Reacting Jets in Compressible Vitiated Crossflow with Negligible Swirl

Abstract

Combustion will likely continue to be utilized over the next century to meet the world’s energy needs. As increasingly stringent requirements on emissions, particularly of oxides of nitrogen (NO and NO2) are imposed on power plants due to their harmful effects on the environment, advanced combustor strategies to limit NOX productions are needed. One such advanced concept involves axially staging the fuel to create a distributed combustion system. The fundamental problem for staged combustion involves the injection of a reacting jet into crossflow. This canonical problem is modified for this dissertation through injection of a reacting premixed natural gas and air jet into a compressible vitiated crossflow with negligible swirl. In addition, the experimental efforts for this work were conducted at elevated inlet air temperature and combustor pressure.The development and performance of a perforated plate burner (PPB) to provide vitiated crossflow and operating using premixed natural gas (NG) and air at engine-relevant conditions is discussed. A significant benefit of using burners with simplified flow fields, such as the PPB, for experimental studies in the laboratory is the potential for decoupling the complex fluid dynamics in typical combustors from the chemical kinetics. The stable operation of the PPB within a high-pressure test rig was validated: successful ignition, effective use of redlines for flashback mitigation, and long duration steady-state operation in both piloted and non-piloted modes were all observed. Exhaust gas emissions measured using a Fourier-transform infrared (FTIR) spectrometer showed very good performance of the PPB in terms of the combustion efficiency and low levels of NOX in non-piloted operation that were generally within 3 ppm.Emissions measurements of the premixed reacting jet in vitiated crossflow were obtained for a variety of conditions and a significant NOX reduction was achieved when the staged combustor exit Mach number was increased and the axial residence time was decreased. Based on this preliminary investigation, a test matrix was developed to independently vary the exit Mach number for a constant axial residence time by using modular rig hardware to change the length of the axial combustor. Up to 70% reduction in NOX produced by the axial stage was observed when the combustor exit Mach number was increased from about 0.26 to 0.66 at a constant residence time of 1.4 ms. NOX reduction based on variation in the Mach number and at a constant residence time has not been previously reported in the literature to the best of our knowledge. This decrease in NOX is hypothesized to be due to the lower static temperature of a compressible flow and potentially better mixing of the jet with the crossflow due to the interaction occurring at high speeds.Based on the strong effect of Mach number for NOX reduction even at a constant residence time, further investigation using laser-based diagnostics is needed to provide insight on physical processes controlling this phenomenon. An optically-accessible secondary combustion zone was developed and fabricated to study the flame position and structure of reacting jets injected into a high-speed vitiated crossflow. The windowed combustor was capable of long-duration, steady-state operation despite a trifecta of: elevated pressures, high combustion gas temperatures, and high-speed reacting flows. High-speed imaging using OH* and CH* chemiluminescence was used to validate operation of the optically-accessible secondary combustion zone.High-repetition-rate (1 – 10 kHz) planar laser-induced fluorescence (PLIF) imaging of OH and CH were performed on both premixed NG-air reacting jets and premixed NG-hydrogen-air reacting jets to investigate the flame structure of the reacting jet within a high-speed crossflow. OH-PLIF was performed in the A-X electronic system using excitation at near 283 nm in the (vʹ = 1, v″ = 0) band and near 311 nm in the (vʹ = 0, v″ = 0) band. The crossflow velocity and equivalence ratio were observed to have a strong impact on the stabilization of the reacting jet flame. Additional insight on the stabilization mechanism was obtained using 50 kHz OH* chemiluminescence imaging. CH-PLIF was performed in the C-X electronic system using R-branch excitation near 311 nm in the (vʹ = 0, v″= 0) band. The CH-PLIF images indicated local stoichiometric regions near the leeward side of jet injection and in regions where significant interaction of the fuel rich jet with the vitiated crossflow is expected. In addition, the CH-PLIF images showed evidence of broken, thickened, non-premixed reaction layers.