Vladimir M. Dulin

Comparative analysis of low- and high-swirl confined flames and jets by proper orthogonal and dynamic mode decompositions

Low-order coherent structures of non-reacting and combusting low- and high-swirl (swirl rates S = 0.41 and S = 1.0) jet flows in an open-ended cylindrical model combustor have been studied using the proper orthogonal decomposition (POD) and the dynamic mode decomposition (DMD) of high-repetition stereoscopic particle image velocimetry (PIV) velocity measurements. Lean methane-air mixture with the equivalence ratio of 0.6 was chosen for the reacting case as lean flames are generally less stable and more receptive to active control. The nonreacting and reacting flows at the same swirl rates exhibit qualitatively similar overall features characterized by highly turbulent annular jets enveloping the central retarding zone (for S = 0.41) or a recirculating bubble (for S = 1.0). At low swirl the dominant coherent structures appeared in the form of tilted ring vortices, evolving into helices as the swirl rate increases. The high-swirl flows are characterized by a pair of well-organized counter-rotating co-windin...

PIV and OH PLIF study of impinging propane-air jet-flames

The paper reports on the study of flow structure of a reacting propane-air jet, issued from contraction nozzle and impinged on flat metallic surface, by using the particle image velocimetry technique. Flows with different nozzle-to-surface distance H/d were studied. The Reynolds number Re was in the range of 500÷5500, equivalence ratio Φ was varied from 0.8 to 1.4. Velocity field was measured for a conical premixed flame for Re = 1500 and Φ = 0.9. A region with flow recirculation was detected between the flame cone and the impingement surface for the case of H/d = 4. This flow feature may result in a reduced local heat transfer.

3D velocity measurements in a premixed flame by tomographic PIV

Tomographic particle image velocimetry (PIV) has become a standard tool for 3D velocity measurements in non-reacting flows. However, the majority of the measurements in flows with combustion are limited to small resolved depth compared to the size of the field of view (typically 1 : 10). The limitations are associated with inhomogeneity of the volume illumination and the non-uniform flow seeding, the optical distortions and errors in the 3D calibration, and the unwanted flame luminosity. In the present work, the above constraints were overcome for the tomographic PIV experiment in a laminar axisymmetric premixed flame. The measurements were conducted for a 1 : 1 depth-to-size ratio using a system of eight CCD cameras and a 200 mJ pulsed laser. The results show that camera calibration based on the triangulation of the tracer particles in the non-reacting conditions provided reliable accuracy for the 3D image reconstruction in the flame. The modification of the tomographic reconstruction allowed a posteriori removal of unwanted bright objects, which were located outside of the region of interest but affected the reconstruction quality. This study reports on a novel experience for the instantaneous 3D velocimetry in laboratory-scale flames by using tomographic PIV.

Helical modes in low- and high-swirl jets measured by tomographic PIV

ABSTRACT We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.

A swirling jet with vortex breakdown: three-dimensional coherent structures

The paper reports on shape of a three-dimensional coherent structure in a velocity field of a high-swirl turbulent jet with the bubble-type vortex breakdown. A set of the 3D instantenous velocity fields was measured by using the tomographic particle image velocimetry (tomographic PIV) technique and processed by the proper orthogonal decomposition (POD) method. The detected intensive coherent velocity component corresponded to a helical vortex core of the swirling jet and two secondary spiral vortices. The entire coherent structure was rotating around the jet axis in compliance with the direction of the flow swirl. From the 3D data it is concluded that the dynamics of the strsucture can be described by a traveling wave equation: Re[A(y, r)·ei(mθ + ky - ωt)] with the number of the spiral mode m = +1 for positively defined k and ω.

Study of vortex core precession in combustion chambers

The article presents the results of experimental investigation of swirling flow of lean propane/air flame in a model combustion chamber at atmospheric pressure. To study the unsteady turbulent flow, the particle image velocimetry technique was used. It was concluded that dynamics of high swirl flows with and without combustion was determined by a global helical mode, complying with a precessing double-spiral coherent vortex structure. The studied low swirl flame had similar size and stability characteristics, but amplitude of the coherent helical structure substantially oscillated in time. The oscillations were associated with intermittently appearing central recirculation zone that was absent in the nonreacting flow. It is expected that the low swirl flow without the permanent central recirculation zone should be more sensitive to an external active control. In particular, this result may be useful for suppression of thermoacoustic resonance in combustion chambers.

Coherent Structures in a Turbulent Swirling Jet Under Vortex Breakdown. 3D PIV Measurements

The current study reports on spatial structure of a global mode of self-sustaining oscillations in a turbulent swirling jet under vortex breakdown conditions. Ensembles of 2D and 3D velocity fields were measured by stereoscopic and tomographic PIV systems, respectively, and were analysed via proper orthogonal decomposition. For the 2D PIV, the spatial resolution was sufficient to resolve most of the turbulent kinetic energy of the turbulent flow. The resolution in the case of tomographic PIV was lower, but the 3D instantaneous velocity fields unambiguously revealing that the global mode corresponds to a spiralling structure, counter-winded to the direction of the jet swirl.

Investigation of mass transfer in swirling turbulent flames

The present paper reports on analysis of flow structure and turbulent transport in swirling flames. The particle image velocimetry and spontaneous Raman scattering techniques were used for the measurements of 2D velocity and density distributions. The focus was placed on comparison between low- and high-swirl flows. A pronounced bubble-type vortex breakdown with strong flow precession took place in the latter case.

On formation of a stagnation zone in the flow between conical flame and flat obstacle

The structure of a jet flow formed by the combustion products of conical propane-air flame and impinging onto a normally oriented flat cooled surface is studied experimentally. The velocity field is measured by the particle image velocimetry technique. Based on the non-intrusive measurements, formation of a recirculation zone in the flow between the flame cone and surface has been detected for the first time. Mechanism for the observed phenomenon is proposed. Presence of the low-intensity recirculation bubble on the jet axis can explain the effect of a heat transfer decrease near the stagnation point on the surface, observed in the previous studies.

Self-oscillations in a jet flow and gaseous flame with strong swirl

Investigation results on unsteady flow dynamics in a gaseous jet flame with strong swirl, vortex breakdown, and precession of a vortex core obtained by panoramic optical methods are presented, as well as the results of theoretical analysis of the fastest growing modes of hydrodynamic instability. Characteristics of the most unstable self-oscillating mode in the initial region of the turbulent strongly swirling propane-air jet burning in the atmospheric air in the form of a lifted flame are determined. Analysis of data by principal component analysis and linear stability analysis revealed that evolution of the dominant self-oscillating mode corresponds to quasi-solid rotation with constant angular velocity of the spatial coherent structure consisting of a jet spiral vortex core and two spiral secondary vortices.