An experimental study of transition to turbulence in vertical mixed convection flows

Results are reported from an experimental study of a mixed convection flow, that is combined forced and natural convection, undergoing transition to turbulence, adjacent to a vertical, uniform-heat-flux surface. Small and aiding forced convection effects were studied. The measurements, in air, were made at pressure levels ranging from 4.4 bars to 8 bars, at flux levels q in the range 14-1,300 W/m3. The imposed free-stream velocities U∞ were around 5 cm/s. One objective was to determine any quantitative parameters that would predict the bounds of the transition region. Another was to measure disturbance growth characteristics during transition. Results show that, at a given U∞ the beginning of transition is not correlated by the local Grashof number Grx* alone. An additional dependence on both the downstream location and pressure level was found. Thermal and velocity transitions were found to begin when the mixed convection parameter εM reached a value of 0.18. Transition was found to be complete when the nondimensional convected energy in the boundary region βq x/5k, reached a value of 7.10. These experimental results confirm the prediction of linear stability analysis, that aiding mixed convection stabilizes the flow, compared to pure natural convection flow. The data also support a physical explanation of these mechanisms