COMPUTATIONAL FLUID-DYNAMICS STUDIES OF THE EFFECTS OF TEMPERATURE-DEPENDENT PHYSICAL-PROPERTIES ON FIXED-BED HEAT-TRANSFER

The influence of the temperature profile on the fluid flow and heat tr ansfer in a fixed bed of tube to particle ratio of 2.86 was studied by solving the 3D Navier-Stokes and energy equations by means of a comme rcial finite element code, ANSYS/FLOTRAN. The geometry model, represen ting a fixed bed, consisted of an arrangement of eight spheres in a tu be. The difference in heat-transfer parameters between a wall-cooled a nd wall-heated reactor was studied, using air as a fluid. The dimensio nless wall heat-transfer coefficient, Nu,, and the radial effective co nductivity ratio, k(r)/k(f), were evaluated from the calculated temper atures at different locations in the bed by comparing these with the a nalytical solution of a two-dimensional pseudohomogeneous model, using a nonlinear least-squares analysis. Results were obtained for Reynold s numbers in the range 9-1450. Though for high Re there was no real di fference between a wall-cooled or a wall-heated tube, for low Re a sig nificant difference was found. The effect of the viscosity, conductivi ty, and density variations, as a consequence of temperature variations , was studied using a hydrocarbon mixture as the fluid. Results indica ted that the temperature profile had an influence on the fluid and hea t flow and thus on the effective parameters, although at high Re numbe rs the influence became less.