Unraveling Flashback Phenomena of Turbulent premixed Hydrogen-Natural Gas-Air Flames

Abstract

The growing energy demand and climate change poses a need for alternative energy generation in terms of renewable resources. Renewable energy resources are characterised by their intermittent behaviour. A backup power supply is required that can deliver electricity when the supply from the renewables is not sufficient. Gas turbines operating with hydrogen is an attractive option, since hydrogen combustion has zero carbon emissions and can be used as an energy storage when the supply of renewable energy sources are abundant. However, hydrogen combustion poses several challenges. A hydrogen flame has a higher flame temperature than natural gas, leading to more NOe production. To
reduce these NOe emissions, gas turbines operate in lean premixed conditions. This creates a risk of flame flashback, in particular boundary layer flashback, which can lead to severe damage to the gas turbine.
Recent research performed on boundary layer flashback revealed that two flame configurations, i.e. unconfined and confined, showed fundamentally different flashback phenomena. Unconfined flame flashback refers to the situation where an initially stable flame anchored at the burner rim eventually moves into burner tube. When a flame is partially of completely surrounded by walls and propagates and then starts propagation along the wall, it is called confined flame flashback. Previous studies have focused on one of the two flashback processes at a time. However, the transient flashback process between unconfined and confined flame flashback is not well understood. Research has shown that hydrogen is much more prone to flashback compared to natural gas. This has been attributed to the difference in flame speed between natural gas and hydrogen, but the exact reason for the difference in flashback behaviour between natural gas flames and hydrogen flames has yet to be found.
In this study, a quartz Bunsen burner is used to investigate the flashback phenomena of turbulent premixed hydrogen-natural gas-air flames. To gain more insight in the flashback phenomena, three experiments have been performed. First, flashback maps are obtained to determine the flashback limits of the quartz Bunsen burner. Secondly, the influence of a flame on the flow was investigated using turbulent statistics. Finally, both unconfined and confined flashback are visualised, thereby capturing the transient flashback process between these two configurations. This has been done for a stoichiometric natural gas flame and a lean hydrogen flame. Laser diagnostics like Particle Image Velocimetry (PIV) and Miescattering are used to obtain the turbulent flow statistics and to visualize the instantaneous flashback process. The results show that regions with negative velocity fluctuations in the unburned mixture are the predominant physical mechanism for the start of unconfined flashback and for the transient flashback process of the flame propagating into the burner. However, the start of a flashback event depends on the combination of several parameters: the bulk velocity, the position of the flame front before it interacts with a region with negative velocity fluctuations, the magnitude of the negative velocity fluctuations and whether a region with positive velocity fluctuations is absent after interaction of the flame with the region consisting of negative fluctuations. So, unconfined flashback is rather a statistical phenomena, where the chance of the occurrence of a flashback event is increasing for a decreasing bulk flow velocity. Experiments showed that the transient process between unconfined and confined flashback is very short and fast. After a distance of approximately 5 mm upstream of the burner rim, a backflow region starts to develop in front of the flame, which denotes the start of confined flashback. The time needed for a natural gas flame propagating upstream from the burner rim to reach a confined configuration is approximately 17 ms and only 5.6 ms for the hydrogen flame. The suggested physical mechanisms leading from unconfined flashback to confined flashback are the convex shape of the flame towards the reactants during upstream flame propagation and the reduced cross-sectional flow area of the burned gases at the flame tip. Due to the created backflow in front of the flame and the abovementioned mechanisms, the upstream flame propagation is strongly enhanced, which explains why the flashback propensity for confined flames is much higher than for unconfined flames. The experiments showed that the hydrogen flame propagates closer to the wall than the natural gas flame, indicating higher backpressure effects. The hydrogen flame is thermaldiffusive unstable and the convex shape of the flame tip during flashback strongly enhances the local flame speed and thus the upstream propagation velocity. In contrast, the hydrodynamic instability encountered in the natural gas flame only retards the flow in front of the flame tip, but does not affect the flame speed. This might explain the difference in flashback behaviour between natural gas and hydrogen flames.