HEAT-TRANSFER AND FLOW OF BULK SOLIDS IN A MOVING-BED

The indirect heat transfer of steam-heated tube bundles in a moving be d has been examined in an experimental apparatus. Heat transfer in sin gle tubes is typified by a characteristic flow of the bulk solids alon g the outer tube wall surface. A cuneiform rest zone is created at the upper tube wall (stagnation point), in which the particles remain for a long time. An 'insulating' effect is exhibited by the dammed bulk z one and is reponsible for the poor heat transfer in this area.Near the sides of the lateral tubes heat transfer is good and increases with i ncreasing mass flux and bulk solids velocity. Bubbling occurs at the l ower tube wall and the heat transfer again decreases due to the small number of wall-particle contacts. The experimentally confirmed 'trace theory' describes the temperature profile at the outlet of a moving be d heat exchanger, being characterized by very good cross-mixing of the bulk solids which allows the integral heat transition to be calculate d. A modelling approach to the heat transfer and bulk solids movement in the moving bed provides a physical model which describes the depend ence of the heat transfer at a single tube on the flow profile between two neighbouring tubes. In order to determine the flow profile, the c ontinuity equation is solved vectorially, allowing an analytical relat ionship of the velocity profile between two tubes to be obtained via t he coaxiality of stress and deformation. To allow such a calculation, the heat-transfer model makes use of the residence and contact time be haviour resulting from the velocity profile, with the different compon ents of heat transfer at a tube being determined from the friction pro perties of the specific bulk material. Calculation of the integral hea t transfer in the moving bed may be achieved via heat transfer at a si ngle tube. By using the theory of 'extended contact time', the total r esidence time of the bulk at the first tubes may be considered as a ca se history for the other tubes. The integral overall heat-transfer coe fficients of moving bed heat exchangers thereby determined have been v erified experimentally.