The effect of time-dependent partial premixing in radiating flamelets

An understanding of the effect of partial premixing on flame characteristic s is important to improve the design of many practical applications. In thi s study, a numerical investigation is carried out to examine the effect of time-dependent partial premixing in counterflow diffusion flames. The objec tive is to help understand the effect of turbulent fluctuations on a flamel et embedded in the how held. Premixing of both air with the fuel stream and of fuel with the air stream is considered. The equivalence ratio of the pa rtially premixed reactants is varied as a function of time. As expected, un steady partial premixing results in a fluctuating double-flame configuratio n: a premixed flame and a diffusion flame. The maximum flame temperature, h eat release rate and radiative heat loss are used to describe the flame res ponse to fluctuations in partial premixing. The increase in partial premixi ng raises the diffusion flame temperature. This increase in temperature is greater at higher strain rates due to closer proximity of the premixed flam e. The flame dynamics to changes in partial premixing is influenced by simi lar physical processes as those responsible for flame dynamics of pure diff usion flames subjected to unsteady effects. At low fluctuation frequencies, the flame responds quasi-steadily to changes in partial premixing. However , the amplitude of the flame response decreases gradually with an increase in the fluctuation frequency. At large frequencies, the flame is effectivel y insensitive to changes in partial premixing. A modified Strouhal number ( based on the ratio of imposed fluctuation frequency and strain rate) is def ined to predict the flame response to imposed fluctuation in partial premix ing and to identify a regime where transient effects are important. (C) 200 0 by The Combustion Institute.