INVESTIGATION OF HELIUM FLOW LAMINARIZATION AT HIGH TEMPERATURES AND HIGH PRESSURES IN A GRAPHITE FLOW CHANNEL
Fundamental forced convection heat transfer experiments have been performed with nitrogen and helium flowing upward through a 16.8 mm diameter flow channel in a 2.7m long graphite test section. Flow regimes include turbulent, transitional and laminar flows with the inlet Reynolds numbers ranging from 1,500 to 14,000. Experiments were performed at different helium and nitrogen temperatures and pressures up to 620 °C and 61 bar, respectively, various flow rates (50-500 SLPM), and heater power up to 6.5 kW. The analyses of the experimental data showed significant reductions in the Reynolds number of up to 50% over the 2.7 m test section between the inlet and . Flow laminarization caused by intense heating was defined to occur when the local Nusselt number decreased 20% below the Nusselt number given by the modified Gnielinski correlation. In this study, flow laminarization criteria were considered based on a dimensionless acceleration parameter (Kν) and buoyancy parameter (Bo*). Experiments involved high heating rates leading to the wall-to-bulk temperature ratios ranging from 2 to 1.05 (T in K), dimensionless heat flux (q+) from 1×10-5 to 3×10-4 and buoyancy parameter (Bo*) from 3×10-7 to 8×10-5. The latter range covers a value of Bo≈6×10-6; previously suggested to correspond to the onset of the influence of the buoyancy force.