Numerical Simulation On Deposition And Convective Heat Transfer Characteristics For Flue Gas Flowing Over Spirally Corrugated Tubes

Spirally corrugated tube with its excellent heat transfer performance is widely used in boilers and other industrial equipment.When the flue gas flows through the boiler heating surface(tube bundles),the fly ash particles will be deposited on the surfaces of the heat transfer tube bundles,seriously affecting the safe and efficient operation of the boiler.Therefore,studying the particles deposition and convective heat transfer characteristics can provide reference for predicting the deposition and heat transfer performance of tube bundles.Computational fluid dynamics(CFD)software was used to simulate the particles deposition and convective heat transfer of the ash-deposited tubes for the flue gas traversing spirally corrugated tube bundles.Gas-solid two phase flow model was established based on discrete phase model(DPM),regarding the particles as discrete phase and flue gas as continuous phase,and the stochastic tracking model was also introduced to reflect the influence of the flue gas turbulence on particles.According to the characteristics of the particle deposition,the deposition model was introduced by user defined function(UDF),which can be inserted in the software so as to be called in real time.The particle trajectory,deposition rate and contours of the deposition distribution were all calculated.In addition,based on the calculated results of the deposition rate,the model of the geometry ash-deposited spirally corrugated tube bundles were constructed and the convective heat transfer characteristics were numerically simulated.On the base of the ratios of the heat transfer coefficient of ash-deposited tube bundles,the convective heat transfer characteristics of the spirally corrugated tube were discussed and analyzed.The main work and conclusions are as follows:(1)Investigated the effects of gas-solid two phase flow parameters on deposition characteristics both of the spirally corrugated tube bundles and bare tube bundles.The results show that the deposition rate decreases significantly and the advantage of reducing fouling weakened as gas velocity increased as for the spirally corrugated tube bundles.The deposition rate for the spirally corrugated tube bundles decreases about 5%,compared to the bare tube bundles when the flue gas velocity is in the range of 7 m·s-1to 10m·s-1.However,when flue gas velocity exceeds 10m·s-1,there is no obvious difference between spirally corrugated tube bundles and bare tube bundles.The deposition rate increases with the particle diameter and the particle concentration in flue gas.The particle diameter greatly affects the local distribution of deposition rate,and the peak value of depositionrate for the 35μm and 85μm particles occurs near the central angle of ±30°and 0° of the tube respectively.(2)Studied the effects of structural parameters of tube bundles on deposition characteristics both of the spirally corrugated tube bundles and bare tube bundles.The deposition rate slightly decreased as the longitudinal pitch increasing.When the longitudinal pitch is 2.0,2.5 and 3.0,the deposition rate for the spirally corrugated tube bundles decreases 3.1%,3.4% and 4.3% respectively,compared to the bare tube bundles.The deposition rate slightly increases as the transverse pitch increasing.When the transverse pitch is 1.5,2.0,the deposition rate for the spirally corrugated tube bundles decreases 6.5%,7.6% respectively,compared to the bare tube bundles.The deposition rate of the spirally corrugated tube bundles is decreased with the pitch or corrugated depth increased,the effect of the pitch on the particle deposition rate is more obvious than that of corrugated depth.(3)Investigated the effects of gas-solid two phase flow parameters on the convective heat transfer characteristics of spirally corrugated tube(ash-deposited).It indicates that the thickness of the deposition layer decreases and the ratio of the heat transfer coefficient of the tube bundles is significantly improved as the flue gas velocity increasing.On the condition that the flue gas velocity is increased from 7m·s-1to 12m·s-1,the ratio of the heat transfer coefficient of the spirally corrugated bundles is increased by 16.6%.As for larger particles and higher particle concentration in flue gas,the thickness of the deposition layer is increased,and then the ratio of the heat transfer coefficient of the tube bundles is decreased.(4)Analyzed and discussed the relationship between the structural parameters of the tube bundles and the convective heat transfer characteristics of spirally corrugated tube(ash-deposited).The ratio of the heat transfer coefficient of the tube bundles increases as the longitudinal pitch increasing.When the longitudinal pitch is increased from 2.0 to 3.0,the ratio of the heat transfer coefficient of the tube bundles is increased by 5%.the ratio of the heat transfer coefficient of the tube bundles decreased as the transverse pitch increasing.When the transverse pitch is increased from 1.5 to 2.0,the ratio of the heat transfer coefficient of the tube bundles is increased by 10%.Increasing the pitch and corrugated depth,the ratio of the heat transfer coefficient of the tube bundles is increased.What’s more,the pitch and corrugated depth increases from 0.375 to 0.875 and 0.01 to 0.05 respectively,the ratio of the heat transfer coefficient of the tube bundles is increased by15% and 4% correspondingly.(5)Compared and analyzed the distribution and difference of the deposition rates between different compositions of particles,it shows that higher silica-alumina ratio and alkali ratio of the ash particles are easier to deposit on the surface of tube bundles,which could thicken the deposition layer,and the ratio of the heat transfer coefficient of the tube bundles decreases.