(102a) Influence of Interparticle Forces and Particle Collision Properties on the Fluidization Behavior at Elevated Temperatures

Recent experiments have shown an influence of temperature on the minimum fluidization conditions, even when the gas phase density and viscosity have been kept constant (Campos Velarde et al., 2016). Correlations that are available in the open literature, for predicting the minimum fluidization velocity and the bed voidage at minimum fluidization conditions at elevated temperatures, fail to accurately describe the experimental data, in particular how the bed porosity at incipient fluidization conditions is changing with temperature.

It is believed that at higher temperatures inter-particle forces become more influential. Inter-particle forces are already known to be important in the fluidization of very fine powders, and may incur detrimental effects in industrial processes such as the formation of particle agglomerates and reduced particle mixing with a strong influence on the solids circulation patterns. The experimental results by Campos Velarde (2016) indicate that inter-particle forces may also become important for larger particles at increased temperatures. In this work, we characterize the effects of the inter-particle forces using numerical simulations with a Discrete Particle Model (DPM). Moreover, it is known from the literature that in addition to inter-particle forces, also particle collision properties can change with temperature. Different materials behave differently with changing temperature, and in this work we will also show how the change of particle collision properties can influence the fluidization behaviour.

An in-house DPM has been used to perform investigations into the minimum fluidization velocity (U_mf), bed porosity (ɛ_mf) and average particle velocities for different fluidization conditions. DPM is an Euler-Lagrange type model with a discrete description of the solids phase and a continuous description of the gas phase. Particle-particle collisions are dealt with deterministically, using a soft-sphere collision model which allows multiple simultaneous contacts between several pairs of particles. The motion of each individual particle is tracked and described with Newton’s second law. The additional force that represent the inter-particle interactions is derived from the Lennard-Jones potential and is described with two constants, one for attractive part (Van der Waals) and one for the repulsive part.

By comparing simulation results for fluidization at different temperatures, with different fluidization gas properties and different particle collisional properties, with unique experimental data of high-temperature fluidization by Campos Velarde (2016), we investigate the influence of the inter-particle forces and the particle collisional properties on U_mf, ɛ_mf, flow fields and bubble properties to further elucidate the dominating phenomena prevailing at high-temperature fluidization.
REFERENCES

VELARDE, I.C, GRIM, R., GALLUCCI, F., VAN SINT ANNALAND, M., (2016), “Influence of Temperature on Minimum Fluidization Properties of Gas-Solid Fluidized Beds”, Powder Technology, Submitted.

DELIMONT, JACOB M. "Experimental Investigation of Temperature Effects on Microparticle Sand Rebound Characteristics at Gas Turbine Representative Conditions." (Doctoral dissertation) 2014

MIHAJLOVIC, M. ROGHAIR, I. VAN SINT ANNALAND, M. “High temperature fluidization - influence of inter-particle forces on fluidization behaviour” In J. E. Olsen, & S. T. Johansen (Eds.), Progress in Applied CFD – CFD2017 (pp. 107-114). Oslo: SINTEF Academic Press, 2017