Azimuthal Instability Research Articles

Control of Azimuthal Combustion Instability Through the Injector Mounting Surface of Annular Combustors

A three-dimensional combustion instability model is presented to study the effects of the wall impedance distributions of annular combustors on thermoacoustic unstable modes, consequently seeking an effective passive methodology to suppress azimuthal combustion instability. The eigenfunction is obtained by use of Green’s function in an enclosed space, whereas the scattered disturbances at flames, the perforated screen where the injectors are installed, and the side perforated liner are characterized by the monopoles, respectively. Results show that compared to the side perforated liner with the restriction of radial size, the impedances over the injector mounting surface provide more acoustic energy dissipation to suppress the azimuthal combustion instabilities. Two patterns of impedance distribution, uniform and radial nonuniform, over the injector mounting surface are then investigated through the parametric variations and associated acoustic energy analysis. A stable combustion system can be realized in combination with homogeny or difference between the spatial impedance distributions and associated optimum perforated parameters, such as perforation ratio, Mach number of the mean bias flow, and aperture radius. It allows mitigating possible design limitations with the appropriate parameter combinations. The present investigation thereby offers an alternative model and methodology for suppression of azimuthal combustion instabilities in gas turbines.

Assessment of the LES-FGM framework for capturing stable and unstable modes in a hydrogen / methane fuelled premixed combustor

The main objective of this paper is to assess the capability of compressible Large Eddy Simulations (LES) to capture azimuthal combustion instability. The thickened flame model coupled with Flamelet Generated Manifold (FGM) tabulated chemistry is used as the combustion model. LES of an annular combustor is performed for five cases featuring stable and unstable combustion of hydrogen-methane mixtures. The unstable modes feature azimuthal instabilities and this annular combustor is used to test the LES-FGM framework. A consistent methodology is applied across all cases. It is found that LES predicts azimuthal modes for stable cases but these modes are weak and intermittent with pressure fluctuation amplitudes within the order of experimental noise. In addition, the unstable cases capture azimuthal modes that have approximately the same frequency as that of the experiment though the amplitudes of the modes are over-predicted. This suggests that the described LES-FGM framework is able to predict the onset of thermoacoustic instabilities and their qualitative changes with addition of hydrogen.

Reduced-order particle-in-cell simulations of a high-power magnetically shielded Hall thruster

High-power magnetically shielded Hall thrusters have emerged in recent years to meet the needs of the next-generation on-orbit servicing and exploration missions. Even though a few such thrusters are currently undergoing their late-stage development and qualification campaigns, many unanswered questions yet exist concerning the behavior and evolution of the plasma in these large-size thrusters that feature an unconventional magnetic field topology. Noting the complex, multi-dimensional nature of plasma processes in Hall thrusters, high-fidelity particle-in-cell (PIC) simulations are optimal tools to study the intricate plasma behavior. Nonetheless, the significant computational cost of traditional multi-dimensional PIC schemes renders simulating the high-power thrusters without any physics-altering speed-up factors unfeasible. The novel reduced-order ‘quasi-2D’ PIC scheme enables a significant reduction in the computational cost requirement of the PIC simulations. Thus, in this article, we demonstrate the applicability of the reduced-order PIC for a cost-efficient, self-consistent study of the physics in high-power Hall thrusters by performing simulations of a 20 kW-class magnetically shielded Hall thruster along the axial-azimuthal and radial-azimuthal coordinates. The axial-azimuthal quasi-2D simulations are performed for three operating conditions in a rather simplified representation of the thruster’s inherently 3D configuration. Nevertheless, we have resolved self-consistently an unprecedented 650 µs of the discharge evolution without any ad-hoc electron mobility model, capturing several breathing cycles and approximating the experimental performance parameters with an accuracy of 70%–80% across the operating conditions. The radial-azimuthal simulations, carried out at three cross-sections corresponding to different axial locations within the discharge channel, have casted further light on the evolution of the azimuthal instabilities and the resulting variations in the electrons’ cross-field mobility and the plasma-wall interactions. Particularly, we observed the development of a long-wavelength, relatively low-frequency wave mode near the exit plane of the thruster’s channel that induces a notable electron transport and a significant ion heating.

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Context. Asteroseismic measurements of the internal rotation rate in evolved stars pointed to a lack of angular momentum (AM) transport in stellar evolution models. Several physical processes in addition to hydrodynamical ones were proposed as candidates for the missing mechanism. Nonetheless, no current candidate can satisfy all the constraints provided by asteroseismology. Aims. We revisit the role of a candidate process whose efficiency scales with the contrast between the rotation rate of the core and the surface. This process was proposed in previous works to be related to the azimuthal magneto-rotational instability. Methods. We computed stellar evolution models of low- and intermediate-mass stars with the parametric formulation of AM transport proposed in previous works for this instability until the end of the core-helium burning for low- and intermediate-mass stars, and compare our results to the latest asteroseismic constraints available in the post-main sequence phase. Results. Both hydrogen-shell-burning stars in the red-giant branch and core-helium-burning stars of low- and intermediate-mass in the mass range 1 M⊙ ≲ M ≲ 2.5 M⊙ can be simultaneously reproduced by this kind of parametrisation. Conclusions. Given the current constraints from asteroseismology, the core rotation rate of post-main sequence stars seems to be well explained by a process whose efficiency is regulated by the internal degree of differential rotation in radiative zones.

Vortex Solitons in Quasi-Phase-Matched Photonic Crystals.

We report solutions for stable compound solitons in a three-dimensional quasi-phase-matched photonic crystal with the quadratic (χ^{(2)}) nonlinearity. The photonic crystal is introduced with a checkerboard structure, which can be realized by means of the available technology. The solitons are built as four-peak vortex modes of two types, rhombuses and squares (intersite- and onsite-centered self-trapped states, respectively). Their stability areas are identified in the system's parametric space (rhombuses occupy an essentially broader stability domain), while all bright vortex solitons are subject to strong azimuthal instability in uniform χ^{(2)} media. Possibilities for experimental realization of the solitons are outlined.

Parametric investigation of azimuthal instabilities and electron transport in a radial-azimuthal E × B plasma configuration

Partially magnetized low-temperature plasmas (LTP) in an E × B configuration, where the applied magnetic field is perpendicular to the self-consistent electric field, have become increasingly relevant in industrial applications. Hall thrusters, a type of electrostatic plasma propulsion, are one of the main LTP technologies whose advancement is hindered by the not-fully-understood underlying physics of operation, particularly, with respect to the plasma instabilities and the associated electron cross field transport. The development of Hall thrusters with unconventional magnetic field topologies has imposed further questions regarding the instabilities' characteristics and the electrons' dynamics in these modern cross field configurations. Accordingly, we present in this effort a detailed parametric study of the influence of three factors on the plasma processes in the radial-azimuthal coordinates of a Hall thruster, namely, the magnetic field gradient, secondary electron emission, and plasma number density. The studies are carried out using the reduced-order particle-in-cell code developed by the authors. The setup of the radial-azimuthal simulations largely follows a well-defined benchmark case from the literature in which the magnetic field is oriented along the radius, and a constant axial electric field is applied perpendicular to the simulation plane. The salient finding from our investigations is that, in the studied cases corresponding to elevated plasma densities, a long-wavelength azimuthal mode with the frequency of about 1 MHz is developed. Moreover, in the presence of strong magnetic field gradients, this mode results from an inverse energy cascade and induces a significant electron cross field transport as well as a notable heating of the ions.

Concept of the generalized reduced-order particle-in-cell scheme and verification in an axial-azimuthal Hall thruster configuration

The reduced-order particle-in-cell (PIC) scheme is a novel modeling approach that enables computationally efficient electrostatic kinetic simulations of plasma. In our previous publications, we demonstrated that a proof-of-concept implementation of this novel PIC scheme resolves the multi-dimensional plasma processes and their interactions in a Hall thruster in a manner close to traditional electrostatic PIC codes. In this work, we extend our efforts on this topic and present a mathematically mature formulation for the dimensionality reduction of Poisson’s equation in the Vlasov–Poisson system, which enables the generalized reduced-order ‘quasi-multi-dimensional’ PIC scheme. The applicability of the dimensionality-reduction approach to solve general 2D Poisson problems is numerically verified. Next, we present several reduced-order quasi-2D (Q2D) simulations of a well-defined axial-azimuthal simulation case from the literature using approximation orders of the 2D problem whose computational costs are 2%–15% of a full-2D simulation. It is shown that these reduced-order simulations allow us to recover the same characteristics, behaviors and effects reported in the literature regarding the azimuthal instabilities in Hall thrusters. Moreover, in terms of the time-averaged plasma properties, it was found that, when increasing the approximation order, the error associated with the Q2D simulations’ predictions decreases from 15% to 4% for the electric field and from 20% to 2% for the ion number density. We have additionally discussed a series of sensitivity analysis results, including the influence of the initial number of macroparticles per cell on the predictions of the Q2D simulations. According to the detailed results and analyses presented, we conclude that the generalized reduced-order PIC scheme serves as a rigorous foundation for eventual cost-effective and comprehensive three-dimensional kinetic studies of the physics in Hall thrusters and similar electrostatic plasma technologies.

Nonlinear characterization of azimuthal combustion instability exhibiting flame transient phenomena

This article characterizes the nonlinear features observed in an annular combustor exhibiting self-excited azimuthal thermoacoustic instability. The annular combustor consists of 12 burners with flames stabilized by bluff bodies protruding into the combustion chamber. This burner configuration closely resembles the flame stabilization geometry and flame shape in ramjet and afterburners configurations. As a first step, two different bluff bodies stabilize the flame: 1) circular and 2) conical and associated instability dynamics are characterized. The combustor exhibits intermittent oscillation in pressure with dominant 1A-1L (first azimuthal - first longitudinal) acoustic mode at about 630 Hz. The duration of periodic oscillation in the intermittent pressure fluctuation varies non-monotonically with the airflow rate. Further, relatively large amplitude pressure fluctuations are observed for intermediate values of airflow rate in both flame stabilization. During these large-amplitude pressure fluctuations, localized flame extinction in certain burners is observed, leading to partial blow-off operation of the combustor. This localized flame extinction is subsequently followed by either successful or unsuccessful re-ignition, leading to introduction of slow time scale heat release rate modulation in the problem. Further, this slow time scale heat release rate modulation is stochastic, contributing to the appearance of intermittent oscillation in pressure.

Longitudinal mode-coupling instabilities of proton bunches in the CERN Super Proton Synchrotron

In this paper, we study single-bunch instabilities observed in the CERN Super Proton Synchrotron (SPS). According to the linearized Vlasov theory, radial or azimuthal mode-coupling instabilities result from a coupling of bunch-oscillation modes, which belong to either the same or adjacent azimuthal modes, respectively. We show that both instability mechanisms exist in the SPS by applying the Oide-Yokoya approach to compute van Kampen modes for the realistic longitudinal impedance model of the SPS. The results agree with macroparticle simulations and are consistent with beam measurements. In particular, we see that the uncontrolled longitudinal emittance blowup of single bunches observed before the recent impedance reduction campaign (2018--2021) is due to the radial mode-coupling instability. Unexpectedly, this instability is as strong as the azimuthal mode-coupling instability, which is possible in the SPS for other combinations of bunch length and intensity. We also demonstrate the significant role of rf nonlinearity and potential-well distortion in determining these instability thresholds. Finally, we discuss the effect of the recent impedance reduction campaign on beam stability in single- and double-rf configurations.

Numerical investigation of azimuthal thermoacoustic instability in a gas turbine model combustor

Self-excited spinning mode azimuthal instability in an annular combustor with non-swirling flow is investigated using large eddy simulation (LES). Compressible Navier–Stokes equations are solved with a flamelet combustion model to describe the subgrid chemistry-turbulence interactions. Two flamelet models, with and without heat loss effects, are compared to elucidate the non-adiabatic wall effects on the thermoacoustic instability. The azimuthal modes are captured well by both models with only marginal differences in the computed frequencies and amplitudes. By comparing with the experimental measurements, the frequencies given by the LES are approximately 10% higher and the amplitudes are well predicted. Further analysis of the experimental and LES data shows a similar dominant anti-clockwise spinning mode, under which a good agreement is observed for the phase-averaged heat release rate fluctuations. Dynamic mode decomposition (DMD) is applied to shed more light on this spinning mode. The LES and experimental DMD modes reconstructed for their azimuthal mode frequencies agree very well for the heat release fluctuations. The DMD mode structure for the acoustic pressure from the LES shows a considerable non-zero profile at the combustor outlet, which could be essential for azimuthal modes to establish in this annular combustor. Finally, a low-order modelling study was conducted using an acoustic network combined with the flame transfer function extracted from LES. The results show that the dominant mode is associated with the plenum showing a first longitudinal and azimuthal mixed mode structure. By tuning the plenum length to match the effective volume, the predicted frequency becomes very close to the measured value.

Simultaneous measurements of axial motion and azimuthal rotation of non-uniformities (“spokes”) in a Hall thruster

Low-frequency instabilities are often present in Hall thrusters and are associated with axial and/or azimuthal oscillations of plasma non-uniformities. The axial oscillations are related to the so-called breathing mode (periodic depletion of the neutral atom density due to ionization) and are associated with large amplitude current oscillations. The low-frequency azimuthal instabilities are characterized by local non-uniformities of the light emission rotating in the azimuthal direction and are generally called “rotating spokes.” The possibility of coexistence of these two modes has been discussed in the literature but without clear experimental evidence of their correlation. In this paper, we present for the first time simultaneous measurements of the axial and azimuthal positions of the spoke. These measurements have been obtained with a high-speed camera using a triangulation method based on parallax. This method has proven to be sufficiently sensitive to track the 3D position (r–θ–z) of local non-uniformities of the light emission. The optical method has been synchronized with measurements of the current on a segmented anode. The results show that under some conditions, breathing oscillations and spoke rotation in the E×B direction are coupled. During the current rise, the spoke moves from the anode region toward the channel exhaust region while rotating in the E×B direction and seems to follow the movement of the front of neutral atoms progressively filling the channel. About 60% ± 20% of the total anode current is carried by the spoke.

Swirling spray flames dynamical blow out induced by transverse acoustic oscillations

Recent experiments on a laboratory scale annular system comprising multiple injectors (namely, MICCA-Spray), indicate that combustion instabilities coupled with azimuthal modes may induce large amplitude oscillations, which under certain conditions, lead to blow out of some of the flames established in the system, a phenomenon designated as dynamical blow out (DBO). An attempt is made in the present investigation to reproduce this phenomenon in a linear array of injectors (namely, TACC-Spray), where the acoustic field is externally applied to flames established by injector units that are identical to those used in the annular combustor. The acoustic field is generated by driver units placed on the lateral sides of a rectangular cavity. The pressure level induced in TACC-Spray can reach a peak value of 1700 Pa in a frequency range extending from 680 to 780 Hz, which corresponds to the typical frequency of azimuthal instabilities observed in the annular system. A theoretical model based on dimensional analysis serves to guide the choice of operating conditions that may lead to the DBO phenomenon. Experiments carried out in TACC-Spray and MICCA-Spray are then used to determine the DBO boundary, define the conditions that need to be fulfilled to observe this phenomenon, and gather high-speed visualizations providing some insights on the mechanisms that induce blow out.

Effect of the flame motion on azimuthal combustion instabilities

In the analysis of combustion instabilities in the annular combustors, especially for the azimuthal combustion modes, the flame motion is usually neglected and the flame is typically regarded as a discontinuity at rest. However, this simplicity may cause severe prediction errors for certain conditions. In this paper, the influence of the flame motion on the entropy, vortex, acoustic, and intrinsic modes in the annular combustion chamber is investigated by constructing an analytical model with the flame motion. Meantime, the influence of flame model parameters is also explored. The flame motion can affect both the frequency and growth rate of the entropy mode, but the growth rate is influenced more significantly. This effect can be damped or enlarged by the inlet Mach number depending on which parameter of flame model remains unchanged. The effect of the flame motion on vortex, acoustic, and intrinsic modes remains small when the inlet Mach number is very small. However, the increase of inlet Mach number can enlarge this effect.

Evolution of flow structure from a coaxial synthetic jet

• Influence of phase differences (∅) on novel coaxial synthetic jet (CSJ) is studied. • Azimuthal instability widens the jet for ∅ = 45°, and 90°. • Vortex pairing results in a strong jet for ∅ = 180°. • Entrainment ability of CSJ is high compared to single cavity synthetic jets. • Modulation of CSJ can be achieved with ∅ without investing any extra power. In this work, we propose and demonstrate a novel coaxial synthetic jet (CSJ) in which two cavities are arranged coaxially with 0° orientation angle with inner and annular orifices of equal hydraulic diameter. The flow behavior of the CSJ is studied at a low Reynolds number; Re = 135. The Navier-Stokes equations are solved (Direct Numerical Simulation) with the finite volume solver of openFoam, an open-source platform for numerical simulations. During the evolution of CSJ, two vortex rings IVR (inner vortex ring), and AVR (annular vortex ring), which are single toroid and double toroid in nature, are emanated from inner and annular orifices, respectively. Both the inner and annular jets are operated at equal mass fluxes. The evolution and interaction of the IVR, and AVR are studied by providing five-phase differences (∅ = 0°, 45°, 90°, 135°, and 180°) between the inner and outer diaphragms, and also its influence on the nature of the CSJ is highlighted. The phase-averaged and time-averaged flow fields are analyzed, and the CSJ is compared with the single cavity synthetic jet (SJ) at Re = 135, which is obtained by operating the inner cavity and annular cavity alone. The CSJ with ∅ = 0° reveals a wide and strong jet as compared to the single cavity SJ. The pairing of IVR and AVR results in a narrow and strong jet for ∅ = 180° as compared to ∅ = 0°. Merging and pulling mechanisms in case of ∅ = 45° and 90° enhances the azimuthal instability resulting in a wide and strong jet as compared to ∅ = 0°. A significant increase in the integrated jet area is observed for CSJ, which evidences the increase in entrainment ability of the CSJ. This novel device may find wide applicability for various engineering applications.

Azimuthal Instabilities of an Annular Combustor With Different Swirling Injectors

Abstract Experiments are carried out on the laboratory-scale MICCA-Spray annular combustor to examine the effects of swirlers on combustion instabilities. This system comprises 16 spray-swirl injectors and gives rise to instabilities coupled by azimuthal modes. Five types of swirlers producing clockwise rotation and varying in swirl numbers and pressure drops are considered. These swirlers can be broadly categorized into two groups: lower-swirl and higher-swirl groups, based on their swirl numbers. An arrangement where clockwise and counterclockwise swirlers alternate is also studied. Experiments are performed systematically with liquid heptane at five levels of thermal power and six equivalence ratios. Results reveal that none of the swirlers in the lower-swirl category exhibit instability in the operating region considered, whereas the higher-swirl units feature strong azimuthal instabilities that trace an overall limit cycle envelope with a few short and random bursts. Among the higher-swirl group, a higher pressure drop swirler is associated with a broader instability map. This shows that the transition to instability mainly depends on the swirl number through its effect on the flame structure and that the pressure drop adds to further variations in amplitude and frequency of oscillation. The spin ratio time series indicate that the modes are of mixed type and that their distribution depends on the operating condition. On specifically comparing the spin ratio distribution between a full set of clockwise rotating (CR) swirlers and a configuration where clockwise and counterclockwise rotating swirlers (CCR) are alternatively placed, it is found that there is no definite statistical preference for spin ratio linked to the effect of bulk swirl. In some cases, however, the CCR configuration promotes a broader distribution of spin ratios centered around the standing mode (sr = 0) while the CR setup favors azimuthal modes spinning in the counterclockwise direction. An attempt is made to interpret the occurrence of instabilities by making use of flame describing functions (FDFs) measured in a single-injector combustor. It is found that the FDFs corresponding to the two swirler categories (lower-swirl and higher-swirl) are relatively distinct. The observed behavior is tentatively interpreted using an instability analysis in which the injector and upstream plenum are represented by an impedance that shifts the band of instability. The unstable behavior is then linked to the relative position of the FDF phase with respect to the instability band in the frequency range corresponding to the expected azimuthal mode frequency. The phase and gain of the FDF notably depend on the swirl number, and it is possible to distinguish, for the present configuration, a category of low swirl number injectors inducing stable operation and another category of high swirl number units leading to oscillations.

Hysteresis and Bi-Stability in Transversely Excited Swirling Flows

Abstract We investigate the effect of transverse acoustic excitation on non-reacting swirling jets. The work is motivated by the azimuthal instabilities in annular gas turbine combustor which are one of the major challenges in aero-engines. We have designed and fabricated a multi-nozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual co-rotating radial swirler configuration. Two compression drivers placed on either side of the combustion chamber are used to generate acoustic fields in the direction transverse to the flow. Simultaneous 2D PIV and high-frequency pressure measurements are conducted to measure the time-averaged velocity field and the chamber acoustics, respectively. It is observed that once the swirling jet is excited with a transverse acoustic forcing, it instantaneously transitions to a wall-jet state. In wall-jet state, the flow moves radially outwards and remains attached to the walls on either side of the nozzle, and is characterised by a recirculation zone with a strong negative axial velocity. In our experiments we demonstrate that transverse acoustic excitation can lead to a bistable state in swirling flows. We investigated the acoustic response to low-amplitude forcing on the combustion chamber by performing a forced acoustic response analysis using COMSOL. It is observed that acoustic forcing leads to a peak response of the radial and azimuthal velocities at the flare of the swirler, which could induce an axisymmetric deflection of the jet shear layer pushing it into a wall-jet state.

Self-Excited Second-Order Azimuthal Thermoacoustic Instabilities in an Annular Combustor With Oblique-Injecting Swirling Burners

Abstract In this paper, we experimentally investigate the thermoacoustic instability issue in an annular combustor with 16 oblique-injecting premixed swirling burners. It is demonstrated that there exist three dominant modes in a narrow operating range: a Helmholtz mode, a first-order azimuthal mode, and a second-order azimuthal mode. Their modal frequencies are consistent with the simulating prediction of a Helmholtz solver. Our present investigations are more focused on the second-order azimuthal modes which are comparatively infrequently observed in the experiments of model annular combustors. The dynamic mode decomposition approach is used to postprocess the high-speed flame images, revealing the primary dynamic structure of the flame responses for the three self-excited thermoacoustic modes. A pressure field analyzing ansatz has been involved to feature the self-excited azimuthal instabilities, including their dynamical nature (standing, spinning, or mixed) and the time-varying pressure antinodes. Results indicate that the first-order and second-order azimuthal modes both exhibit a standing nature with relatively fixed pressure antinodes. Additionally, in a transition case where these two azimuthal modes co-exist, the first-order azimuthal mode behaves as a weakly oscillating standing mode whose pressure antinodes exhibit a fat-tailed distribution. Exceptionally, the second-order azimuthal mode is split into a pair of nondegenerate modes with two close frequencies. And the split pairs are found to yield distinct pressure antinodes that are orthogonal to each other.

Influence of mass flux ratio on the evolution of coaxial synthetic jet

This paper highlights a direct numerical simulation study on the flow field of a coaxial synthetic jet (CSJ) generated from two independently controlled synthetic jet actuators, which are combined coaxially with 0° orientation angle. The jet is issued into a quiescent environment from inner and annular openings (orifices) with equal hydraulic diameters, employing an oscillating boundary. Seven different mass flux ratios (Mr) such as 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, and 3.0 are considered for the study. The average velocity (Uavg) of inner jet, measured at orifice exit, is kept at 0.7 m/s (Reynolds number, Re=135), and the same is varied for the annular jet to achieve the desired Mr s. The influence of Mr s on the vortex rings, evolved from inner and annular orifices, along with their dynamics, is predicted by furnishing the instantaneous flow field. Also, we examine the effect of Mr s on the mean flow parameters of the CSJ. Moreover, the CSJ flow field is compared with the inner cavity synthetic jet (SJ), and annular cavity SJ under identical conditions, to demonstrate the superior performance of the CSJ over the single cavity SJs. For CSJ, the azimuthal instability of the evolved vortex rings can be triggered by decreasing the Mr, which results in a wide jet. For Mr≥1.0, the CSJ retains its axisymmetric nature, and the interaction of vortex rings emanating from the inner and annular cavities influences the strength and spreading of the CSJ. The modal decomposition of the instantaneous flow field is also performed using proper orthogonal decomposition method to gain insight of the coherent vortical structures present in the modes. The study will be useful for deploying such novel coaxial synthetic jets in various applications.

Eigenvalue Bounds in an Azimuthal Instability Problem of Inviscid Swirling Flows

We consider the eigenvalue problem of azimuthal instability of inviscid swirling ows between coaxial cylinders. It is shown that the complex eigenvalues corresponding to unstable azimuthal normal modes lie inside a semi-ellipse type region whose major axis coincides with the range of the angular velocity of the basic ow while its minor axis depends on the minimum Richardson number, the azimuthal wave number, and the width of the annular region between the coaxial cylinders.

Effect of recess length on flow dynamics in gas-centered liquid-swirl coaxial injectors under supercritical conditions

The flow dynamics in gas-centered, liquid-swirl coaxial injectors at supercritical conditions are comprehensively investigated using the large eddy simulation technique. This class of injectors has been used in many high-performance propulsion and power-generation systems. Gaseous oxygen is axially injected into the center post of the injector at a temperature of 687.7 K, while liquid kerosene is tangentially introduced into the coaxial annulus at a temperature of 492.2 K. The operating pressure of 25.3 MPa substantially exceeds the thermodynamic critical pressures of oxygen and kerosene. Detailed flow structures and mixing layer characteristics are examined, with special attention to the effect of the distance between the end of the center post and the entrance of the taper region, that is, the recess length. Six different cases ranging from no recess to fully recessed are investigated. Various controlling mechanisms of the injector flow dynamics, including recirculating motion near the fuel injection slit, shear-layer instabilities in the recess region, and vortex expansion and merging in the taper region are considered. Dominant frequencies of the axial and azimuthal instability modes are characterized and compared to empirical correlations. Dynamic analyses are conducted for the overall computational domain, as well as individual regions. Among the six cases, only the fully recessed Case 1 exhibits strong pressure oscillations over the entire injector, which are found to be closely related to the longitudinal acoustic oscillations in the center post. In cases with partial or no recess, Case 2 with a long recess achieves the best mixing, and Case 6 with zero recess leads to the worst mixing.