ON THE ORIGIN OF RETURN FLOWS IN HORIZONTAL CHEMICAL-VAPOR-DEPOSITIONREACTORS

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 experimental ly for reactors with either top or bottom heating, is shown to be the result of changes in the relative magnitude of vertical and axial pres sure gradients. A criterion for evaluating whether or not recirculatio ns will arise is developed in terms of Reynolds and Grashof numbers fo r the physical system. The predictions based upon this criterion are s hown to be consistent with published experimental data and numerical s imulations. 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 a bove and below a substrate placed in midstream.