HEAT EXCHANGE BETWEEN A FLUIDISED BED AND AN IMMERSED CYLINDER: ESTIMATION OF LOCAL HEAT TRANSFER COEFFICIENT
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media (T2-7P)
Keywords: surface-to-bed heat transfer, fluidised bed, particle convection
The fluid dynamic field around an immersed surface plays a fundamental role in determining the rates of heat and mass transfer in fluidized beds, whose high values are among the most relevant features of fluidization technology and are at the basis of many industrial applications such as heat exchangers, membrane reactors, surface treatments, coating and decoating processes etc..
The bed structure near an immersed surface mainly consists of a region characterized by gas velocity and void fraction higher than the bulk of the bed, leading, in some cases, to the formation of indigenous bubbles and whose characteristics strongly depend on the shape of the submerged object.
The local fluid dynamic field can be studied using direct (i.e. optical investigations, PEPT techniques) or indirect (i.e numerical simulations, abrasion measurements, heat and mass transfer measurements) methods. Among them, the measurement of heat transfer coefficient seems to be the most reliable and inexpensive: the correlation between the local structure of the bed and the heat transfer coefficient requires the use of proper heat transfer models.
Although the effects of the exchange surface shape on heat transfer rates are poorly described in literature, experimental evidences clearly pointed out a close correlation due to the assessment of different fluid dynamic conditions around the exchange surfaces.
This work reports experimental results on heat transfer coefficient between bubbling or slugging fluidised beds and a horizontal thermal insulating cylinder on whose surface is inserted an small heat transfer probe. This system allows the evaluation of surface–to-bed heat transfer at different angular positions on the cylindrical surface. Experimental runs have been carried out by varying gas velocity from fixed bed conditions to about 0.6 m/s. Moreover, experimental analysis has been extended to several bed materials of industrial interest as silica sand, corundum, carborundum, and FCC as well as to glass beads of different sizes.
Experimental results show that the heat transfer coefficient is the highest on the lateral surface of the cylinder and the lowest on its upper face. This evidence can be explained by considering that heat transfer coefficient in fluidised beds mainly depends on particle convection which, in turns, increases with local surface renewal frequency and solid concentration. The differences in local fluid dynamic field give rise to the observed angular depedence of heat transfer coefficient.
Presented Monday 17, 13:30 to 15:00, in session Transport Phenomena in Porous/Granular Media (T2-7P).
