OCR Text |
Show z - mass of diluent (3) mass of stoichiometric mixture This parameter can be expressed in terms of the mass fractions of diluent in the fuel and air streams, Y dil,F and Y dil.A, respectively, as Z = fs Y dil.F + (1- fs) Y dil,A 1 - Y dil.F 1 - Y dil.A (4) where fs is the stoichiometric mixture fraction (mass of fuel per mass of stoichiometric mixture). To simulate conventional (windbox) FGR, Y dil.F is set to zero, and Y dil.A is varied to produce a range of values typical of FGR. To simulate FIR, Y dil.A is set to zero, and Y dil.F is varied to produce the desired range of Z values. Nonreacting Isothermal Flow To help understand the counterflow flames, it is instructive first to consider the case of a nonreacting flow. Calculations were performed for fixed inlet velocities (cf. Fig. 1) of Uo = UL = 50 crn/s. Methane was introduced at x = 0 and air at x = L. Nitrogen, the diluent, was added either at x = 0, fuel dilution, or x = L, air dilution, to provide a diluent fraction Z = 0.15. Significant differences in the species and velocity distributions result when N2 is added to the fuel stream compared to the cases of no dilution or air-side dilution. With fuel-side dilution, the CI-4 mole fraction drops from unity (no dilution) to approximately 0.39 for Z = 0.15, with corresponding N2 mole fractions of zero and 0.61. Since N2 is a major constituent of air, air-side N2 addition has only a minor effect on the species concentrations in the oxidizer stream; oxygen concentrations fall from 0.21 to 0.18, while N2 concentrations rise from 0.79 to 0.82 when air is diluted to Z = 0.15. These differences in concentrations at the boundaries (x = 0 and L) result in significant differences in molecular transport and also alter the momentum flux of the reactant streams, since the densities are altered with N2 addition while the velocities are chosen to be fixed. The net influence of these effects is illustrated in Fig. 2, where the local flow velocities are shown as functions of distance from the stagnation plane for the two cases of air-dilution and fuel-dilution, both with Z = 0.15. In Fig. 2, a positive velocity indicates a flow from left to right, 4 |