ON THE ORIGIN OF RETURN FLOWS IN HORIZONTAL CHEMICAL-VAPOR-DEPOSITION REACTORS

The mechanism underlying the onset of recirculations, so-called return flows, in the entrance region of horizontal chemical vapor deposition reactors is analyzed. This phenomenon, which is observed experimentally for reactors with either top or bottom heating, is shown to be the result of changes in the relative magnitude of vertical and axial pressure gradients. A criterion for evaluating whether or not recirculations will arise is developed in terms of Reynolds and Grashof numbers for the physical system. The predictions based upon this criterion are shown to be consistent with published experimental data and numerical simulations. The impact of pressure distribution changes on the use of simplified, parabolic fluid flow simulations is discussed. Finally, it is shown that variations in relative magnitudes of axial and vertical pressure gradients may also lead to an unequal distribution of flow above and below a substrate placed in midstream.