Axial dispersion in shell-and-tube heat exchangers

The effect of the deviation of the actual th ree-dimensional flow field on the shell-side from the frequently assumed one-dimensional uniform axial plug flow can be taken into account by superimposed axial dispersion in the fluid. The measure for axial dispersion is the Peclet number which can vary from infinite (no dispersion) to zero (complete axial mixing). For the fast and more reliable calculation of transient processes with the axial dispersion model, the Peclet number has to be known. A residence time distribution measurement technique for the determination shell-side dispersive Peclet numbers is described and used to determine Peclet numbers for different shell-to-baffle clearances, numbers of baffles, and axial plug flow Reynolds numbers, Measurements with water reveal that Peclet numbers from 1 5 to 160 can occur and axial dispersion cannot be neglected in many cases. (C) 2000 Editions scientifiques et medicales Elsevier SAS.