Study On Overall Flow Field Simulation And Local Performance Of Combined Plate Air Preheater

Heating furnace air preheater is an important energy recovery device,which is used for heat recovery of heating furnace tail gas and preheating of combustion supporting air in refinery.Therefore,the heat exchange efficiency of air preheater is very important,which determines the utilization degree of flue gas waste heat and has an important impact on the overall energy consumption of refinery.The research object of this thesis is the combined plate air preheater of coking heating furnace in a refinery.At present,there is a problem of flue gas temperature deviation on both sides of the equipment.This thesis studies the equipment from two aspects: global flow field simulation and local flow field simulation,analyzes the causes of the problems,and optimizes the preheater structure to improve the heat exchange efficiency.In view of the complex structure of plate air preheater and the difficulty in constructing the complete structural grid,this thesis adopted the porous medium model to simplify the structure of each heat transfer unit in the high and low temperature sections of air preheater and constructed the simplified geometric models of flue gas side and air side.In the process of solving the parameters of porous media,a local model of heat exchange unit was built,the flow characteristics in the heat exchange unit are studied,and the relationship between flow velocity and pressure drop was obtained.Numerical simulation was carried out on the overall flow field of the flue gas side and the air side.The simulation results showed that the boundary layer separation occurred at the flue elbow on the flue gas side,and the downstream section was too short,resulting in uneven velocity distribution at the outlet of the flue.The boundary layer separation occurred when the fluid diverges at the inner wall of the rectangle-circle connector,and a wide range of eddy currents were generated downstream.The existence of vortex zone made the fluid velocity near the inner wall of the inlet of the heat exchange unit of the air preheater on both sides low.At the same time,the velocity distribution on the whole inlet surface was uneven,and the velocity unevenness on the left inlet surface was higher than that on the right.The uneven distribution of flue gas velocity on the inlet surface might lead to the difference of heat exchange efficiency in the heat exchange unit,resulting in the deviation of flue gas temperature on both sides.In addition,the working conditions of inconsistent flow resistance of heat exchange units on both sides were investigated.The results showed that changing the flow resistance will lead to the deviation of flue gas flow in air preheaters on both sides,and then affect the flue gas temperature.Eddy current was generated in the air side steering chamber,which made the air flow rate at the bottom of each heat exchange unit low,especially the first heat exchange unit in the high temperature section.The structure of the steering chamber near the heat exchange unit was optimized,and the flow field simulation results showed that the inlet velocity distribution of the unit was greatly improved.The fin structure also has a great influence on the heat exchange efficiency of the air preheater.In this thesis,the local model of air side plate fin unit in low temperature section was constructed,the flow field and temperature field in the model were simulated,and the effects of flow velocity and various fin structures(fin spacing,fin thickness and fin height)on the flow and heat transfer performance of heat exchange unit were investigated.Among the fin structure parameters studied,the highest comprehensive heat transfer capacity could be obtained when the fin spacing was 19 mm,the fin thickness was 2.5 mm,and the fin height was 20 mm.