Unsteady Flamelet Modeling of Soot Formation in Turbulent Diffusion Flames
H. PITSCH, E. RIESMEIER, N. PETERS
Combustion Science and Technology
Unsteady flamelet modeling of steady turbulent diffusion flames has been shown in recent studies to yield good predictions for temperature, concentrations of major chemical components, including intermediates, as well as OH radical and NO concentrations [1, 2] . It has been shown that transient effects in steady jet diffusion flames have to be considered, if slow physical processes, such as radiation, or slow chemical processes, like the formation of NO x , are important. In the present study
... e formation of soot in a steady turbulent C 2 H 4 /air jet diffusion flame is modeled using the unsteady flamelet concept. The chosen configuration has experimentally been investigated by Kent et al. [3, Flame A]. In this experiment the nozzle diameter was 3 mm and the jet exit velocity 52 m/s, which leads to a fuel Reynolds number of 14660. Since the formation of soot is a slow process and both gas and soot radiation have a strong influence on the temperature in this flame unsteady effects are expected to be important in the investigated configuration. The unsteady flamelet model applied in the present study is described in detail in Ref. . The flamelet equation for the temperature is where t denotes the time, Z the mixture fraction, T the temperature, χ the scalar dissipation rate, ρ the density, c p the specific heat capacity at constant pressure,q R the rate of radiative heat loss per unit volume. N is the number of chemical species, h k the enthalpy, andṁ k the chemical production rate per unit volume of species k. H accounts for the enthalpy flux by mass diffusion. The flow field has been calculated using the FLUENT code. To incorporate transient effects into the flamelet calculations, an unsteady flamelet has been solved interactively with the CFD solution. Since the boundary conditions of the flamelet, which are the temperatures and the composition of the fuel and the oxidizer stream, as well as the pressure, remain constant throughout the calculation, the only varying parameter influencing the flamelet solution is the scalar dissipation rate describing the impact of the turbulent flow field on the diffusion flame structure. The unsteady flamelet has been calculated as a function of the flamelet time, which is related to the distance from the nozzle x as where Z is the Favre average of the mixture fraction and u(x) Z = Z st is the axial velocity component at the radial position, where Z = Z st , and the index st refers to stoichiometric conditions. Following Ref.