Stabilization Mechanisms of Swirling Premixed Flames With an Axial-Plus-Tangential Swirler

The stabilization of premixed flames within swirling flows produced by an axial-plus-tangential swirler is investigated in an atmospheric test rig. In this system, flames are stabilized aerodynamically away from the solid elements of the combustor without help of any solid anchoring device. Experiments are reported for lean CH4/air mixtures, eventually also diluted with N2, with injection Reynolds numbers varying from 8 500 to 25 000. Changes of the flame shape are examined with OH* chemiluminescence and OH laser induced fluorescence measurements as a function of the operating conditions. Particle image velocimetry measurements are used to reveal the structure of the velocity field in non-reacting and reacting conditions. It is shown that the axialplus-tangential swirler allows controlling the flame shape and the position of the flame leading edge with respect to the injector outlet. The ratio of the bulk injection velocity over the laminar burning velocity Ub/SL, the adiabatic flame temperature Tad and the swirl number S0 are shown to control the flame shape and its position. It is then shown that the axial flow field produced by the axial-plus-tangential swirler is different from those produced by axial or radial swirlers. It takes here a W-shape profile with three local maxima and two minima. The mean turbulent flame front also takes this W-shape in an axial plane, with two lower positions located slightly off-axis and corresponding to the positions where the axial flow velocity is minimum. It is finally shown that these positions can be inferred from axial flow velocity profiles under non-reacting conditions. ∗Address all correspondence to this author. INTRODUCTION Swirling flames are widely used in industrial combustors such as boilers or gas turbines. By imparting a sufficient rotation to the injected flow, a reversal flow often designated as Internal Recirculation Zone (IRZ) develops along the burner axis. Under reactive conditions, hot burnt gases are recirculating inside the IRZ. This recirculation allows sustaining a region filled with hot gases near the burner outlet with low velocities enabling to aerodynamically stabilize the flame away from the solid components of the combustor. Swirling flows expanding at the combustor dump plane also feature outer recirculation zones (ORZ) helping stabilizing the flame. The structure of swirling flows is characterized by highly sheared layers increasing the turbulence level, enhancing mixing and making the flame more compact. Swirling flows allow extending the lean flame blow-off limits [1] and serve to stabilize flames in regions where stretch rates are high [2]. Swirling injectors are widely used technologies to help flame stabilization and increase the volumetric power density of flames [3]. In many labscale experiments, swirling injectors are combined with a central bluff body to help flame stabilization, but in high power applications, the proximity of the flame with this solid component may lead to damages and this additional component is removed ([4, 5, 6]). Stabilization mechanisms of swirling flames have been the topics of many reviews [7, 8]. It is found that the swirl number, the flow confinement and the diverging cup at the injector outlet are the main geometrical parameters altering the flame topology [9, 10]. Heat losses to the chamber walls are also known to modify the flame shape [11, 12]. The swirl number is the most important parameter. It is defined as S1 = Gθ/(RGz), where Gθ is the axial flux of tangential momentum, Gz the axial momentum flux and R is a characteristic radius of the injector. The pressure term in the axial momentum flux Gz is generally neglected and S1 is given by [13]: S1 = ∫ uθ uzrdr r1 ∫ uz rdr (1) where uz and uθ are the axial and azimuthal velocity components and r1 is the injector outlet radius. The swirl number S1 remains constant in a tube of constant cross section for an inviscid flow. In practice, it is generally determined at the outlet of the injector. It is known that the swirl number controls the dynamics and topology of swirling flames [7, 13, 14]. To obtain a stable IRZ, the swirl number has to be above a critical threshold value. This value is often found to be around Sc ∼ 0.6 for radial vanes, but this threshold is not absolute and is sensitive to geometrical details of the injector and of the combustion chamber [15, 16, 17]. It also depends on the structure of the flow field inside the injector and in the injection slits [15, 16]. In many investigations, the swirl number is however not measured but estimated from algebraic expressions in terms of geometrical parameters of the injector. The flow fields produced by different swirling injectors appear to be qualitatively similar for a given swirl level. Syred et al. [18] showed that an IRZ is obtained with a wide set of technologies and that the swirler type makes little difference for the overall recirculating mass flow rate. Gupta [13] however showed that the pressure losses may largely differ and that for swirl numbers S1 ≤ 0.70, straight annular vane type swirlers are the more efficient, whereas for higher swirl numbers the use of swirler with curved vanes or swirlers with tangential inlets is more efficient. The choice of the swirler design thus depends on the acceptable pressure drop and on the space available for the swirler. Axial swirlers are more compact than radial or axial-plus-tangential swirlers. In most practical systems, the swirl number remains fixed [14, 19], but some swirlers have been designed to allow changes of the swirl level [1, 20, 21]. This can be achieved with two different types of technologies. The first types use modifications of the swirler geometry [22]. The second types act on the flow distribution between different injection inlets. Movable block swirlers belong to the first types. They are used in some labscale burners to change the angle of the flow at the exit of radial or axial swirling vanes [23]. Recent systems were also developed with a step motor control to continuously change the vane angle during operation of the combustor. In other combustors, fluidic systems are preferred. One possibility is the use of axial-plus-tangential swirlers. They allow to continuously change the mass flow rates injected tangentially ṁθ and r0/2 175 mm