Insights into the dynamics of spray–swirl interactions

The near-field breakup and interaction of a hollow-cone liquid sheet with coannular swirling air flow have been examined using high-speed diagnostics. Time-resolved PIV (particle image velocimetry; $3500~\text{frames}~\text{s}^{-1}$) is employed to capture the spatio-temporal behaviour of the swirling air flow field. The combined liquid–gas phase interaction is visualized with the help of high-speed ($20\,000~\text{frames}~\text{s}^{-1}$) shadowgraphy. In this study, the transition from weak to strong spray–swirl interaction is explained based on the momentum ratio. Proper orthogonal decomposition (POD) is implemented on instantaneous PIV and shadowgraphy images to extract the energetic spatial eigenmodes and characteristic modal frequencies. The POD results suggest the dominance of the KH (Kelvin–Helmholtz) instability mechanism (pure axial shear, axial plus azimuthal shear) in swirl–spray interaction. In addition, linear stability analysis also shows the destabilization of the liquid–air interface caused by KH waves ($\unicode[STIX]{x1D706}_{p}$), which arises from the formation of a vorticity layer of thickness $\unicode[STIX]{x1D6FF}_{g}$ near the liquid–air interface. The frequency values obtained from the primary KH wavelength ($\unicode[STIX]{x1D706}_{p}$) exhibit good agreement with the POD modal frequencies. Scaling laws are proposed to elucidate the relationships between the global length scales (breakup length, spray spread) and the primary wavelength. The breakup length scale and liquid sheet oscillations are meticulously analysed in the time domain to reveal the breakup dynamics of the liquid sheet. Furthermore, the large-scale coherent structures of the swirl flow exhibit different sheet breakup phenomena in the spatial domain. For instance, flapping breakup is induced by the central toroidal recalculation zone in the swirling flow field. Finally, the ligament formation mechanism and its diameter, i.e. the size of first-generation droplets, are measured with phase Doppler interferometry. The measured sizes scale reasonably with KH waves.

Reaction Zone Characterization in a Gas Turbine Model Validation Combustor

The Enclosed Sydney Swirl Burner (ESSB), a half-scale version of the Sydney Swirl Burner coupled to an optically accessible combustion chamber, was recently constructed at the National Energy Technology Laboratory for the purpose of generating global emissions and model validation data in a configuration relevant to industrial and gas turbine combustion. The ESSB is capable of diffusion flame combustion of CH4/H2/inert fuel mixtures in highly swirling air flow over a bluff body, and can produce a wide variety of flame types and structures for study. Based on stability characteristics and global emissions data, three flames were chosen for reaction zone characterization: a non-swirling 1:1 H2:CH4 flame, a high-swirl 1:1 H2:CH4 flame, and a lifted, V-shaped flame of CH4 with a swirling air flow. Reaction zone characterization is performed via planar OH-PLIF measurements taken at multiple locations within the square cross-section of the ESSB. Mean flame surface locations are described, and maps of flame front probabilities are generated for each of the flames. Measurements indicate quenching in the high strain region in the neck above the bluff body for the non-swirling flame, wall-quenching for the swirling flames, and OH production below the lifted flame that helps sustain the reaction zone. The OH-PLIF data, as well as global emissions and thermal boundary condition measurements for these flames, are freely available for model validation purposes.

Flow Field of a Model Gas Turbine Swirl Burner

The flow field of a lab-scale model gas turbine swirl burner was characterised using particle imaging velocimetry (PIV) at atmospheric condition. The swirl burner consists of an axial swirler, a twin-fluid atomizer and a quartz tube as combustor wall. The main non-reacting swirling air flow without spray was compared to swirl flow with spray under unconfined and enclosed conditions. The introduction of liquid fuel spray changes the flow field of the main swirling air flow at the burner outlet where the radial velocity components are enhanced. Under reacting conditions, the enclosure generates a corner recirculation zone that intensifies the strength of the radial velocity. Comparison of the flow fields with a spray flame using diesel and palm biodiesel shows very similar flow fields. The flow field data can be used as validation target for swirl flame modelling.