Fundamentals of turbulent gas-solid flows applied to circulating fluidized bed combustion

Abstract

A summary is made of the present state of knowledge of turbulent gas-solid flow modeling and in particular its application to circulating fluidized bed combustion chambers. Models are presented to close the set of equations describing isothermal non-reacting turbulent gas-particle flows applied to fluidization, and it is shown under which assumptions the models can be derived. With the kinetic theory of granular flow, transport equations for the velocity moments and closure laws for the stress tensor and the energy flux are derived for the particle phase. Closure equations for the drift velocity and for the fluid-particle velocity correlation tensor are presented, first based on algebraic models and, second, based on transport equations with the fluid-particle joint probability density function. An alternative derivation of the fluid-particle velocity covariance transport equation is compared to the formulation based on the fluid-particle joint probability density function. Two-way coupling is discussed, and a transport equation for the second-order velocity moments is used to derive a two-equation model accounting for the modulation of gas phase turbulence by particles. Boundary conditions for the set of equations describing a turbulent gas-solid flow are discussed. Provided that the domain of applicability of the models is known, a discussion on the usefulness of the models is given, as well as an application to fluidization and especially to circulating fluidized bed combustors. Prospects for improvement of the existing models are presented.