Dynamic Mesh Simulation Of Deposition Characteristics Of Fly Ash With High Alkali Content

The issue of particulate deposition on the heat exchange surface of heat exchangers exists widely,which not only weakens the heat transfer efficiency greatly,but also threatens the safe operations of heat exchangers in the meantime.Especially when fossil fuels and solid-liquid waste with high content of alkali metal composition are used,the fly ash with high alkali metal derived after combustion will enter the heat transfer equipment with flue gas,which causes the particulate deposition more serious.This is because the alkali metal composition will significantly reduce the melting point of fly ash and increase the sticking efficiencyIn this paper,fly ash particles with high alkali content derived by waste liquid incineration are taken as research object,the dynamic deposition characteristics including transport,impaction,sticking and distribution of deposition are studied based on dynamic mesh technology when fly ash particles flow across tube bundle of heat exchangers.The main contents and conclusions are as follows:According to the differentiation deposition mechanisms of fly ash particles under low temperature and high temperature condition,the melt fraction model suitable for high temperature condition and critical velocity model suitable for medium and low temperature condition are combined to establish the deposition coupled model.Considering the effect of erosion on deposition of fly ash particles,the tangential slip model and aerodynamic removal model are added to supplement the deposition model.Considering the influence of deposition growth on the movement of flue gas and deposition characteristics,dynamic mesh model is employed to predicted the change of deposition characteristics with time,and the boundary of deposition layer is updated according to the calculation results derived from the deposition coupled model.The deposition coupled model established in this paper is compared with the experimental data to verify the accuracy of the model.The influence of six key factors on deposition characteristics of fly ash with high alkali metal including particle size,inlet flue gas velocity and temperature,longitudinal tube spacing,spanwise tube spacing and tube geometry are investigated.The results show that the distribution of deposition formed by small particles is relatively uniform,and the impaction rates of small particles decrease as deposition grows.With the increase of particle size,the deposition gradually concentrates on the windward side.Increasing the inlet flue gas velocity has little effect on the distribution of deposition and the impaction rates increase with the inlet flue gas velocity.As inlet flue gas temperature rises,the impaction rates decrease first and then rise slightly.The distribution of deposition is relatively close.As longitudinal tube spacing rises,the impaction rates of fly ash particles increase and the distribution of deposition on the windward side of rear tubes tends to be symmetrical.With the increase of spanwise tube spacing,the impaction rates of fly ash particles reduce first and then rise.The deposition peak value is significantly higher than the other cases when s2/D equals to 1.5.The impaction rates of fly ash particles on the elliptical tube bundle are lower than that of circular tube,the distribution of deposition mainly concentrates on the windward side of the first-row tube.The heat transfer,resistance characteristics and comprehensive heat transfer performance?PEC?under different conditions are evaluated.With the increase of inlet flue gas velocity,Nusselt number and pressure drop gradually increase,the PEC gradually improves.Increasing the inlet flue gas velocity and spanwise tube spacing will lead to the decrease of Nusselt number,pressure drop and PEC.When s₁/D equals to 2,the PEC reaches to maximum.When flue gas flows across the elliptical tubes,the pressure drop,Nusselt number and PEC are lower compared with oval tubes.