The effects of flame on the turbulence properties of unburned gas upstream of the flame were investigated. Fluctuating velocity field was diagnosed by two-color laser velocimeter and velocity data were analyzed to obtain turbulence statistics. Present study is partly motivated to test the validity of assumptions that are commonly made in the models of turbulent premixed flames. To identify the effects of the flame, all the test conditions made identical except the flame. Same set of measurements was made in both combusting and cold flow arrangement at the same locations. Turbulent statistics of both measurements were compared to each other.
Methane-air premixed flame anchored at 15mm diameter nozzle was used for the experiments. At the jet exit, turbulent Reynolds number was 7,000 and flow was fully developed turbulent pipe flow. Two mixture ratios at equivalence ratio of φ=0.6 and 1.0 were selected to determine the influence of equivalence ratio. Hydrogen pilot flame stabilized the flame at elevated Reynolds number flow.
Effects of flame on upstream flow field resulted in flat mean velocity profile and low rms fluctuations near the flame front. Reynolds stress distribution showed significant effects of the flame and the turbulent mixing decreases near the flame. The effective viscosity was evaluated in cold and combusting flow. The effective viscosity in combusting flow was 5~9 times of its cold flow counterpart and suggests the flaw in using the cold flow empirical correlations in combusting flow modeling. PDF shows nearly Gaussian distribution with slight negative skewness in cold flow measurements and positive skewness in combusting flow. The tendency of probability density decrease with radial distance reversed by the flame in flame on case. JPDF contour shows significantly higher peak density in combusting flow than cold flow. Autocorrelation was evaluated from the velocity data and integral length scale was obtained using integral time scale and Taylor's frozen field hypothesis. Both of the integral scales decreased with radial distance from the centerline in cold flow, whereas the integral length scale increased with radial distance in combusting flow with the effects of the flame is more pronounced at equivalence ratio of φ=0.6.
Based on the present finding, the effects of the flame on upstream flow field are significant in contrast to the common belief and must be accounted for in any modeling attempt of turbulent premixed flames.
