Particle dynamics in thermal plasma reactor for the conversion of UF6 to U3O8
Poster
Abstract
The main focus of this work is to study the behavior of U3O8 & UO2F2 particles of micron size (5 to 500 micro meter) formed in the thermal plasma reactor during the conversion of UF6 to U3O8 that experiences large variations of surrounding plasma temperature and velocity fields [1-14]. These particles are then transported in the plasma atmosphere due to four major forces: Viscous drag force, Basset history term, Thermophoresis effect, and Turbulent dispersion. The drag forces exerted on the micron size particles give rise to a creeping flow, and semi-empirical relation is adopted for determining the drag coefficients considering two important aspects: the variable property effect, that takes into account the temperature drop in the boundary layer and hence the property variations within the boundary layer. The second one, the non-continuum effect, is due to almost same order of magnitude of the particle diameter as the molecular mean free path lengths of the plasma constituents in thermal plasmas at atmospheric pressure. Further, the effect of particle density (ρ_P), particle diameter (D_P), and particle relative velocity (V_g - V_P) are taken into account to estimate the acceleration of the particles caused by the viscous drag force, and the analysis indicates that due to viscous drag force UO2F2 particles accelerated at faster rate compared to U3O8 particles mainly because of the low particle density of UO2F2 (6370 kg/m^3) compared to U3O8 particles (8300 kg/m^3). Here, V_g is the plasma-fluid velocity and V_p is the particle velocity in the plasma-fluid medium ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109-159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307-359)). Next, the acceleration of the particle due to Basset history term is examined under strong unsteady state conditions and it is observed that the influence of Basset history term become important for large particle diameter ( D_P > 100 micro meter) and for small relative velocities [ (V_g - V_P) < 1 m/sec ]. The small relative velocity condition is generally experienced during the final stage of processing, and is particularly important for materials treated over long distances, where Basset history term plays a very significant role. Furthermore, the transport of U3O8 & UO2F2 particles due to a temperature gradient in the surrounding thermal plasma atmosphere is investigated caused by the thermophoresis effect which is strongly related to the free-stream temperature of the plasma-fluid (T_∞) and to the particle size (D_P). The choice of mean free path is cumbersome because thermal plasma represents a multi-component mixture with strongly varying properties. Thus, effective mean free path is used to estimate the acceleration of the particles due to thermophoresis effect. For particle-dispersion in turbulent plasma flows, the turbulent fluctuations are characterized by the turbulence kinetic energy (K) and dissipation rate (ε), and hence, the particle trajectories are computed by tracking them as they interact with a sequence of turbulent eddies through constant flow properties. The particle relaxation time, which is an indicator of how quickly the particle surface temperature and particle velocity relaxes to local plasma-fluid temperature and velocities have been estimated assuming Stokesian plasma flow around a spherical particle, and the analysis suggests that for rapidly changing temperature field the particle relaxation time is nearly 100 times longer than the rapidly changing velocity field. The analysis of particle relaxation time and particle Stokes number further indicates that the particles are likely to follow the motion of turbulent eddies, and have successfully attained the velocities equivalent to the plasma-fluid and do not lag behind the fluid motions.
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