Investigation of Turbulent Lifted Jet Flame Stabilization Using Experimental Methods and Simulations

The stability and structure of lifted jet flames are studied experimentally and computationally. A high speed charge-coupled device (CCD) camera with an intensifier system is used for the methane air lifted jet flame base visualizations. CH* distributions in reaction zones at different Reynolds (Re) conditions have been recorded. Reattachment and blowout processes are investigated. It is found that the heat releases at the flame base always remain high and stable; whereas, it decreases significantly downstream. Also, the shapes of the flame base mimic a partially premixed condition. As the Reynolds number is increased, flame holes near the stabilization zone become more prevalent. These holes perturb the flame base and make it unstable. Throughout the reattaching transition, local high heat release at the flame base is responsible for the flame stabilization until the flame transitions to an attached flame. During blowout, heat release at the flame edge decreases gradually resulting in flame extinction.;The application of simultaneous particle image velocimetry (PIV) with flame chemiluminescence enabled the study of the velocity variations at the flame edges. The streamline divergences at flame edges not only indicate flame base positions, but also are consistent with the streamline patterns of triple or edge flames. The measurements find two velocities in flame edges are indicators of stabilizing flames. One is the flame edge velocity, which is close but larger than the corresponding laminar flame speed, and the other is the flame propagation speed. The two velocities ratio averages are similar in magnitude to the theoretical estimate of the triple flame speed as the square of the density ratio across the flame.;In simulations, two DNS methods were used to study shear layer flames with some degree of chemical complexity and a transitional jet flame with simple chemistry. In the shear layer flame simulations, heat release rates exhibit triple flame structures. The sharp heat release gradients between flame edges and flames downstream are consistent with the experimental flame base visualization. The displacement speed profiles, as well as the streamline divergence characteristics, show that the flame base is always a triple flame shape during the unsteady flame evolution despite the flame structure distortion by the flowfield.;Another DNS simulation of a lifted jet flame under transitional flow also demonstrates the presence of triple flames at the leading edge of the lifted flames. Moreover, all the flame edges are located in the laminar flow region and their position fluctuations are hardly observed. Because of the characteristic of transition flow, flames at exterior edges are almost laminar and premixed lean flames persist further downstream. Meanwhile, towards the center of the jet, the rich premixed branch appears intermittently because of unsteady flow conditions and the presence of high shear.;Furthermore, the RANS method with the eddy dissipation model (EDM) is implemented to investigate the lifted jet flame structures. The mean values of the normalized flame index clearly show a triple flame structure with the lean premixed, the rich premixed, and diffusion branches. Both co-flow and free jet conditions show the velocity variations through flame edge decreasing firstly then increasing significantly, which also support the triple or edge flame stabilization mechanism. That is also consistent with the experimental findings and the DNS simulations. The simulation lift-off heights agree well with the experimental results and the EDM model is very efficient to model lifted jet flames.