Numerical Simulation Of High Temperature Phase Change Regenerator Modulating SCR Inlet Temperature

The air pollution caused by automobile exhaust needs to be solved urgently.Countries have increasingly stringent requirements for the emission limits of gaseous pollutants and particulate matter from diesel engines.In the face of stringent emission standards,diesel engine cold start and exhaust transient temperature control under low exhaust temperature conditions have become an important breakthrough to improve the efficient NOx conversion of SCR post-treatment system under all working conditions.In view of the above problems,this subject designs the medium and high temperature phase change accumulator modulation SCR inlet temperature control system to recover the excess heat under high temperature conditions,which is used to heat the tail gas under engine cold start and low exhaust temperature conditions,so as to effectively control the inlet temperature of SCR reactor.The phase change heat storage device can meet the requirements of load fluctuation within a certain range,avoid the low inlet temperature of tail gas post-treatment system,and match the tail gas temperature with the high efficiency window of catalyst.In order to design a practical and reasonable SCR inlet temperature control system of diesel engine,the exhaust gas state and energy are analyzed with the6-cylinder diesel engine as the analysis object.The results show that the available power of exhaust gas accounts for about one-third of the effective power of diesel engine,and there is a large proportion for recycling.Through the analysis of general design principles,the overall design of phase change heat storage heat exchanger is carried out,the available phase change material solar salt is selected according to the actual working conditions,the heat storage capacity of the heat accumulator is theoretically calculated,and the overall size of phase change heat storage body is determined to be 136 mm in diameter × 400mm。In order to study the heat storage and release characteristics of phase change heat accumulator,the coupling heat transfer model of phase change heat transfer unit tube and thermal fluid is established.The distribution law of heat transfer coefficient and heat transfer coefficient of solid-liquid heat storage unit under different phase change materials are simulated and analyzed.The results show that:1.For the smooth tube casing unit filled with solar salt material,during the heat storage process,due to the small thermal conductivity of the heat storage material and the large thickness of the heat storage layer,the global temperature difference of the heat storage body is large,reaching more than 200 K.2.for the light tube sleeve element of foam metal matrix Solar-salt composite filled with enhanced thermal conductivity,the overall temperature difference of the regenerator greatly decreases during the thermal storage process,and decreases to60 K.The melting time of phase change material is more concentrated and the heat storage efficiency is improved.3.for the fin tube sleeve unit of foam metal matrix Solar-salt composite phase change material filled with enhanced thermal conductivity,the fin structure strengthens the heat transfer between heat flow and phase change material,which is limited by the thermal conductivity of the thermal storage material,the global temperature difference of the phase change heat storage body is 103 K,the melting time of the phase change material is nearly 100 s,the phase transition time is relatively concentrated,and the heat storage efficiency is relatively high.4.for the fin tube sleeve element of foam metal matrix Solar-salt composite phase change material filled with enhanced thermal conductivity,phase change material is heated at high temperature in the phase change exothermic process,but the heating capacity of the near wall gas is strong,but the duration is short.The temperature of the heat storage material near the inner wall of the heat exchange unit tube can be maintained at 493 k for a long time.When the exhaust near the wall reaches the outlet,it is heated to about 473 k.By analyzing the heat transfer performance of heat storage body under different structural parameters,the parameters of phase change heat storage with optimized structure are determined.Finned tube straight through type is adopted,which is circularly distributed in the regenerator.The outer diameter of the regenerator is d =136mm,the diameter of the inner flow channel is d = 12 mm,the center distance of the inner flow channel is p = 24 mm,and the pipe length is L = 400 mm.There are annular fins in the flow channel in HTF,with fin height h = 2mm,thickness t = 2mm and spacing S = 50 mm.The numerical simulation of the heat storage and heat release process of the regenerator is carried out by fluent.The results show that for the heat storage process,compared with the single channel finned tube to complete the heat storage process in 1300 s,the optimized multi-channel finned tube can heat the regenerator to more than 520 k in only 848 s when the heat flow tail gas is 673 k,saving34.7% of the time.For the heat release process,for the regenerator with an initial temperature of 520 k,the multi-channel sleeve fin regenerator can stably heat the tail gas from 423 K to more than 490 k for 1800 s.Finally,the performance of the temperature control system is analyzed based on the WHTC test cycle.The results show that through the necessary thermal insulation of bypass pipeline and thermal insulation packaging of heat accumulator,the exhaust gas temperature at the outlet of phase change accumulator can be stably controlled at220 ℃ ～ 300 ℃ in cold start test cycle and hot start test cycle,and the efficient purification of NOx can be realized in this temperature range.