Structure of Turbulent Nonpremixed Syngas Jet Flames in a Heated Coflow at Elevated Pressure

This study investigates the effects of coflow preheat temperature and elevated pressure on the structure of turbulent nonpremixed syngas jet flames representing conditions relevant to the integrated gasification combined cycle (IGCC) combustion systems. Experiments were conducted using the KAUST high temperature and pressure duct (HTPCD). The study begins by establishing a stability curve for different nitrogen dilution ratios versus blowoff jet velocity, revealing a nonlinear relationship between nitrogen content in the jet and blowoff velocity; the blowoff velocity decreases from approximately 76 m/s for undiluted syngas to approximately 32 m/s at 45% nitrogen dilution. Selected flame conditions, with varying jet velocities (25-45 m/s) and nitrogen content (15-30%), are analyzed under four different pressure and coflow temperature conditions: 1 bar at 295 K, 1 bar at 373 K, 5 bar at 295 K, and 5 bar at 373 K. Direct flame imaging and OH-PLIF techniques are used to examine the appearance, structure, OH corrugation, and thickness of the flame. The results demonstrate that flames with higher jet velocities are more significantly affected by elevated coflow temperatures, resulting in shorter luminous flame lengths and increased corrugation. In contrast, for lower jet velocities (25 m/s), luminous flame length and corrugation are largely unaffected by preheat temperature, owing to a competing reduction in Reynolds number caused by the lower gas density at elevated temperatures. Increasing the pressure to 5 bar enhances the corrugation of the flame in all conditions and reduces the visible length of the flame, regardless of the conditions of the jet. Furthermore, increasing either jet velocity or nitrogen dilution ratio decreases the thickness of the OH layer, although this effect diminishes in high-pressure flames. Coflow preheating consistently increases OH layer thickness (from approximately 1.4 mm to 1.8 mm at atmospheric pressure), while elevated pressure reduces it (from approximately 1.8 mm at 1 bar to approximately 1.2 mm at 5 bar). Higher coflow temperatures also increase the thickness of the OH layer regardless of pressure. One-dimensional (1-D) counterflow diffusion flame simulations provide chemical-kinetic support for the observed experimental trends, particularly the widening of the OH distribution at higher air preheat temperatures.