Modeling of axial bed-to-wall heat transfer in a CFB combustor with abrupt riser exit geometry

The shape of the riser exit geometry in a circulating fluidized bed (CFB) combustor influences the hydrodynamics of the riser column in the exit region and into the riser column to a considerable length. In the present work, the change in axial hydrodynamics for two different riser exit shapes under different operating conditions is examined to predict the corresponding axial heat transfer coefficients. The influence of the two riser exit shapes (smooth and abrupt) on the axial voidage profile is estimated using the core-annulus mass flux balance model and the corresponding axial bed-to-wall heat transfer coefficient is estimated using the cluster renewal mechanistic model. The results are reported for different hydrodynamic relations and operating parameters. The analysis provides fundamental understanding on the influence of the riser exit geometry on the axial heat transfer characteristics of the CFB combustor.