Phase averaging of the precessing vortex core in a swirl burner under piloted and premixed combustion conditions☆
Abstract
The flow patterns produced in and past the exhaust of a 100-KW swirl burner have been investigated experimentally under piloted premixed combustion conditions. The well-known three-dimensional time-dependent instability called the precessing vortex core (PVC) dominates the flow and mixing patterns. The PVC and its associated cycle time were used to trigger a three-component laser anemometry system. Successive cycles were overlaid and phase averaged to give a three-dimensional picture of the rotating flow fields. Measurements were obtained over successive slices of the flow, extending to X/De = 2.5 past the burner exit. A description of the flow was thus obtained in terms of phase averaged tangential, axial and radial velocities in tangential/radial and axial/radial planes. The results confirm previously reported work on the same burner operated isothermally and show that the center of the vortex flow is displaced from the central axis of the burner, creating the PVC phenomena as the center of the vortex precesses around the central axis of symmetry. As a consequence of this displacement the reverse flow zone (RFZ) is also displaced, while also partially lagging behind the PVC by up to 180°. The RFZ acts as a feedback mechanism for the PVC phenomena. As a consequence of the displaced vortex centre, flow between the PVC centre and the wall is squeezed. Thus, due to angular momentum flux consideration, it produces a considerable increase in tangential velocity and gives the characteristic PVC signal. The displaced RFZ is both rotating through a region of forward flow whilst also being of an intermittent nature, giving rise to the excellent flame stabilisation and mixing characteristics of these types of burners. Similar results were obtained for isothermal and premixed combustion conditions providing the flame was stabilised close to the burner exit nozzle.
References (15)
- N. Syred et al.
- N. Syred et al.
Combust. Flame
(1974) - K. Sato et al.
- A.K. Gupta et al.
Swirl Flows
(1984) - T. O'Doherty et al.
J.I. Mech. Eng.
(1993) - A.G. Spruyt
- T.C. Claypole et al.
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Investigation of transient PVC dynamics in a strongly swirled spray flame using high speed planar laser imaging of SnO<inf>2</inf> microparticles
2021, Combustion and FlameThe Precessing Vortex Core (PVC) dynamics is investigated in a laboratory scale swirl stabilized spray combustor. This helical structure commonly found in swirling flows used to stabilize flames in many combustion systems has a notable impact on the structure of the flow, spray and flame formed by the injector unit. The dynamics of the PVC is examined by combining laser sheet illumination of flame front tracers filmed with a high speed camera, Proper Orthogonal mode Decomposition (POD) and wavelet analysis. In distinction with most applications of laser sheet illumination, in which the flow is seeded with oil droplets that vanish at a relatively low temperature when crossing the flame front, the present implementation relies on micronic tin oxide (SnO) particles that vanish at much higher temperatures. This is well suited to flows where fresh reactants are already at high temperature or where hot gases are recirculated as is the case in most swirling injection configurations. A first set of experiments carried out with stagnation point premixed flames is used to determine the temperature () at which the tin dioxide particles vanish and to identify the chemical reaction that controls this process. Because the light scattered by the solid particles is quite intense, the method can be used at very high repetition rates. It is then applied at a 100 kHz frame rate to a swirling injection configuration in a situation where the flame is well stabilized and the combustor does not exhibit thermo-acoustic instabilities. It is found that the PVC takes the form of a double helix with brief intermittent switching to a single, followed by a triple helicoidal geometry. The POD-wavelet analysis indicates that these moments correspond to flashback events that randomly occur and during which the PVC characteristics change for a few milliseconds at a time. In a second set of experiments the chamber length is augmented giving rise to a combustion instability coupled by a longitudinal acoustic mode at a frequency that is an order of magnitude lower than the PVC frequency. The planar slices of the flame reveal a large cyclic motion of the lower edge of the flame in the inner recirculation zone. It is found that the PVC is modulated both in amplitude and frequency, the modulating frequency corresponding to that of the thermoacoustic oscillation. The double helix PVC is also observed to vanish in a random manner.
Renormalisation of particle distributions in an initially-biased turbulent jet by swirl and radial injection
2021, International Journal of Multiphase FlowThe distribution of particles in a jet with an initial bias is assessed using planar nephelometry, which is a planar imaging technique using Mie-scattering. Initial biases in particle distributions can have large and significant effects, particularly in solid fuel combustion systems, where inhomogeneities will affect the local and global performance of a flame. In this study, a two-phase jet flow with a controlled initial bias is generated at a Stokes number, based on the large-eddy time-scale, of and various strategies are explored to correct this bias. The investigation examines the usefulness of injecting air at various momentum ratios and through various configurations upstream from the nozzle exit plane. The study examines the differential roles of swirl (tangential) and radial injection of air at various locations from the flow exit. Radial injection of momentum is found to be superior to injection of swirl, consistent with swirl being a magnifier of eccentricities of a flow. Radial injection upstream from the nozzle can achieve a re-homogenised flow to develop prior to the exit, at least for the present Stokes numbers, but can also lead to downstream biases in particle distribution for some conditions. Injecting momentum directly at the nozzle exit does not allow a symmetrical distribution to be achieved, but does have an observed influence on the emerging flow-field.
Revealing the details of vortex core precession in cyclones by means of large-eddy simulation
2020, Chemical Engineering Research and DesignCitation Excerpt :Confined vortex flow finds significance in numerous industrial processes. Such flows are used for mixing (Levy and Albagli, 1991; Liu et al., 2017) as well as for the separation of two or more phases with different densities (Sommerfeld and Qiu, 1993), and also for flow stabilization (as in swirl burners (Froud et al., 1995)). One popular application of swirl-dominated flow and the subject of this study is the cyclone separator.
We investigate the three-dimensional, time-dependent behaviour of coherent flow structures inside two cyclone separator geometries, a Stairmand cyclone and a swirl tube, operating in the turbulent regime. For this, large-eddy simulations (LES) with a standard Smagorinsky subgrid-scale model are carried out on the single-phase flow employing a non-orthogonal curvilinear hexahedral mesh. Reynolds numbers (Re) are in the range 9·104–4·105. The simulations successfully capture the quasi-periodic precessing vortex core (PVC) phenomenon. The spectra derived from the temporal signals of different flow variables viz. static pressure, tangential velocity, axial velocity and radial velocity in the Stairmand cyclone exhibit two distinct frequencies at all Re, whereas the spectra for the swirl tube show a single peak. These spectra strongly resemble those reported in the experimental literature. The periodicity pertaining to the inner vortex about its rotational axis is also analyzed. Next, the effect of Re on the two distinct spectral peaks have been evaluated for the Stairmand cyclone. Lastly, we present the effect of Re on mean and fluctuating fields and their variations along the geometrical axis of the cyclone.
On the emerging flow from a dual-axial counter-rotating swirler; LES simulation and spectral transition
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Large eddy simulation and Reynolds-averaged Navier–Stokes based modelling of geometrically induced swirl flows applied for the better understanding of Clean-In-Place procedures
2017, Food and Bioproducts ProcessingIn this work we numerically studied the potential of a geometrically induced swirl flow generated by means of flow passing through a four-lobed swirl pipe on the intensification of Clean-In-Place (CIP) procedures in closed processing systems without the need of increasing flow velocity. We aimed at revealing the time-averaged and unsteady property of the turbulent swirling flow and its strengthened hydrodynamic impact on the internal pipe surface where cleaning takes place. We performed modelling based on Reynolds-averaged Navier–Stokes equations (RANS) and large eddy simulation (LES) of turbulent swirl flows with different swirl intensities. The LES model presented a more vigorous and dynamic swirling flow than the RANS approach. The LES successfully identified four vortex core regions within and downstream of the swirl pipe which the RANS approach failed to capture. The RANS and LES results showed that, without the need of increasing the overall flow velocity, the four-lobed swirl pipe is able to increase not only the mean wall shear stress to the downstream but also the fluctuation rate of the wall shear stress especially in the locations further downstream of the swirl pipe where the four cortex cores have interacted and dissipated. As the increase of either the mean or the fluctuation rates of wall shear stress contribute to pipeline cleaning, the numerical work has demonstrated that, with the local introduction of the swirl pipe, the efficiency of CIP procedures in closed processing systems would be improved significantly thus reducing time and costs for the CIP process.
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Presented at the Twenty-Fifth Symposium (International) on Combustion, Irvine, California, 31 July–5 August 1994.