“ Trends in the Development of Machinery and Associated Technology ” TMT 2008 , Istanbul , Turkey , 26-30 August , 2008 NUMERICAL ANALYSIS OF THE EFFECT OF OPERATIONAL PARAMETERS OF HEAT TRANSFER SURFACES IN CFBS
The design or scale-up of heat transfer systems in fluidized beds require estimation of the effective heat transfer coefficient on heat transfer surfaces in contact with the fluidized medium. In this study, the effects of operational parameters on bed-to-wall heat transfer in CFBs is investigated such as solids volume fraction, particle diameter, suspension density, superficial velocity and solid circulation rate, using modified cluster renewal model. In the modified cluster renewal model, the gas phase properties are used for the dilute phase and similarly the solid phase properties are used for the cluster phase. The gas phase and the solid phase properties are considered in the control volume which is in the vicinity of the heat transfer surface.
Experiments on the local heat transfer characteristics of a circulating fluidized bed
Abstract The role of particle diameter in the heat transfer of a gas–solid suspension to the walls of a circulating fluidized bed was studied for particles of uniform size. This work reports and analyzes new experimental results for the local bed to wall heat transfer coefficient, not including the radiation component, in a long active heat transfer surface length laboratory bed, which extend previous findings and clear up some divergences. The research included determining the effects of extension and location of the heat transfer surface, circulating solids mass flux and average suspension density. An experimental set-up was built, with a 72.5 mm internal diameter riser, 6.0 m high, composed of six double pipe heat exchangers, 0.93 m high, located one above the other. Five narrow sized diameter quartz sand particles − 179, 230, 385, 460 and 545 μm − were tested. Temperature was kept approximately constant at 423 K and the superficial gas velocity at 10.5 m/s. The major influence of suspension density on the wall heat transfer was confirmed, and contrary to other authors, a significant effect of particle size was found, which becomes more relevant for smaller particles and increasing suspension density. It was observed that the extension of the heat transfer surface area did not influence the heat transfer coefficient for lengths greater than 0.93 m.The heat transfer surface location did not show any effect, except for the exchanger at the botton of the riser. A simple correlation was proposed to calculate the heat transfer coefficient as a function of particle diameter and suspension density.
Laminar heat or mass transfer in rectangular channels and in cylindrical tubes for fully developed flow: comparison of solutions obtained for various boundary conditions
Abstract A comparison is made of results obtained with various boundary conditions at the wall of the channels. By comparing blood Sherwood numbers for finite wall resistance with those for constant concentration boundary condition only a small variation is found for various points along the blood channel.