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TECHNICAL PAPERS: Internal Combustion Engines

# Sensitivity Analysis of $NOx$ Formation Kinetics in Pilot-Ignited Natural Gas Engines

[+] Author and Article Information
Huateng Yang

Department of Mechanical Engineering,  The University of Alabama, Tuscaloosa, AL 35487

S. R. Krishnan

Energy Systems Division,  Argonne National Laboratory, Argonne, IL 60439

K. K. Srinivasan

Department of Mechanical Engineering,  Mississippi State University, Starkville, MS 39762

K. C. Midkiff

Department of Mechanical Engineering,  The University of Alabama, Tuscaloosa, AL 35487cmidkiff@eng.ua.edu

J. Eng. Gas Turbines Power 129(1), 261-270 (May 31, 2006) (10 pages) doi:10.1115/1.2360601 History: Received May 23, 2005; Revised May 31, 2006

## Abstract

A sensitivity analysis of $NOx$ formation in pilot-ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The $NOx$ formation mechanism employed in this study incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to $NOx$ formation, and identifies the rate-controlling $NOx$ formation reactions in different high-temperature regions—the burning pilot spray, the premixed flame associated with the gaseous fuel-air mixture, and the burned combustion products. The formation rates for reactions involving $NOx$ are also investigated to reveal the primary $NOx$ formation paths. Results show two main $NOx$ formation paths both in the pilot spray (also called the packets zone) and the burned zone. The rate-limiting reactions for the packets zone are $O+N2=NO+N$ and $N2+HO2=NO+HNO$. Rate-limiting reactions for the burned zone are $N2O+M=N2+O+M$ and $N2+HO2=NO+HNO$. Since the aforementioned reactions significantly influence the net $NOx$ prediction, it is important that the corresponding reaction rates be determined accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in $NOx$ modeling, a transient relative error is estimated to evaluate the validity of the assumption. The relative error in $NOx$ prediction with and without the use of the steady-state assumption is small, of the order of 2%. This work also confirms that sensitivity analysis can provide valuable insight into the possible $NOx$ formation pathways in engines and improve the ability of current prediction tools to obtain more reliable predictions.

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## Figures

Figure 7

Temperature and equivalence ratio history of flame zone

Figure 1

Conceptual schematic of zone evolution (Krishnan (20))

Figure 2

Crank resolved NOx prediction (EZM and SEZM) versus experimental NOx measurement at an injection timing of 30 BTDC

Figure 3

QSSA error for five species and temperature (TP (1,1)) for packet (1,1) versus crank angle

Figure 4

QSSA error for five species for the burned zone and total engine NO prediction versus crank angle

Figure 5

Comparison of instantaneous NOx predicted from the flame zone with and without QSSA

Figure 6

Temperature and equivalence ratio history of the first injected packet in the packets zone

Figure 8

Temperature and equivalence ratio history of burned zone

Figure 9

Comparison of NOx prediction with and without QSSA

Figure 10

Comparison of NOx formed from packets zone and flame zone

Figure 11

Normalized sensitivity coefficients of important reactions for the first injected packet at 360 crank angle degrees

Figure 12

Normalized sensitivity coefficients of important reactions for the first injected packet versus crank angle

Figure 13

Rate of progress of important reactions for the first injected packet versus crank angle

Figure 14

Normalized sensitivity coefficients of important reactions for the burned zone at 360 crank angle

Figure 15

Normalized sensitivity coefficients of important reactions for the burned zone versus crank angle

Figure 16

Rate of progress of important reactions for the burned zone versus crank angle

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