Numerical Investigation of the Time Scales of Single Droplet Burning

The effect of gas phase velocity fluctuations on single droplet burning is investigated numerically. The main objective of this study is to understand the effect of gas phase turbulence on nitric oxide formation in single droplet flames. Since the interaction of gas phase velocity fluctuations with droplet burning is of sequential character, a separate investigation of droplet momentum coupling and droplet burning is performed. Momentum coupling controls droplet relaxation against changes of the gas phase velocity along the droplet trajectory and, thereby, determines to what extend gas phase velocity fluctuations translate into droplet slip velocity fluctuations. This coupling effect acts as a high pass filter with a cutoff frequency determined by the droplet Reynolds number and diameter. In the simulation of single droplet burning detailed models for chemical reaction, diffusive species transport and evaporation are used. A significant effect of slip velocity fluctuations on the mean values of NO formation rate is observed. The effect of slip velocity fluctuations on the mean NO formation rate is frequency dependent. The frequency response of the droplet flame is similar to that of a low pass filter. The droplet flame time scale characterizing the response to slip velocity fluctuations is found to correlate with chemical time scales. This time scale is not affected by droplet diameter.