Study On Condensation Heat Transfer Characteristics Of Wet Flue Gas

Biomass has high water content,and a large amount of wet flue gas will be generated during direct combustion power generation.However,due to low-temperature corrosion and other reasons,this part of flue gas’s exhaust temperature is mostly above 120℃,and only a small number of coal-fired power stations are equipped with low-temperature economizers to reduce the exhaust gas temperature to about 90°C,wasting a lot of waste heat,especially the latent heat.Recycling this latent heat through the condensing heat exchange equipment and reducing the exhaust temperature can effectively improve the energy utilization rate of biomass fuel.In this paper,the condensation heat transfer characteristics of wet flue gas outside a horizontal single pipe are studied by experimental and numerical simulation methods.The specific research contents are as follows:First,through elemental analysis of biomass straws in Northeast China,the raw material flue gas composition is RO2,N2,O2,H2O,and the proportions are 11.59%,67.79%,6.01%,and 14.62%.And through the contact angle test of common heat exchanger materials of titanium tube,fluoroplastic,20 steel,316L steel,and 2205 steel,the hydrophilicity of different materials is analyzed.A visual test system for low-temperature flue gas condensation heat transfer is established to study the effects of different flue gas velocity,cooling water flow,subcooling,water vapor volume fraction and other parameters on the heat transfer efficiency of the above-mentioned five materials in a single tube.The results show that 20#steel has high hydrophilicity,and its heat exchange stability is easily affected by the thermal resistance of the condensate.The titanium tube has good heat transfer performance and is suitable for condensing heat exchanger.The average heat transfer coefficient of the titanium tube can reach 1693.4W/m~2?K under the condition of drop condensation.Then,based on experimental phenomena and results,a numerical model of the multi-component film condensation heat of the wet flue gas was established.The VOF model is used to calculate the heat transfer process of the two-phase flow.The component transport equation is used to calculate the multicomponent diffusion and convection process.The CSF is used to calculate the force between the phases.The LEE condensation model calculates the condensation heat transfer process,compiles the saturated steam model through UDF,and judges the wall’s condensation conditions.The heat transfer process of film condensation is simulated by solving mass,momentum,energy and diffusion equations simultaneously.The model verification shows that the error between simulation and test data is less than 10%.Finally,based on the numerical calculation results of film condensation outside the horizontal tube,analyze the influence of different inlet flue gas Reynolds number,inlet water vapor volume fraction and wall subcooling on the condensation heat transfer process outside the single horizontal tube,and analyze the changes of local Nusselt number and local condensation rate outside the horizontal tube.The response surface method is used to optimize the condensation heat transfer’s performance outside the horizontal tube.The Adeq Precisior value is far greater than 4,indicating that the model is suitable for performance prediction and design optimization.The influence of wall supercooling degree and inlet gas velocity on the average condensation heat transfer coefficient is significantly interactive.The optimal reaction conditions are given,and the error between the value calculated by the numerical calculation model and the model’s predicted value is 5%.Provide theoretical basis and numerical support for the design of low-temperature flue gas condensing heat exchanger.