Investigation of turbulent flow in square duct with heated foil thermometry

Abstract

Fully developed turbulent flow of water in a square duct has been studied with thermography of the 25 μm heated steel foil that covers part of the duct wall. Thermal fluctuations observed in the specific case fall in-between the ideal isothermal boundary condition, which is achieved with a thick heater cooled with water, and ideal isoflux boundary condition, which could be theoretically achieved with an infinitely thin heated foil. Results in the present paper are obtained at fixed Reynolds number 10000 and heat flux at the thin foil around 6 kW/m2, which was considered to be the upper limit, where the water can be treated as a passive scalar at Prandtl number 5.4 with temperature increments around 5 °C above the inlet value. Main experimental results are 2D temperature fields on the outer side of the foil, which were averaged over time to predict the mean foil temperature, temperature fluctuations and the corresponding spectra. These parameters provide accurate description of the turbulence near the heated foil. Simple geometry of the experiment and fully developed velocity field are convenient for accurate numerical simulation, which was performed with wall-resolved Large Eddy Simulation approach. Average magnitude of the measured and computed temperature fluctuations was roughly 0.5 °C and 0.6 °C, respectively, which corresponds to about 10% of the temperature increment. Measured thermal streak spacing was around 60 wall units, and was together with the power spectra of the temperature fluctuations in close agreement with the LES numerical predictions that included the foil with conjugate heat transfer. Despite the square cross-section of the duct, which contains around dozen thermal streaks on each side of the duct, the spectra and the streak spacing remain similar as in the channel flow between the parallel planes.