THE TRANSIENT-RESPONSE OF STRAINED LAMINAR-PREMIXED FLAMES

Modeling and simulation of turbulent combustion in premixed gases, for relatively large-scale and low-intensity turbulence, have traditional ly been based on the assumption that the flame response to strain is i nstantaneous. In this paper, we revisit the validity of this assumptio n by examining the time-dependent response of a premixed laminar flame when subjected to a sudden change in strain and a periodic strain. We find that at unity Lewis number and for a stepwise increase in strain , the settling time of the flame varies between the chemical time, the flame time and the flow time as the Karlovitz number changes from low to intermediate to high values, respectively, over the entire range o f flame temperatures. At nonunity Lewis numbers, the settling time cha nges from the flame time to the flow time as the strain jump increases from intermediate to high Karlovitz numbers and over the entire range of flame temperatures. For given Lewis and Karlovitz numbers, the set tling time decreases as the flame temperature increases. Thus, in a fl amelet or thin flame modeling, and over the entire range of Lewis numb er, the response of a premixed flame can be considered instantaneous o nly for high flame temperatures. The same is found to be true for inte rmediate flame temperatures when the Lewis number is unity. Otherwise, for low and intermediate flame temperatures, and nonunity Lewis numbe rs, corrections reflecting the lag between the how and the flame shoul d be considered. The response of the flame to oscillating strains whos e maximum value is below unity Karlovitz number is also investigated f or two values of the flame temperatures. It is found that the average burning velocity is close to the burning velocity at the average strai n. For low frequency oscillations, the phase shift between the strain and the burning velocity is close to 0 for L(e) < 1 and near 90 degree s for L(e) greater than or equal to 1. For high frequency oscillating strains, and over the entire range of Lewis number and flame temperatu re, the phase shift is of order of 140 degrees.