A methodology is presented in this paper on the modeling of $NOx$ formation in diffusion flame combustors where both droplet burning and partially premixed reaction proceed simultaneously. The model simulates various combustion zones with an arrangement of reactors that are coupled with a detailed chemical reaction scheme. In this model, the primary zone of the combustor comprises a reactor representing contribution from droplet burning under stoichiometric conditions and a mixing reactor that provides additional air or fuel to the primary zone. The additional flow allows forming a fuel vapor/air mixture distribution that reflects the unmixedness nature of the fuel injection process. Expressions to estimate the extent of deviation in fuel/air ratios from the mean value, and the duration of droplet burning under stoichiometric conditions were derived. The derivation of the expressions utilized a data base obtained in a parametric study performed using a conventional gas turbine combustor where the primary zone equivalence ratio varied over a wide range of operation. The application of the developed model to a production combustor indicated that most of the $NOx$ produced under the engine takeoff mode occurred in the primary as well as the intermediate regions. The delay in $NOx$ formation is attributed to the operation of the primary zone under fuel rich conditions resulting in a less favorable condition for $NOx$ formation. The residence time for droplet burning increased with a decrease in engine power. The lower primary zone gas temperature that limits the spray evaporation process coupled with the leaner primary zone mixtures under idle and low power modes increases the $NOx$ contribution from liquid droplet combustion in diffusion flames. Good agreement was achieved between the measured and calculated $NOx$ emissions for the production combustor. This indicates that the simulation of the diffusion flame by a combined droplet burning and fuel vapor/air mixture distribution offers a promising approach for estimating $NOx$ emissions in combustors, in particular for those with significant deviation from traditional stoichiometry in the primary combustion zone.

1.
Lefebvre
,
A. H.
,
1984
, “
Fuel Effects on Gas Turbine Combustion-Liner Temperature, Pattern Factor, and Pollutant Emissions
,”
AIAA J. Aircraft
21
, No.
11
, pp.
887
898
.
2.
Plee
,
S. L.
, and
Mellor
,
A. M.
,
1979
, “
Characteristic Time Correlation for Lean Blowoff of Bluff-Body-Stabilized Flames
,”
Combust. Flame
,
35
, pp.
61
80
.
3.
Fletcher, R. S., and Heywood, J. B., 1971, “A Model for Nitric Oxide Emissions From Aircraft Gas Turbine Engines,” AIAA Paper No. 71–123.
4.
Rizk
,
N. K.
, and
Mongia
,
H. C.
,
1993
, “
Semianalytical Correlations for NOx, CO, and UHC Emissions.
ASME J. Eng. Gas Turbines Power
,
115
, pp.
612
619
.
5.
Rizk, N. K., and Smith, D. A., 1994, “Regional and Business Aircraft Mission Emissions,” ASME Paper No. 94-GT-300.
6.
Kelkar, A. S., Ramakrishna, Ch., Sivathanu, Y. R., and Gore, J. P., 1996, “Temperature and Velocity Statistics of Lean Premixed Jet Flames for NOx Calculations,” AIAA Paper No. 96-0818.
7.
Yule
,
A. J.
, and
Bolado
,
R.
,
1984
, “
Fuel Spray Burning Region and Initial Conditions
,”
Combust. Flame
,
55
, pp.
1
12
.
8.
Law
,
C. K.
, and
Chung
,
S. H.
,
1980
, “
An Ignition Criterion for Droplet in Sprays
,”
Combust. Sci. Technol.
,
22
, pp.
17
26
.
9.
Cooper, L. P., 1980, “Effect of Degree of Fuel Vaporization Upon Emissions for a Premixed Partially Vaporized Combustion System,” NASA Technical Paper No. 1582.
10.
Pompei
,
F.
, and
Heywood
,
J. B.
,
1972
, “
The Role of Mixing in Burner-Generated Carbon Monoxide and Nitric Oxide
,”
Combust. Flame
,
19
, pp.
407
418
.
11.
Fric, T. F., 1992, “Effects of Fuel-Air Unmixedness on NOx Emissions,” AIAA 92-3345.
12.
Dimotakis
,
P. E.
, and
Miller
,
P. L.
,
1990
, “
Some Consequences of the Boundedness of Scalar Fluctuations
,”
Phys. Fluids
,
2
, No.
11
, pp.
1919
1920
.
13.
Nicol, D. G., Malte, P. C., and Steele, R. C., 1994, “Simplified Models For NOx Production Rates in Lean-Premixed Combustion,” ASME Paper No. 94-GT-432.
14.
Rizk, N. K., and Mongia, H. C., 1994, “Emissions Predictions of Different Gas Turbine Combustors,” AIAA Paper No. 94-0118.
15.
Westbrook
,
C. K.
, and
Pitz
,
W. J.
,
1984
, “
A Comprehensive Chemical Kinetic Reaction Mechanism for Oxidation and Pyrolysis of Propane and Propene
,”
Combust. Sci. Technol.
,
37
, pp.
117
152
.
16.
Glassman, I., 1977, Combustion, Academic Press, San Diego, CA.
17.
Toof
,
J. L.
,
1986
, “
A Model for the Prediction of Thermal, Prompt, and Fuel NOx Emissions from Combustion Turbines
,”
ASME J. Eng. Gas Turbines Power
,
108
, pp.
340
347
.
18.
Rizk, N. K., 1995, “Calculation Method For NOx Production in Gas Turbine Combustors,” AIAA Paper No. 95-0282.
You do not currently have access to this content.