| The exhaust gas at the outlet of the sizing machine has a high temperature and contains a large amount of oil,particles,fibers and other impurities.The commonly used heat exchanger structure and gas side enhanced heat transfer technology are easy to block the flow channel at the flue gas side,which not only greatly reduces the waste heat recovery efficiency,but also leads to the shutdown of the heat exchanger after a certain period of operation.Therefore,a new spiral fin heat pipe heat exchanger structure is proposed.On the one hand,the smooth tube is used in the flue gas side to replace the flat fin tube for heat exchange,and the grease impurities are smoothly discharged along the smooth tube wall under the action of gravity;On the other hand,spiral fins are added to the air side instead of flat fins to enhance heat transfer.Firstly,the heat pipe element,the smooth tube bundle in the evaporation section and the finned tube bundle in the condensation section are designed and calculated according to the improved structure.The calculation results show that the working medium of the heat pipe is water and the pipe is low carbon steel,the liquid filling rate is 25%,the number of heat pipes is 24,a total of four rows,six heat pipes in each row,the length of smooth tubes at the evaporation end is 0.8m,and the length of finned tubes is 0.35m;The exhaust gas flows in countercurrent with fresh air.In addition,the spiral fin heat pipe heat exchanger has a total length of 0.5m,a total width of 0.35 m and a total height of 1.2m.The air flow rate in the flow channel is 2m/s and the exhaust gas flow rate is 3m/s.The exhaust gas temperature drops from 168℃ to 145℃,and the air temperature rises from 35℃ to 80℃.Secondly,the mathematical model of the condensation section of the spiral finned heat pipe heat exchanger is established,the heat transfer process outside the tube is numerically simulated by using FLUENT software,and the flow field distribution characteristics of the air side and the flue gas side are analyzed.It is found that the key to improve the overall performance of the heat exchanger and enhance heat transfer is to reduce the vortex area behind the tube,reduce the air flow boundary layer and enhance the disturbance of the tube bundle to the air.In addition,the heat transfer and resistance performance of spiral finned tube is compared with that of smooth tube and flat finned tube bundle with the same fin parameters,and the following conclusions are drawn:among the three types of fins,the performance of spiral finned tube bundle is the best.Compared with the smooth tube,the Nusselt number Nu of the spiral finned heat pipe bundle is increased by an average of 4 times,the Euler number EU is increased by an average of about 87.9%,and the comprehensive evaluation factor j/f is increased by an average of about 202.3%;Compared with the flat fin,the Nusselt number Nu of the spiral fin heat pipe bundle increases by 4.5% on average,the Euler number EU decreases by 5.4% on average,and the comprehensive evaluation factor j/f increases by 9.27% on average.Therefore,it is reasonable to use spiral fins instead of flat fins to strengthen heat transfer on the air side of heat pipe heat exchanger for waste heat recovery of sizing machine.Finally,the influence of arrangement parameters and spiral fin parameters on heat transfer and flow performance of the heat exchanger is analyzed,and the heat pipe heat exchanger is optimized.Through the calculation and analysis of Nusselt number Nu,heat transfer factor j,pressure drop Δp,Euler number Eu,resistance factor F and comprehensive evaluation factor j/f of different parameter combination models with inlet wind speed of 2~8 m/s,fin spacing of3~6mm,fin thickness of 0.8~1.4mm,fin height of 8~14mm,transverse spacing of 50~110mm and longitudinal tube spacing of 60~120mm.The optimized spiral fin heat pipe heat exchanger is manufactured and installed to recover the waste heat generated by a 110 k W setting machine.Through experimental measurement and calculation,the error between the measured value and the simulated value of heat exchange is 1.7%~9.7%,and the error between the two on the exhaust side is 2%~8%,which is consistent.The energy saving and economic analysis of the overall scheme is carried out according to the field operation.It is considered that the use of spiral fin heat pipe heat exchanger can save about 10% of natural gas,and the investment cost can be recovered in 1.1years. |
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