Observed flow instability and measured nusselt numbers for buoyancy assisted and opposed laminar flows in a vertical annulus

In the present work, the annulus consisted of a uniformly heated stainless steel tube, 1.27 OD and 90 cm long, and an outer, insulated plexiglas tube, 2.54 cm ID, resulting in a diameter ratio of 2.0. All experiments were conducted with the inlet Prandtl number maintained approximately at 7.02, which corresponds to a water temperature of 293 K. Additionally, the inlet Reynolds number was varied from 330 to 110 for upflow and from 100 to 1133 for downflow, while the heat flux was increased in small increments up to 7.8x10/sup 4/ W/m/sup 2/ and 7.23x10/sup 4/ W/m/sup 2/ for upflow and downflow, respectively. The values of the heat flux to flow rate ratio were adjusted so that a sufficient density difference and thus buoyancy was introduced to distort the velocity profile and to initiate flow instability. The flow field in the annular gap was observed by injecting a dye solution into the water stream. The images of the flow field were documented photographically and recorded on a video tape. Subsequently, the video tape was used to determine the location of the onset of flow instability and to examine the flow mixing occurring downstream. The occurrence of flow instability was also detected from the random oscillations of the temperatures measured at the heater surface and those of the water bulk near the outer wall. The water bulk and heater's surface temperature were measured at seven axial locations (10, 20, 30, 40, 50, 60, 70 and 80 cm from the bottom of the heater)