Experimental Study of the Interaction of Water Sprays With Swirling Premixed Natural Gas Flames

Water injection is often used to control NOx emissions or to increase power output from nonpremixed combustion of gaseous and liquid fuels. Since the emission level in premixed natural gas combustion is significantly lower than for nonpremixed combustion, water injection for emission reduction is usually not an issue. However, the increasing share of fluctuating power output from renewables motivates research activities on the improvement of the operational flexibility of combined cycle power plants. One aspect in that context is power augmentation by injection of liquid water in premixed combustors without drawbacks regarding emissions and flame stability. For research purposes, water injection technology has therefore recently been transferred to premixed combustors burning natural gas. In order to investigate the influence of water injection on premixed combustion of natural gas, an atmospheric single burner test rig has been set up at Lehrstuhl fur Thermodynamik, TU Munchen. The test rig is equipped with a highly flexible water injection system to study the influence of water atomization behavior on flame shape, position, and stabilization. Presented investigations are conducted at gas turbine like preheating temperatures (673 K) and flame temperatures (1800-1950 K) to ensure high technical relevance. In this paper, the interaction between water injection, atomization and macroscopic flame behavior is outlined. Favorable and nonfavorable operating conditions of the water injection system are presented in order to clarify the influence of water atomization and vaporization on flame stability and the emission behavior of the test rig. Water spray quality is assessed externally with a Malvern laser diffraction spectrometer whereas spray distribution in the test rig is determined by means of Mie scattering images at reacting conditions. The flame shape is analyzed using time-averaged OH* chemiluminescence images while the efficiency of water injection at various operating points is evaluated using global emission concentration measurements. Finally, the important influence of the water injection system design on the combustor performance will be shown using combined Mie scattering and OH* chemiluminescence images. At constant adiabatic flame temperatures, a stable flame could be established for water-to-fuel ratios of up to 2.25. While only minor changes could be detected for the heat release distribution in the combustion chamber, the water distribution changes significantly while increasing the amount of water. Finally, changes in NOx emission concentrations can directly be related to water droplet sizes and the global water distribution in the combustion chamber.