Gas-particle flow inside cyclone diplegs with pneumatic extraction

Abstract

This paper presents a detailed flow analysis inside cyclone diplegs equipped with bottom gas extraction. Up to now, little experimental work has been done related with these devices, specially used for hot gas cleaning applications, such as pressurized fluidized bed combustion (PFBC). Tests varying inlet solid loading and gas extraction flow rate were carried out in a one-fifth cold-flow model of a PFBC cyclone. Velocity and pressure measurements along cyclone dipleg have been carried out. Results from experimental measurements reveal that the swirling flow penetrates inside the dipleg, deeper than can be anticipated by literature predictions on conventional cyclone designs. This effect is caused by the high inlet velocities of this kind of cyclones and the gas suction at dipleg bottom. Dipleg pressure coefficient is shown to be a function of the inlet solid loading and the tangential to axial velocity ratio. Although for standard cyclone designs it has been claimed that underneath the vortex end there is a stagnant region where solid recirculation and reentrainment can occur, it has been experimentally verified that even with a small percent of gas extracted at the bottom a substantial tangential velocity component is induced at dipleg bottom. This velocity assures solids conveyance to the extraction device. As solid loading is increased, an upward movement of the vortex end is detected from wall pressure measurements. The solid loading causes an important decrease of the vortex energy and, consequently, a weakening of the dipleg tangential velocity. A new method for measuring the vortex penetration in dipleg is presented.