Research On Condensing Heat Transfer Performance And Structure Optimization Of Boiler Air Preheater

The air preheater is an important auxiliary device of the boiler system,which is used to increase the inlet air temperature of the boiler by utilizing the heat of the flue gas.The two-stage air preheater technology proposed in recent years can achieve a greater degree of waste heat recovery by considering the heat exchange process in different flue gas temperature ranges.However,the research on the condensation heat transfer of water vapor flue gas in the low temperature section is not enough.Therefore,in order to improve the heat exchange efficiency of the air preheater,further research work has important scientific significance and engineering value for energy saving and emission reduction of boiler systems.In this paper,the numerical method is mainly used to study the condensation heat and mass transfer process of the two-stage air preheater,reveal the condensation heat transfer law and optimize the heat exchange structure.The main work and conclusions are as follows:(1)A numerical calculation model was established to describe the condensation heat transfer process of the non-condensable gas-water vapor mixture along the vertical tube,and the reliability of the model was verified by the Kuhn condensation test tube center mixed gas temperature,tube wall temperature and other parameters.Through numerical simulation,the condensing heat and mass transfer laws of non-condensable gas-water vapor mixture flowing in the vertical tube and outside the tube were respectively studied.The results show that in both cases,the mass transfer rate gradually decreases from the tube wall to the center of the mixture,and the thickness of the condensate film gradually increases along the tube length;The difference is that in the case of condensation in the tube,the condensation occurs closer to the nozzle,and the initial liquid film is thicker.And In the case of condensation outside the tube,the growth rate of the thickness of the condensate film along the length of the tube is higher than that of the condensation inside the tube.And under the same working conditions,the heat transfer coefficient outside the mixed gas pipe is higher than that in the mixed gas pipe.(2)The structure of a two-stage air preheater was designed,in which the flue gas in the low temperature section was condensed outside the tube for heat exchange.And the influence of inlet parameters on the condensation heat and mass transfer process in the low temperature section was investigated by numerical simulation.Considering the condensation heat transfer efficiency and avoiding the dew point wall temperature in the high temperature section,it was proposed that the operating temperature range of the inlet flue gas in the low temperature section was 345K-355K.Studies have shown that when the operating conditions deviate from the design conditions and the air outlet temperature in the low temperature section does not meet the requirements,it can be adjusted by reducing the air flow or increasing the inlet flue gas temperature.Although increasing the inlet flue gas flow can promote sensible heat transfer,the condensation heat exchange efficiency is reduced,and the overall heat exchange effect is not significantly improved.(3)Aiming at the shell-side flow field in the low temperature section of the air preheater,the horseshoe vortex phenomenon in the turning chamber was studied,and the influence of the vortex distribution and vortex intensity on the condensation heat transfer performance was discussed.It was found that the horseshoe vortex generated by the change of the flow direction of the flue gas in the turning room would cause the flue gas temperature and the water vapor content in the flue gas to be uneven,thus affecting the heat transfer effect;The structure of the steering chamber was changed and the deflector was set to reorganize the flue gas flow field,so as to avoid the local vortex structure with excessive strength and area,and improve the heat exchange effect.After the structure was optimized,the heat exchange was increased by 3 w/m~2.(4)Based on the overall optimization of the steering chamber structure,the effects of the deflector displacement,radius,thickness and ovality on the condensation heat transfer were further investigated.And the results show that the displacement of the guide plate in the steering chamber is better when the displacement is 125 mm,the radius is 125 mm and the thickness is6 mm,and the overall heat and mass transfer performance is better when the ratio of long axis to short axis is 1.5.