Spatio-temporal characteristics of temperature fluctuations in turbulent non-premixed jet flames

We report new results from high-repetition-rate (10 kHz) 2D temperature imaging to characterize the temporally-fluctuating nature of the well-known DLR series of turbulent non-premixed jet flames. Specifically, we present temporally-based statistics of temperature fluctuations at multiple simultaneous spatial positions with a specific focus on characterizing the effects of Reynolds number. DLR B (Re = 22,800) was found to exhibit a much higher probability of large temporal gradients as compared to DLR A (Re = 15,200). While the higher Reynolds number should promote higher fluctuations, the levels were commensurate with an increased effect due to high levels of local flame extinction and re-ignition in DLR B as compared to DLR A. The joint probability density function between the temperature fluctuation and the natural logarithm of the square of the temporal gradient was examined across flame conditions. At an axial position of forty nozzle diameters downstream (x/d = 40), DLR A exhibited statistical independence, while DLR B exhibited a negative correlation between the two quantities. These results suggest a Reynolds number-dependence on the spatial development of statistical independence. Integral time scales also are reported for both flames as a function of axial and radial position. The integral time scales are seen to increase with increasing axial and radial position and decrease with increasing Reynolds number as expected from theory.