Estimation of Effective Heat Transfer Coefficient for Spherical Particles in a Vertical Airlift Conveyor
Vertical airlift conveyors are used industrially to transport particulate material for distribution to storage or for feeding to downstream processing equipment. In some cases, the conveyor is also used as a heat exchanger, usually a cooler, to remove heat from the particles during transport. In the system considered here, the (hot) particles entering the air stream in the conveyor are initially at rest and are accelerated as they move upwards, usually attaining a steady slip velocity before they leave the conveyor.
In previous work, heat transfer from the particles to the conveying air was estimated by a method which sequentially combined an analytical determination of the slip velocity profile and a numerical stepwise computation of the particle temperature along the conveyor .In this paper, mean values of heat transfer coefficient have been obtained by integrating and averaging the local heat transfer coefficient over the full height of the conveyor, with respect to slip velocity. The mean values have then been used with a lumped capacitance model for the coupled air particle system to find temperature profiles in the conveyor, with the particular objective of estimating exit temperatures. Estimated temperatures along the conveyor height, using mean heat transfer coefficients and the analytical model, agree very closely with the values obtained using a numerical model in which heat transfer coefficient was estimated at each computation step.