Investigation On Heat Transfer Characteristics And Pressure Drop In A Downflow Circulating Fluidized Bed Evaporator

In this study,an experimental device is designed and built to investigate the heat transfer characteristics and hydromechanics in a downflow circulating fluidized bed evaporator.POM1,POM2,POM3,POM4,GB and SiC are used as the inert solid particles.A range of experimental investigations are performed by varying the amount of added particles（0.5%–2.0%）,heat flux(8–16 kW·m^-2)and circulation flow velocity（0.56–1.78 m/s）.Results show all three kinds of particles added can enhance the heat transfer.The maximum enhancing factors are 78.5%,68.8%,and 72.1%within the range of experiments for POM1,glass bead and SiC particles,respectively.The flow boiling heat transfer coefficient initially increases and then decreases or fluctuates with the increase in the amount of added particles.The circulation flow velocity has two aspects of influence on the flow boiling heat transfer.The enhancing factor increases with the increase in circulation flow velocity at low heat flux,but initially increases and then decreases at high heat flux.The enhancing factor decreases with the increase in heat flux because of the generation of vapor film on the heating wall.The results of the effect of particles size on flow boiling in a downflow circulating fluidized bed evaporator are shown as follows:For POM particles with different particle sizes,the flow boiling heat transfer coefficient presents different trends with the increase in the amount of added particles.For POM particles with different particle sizes,the flow boiling heat transfer coefficient presented different trends with the increase in the amount of added particles.The flow boiling heat transfer coefficient of POM4 particles increases with the increase in the amount of added particles.With the increase in heat flux,the enhancing factor of POM particles decreases,except that of POM4 particles.The pressure drop first increases rapidly and then decreases or fluctuates with the increase in the amount of added particles.The increase in circulation flow velocity leads the increase of pressure drop.Heat flux makes little effect on pressure drop.It is found that the addition of particles has both positive and negative effects on the pressure drop.The pressure drop of two-phase and three-phase flow increases with the increase in flow circulation velocity.The pressure drop ratio fluctuates with the increase in flow circulation velocity.When the amount of added particles was low,the pressure drop ratio of POM1 particles is higher than that of other POM particles.When the amount of added particles is high,the pressure drop ratios of the four POM particles are similar.Heat flux has little effect on pressure drop.