Non-reacting Conditions Research Articles

Surface reconstruction of ruddlesden–popper-based oxides in nonreactive environments and under electrochemical conditions for the oxygen evolution reaction

Perovskite oxides have emerged as catalysts for electrochemical water splitting, and surface reconstruction is crucial for their electrocatalytic performance. In this study, we report the surface reconstruction of Ruddlesden–Popper-based oxides under nonreactive and electrochemical conditions. The rapid migration of Sr ions from the bulk to the surface results in a porous structure, increase the number of oxygen vacancies, and produces an A-site cation deficiency. Collectively, these modifications significantly enhance the electrocatalytic activity and stability for water oxidation. In addition, the engineered electrocatalysts have a low overpotential of 350 mV at 10 mA cm−2, alone with a Tafel slope of 77 mV dec−1. They also exhibit outstanding durability, maintaining a current density of 5 mA cm−2 over 6 h.

Influence of blowing ratio on double-wall cooling characteristics under the condition of high-temperature flue gas

Double-wall cooling has been extensively investigated due to its exceptional cooling performance. However, most previous studies on the cooling characteristics of double-wall geometry have been conducted under non-reacting conditions, neglecting the influence of high-temperature flue gas radiation. In this paper, an experiment investigation was carried out to evaluate the impact of blowing ratio <italic>M</italic> and coolant-to-gas temperature ratio on the cooling characteristics of effusion/impingement cooling at reacting flow conditions. The experiments were performed at atmospheric pressure with flame temperatures of 1,800 and 1,900 K respectively. Flue gas temperature was measured by S-type thermocouples, while an infrared camera and N-type thermocouples were employed for the gas-side wall surface temperature distribution measurements. The results demonstrate that the laterally averaged cooling effectiveness of the effusion/impingement cooling system increases with the increase of <italic>M</italic>. However, beyond a blowing ratio of 5.1, further increments in <italic>M</italic> do not significantly affect the cooling effectiveness. Moreover, it is observed that the laterally averaged cooling effectiveness gradually improves along the flow direction but at a reduced rate. Additionally, no significant changes in cooling effectiveness are observed after <italic>X</italic>/<italic>D</italic> > 50. Furthermore, when considering a blowing ratio of 4, higher coolant temperatures result in higher cooling effectiveness compared to lower coolant temperatures.

Sub-filter-scale shear stress analysis in hypersonic turbulent Couette flow

Direct numerical simulations of hypersonic turbulent Couette flows are performed for top-wall Mach numbers of 6, 7 and 8, inspired by non-reactive high-enthalpy wind tunnel free-stream conditions, with the goal of analysing the physical processes driving the sub-filter-scale (SFS) stresses to inform development of large-eddy simulation techniques for hypersonic wall-bounded flows. Semi-local scaling laws collapse mean profiles and second-order turbulent statistics well, in spite of the strong wall-normal gradients of temperature and density. On the other hand, the SFS shear stresses exhibit an unexpected profile characterized by a region of pronounced shear stress deficit, which becomes more pronounced for higher Mach numbers. Instantaneous visualizations suggest that the SFS shear stress deficit is induced by the counter-gradient resolved momentum transport driven by residual velocity motions at the interface between high-density low-speed streaks being ejected away from the wall, and low-density high-speed ones replacing the displaced fluid, qualifying this as a compressibility effect. It is shown that the SFS shear stresses are primarily driven by second-order interactions between residual velocities, in spite of their triply nonlinear nature. This, in turn, motivated a statistical quadrant analysis revealing the presence of a SFS shear stress deficit, that is, SFS processes driving momentum transport of the resolved field towards the top wall. Additionally, higher-Reynolds-number simulations reveal that there is an upper limit of spatial filter widths to resolve large-scale structures, and such deficit is observable at any Reynolds number when a reasonable spatial filter is applied.

Effects of fuel injection pressure and quantity on low octane gasoline sprays in a simulated high-pressure ambient environment of a gasoline compression ignition engine

Gasoline Compression Ignition technology using low-octane gasoline-like fuel can lead to very low NOx and Soot emissions and enhanced thermal efficiency than conventional diesel engines. The difference in test fuel properties is a significant reason for the difference in the fuel–air mixture formation in Gasoline Compression Ignition engines offering these advantages. In this study, microscopic and macroscopic spray characterisation of baseline diesel, 50 % v/v diesel blended with 50 % v/v gasoline (G50), and 20 % v/v diesel blended with 80 % v/v gasoline (G80) are conducted in a high-pressure ambient environment relevant to Gasoline Compression Ignition engines. The study was performed in a constant volume spray chamber in non-reacting conditions using Phase Doppler Interferometry and Diffused Backlit Illumination techniques. The test fuels were injected into a 30 bar ambient pressure environment at 400, 500, and 700-bar fuel injection pressures. The liquid spray penetration length of gasoline-diesel blends was slightly shorter than baseline diesel, but their spray cone angles and entrained air mass were significantly higher. The average axial droplet velocity was higher, and the Sauter Mean Diameter was lower for gasoline-diesel blends than baseline diesel 40 mm away from the injector nozzle exit, indicating superior mixture formation for the gasoline-diesel blends. The effect of droplet collision and injected fuel quantity on droplet size distribution was assessed in all test fuels. Droplet collisions increased the Sauter mean diameter of the spray at farther distances. Smaller fuel injection quantities decreased the Sauter mean diameter of the spray, increasing the probability of unburnt hydrocarbon emission producing ultra-lean regions in the engine combustion chamber.

Experimental characterization of direct injection liquid ammonia sprays under non-reacting diesel-like conditions

Ammonia is seen as a promising fuel to replace fossil fuels internal combustion engines, given the advantages of zero carbon emission and extensive experience in its synthesis at large scale. The ignition of ammonia in direct injection engine is still a challenge due to its unfavorable combustion characteristics, but the investigation of ammonia spray characteristics is an important step to shed light on the optimum combustion strategy. In this paper, the evolution of direct injection ammonia sprays was measured at engine-like conditions generated in a constant pressure facility by means of diffused back-illumination imaging and schlieren. In a first step, ammonia spray behavior is compared against that of Diesel fuel. Experimental results show a slower penetration of ammonia mainly due to a lagging starting penetration phase, which cannot be later recovered. In terms of liquid length, initial stages show the same pattern as for ammonia, while a shorter stabilized liquid length can be observed. Parametric trends over different operating conditions show a similar behavior to Diesel against ambient density and temperature, as well as with injection pressure and nozzle diameter. Such results, backed up by the application of a 1D spray model developed for Diesel-like fuels, suggest that ammonia sprays under non-reacting conditions have a mixing-controlled evolution.

Mixing measurement on hydrogen jet by LIBS under various injection strategies

Favorable fuel properties of hydrogen for combustion system opens a new avenue to achieve deep decarbonization in transportation sector. Meanwhile, various technical challenges including low volumetric energy density, nitrogen oxide emissions, and backfire impede successful implementation of hydrogen in combustion engines. To overcome those issues, stratified charge combustion (SCC) is considered to optimize engine performance and emissions. As direct injection approach is utilized in SCC mode, ensuring quality of air-fuel mixing in combustion chamber is crucial. This study aims to understand hydrogen mixing behavior by quantitative measurement on equivalence ratio using laser-induced breakdown spectroscopy (LIBS). This measurement has been conducted under non-reacting conditions, employing realistic injection strategy, multiple injection, so the influences of dwell time, and split ratio are addressed. In addition to LIBS, this study includes a morphological analysis of the hydrogen jet, which was visualized using high-speed schlieren imaging. The experimental data showed that shorter first injection duration was resulted in an increased jet width at the spark plug location, which reduced the equivalence ratio within the center of the jet. Longer dwell time also demonstrated wider jet width at the spark plug location, reducing the equivalence ratio within the center of the jet.

Near-infrared imaging-based diffusion coefficient mapping for acid–base reactions

This study describes a method based on near-infrared concentration imaging and advection–diffusion–reaction modeling for the simultaneous determination of the diffusion coefficient distributions of acids, bases, and salts during the neutralization reaction. The reactions between parallel streams of aqueous solutions of H2SO4 and NaOH, and HCl and KOH through a microfluidic channel are investigated. The measured concentration profiles were approximated by a Gaussian or error function, to calculate the advection, diffusion, and reaction terms analytically and robustly. The diffusion coefficients differ significantly within the channel; those at the reaction interface are the largest than the literature values measured under non-reactive conditions, indicating temperature and concentration dependence, and anomalous features. The proposed method facilitates the in-situ visual and quantitative analysis of diffusion reactions.

Exploiting the potential of large eddy simulations (LES) for ducted fuel injection investigation in non-reacting conditions

The diesel combustion research is increasingly focused on ducted fuel injection (DFI), a promising concept to abate engine-out soot emissions in compression-ignition engines. A large set of experiments carried out in constant volume vessel and numerical simulations, at medium-low computational cost, showed that the duct adoption in front of the injector nozzle activates several soot mitigation mechanisms, leading to quasi-zero soot formation in several engine-like operating conditions. However, although the simplified CFD modelling so far played a crucial role for the preliminary understanding of DFI technology, a more accurate turbulence description approach, combined with a large set of numerical experiments for statistical purposes, is of paramount importance for a robust knowledge of the DFI physical behaviour.In this context, the present work exploits the potential of large eddy simulations (LES) to analyse the non-reacting spray of DFI configuration compared with the unconstrained spray. For this purpose, a previously developed spray model, calibrated and validated in the RANS framework against an extensive amount of experimental data related to both free spray and DFI, has been employed. The tests have been carried out considering a single-hole injector in an optical accessible constant volume vessel, properly replicated in the simulation environment. This high-fidelity simulation model has been adapted for LES, firstly selecting the best grid settings, and then carrying out several numerical experiments for both spray configurations until achieving a satisfying statistical convergence. With this aim, the number of independent samples for the averaging procedure has been increased exploiting the axial symmetry characteristics of the present case study.Thanks to this approach, a detailed description of the main DFI-enabled soot mitigation mechanisms has been achieved, shrinking the knowledge gap in the physical understanding of the impact of spray-duct interaction.

The mixing field and flame structure near the reaction zone of turbulent planar flames at different levels of mixture inhomogeneity

Turbulent flames with compositionally inhomogeneous mixtures are commonly used in many combustion systems. In this work, turbulent planar jet flames issued from a concentric flow slot burner, CFSB, were used to study the impact of mixture inhomogeneity near the flame sheet. The CFSB burner can control the mixing inhomogeneity by changing the mixing length “L” between the concentric fuel and air slot nozzles. At various levels of mixture inhomogeneity, the mixing field, presented by mixture fraction, Z, distribution, and the flame structure was investigated via simultaneous Rayleigh and OH-PLIF imaging technique. Our previous study investigated the mixing field immediately downstream of the burner exit in non-reacting conditions. The PDFs of Z showed that the mixing field covered a wide range of mixture fractions, where the high flame stabilization occurred when a large portion of the PDF(Z) was located within the fuel flammability limits. This work showed that further downstream, the highly stabilized flames were also obtained when the range of fluctuations in the mixture fraction was close to the stoichiometric mixture fraction. Moreover, plotting the mixing field using the scatter plot within the mixing regime diagram for various flame conditions showed that the mixing field downstream of the burner exit consistently follows the mixing diagram classification. Moreover, a close inspection of the flame structure showed that the flame sheet varies from thick, and corrugated at a low and high levels of mixing to thin flame sheet but less corrugated and that at a particular mixing normalized length L/D = 7. The flame corrugation data showed that the flame stability occurs at a minimum corrugation factor.

Influence of fuel characteristics on the alternative jet fuel atomization at non-reacting conditions

Synthetic paraffinic kerosene jet fuel derived from natural gas (NG-SPK) is gaining importance as a low-emission alternative fuel for aviation combustors. In this context, the influence of fuel properties on the atomization of NG-SPK jet fuel at non-reacting, evaporating conditions is investigated. The spray results of NG-SPK jet fuel are compared with the reference, Jet A-1 fuel. This study considers a simplex atomizer, commonly deployed as a pilot injector in aviation gas turbine combustors for ignition at sea level and in high-altitude relight conditions. The local atomization characteristics like droplet concentration, mean diameters, and mean axial droplet velocity is obtained experimentally using phase Doppler anemometry technique at different ambient gas pressures of 100, 500, and 900 kPa while maintaining the ambient temperature at 400 K. The fuel dispersion parameters are measured at two nozzle pressure differentials of 300 and 900 kPa. The NG-SPK jet fuel exhibited higher droplet concentrations for the conditions studied than conventional fuel. Furthermore, the droplet size distribution of NG-SPK jet fuel revealed a higher percentage of smaller droplet diameters than Jet A-1 fuel. This difference in droplet characteristics can positively influence the heat release patterns under reacting conditions.

The effect of toroidal geometrical shape on turbulent oscillations in a bidirectional vortex combustor

The paper reports on detailed studies of a bidirectional swirling flow structure in a toroidal vortex chamber under non-reactive and combustion conditions at the Reynolds numbers Re = 65,000 and Re = 10,000. The explanation of reduced velocity fluctuations near the head wall compared to simple cylindrical bidirectional chambers is also given. The studies were carried out using transient numerical simulations DES (Detached Eddy Simulation). The necessary experimental validation based on PIV (Particle Image Velocimetry) and temperature measurements were performed as well. The results show that fluctuations of all velocity components in the toroidal part of the chamber do not exceed 20% in non-reactive conditions and 10% in the combustion case. Additionally, temperature oscillations are less than 10% of the adiabatic stoichiometric temperature of propane-air mixture combustion. The studies of combustion at different equivalence ratios show that the rich combustion ϕ = 1.5 provides the most favorable conditions in terms of mixture ignition. Moreover, a large-scale backflow structure is formed beyond the outlet nozzle at rich modes that leads to an increase in combustion stability. This feature can be usefully applied in studies of mesoscale bidirectional arrays and developing multiport combustion chambers.

Investigation of Harmonic Response in Non-Premixed Swirling Combustion to Low-Frequency Acoustic Excitations

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion.

A CFD Modelling Approach of Fuel Spray under Initial Non-Reactive Conditions in an Optical Engine

A better understanding of why and how pollutant emissions from compression ignition engines are produced is one of the strategies to reduce them, and to achieve this it is important to understand what happens in the fuel injection inside the combustion chamber and in the combustion process. Experimentally, it is difficult to analyse the fuel spray right at the initial moments when it enters the combustion chamber due to its high velocity. These initial moments of the fuel spray affect its complete development and, consequently, the combustion process inside the chamber. This fact has motivated the approach of this work, in which a parametric study of the spray penetration as a function of variables that can be measured has been proposed. The purpose of this model is to understand which variables of the injection system significantly affect the spray penetration in the initial instants and how they affect it. This study was carried out using diesel and serves as a reference framework for similar studies using pure or blended sustainable advanced fuels. A computational fluid dynamics (CFD) model that determines the spray penetration at initial instants under different injection pressures and nozzle hole diameters is presented in this work. To tune the model, experiments were carried out on an optical engine. The modelled and experimental results exceed 94.8% agreement in all cases studied.

Comparison of non-reactive flow fields in conventional swirl-air combustion and swirl distributed combustion

Flow field distribution in traditional swirl flow and simulated distributed reaction swirl flow configuration was examined at 3 kHz frequency using high-speed Particle Image Velocimetry (PIV). Non-reactive flow field conditions in distributed reaction zones were fostered from the conventional swirl flow case using the designed inlet mixture preparation with CO2 dilution of the inlet airstream that corresponded to the combustion case at a heat release intensity of 5.72 MW/m3-atm. High-speed imaging of flow fields was performed at a framing rate of 3000 frames/s using a Neodymium-doped Yttrium Lithium Fluoride (Nd:YLF) laser beam at 527 nm wavelength to capture the Mie scattering images of the flow field. Mean flow velocity obtained from the post-processing of raw PIV images showed higher momentum flux injection in the swirl distributed flow zone case as compared to the normal swirl flow case. Enhanced velocity fluctuations were also observed in both the axial and radial direction for the distributed swirl flow case resulting in higher turbulence for the examined condition. The distributed swirl flow case showed a prominent corner recirculation zone along with higher flow entrainment within the recirculation zone. Enhanced interaction of average streamlines with vortices was found in the swirl distributed flow case. Increased turbulent fluctuation in the two different spatial directions was responsible for higher Reynolds stress distribution (u′v′) in the distributed flow case, indicating improved mixing behavior at lower global oxygen concentrations from the carbon dioxide (CO2) dilution, and showed different length scales of turbulent eddies. Furthermore, the spatial local Damkӧhler number values for distributed flow case (for its implication in reacting case) were found to be less than unity. The results showed a shorter flow timescale than the corresponding chemical timescales in distributed reaction zone. These results help support substantiating the difference of reaction regimes on the Borghi diagram (plot of velocity fluctuation/ laminar flame speed versus the integral length scale/ flame thickness) between the distributed reaction zone (that possesses thickened reaction zone) and conventional swirl reaction flow regimes having corrugated flamelet zone.

Mean pressure gradient effects on the performance of ramjet cavity stabilized flames

Premixed cavity stabilized flames are experimentally investigated in a high-speed ramjet engine under the influence of varying mean pressure gradients. The ramjet cavity incorporates a backward facing step with an aft ramp for flame stabilization in high Reynolds number. The ramjet engine is subject to varying wall geometry to form converging, diverging, and nominal configurations in order to investigate the effects of mean pressure gradients on engine performance. High-speed particle image velocimetry (PIV) and chemiluminescence imaging diagnostics are simultaneously employed to capture the reacting flow fields and flame dynamics. Imposing a larger favorable pressure gradient is shown to shrink the recirculation zone and alter shear layer dynamics, which leads to increased drag on the cavity. Additionally, inducing a stronger favorable pressure gradient is shown to excite a shear layer instability mode, characterized by a Strouhal number of St=0.1. Proper orthogonal decomposition (POD) results reveal that the instability mode is comprised of large-scale oscillations that occupy the entire cavity flow region, indicating that the excited oscillations are the manifestation of a global vortex shedding instability that occurs under non-reacting conditions. The results demonstrate that the performance and stability of the ramjet cavity flame can be influenced by the mean pressure gradient, which is vital for the design of high-speed air-breathing propulsion systems such as dual-mode scramjets.

Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions

The synthetic diesel fuel (Poly-)oxymethylene ether (OME) has various interesting properties for the transport sector. OME reduces emissions in direct injection (DI) diesel engines, e.g., soot and NOx, and is considered CO2 neutral. While most studies have examined a mixture of OMEs with different chain lengths, recent studies have focused on neat OME3 and OME4. In the literature similar chemical ignition delay times were shown under homogeneous conditions and significant differences were reported in their thermophysical properties. Furthermore, differences between OME3 and OME4 in terms of the mixture formation were shown on a heavy-duty injector under non-reactive conditions. The question of how this mixture formation affects the ignition and flame characteristics of neat OME3 and OME4 spray flames remains unresolved. To address this topic, this study conducts a combined experimental and numerical investigation under inert and reactive spray conditions for OME3 and OME4 on a heavy-duty injector. Large Eddy Simulations (LESs) and high-pressure, high-temperature spray chamber measurements are conducted. The distinct differences between neat OME3 and OME4 are analyzed along the cause–effect chain comprising the mixture formation, ignition behavior and its effects on the overall heat release rate. Based on the differences in the thermophysical properties, e.g., density and vapor pressure, the investigations in inert conditions reveal differences in the liquid penetration and mixture formation. The reactive spray flame measurements and simulations show a significant offset in the ignition delay time for OME3 and OME4. An explanation is found in the significantly different distribution of the scalar dissipation rate due to the differences in the mixture formation. Despite the differing ignition delay times, similar local flame structures are found for both fuels.

Probabilistic derivation of droplet velocity using quadrature method of moments

Abstract Accurately prediction of dispersion and polydispersity of droplet flow is not a trivial task due to the complex behavior of the droplet size distribution (DSD) and the strong state of the instantaneous velocity of a droplet on the shape and size of the droplet. Describing the distribution of sizes and velocities of droplets initially formed in sprays is an essential piece of information needed in spray modeling, which is used to define the initial state of the spray droplets in the downstream two-phase flow fields’ predictive computations. The predictive model for the droplet size and velocity distributions in sprays is formulated as the droplet’s velocity magnitude has a power–law relationship with the droplet in this study. The present model incorporates the deterministic and stochastic aspects of the spray formation process. The quadrature method of moments (QMOM) is applied to solve numerically the transport equations of the probability density function coupled with conserved source terms incompressible Navier-Stokes equations for the liquid phase. The sub-models are connected by different source terms signifying the liquid-gas interaction. Equations of transport for spray moments are derived from DSD, and closure is attained using a gamma distribution. The integer spray moments concerning the volume are used to construct the continuous distribution of QMOM. In contrast, the velocity moments are used to determine the droplet velocity as a constant function of the droplet diameter. The model is first applied to simulate a diesel spray tip penetration under nonreactive conditions with different droplet velocity profiles to validate the approach with experimental data. An additional case of a liquid nitrogen spray is applied to show the gamma distribution’s ability to describe spray drop size distribution. The model generally predicts reasonable agreement with the experimental for both cases.

Uncertainty quantification analysis of Reynolds-averaged Navier–Stokes simulation of spray swirling jets undergoing vortex breakdown

The computational fluid dynamics-based design of next-generation aeronautical combustion chambers is challenging due to many geometrical and operational parameters to be optimized and several sources of uncertainty that arise from numerical modeling. The present work highlights the potential benefits of exploiting Bayesian uncertainty quantification at the preliminary design stage. A prototypical configuration of an acetone/air spray swirling jet is investigated through an Eulerian–Lagrangian method under non-reactive conditions. Two direct numerical simulations (DNSs) provide reference data, coping with different vortex breakdown states. Consequently, a set of Reynolds-averaged Navier–Stokes simulations is conducted. Polynomial chaos expansion (PCE) is adopted to propagate the uncertainty associated with the spray dispersion model and the turbulent Schmidt number, delivering confidence intervals and the sensitivity of the output variance to each uncertain input. Consequently, the most significant sources of modeling uncertainty may be identified and eventually removed via a calibration procedure, thus making it possible to carry out a combustion chamber optimization process that is no longer affected by numerical biases. The uncertainty quantification analysis in the current study demonstrates that the spray dispersion model slightly affects the fuel vapor spatial distribution under vortex breakdown flow conditions, compared with the output variance induced by the selection of the turbulent Schmidt number. As a result, additional high-fidelity experimental and numerical campaigns should exclusively address the development of an ad hoc model characterizing the spatial distribution of the latter in the presence of vortex breakdown phenomenology, discarding any effort to improve the spray dispersion formulation.

Adsorbate Organization Characterized by Sublevelset Persistent Homology.

Interfacial adsorbate organization influences a variety physicochemical properties and reactivity. Surfaces that are rough, defect laden, or have large fluctuations (as in soft matter interfaces) can lead to complex adsorbate structures. This is amplified if adsorbate-adsorbate interactions lead to self-assembly. Although image analysis algorithms are somewhat common for the study of solid interfaces (from microscopy for example), images are often not readily available for adsorbates at soft matter surfaces, and the complexity of adsorbate organization necessitates the development of new characterization approaches. Here we propose the use of adsorbate "density" images from molecular dynamics simulations of liquid/vapor and liquid/liquid interfaces. Topological data analysis is employed to characterize surface active amphiphile self-assembly under nonreactive and reactive conditions. We develop a chemical interpretation of sublevelset persistent homology barcode representations of the density images, in addition to descriptors that clearly differentiate between different reactive and nonreactive organizational regimes. The complexity of amphiphile self-assembly at highly dynamic liquid/liquid interfaces represents a worst-case scenario for adsorbate characterization, and as such the methodology developed is completely generalizable to a wide variety of surface image data, whether from experiment or computer simulation.

Transient dynamics of the precessing vortex core in an intermittently shape-transitioning swirl flame

The transient dynamics of the precessing vortex core (PVC), a globally unstable mode, in an intermittently shape-transitioning swirl flame is investigated by means of simultaneous OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (S-PIV) at a repetition rate of 10 kHz. A time-frequency analysis finds that the PVC is present in steadily lifted flames and at non-reacting conditions, whereas it is suppressed when the flame is steadily attached. While the flame shape spontaneously transitions between these two states, the onset of the growth/decay of the PVC amplitude occurs prior to the flame lift-off/reattachment. The transient formation of the PVC before flame lift-off is quantitatively elucidated by a transient linear stability analysis (TLSA), which well predicts the instability frequency and absolute growth rate. The TLSA reveals that the onset of the flow transition from convective to absolute instabilities, corresponding to the growth of the PVC, is attributed to continuous suppression of the convective perturbation and viscous diffusion, and afterwards the absolute instability is sustained only due to the weakened convection. The suppression of the convection and viscous diffusion terms is mainly due to the changes in the transient mean velocity and density fields within the inner shear layer, which are likely triggered in the first place by unsteady heat loss of the flame to the dump plane of the combustor.