Numerical Simulation Of Heat And Mass Transfer Characteristics Of Condensation In Flue Boiler Gas Of Biomass Boiler

At present,thermal power generation is still the most important form of power generation in China.Coal,oil and natural gas are the most widely used fossil fuels in thermal power generation.Their reserves are very limited,and they are not renewable.In the combustion process,they will produce a large number of harmful gases,which will not only cause great pollution to the environment,but also produce the greenhouse effect,make the global warming,affect the balance of the ecosystem and produce many ecological problems.Biomass energy,as a clean and renewable energy,produces about the same amount of CO₂as plant photosynthesis.The content of N and S in biomass fuel is very low,so the emissions of NO₂ and SO₂ in flue gas after combustion are very small.There are abundant biomass resources for energy utilization in China.Direct combustion power generation technology is the most common form of biomass energy utilization.Compared with traditional fossil fuel,biomass fuel contains more water,and its flue gas after combustion contains more water vapor,so condensing the water vapor in the flue gas at the tail of the boiler can recover more latent heat,which can not only effectively reduce the flue gas temperature and heat loss,but also improve the thermal efficiency of the boiler.Moreover,the condensate can dissolve some harmful gases in the flue gas,which is conducive to the protection of the atmospheric environment.Based on Eulerian multiphase Mixture model in ANSYS Fluent and Lee model as condensation mass transfer model,the condensation heat transfer characteristics of tail gas of biomass boiler flowing through finned tube heat exchanger were numerically simulated.The effects of inlet flue gas temperature and inlet water vapor volume fraction on the temperature field,flow field,liquid water volume fraction,wall condensation rate,average surface heat transfer coefficient and average heat flux of finned tubes were revealed.The flow and heat and mass transfer characteristics of finned tubes with different transverse and longitudinal pitch diameter ratios（S₁/d and S₂/d）were analyzed.The results show that with the increase of flue gas inlet velocity,the outlet temperature of flue gas increases,the wall condensation rate increases,and the amount of condensation water decreases.Meanwhile,the average surface heat transfer coefficient and heat flux of finned tube increase.With the increase of vapor volume fraction in the inlet,the condensation rate on the wall increases,the volume fraction of condensate at the flue gas outlet also increases,and the average flue gas temperature at the outlet gradually increases.The transverse pitch diameter ratio is 3.55,and the transverse tube spacing is 30mm.The longitudinal pitch diameter ratio is4.00 and the longitudinal pipe spacing is 38mm.This article will provide important theoretical guidance for the analysis of condensation heat transfer of finned tube heat exchanger,and the research results have certain engineering application value.