Fluidized Bed Operating Parameters and Eulerian Erosion Models
A numerical two-fluid code, Gemini 2D, was adapted to study gas-solid fluidization in arbitrary 2D domains. The code was also used to study fluid dynamic mechanisms of dense particle flow that cause ductile and brittle erosion of solids.
Simulations of the in-bed fluid dynamics of a small-scale pressurized fluidized bed were made with and without internals. The inflow and outflow system of a small-scale bed was studied, where the air-feed system - incorporating an air inflow system, plenum chamber and distributor plate - was modelled. It was found that the simulated large-scale fluid dynamics was significantly improved when a model was included for the air feed system of the bed. In particular, at atmospheric operating conditions, it was found that regular pressure fluctuations occurred at a constant frequency, in good agreement with experimental observations for a slugging bed behaviour. These pressure fluctuations correlate well with a strong temporal variation in the total inflow of air. Animated sequences of the slugging bed behaviour resemble that of a piston-like motion of the major part of the bed mass. Previous modelling of the same bed when a constant influx of air through the distributor plate is assumed resulted in large discrepancies between simulated and experimental bed behaviour. It is concluded that it is necessary to include complementary operating parameters to describe the present fluidized bed system, as the standard operating parameters such as fluidization velocity and operating pressure are not sufficient.
A simple erosion model was used to study erosion of cooling tubes for a bed containing two tubes. This model, the monolayer kinetic energy dissipation erosion model, was used to study erosion mechanisms when bubbles pass a tube. In accordance with experimental observations, the impact of bubble wakes results in high instantaneous simulated erosion rates. Time-averaged local erosion rates around tubes were also modelled.
This simple Eulerian erosion model was further developed to enable erosion modelling of real engineering metals subject to dense flow impacts of hard particles. The irreversible processes in the simulated flow field in the vicinity of a solid eroding surface can be divided into two fundamental components that can be associated with cutting and deformation wear, respectively. Ductile and brittle erosion were simulated for stationary jet streams of particles impacting a tilted plate. The influence of particle diameter, particle concentration, jet velocity and jet diameter was investigated.
The literature has a great many reports of erosion models that can be applied in the case of dilute flows and in studies of single particle impacts. However, these models - generally Lagrangian - are here argued to be difficult to implement successfully for dense flows causing erosion. Examples of such cases are sandblasting or erosion caused by a large collection of particles, which is common in bubbling fluidized beds.
Eulerian erosion model