Effects of jet-to-disk separation distance on the characteristics of mixed convective vortex flow in an impinging air jet confined in a cylindrical chamber

How the jet-to-disk separation distance H affects various aspects of the laminar mixed convective vortex flow resulting from a low speed air jet impinging onto a heated horizontal disk confined in a vertical adiabatic cylindrical chamber is explored herein by flow visualization combined with temperature measurement. In the present study the air flow rate is varied from 1.0 to 8.0 slpm (standard liter per minute) for two different injection pipes with the temperature difference between the heated disk and air injected into the chamber varied from 0 to 25.0degreesC at four jet-to-disk separation distances. The corresponding jet Reynolds and Rayleigh numbers, respectively, range from 0 to 1082 and from 0 to 63,420. The experimental results clearly reveal a new vortex roll. More specifically, in the upper range of the jet Reynolds number tested here a small tertiary inertia-driven circular vortex roll appears near the upper wall of the chamber. in addition to the previously known primary and secondary inertia-driven rolls. Besides. we also note that a reduction in the jet-to-disk separation distance causes a delayed onset of the inertia-driven rolls except for H greater than or equal to 20.0 mm and in the meantime causes a substantial decrease in the Rayleigh number since Ra alpha H-3, which in turn significantly reduces the size and strength of the buoyancy-driven roll. Moreover, the inertia-driven rolls are bigger for a larger H. At the largest H (=30.0 mm) tested here both the primary inertia-driven roll and buoyancy-driven roll are relatively large and they can contact with each other. Hence no space is available for the secondary inertia-driven roll to appear. Our data also suggest that the onset of the buoyancy roll takes place when the local buoyancy-to-inertia ratio at the edge of the disk Gr/Re-we(2) approximate to 33.0 within the experimental uncertainty. Finally, the temperature data indicate that the steady radial air temperature distributions are non-monotonic which mainly reflects the counter-rotating vortex flow structure of the primary inertia-driven and buoyancy-driven rolls. (C) 2004 Elsevier.Ltd. All rights reserved.