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This article presents an investigation of the swirl number effects on the stability limits of various syngas compositions in an atmospheric premixed variable-swirl burner. Lean blowout (LBO) and flashback (FB) experiments were performed using fuel mixtures containing varying amounts of H2, CO and CH4 at two swirl numbers. Reducing the swirl number from 0.66 to 0.53, reduced the flashback propensity of various syngas/air mixtures but it did not affect the LBO limits considerably. The flow-field in the combustor was studied at S = 0.66 and 0.53 using high-speed particle image velocimetry (PIV) for a mixture of H2/CH4 (50:50) at various equivalence ratios. At both swirl numbers, for the non-reacting flow and low equivalence ratios the flow-field consisted of an inner recirculation zone at the entrance of the combustor, an annular high velocity zone and an outer recirculation zones between the high-velocity zone and the bounding walls. Increasing the equivalence ratio towards the flashback limit, had varying effects on the flow-field, depending on swirl number. At S = 0.66, increasing the equivalence ratio did not have a significant effect on the general features of the flow-field. At S = 0.53 the flow-field consisted of an inner recirculation zone at low equivalence ratios, but as the equivalence ratio was increased, the high velocity zone extended radially towards the center and the recirculation zone disappeared from the flow-field. The high-speed OH*-chemiluminescence images recorded at the onset of flashback revealed significant differences between S = 0.66 and S = 0.53 in the flame stabilization mechanism prior to flashback and flame propagation in the premixing tube. Considering the velocity measurements together with the OH*-chemiluminescence images, it was concluded that at S = 0.66 flashback was caused by CIVB mechanism whereas at S = 0.53 flashback was initiated by the competition between the flame speed and flow velocity in the core flow.

This study examines the influence of vortex core precession on flame flashback of swirl-stabilised hydrogen flames. Theoretical considerations suggest that the angular velocity of a swirling flow is reduced as vortex precession causes it to acquire an eccentric motion around the central axis of the burner. The eccentric motion of the vortex generates a secondary flow, which is thought to reduce the angular velocity and tangential momentum available to the primary flow, and thereby reduce the flashback propensity at the centre of the vortex core. Experiments measuring the influence of the eccentric motion of the flame tip on flame flashback behaviour were conducted using high-speed sequences of OH*-chemiluminescence images. Temporal analysis of a large sample of images revealed the existence of a systematic rotational frequency of the flame tip around the central axis of the burner. Analysis of the radial position of the flame tip in relation to its axial propagation velocity showed that flashback velocity increased as the flame tip eccentricity approached zero and flashback velocity decreased as the eccentricity amplitude of the flame tip reached larger values. This suggested that flame eccentricity caused by vortex core precession may be detrimental to upstream flame propagation and may be effective in inhibiting flame flashback in swirl-stabilised flames. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. (Less)

Autoignition of propane in air was visualised in a turbulent flow reactor using natural OH*-chemiluminescence imaging. The spatial and temporal development of autoignition kernels was studied in an optically accessible tubular section of the reactor. Kernel nucleation, movement and growth affected the location and movement of subsequent autoignition sites, and resulted in stagnation of the incoming flow and flashback. The autoignition delays of the reactants were measured under various conditions of temperature, pressure and equivalence ratio, relevant to micro gas turbines: Temperature T= 803-903 K, pressure p = 0.4-0.6 MPa, equivalence ratio phi = 0.2-0.6, mass flow rate of reactants m(r) = 8-21 g/s, with ignition delays tau between 191 and 498 ms. The effect of diluting the propane + air mixtures with CO2 was investigated for mole fractions of 0

Flame flashback from the combustion chamber to the premixing section is a major operability issue when using high H2 content fuels in lean premixed combustors. Depending on the flow-field in the combustor, flashback can be triggered by different mechanisms. In this work, three flashback mechanisms of H2/CH4 mixtures were visualized in an atmospheric variable-swirl burner using high speed OH* chemiluminescence imaging. The H2 mole fraction of the tested fuel mixtures varied between 0.1 and 0.9. The flow-field in the combustor was varied by changing the swirl number from 0.0 to 0.66 and the total air mass-flow rate from 75 to 200 SLPM (standard liters per minute). The following three types of flashback mechanism were observed: Flashback caused by combustion induced vortex breakdown (CIVB) occurred at swirl numbers ≥0.53 for all of the tested fuel mixtures. Flashback in the boundary layer (BL) and flame propagation in the premixing tube caused by auto-ignition were observed at low swirl numbers and low total air mass-flow rates. The temporal and spatial propagation of the flame in the optical section of the premixing tube during flashback was studied and flashback speed for different mechanisms was estimated. The flame propagation speed during flashback was significantly different for the different mechanisms.