Examining heat transfer in the developing laminar-turbulent mixed convection flow regime with buoyancy effects

This experimental study investigates the hydrodynamic and thermal phenomena in the concurrently hydrodynamically and thermally developing phase of laminar, transitional, and turbulent mixed convection flows of water in a vertical tube with buoyancy effects. A mixed convection experimental test-rig was designed, built, and validated for the experimentation. Experiments were conducted for Reynolds number (Re) varying from 494 to 15053, Grashof number (Gr) from 1.44 x 104 to 1.35 x 107,Prandtl number (Pr) from 3.1 to 7.1, and Richardson number (Ri) from 0 to 1.5 with a tube length-to-diameter ratio of 130, undergoing constant wall heat flux. A facility for flow visualization is also made following the test section for identifying the onset of transition. The flow visualization indicates an earlier transition in buoyancy-opposed flow in comparison to the buoyancy-aided flow. The effects of varying the Gr , Re , at fixed Ri on friction factor (f) and Nusselt number (Nu) were investigated for both the flows. Experimental results showed that at fixed Ri with the increment in Re , the average f gradually reduces in the laminar region, starts increasing in the transitional phase, and further reduces in the turbulent regime. In contrast, the average Nu increases in the laminar region, remains unchanged in the transitional phase, and then increases with the increase in Re in the turbulent region. Further, in the transitional phase, with the increase in Ri , the average f lowers, and the average Nu enhances in buoyancy-aided and opposed flows, respectively. It has also been observed that the transition is delayed with the increase in Ri in both flows and shortens the span of the transitional regime. The transition onset occurs earlier in the opposing flow contrary to the aiding flow for the same Re and Gr .