Experimental study of onset of laminar–turbulent transition in mixed convection in a vertical heated tube

Unsteady phenomena were observed in the case of airflows approaching transition inside a uniformly heated vertical tube. An analysis of the experimental data at three Reynolds numbers (Re = 1000, 1300 and 1600) allowed to establish that: (i) A flow instability onsets at different Grashof numbers (Gr) when the condition Gr/Re > 1500 is achieved. (ii) The FFT of the temperature signal shows a peak at a frequency fD (0.45 Hz) when Gr/Re > 1500. (iii) This instability is associated to a thermal buoyant instability rather than to a thermal shear instability. (iv) This instability forms in the buffer region and then propagates towards the whole section of the tube as the Grashof number is increased.