Flow boiling in a downflow circulating fluidized bed evaporator

Highlights: • A downflow circulating fluidized bed evaporator is built and examined. • The flow boiling heat transfer coefficient and enhancing factor are investigated. • The addition of the solid particles in the downer can enhance heat transfer. • Circulating flow velocity and heat flux influence the enhancing factor. • Three dimensional diagrams are established to select the suitable particles. -- Abstract: In this study, an experimental device is designed and built to investigate the heat transfer characteristics in a downflow circulating fluidized bed evaporator. The tested downer has a length of 1200 mm and a dimension of Ф38 mm × 3 mm. Polyformaldehyde (POM), SiC and glass bead particles 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 that as the addition of the particles destroys the laminar sublayer and increases nucleate sites, the boiling heat transfer coefficient of the three-phase flow is higher than that of the two-phase flow. 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 POM, 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. Three dimensional diagrams are established to determine the optimum particles. The results can provide some references for the industrial application of the circulating fluidized bed heat transfer technology.