Interaction of Flame Flashback Mechanisms in Premixed Hydrogen-Air Swirl Flames

Abstract

An experimental study is presented on the interaction of flashback originating from flame propagation in the boundary layer (1), from combustion driven vortex breakdown (2) and from low bulk flow velocity (3). In the investigations, an aerodynamically stabilized swirl burner operated with hydrogen-air mixtures at ambient pressure and with air preheat was employed, which previously had been optimized regarding its aerodynamics and its flashback limit. The focus of the present paper is the detailed characterization of the observed flashback phenomena with simultaneous high speed PIV/Mie imaging, delivering the velocity field and the propagation of the flame front in the mid plane, in combination with line-of-sight integrated OH*-chemiluminescence detection revealing the flame envelope and with ionization probes which provide quantitative information on the flame motion near the mixing tube wall during flashback. The results are used to improve the operational safety of the system beyond the previously reached limits. This is achieved by tailoring the radial velocity and fuel profiles near the burner exit. With these measures the resistance against flashback in the center as well as in the near wall region is becoming high enough to make turbulent flame propagation the prevailing flashback mechanism. Even at stoichiometric and preheated conditions this allows safe operation of the burner down to very low velocities of approx. 1/3 of the typical flow velocities in gas turbine burners. In that range the high turbulent burning velocity of hydrogen approaches the low bulk flow speed and, finally, the flame begins to propagate upstream once turbulent flame propagation becomes faster than the annular core flow. This leads to the conclusions that finally the ultimate limit for the flashback safety was reached with a configuration, which has a swirl number of approx. 0.45 and delivers NOx-emissions near the theoretical limit for infinite mixing quality, and that high fuel reactivity does not necessarily rule out large burners with aerodynamic flame stabilization by swirling flows.

Experimental investigation of upstream flame propagation during boundary layer flashback of swirl flames

“…The third configuration concerns flashback of swirl flames along a wall, which by geometry combines aspects of boundary layer flashback and flashback along a vortex axis. In swirl combustors featuring a mixing tube with center body, flashback typically occurs along the center body wall [15,[35][36][37][38]; however, boundary layer flashback has also been observed along the mixing tube outerwall in hydrogen-air swirl flames [39]. Flashback along the center body wall has been attributed to CIVB based on the finding of regions of negative axial velocity ahead of the flame tip, identified as boundary layer separation, which pulls the flame upstream [15].…”

Experimental Investigation of the Confinement Effect on Flashback in Multi Combustion Modes Tangential Swirl Burner

“…The changing from the BLF to CIVB was investigated by Baumgartner and Sattelmayer [21] with increasing equivalence ratio and low swirl flow. Other studies reach the same results but in other words where Sayed [22] reaches the combustion induced vortex breakdown to occur more than boundary layer in high swirl flow .The diffuser position between the burner mixing tube and the combustor also can change CIVB flashback to BLF [23]. The second field in investigations was the entry way of fuel or air which mainly affects the CIVB, the using of axial fuel injection which investigated by Mayer [24] improves the resistance to the CIVB compared with trailing injection .…”

Generation of Adverse Pressure Gradient in the Circumferential Flashback of a Premixed Flame

“…This then dominates the flame upstream propagation [45]. More recently, the understanding gained from the above studies on boundary layer, vortex break down and core flow flame flashback was used to develop a flashback resistant hydrogen burner [46].…”