Note on thermocouple measurements for laminar-turbulent transition phenomena in pipe flow

The possibility of using a fine thermocouple to detect the onset of unsteady motion in slightly heated gases is explored. Signals from thermocouple measurements using Chromel-Constantan thermocouples (wire diameter = 12.7 mu m) in slightly heated air flows in a horizontal pipe (pipe diameter 50.8 mm, length/diameter = 29.5) are analyzed to study the laminar-turbulent transition phenomena for Reynolds numbers varying from 1,500 to 4,500 with an increment of 500. The original signal from the thermocouple, fast Fourier transform (FFT) of the original signal, and filtered FFT signals in frequency and time domains are presented for each Reynolds number with temperature differences Delta T = 5 degrees C and 10 degrees C (Delta T = temperature difference between pipe center and room air). The critical Reynolds number is found to occur between Re = 2,500 and 3,000. The secondary flow caused by buoyancy forces is found to suppress the onset of turbulence. Without slightly heating the air, no temperature fluctuations are detected. The experimental data confirm the applicability of the method in studying the laminar-turbulent transition phenomena in pipe flow. The possible difficulties in stability analysis are pointed out.