Experimental investigation of boundary layer flashback in stratified swirl flames

摘要

Hydrogen enrichment of natural gas offers an attractive solution to reduce the carbon footprint and NO_x emissions of gas-turbine power plants; however, the higher flame speed and molecular diffusivity of hydrogen-air may lead to undesirable effects such as flashback. With flashback, an erstwhile stable flame propagates upstream into the fuel-air mixing zone, which can lead to mechanical failure of parts near the flame stabilization location. Stratification of equivalence ratio has been proposed as a method for reducing the propensity of flashback with enriched hydrogen fuels. In the current study, we investigate boundary layer flashback in stratified mixtures of hydrogen-air at atmospheric pressure. A model annular swirl combustor with a solid center-body, an axial swirl-nozzle, and an optically accessible mixing tube is used. Fuel is injected through radially outermost fuel ports on the swirl-nozzle vanes such that the fuel-air mixture is radially stratified. Once a stable flame is generated, flashback is triggered by providing a step increase in global equivalence ratio. High-speed chemiluminescence images and stereoscopic PIV measurements are captured at 4kHz to understand the flame-flow interaction in the mixing tube during flashback. It is observed that at the onset of flashback, a large flame tongue propagates with swirling motion around the center-body. As the flame propagates upstream, intermittent bright flame structures start appearing at the flame front which grow in size with time and may extend up to the outer wall of the mixing tube. For hydrogen-rich fuel (>75%), the flame switches from the center-body to the outer wall and propagates along the outer wall of the mixing tube until it stabilizes on the fuel ports.