Particle Collision Behavior And Heat Transfer Performance In A Na₂SO₄ Circulating Fluidized Bed Evaporator

A set of Na₂SO₄ circulating fluidized bed evaporation apparatus is designed and built to further promote the application of fluidized bed heat transfer and fouling prevention technology in the evaporation process of Na₂SO₄ solution and explore the heat transfer enhancement mechanism.The particle collision behavior and heat transfer performance of the evaporator are studied.Firstly,the characteristic frequency ranges of different phases in the multiphase flow are determined by the time domain and frequency domain analyses of the collision acceleration signal.Then,the particle collision behavior and heat transfer performance in the Na₂SO₄ solution and water are compared.Finally,the effects of operating parameters,such as the amount of added particles,circulation flow velocity,heat flux and particle type,on the particle collision behaviors and heat transfer performance are investigated,and the empirical correlation for convective heat transfer coefficient of Na₂SO₄ solution is established.The results show that the characteristic frequencies of liquid phase and solid phase are 0–1000 Hz and 6000–17000 Hz,respectively,and the particle collision plays a leading role in multiphase flow collision.The characteristic frequency range of the vapor phase is not found,but the generation of the vapor obviously enhanced the collision intensity of other phases.Within the experimental range,the addition of different types of particles can enhance the heat transfer of Na₂SO₄ solution.Ф4.5 mm×3.2 mm polytetrafluoroethylene particles have the best heat transfer enhancement effect,the maximum enhancement factor is 14.6%,and the corresponding operating parameters areε=3%,u=1.78 m·s^-1,q=7.29 k W·m^-2.The convective heat transfer coefficient of Na₂SO₄ solution is lower than that of water,but its heat transfer enhancement is more affected by particle collision behavior.Both the standard deviation of the particle acceleration collision signal and enhancement factor increase with the increase in the amount of added particles.The standard deviation increases with the increase in circulation flow velocity;however,the enhancement factor initially decreases and then increases.The standard deviation slightly decreases with the increase in heat flux at low circulation flow velocity,but initially increases and then decreases at high circulation flow velocity.The enhancement factor decreases with the increase in heat flux.In general,the amount of added particles and circulation flow velocity have obvious effects on particle collision behavior.The standard deviation of particle collision signal increases,which is conducive to enhancing heat transfer.3D diagrams of the effects of the operating parameters on the standard deviation and heat transfer enhancement factor are established.The empirical correlation of convective heat transfer coefficient in Na₂SO₄ solution also built.The model result agrees with the experimental data,which is beneficial to the industry application.