Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts

Abstract

The design of flashback-resistant premixed burners for hydrogen-rich fuels is strongly dependent on reliable turbulent boundary layer flashback limits, since this process can be the dominant failure type for mixtures with high burning velocities. So far, the flashback data published in literature is based on tube burner experiments with unconfined flames. However, this flame configuration may not be representative for the most critical design case, which is a flame being already present inside the duct geometry. In order to shed light on this potential misconception, boundary layer flashback limits have been measured for unconfined and confined flames in fully premixed hydrogen-air mixtures at atmospheric conditions. Two duct geometries were considered, a tube burner and a quasi-2D turbulent channel flow. Furthermore, two confined flame holding configurations were realized, a small backward-facing step inside the duct and a ceramic tile at high temperature, which was mounted flush with the duct wall. While the measured flashback limits for unconfined tube burner flames compare well with literature results, a confinement of the stable flame leads to a shift of the flashback limits towards higher critical velocity gradients, which are in good agreement between the tube burner and the quasi-2D channel setup. The underestimation of flashback propensity resulting from unconfined tube burner experiments emerges from the physical situation at the burner rim. Heat loss from the flame to the wall results in a quenching gap, which causes a radial leakage flow of fresh gases. This flow in turn tends to increase the quenching distance, since it constitutes an additional convective heat loss. On the one hand, the quenching gap reduces the local adverse pressure gradient on the boundary layer. On the other hand, the flame base is pushed outward, which deters the flame from entering the boundary layer region inside the duct. The flashback limits of confined flames stabilized at backward-facing steps followed this interpretation, and experiments with a flush ceramic flame holder constituted the upper limit of flashback propensity. It is concluded that the distribution of the flame backpressure and the flame position itself are key parameters for the determination of meaningful turbulent boundary layer flashback limits. For a conservative design path, the present results obtained from confined flames should be considered instead of unconfined tube burner values.

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

“…Since research on flashback began with the first systematic study by Lewis and von Elbe [1,2], the focus has been on measuring flashback limits in (non-swirling) Bunsen-flame type burners for many years [3][4][5][6][7], including more recent studies carefully testing additional parameters such as confinement, wall temperature and pressure [8][9][10][11][12][13]. High-speed optical diagnostics and advanced simulation tools have been applied more recently to reveal new information about the flame propagation dynamics [14][15][16][17].…”

Segregation-induced fingering instabilities in granular free-surface flows

“…The flame holding behaviour is investigated as a function of injection geometrical configuration, fuel composition and injection angle (Grout et al 2011Kolla et al 2012). The former topic, focusing on the characteristics of the upstream propagation of premixed, preheated hydrogen (H 2 )-air flames in turbulent boundary layers, against the bulk flow direction, constitutes the main concern of the present DNSs and of a parallel experimental effort (Eichler, Baumgartner & Sattelmayer 2011; conducted in the framework of the BIGCO2 Project and of the BIGCCS International CCS Research Center (more information is available at http://www.bigccs.no).…”

Turbulent Flame Speed as an Indicator for Flashback Propensity of Hydrogen-Rich Fuel Gases

“…Nonetheless, the Blasius correlation is expected to provide a reasonably good estimation of the velocity gradient, as suggested in Ref. [15]. In contrast to g c , g f indicates the capability with which the flow can counteract the opposed propagation of the flame front.…”

“…Specifically for the hydrogen-containing or hydrogen-rich mixtures, the issue of boundary layer flashback has been investigated in some recent studies [10,[13][14][15]. From the perspective of fuel compositions, it is found that the flashback behavior is dominated by the hydrogen content [10,16,17].…”

“…The flame propagation is facilitated since the flame front is enriched with the limiting component (hydrogen) and the critical velocity gradient for flashback is found to be increased with decreasing Lewis number [5,18,19]. On the other hand, concerning the validity of the gradient principle, it is proposed that the outer regions of the boundary layer should also be taken into account to better predict the flashback propensity in turbulent boundary layers [14,15]. A confinement of the stable flame is found to modify the pressure boundary conditions and the flashback limits are shifted accordingly.…”