The length of recirculation zone is an important factor in the design of CVD chambers for manufacturing monolithic infrared optical materials. The recirculation length of confined coaxial air jet flows, which consist of a central tubular flow and a surrounding annular flow, into a cold model of CVD chamber at atmospheric pressure and room temperature were studied numerically by a two-dimensional Galerkin finite element method and experimentally by observing the streamlines of methylamino chloride particles. In the experiments, the tubular Reynolds number varies from 180 to 600 and the annular Reynolds number varies from 37.0 to 165.1. The simulation and experimental results show that the length of recirculation zone increased initially with the increase of either tubular or annular Reynolds number. Nevertheless, when either one of the tubular and annular Reynolds numbers was relatively high, or both Reynolds numbers were at relatively medium values, small waves occurred, laminar flow could not be maintained, and the observed recirculation length dropped dramatically from 10.4-15.7 D-h, to 3.7-4.6 D-h, where D-h is the hydraulic diameter of the chamber. Suggestions of the flow conditions and chamber length for the design of CVD chambers are given to make most of the chamber length occupied by the recirculation zone so that an uniform deposition of materials could be obtained.
