OCR Text |
Show Emission Indices To provide an unambiguous calculated measure of the amount of NOx produced in the counterflow-flame simulations, emission indices (mass of NOx emitted from the flame per mass of fuel burned) were calculated, following the procedures of Takeno and Nishioka [10]. Based on the amount of fuel actually consumed in the flame (much of the fuel escapes combustion in the counterflow geometry), the NO emission index is expressed in tenns of the NO production rate, roNO' and the fuel consumption rate, -roF: L JWNOroNOdx = ....::0"--::,-___ _ L JWFroFdx ° where WNO and WF are the nitric oxide and fuel molecular weights, respectively. Dilution Parameter, Z (1) In an industrial burner, the overall stoichiometry is slightly lean, with a typical 02 content of less than 1 % to 3% in the product stream. Percentage flue gas recirculated, %FGR, is typically defined as %FGR = mPr . . ,recrr .100% = mPr,tot mPr . ,fecrr .100% mair + mF (2) where mpr,recir is the mass flowrate of product gases recirculated. In such burners, all of the fuel is burned. In the model counterflow flame, however, not all of the fuel is consumed, since much passes out the sides (cf. Fig. 1) without ever coming close to the flame zone. The same scenario holds for the oxidizer. Because of this, the overall stoichiometry associated with the flowrates of the fuel and oxidizer streams is not meaningful. A meaningful measure of stoichiometry in the counterflow arrangement is to assume that fuel and oxidizer react in stoichiometric (<l> = 1) proportions in the flame zone, a reasonable assumption for pure diffusion flames [11]. Since some of the fuel (and oxidizer) never reaches the flame, neither does some of any diluent introduced with either stream. Therefore, we define a dilution parameter, nominally equivalent to the FOR fraction, as 3 |