Ignition Characteristics

OVERVIEW

Lean premixed combustion technology is widely accepted in gas turbine industry to reduce pollutant emissions. Engines which use this strategy are being installed world-wide. In lean-premix combustors and other types of low-emissions combustors, fuel and air are premixed before combustion. One of the most important concerns is the ignition delay phenomenon in these systems. We have to avoid auto-ignition at all costs in these systems to protect the combustor components as well as to avoid producing unacceptably high levels of pollutant emissions. So, it is very important to understand ignition delay times in lean premixed combustion systems.

In this program, ignition delay times are studied for different fuels such as natural gas fuels, medium BTU fuels and low BTU fuels under different conditions. The effects of temperature, pressure, equivalent ratio, fuel composition and state, and turbulence intensity on ignition delay times are addressed by using experimental and modeling methods. The experimental method is a continuous flow device with a 147-inch-long test section. A series of photodiodes and thermocouples are used as ignition detectors. Modeling methods include chemical kinetics modeling, CFD modeling and statistical optimization. The testing pressures vary from one atmosphere to fifteen atmospheres and temperature can go up to 1300 °F. Liquid fuels can also be tested in this research. An important goal is to establish simple correlations of autoignition delay with fuel composition and state for application to gas turbine applications. Figure 1 shows an overview of the project.

Figure 1. Program Overview

APPROACH

• Experimental Methods: Continuous Flow Reactor. A simplified continuous flow reactor can be found in Figure 2. Figure 3 shows the test rig.
• Numerical Tools: CFD, Chemical Kinetics Studies, and Design of Experiment.
• Test Conditions

Figure 2. A Continuous Flow Reactor

GOALS

The goal of this program is to establish the understanding of autoignition in lean premixed combustion systems as a function of fuel composition for various inlet temperature, pressure, turbulence intensities, and fuel concentrations. This research will:

• Directly apply to challenges facing advanced gas turbine applications;
• Provide insight into the nature of auto-ignition delay in lean, premixed combustion systems by judicious experimental and theoretical analyses;
• Provide needed data as an archive for future research.

RESULTS

CFD Model

To simulate the flow inside the test rig, CFD modeling was accomplished and the results are shown in Figure 4.

Figure 4. CFD Simulation of Flow Pattern

Factorial Model

To evaluate the effects of different parameters on autoignition delay time, a factorial model was built using the design of experiment methodology. The autoignition data in the model were obtained from chemical kinetics modeling using GRI-Mech V3.0 mechanism and Chemkin software package. It was found that temperature, pressure and fuel composition have major effects on autoignition delay times in the studied region. The effect plot in Figure 5 shows the results from the model.

Crossed Model

Another design of experiment model was built to study the effect of both operating parameters and different high-hydrocarbons on autoignition delay times. For fuel composition consideration, methane was set to be 80%-100% by volume, ethane and propane were 0-20%. Again, the data was obtained from chemical kinetics modeling using GRI-Mech V3.0 and Chemkin. Example results were presented in Table 1. It shows the transformed autoignition delay times as function of both operating factors and fuel composition in the same plots.

Table 1. Example Results of Crossed Model

Temperature Effect
Pressure Effect
Equiv Ratio Effect

From the initial results presented above, we can get the following initial conclusions:

• The major factors of autoignition delay time include temperature, pressure and fuel composition (or high hydrocarbon additives to natural gas).
• Equivalence ration has limited effects on auto-ignition delay times.
• Fuel composition has tremendous effects and different higher hydrocarbon has different effect.

RECENT PRESENTATIONS

EFFECTS OF ETHANE AND PROPANE ADDITIVES ON THE AUTOIGNITION BEHAVIOR OF NATURAL GAS FUELS (2004). Presented at the Spring Meeting of the Western States Section/The Combustion Institute, Paper 04S-24, March (J.H. Chen, V.G. McDonell, and G.S. Samuelsen)

PERSONNEL

Investigators: V.G. McDonell and G.S. Samuelsen
Staff: R.L. Hack, M.D. Crespin
Students: D.L. Kramer

SPONSORS: U.S. Department of Energy (University Turbine Systems Research)