Numerical Investigation of the Time Scales of Single Droplet Burning

Abstract

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.

Hydrocarbon Droplet Turbulent Combustion in an Elevated Pressure Environment

“…Cho et al [16], in a numerical study, reported that turbulence intensity of a free airstream has a marginal effect on the burning rate of a stationary droplet under elevated ambient conditions (2.02T c and 0.47P c where T c and P c are the fuel critical temperature and pressure, respectively). Similar conclusion was also reported by Beck et al [17], who investigated numerically the effect of turbulence intensity on a moving burning droplet in a hot (0.88T c and 1.43T c ) airstream at atmospheric pressure.…”

“…The effect of forced flow/convection on hydrocarbon droplet combustion has been studied quite extensively under laminar flow conditions and therefore there is a wealth of knowledge (see, e.g., recent references [1][2][3][4][5][6][7][8][9][10], and references cited therein, M. Birouk ( ) · S. L. Toth Department of Mechanical Engineering, University of Manitoba, Winnipeg, MB R3T 5V6, Canada e-mail: madjid.birouk@umanitoba.ca to cite only a few). However, studies reporting on the effect of a turbulent or an acoustic field are quite limited (e.g., [11][12][13][14][15][16][17]). Birouk et al [11] investigated experimentally the combustion of a suspended hydrocarbon droplet under turbulent flow environment at atmospheric/room conditions.…”

Ethanol turbulent spray flame response to gas velocity modulation

“…They concluded that the mixing of the droplets, and so the evaporation, has an impact on the burning rate and this is related to combustion instabilities. An extensive numerical analysis has been performed by Beck, Koch, and Bauer [8] on the effects of slip velocity fluctuations on a single burning droplet. NO formation was predicted to vary with the forcing frequency, depending on changes in the droplet-to-flame distance.…”