bundle is superior to that of the smooth pipe bundle.(6)Set the Nu value,f value and PEC value as the objective function,and optimize the parameter design of the oil velocity v,relative transverse pitch s1/D and relative longitudinal pitch s2/D that affect the flow and heat transfer characteristics of the pitted tube bundle.According to the system,the optimal combination of parameters of in-line tube bundle is obtained as follows: inlet oil velocity v=1.8m/s,relative transverse pitch s1/D=1.68,relative longitudinal pitch s2/D=1.45,corresponding to Nu=214.99,PEC=1.47.The optimal parameter combination of fork row tube bundle is: inlet oil velocity v=1.8m/s,relative transverse pitch s1/D=1.75,relative longitudinal pitch s2/D=1.33,corresponding to Nu=233.53,PEC=1.46.