Some aspects of heat transfer in rolling mode rotating drums operated at low to medium temperatures

Rotating drums in industries are often operated at elevated temperatures. Heat transfer in such devices therefore plays a prime role. However, the controlling steps in heat transfer have not been fully clarified though a considerable amount of work has been carried out. In this paper, heat transfer in rotating drums operated in a rolling mode is assessed. The focus is on direct heating operations at low to medium temperatures, where thermal radiation is unimportant. It is shown that heat transfer from the covered wall to the particle bed is the dominant mechanism in supplying heat to the bed. Heat transfer between the freeboard gas and the exposed surface of the free cascading layer accounts for only a small portion, in contradiction to our previously held opinion. The heat transfer rate between the freeboard gas and the uncovered drum wall may be comparable to that between the covered drum wall and the particle bed indicating that both steps could be controlling. If heat is supplied to the boundaries of the particle bed rapidly, and macroscopic particle mixing is rapid, then heat transfer within the bed may be controlled by either or both of gas–solid heat transfer and heat conduction within individual particles depending upon the physical properties of both interstitial gas and particles. Although theoretical analyses are principally used in this work, comparison with experiments is also made where appropriate.