arXiv subjects

Akhil Aravind

Publications and source records attributed to Akhil Aravind.

8 recordsLinked to original sources

arXiv2026

Machine Learning based Ensemble Flame Regime Classification for Mesoscale Combustors based on Insights from Linear and Nonlinear Dynamic Analysis

Gaining insights into flame behaviour at small scales can lead to improvements in the efficiency of micro-reactors, compact power generation systems, fire safety technologies, and various other applications where combustion is confined to micro or mesoscales. Flame regimes observed in mesoscale combustors, namely Stable flame, Flames with repetitive extinction and ignition, and Propagating flame, exhibit unique dynamic characteristics that differentiate them from one another. In this study, we systematically examine the various flame regimes observed in mesoscale combustors from both dynamical and statistical standpoints. Our experimental methodology involves stabilizing a flame inside a quartz tube (an optically accessible mesoscale combustor) with an inner diameter of 5 mm. A premixed methane-air mixture is used as fuel, with its equivalence ratio and Reynolds number being the input parameters. Instantaneous OH* chemiluminescence and Acoustic pressure signals, along with high-speed flame imaging, were acquired for combustion dynamics characterization. The objective of this study is to analyze the distinct dynamical signatures associated with these observed flame regimes. For this purpose, Recurrence Quantification Analysis, followed by a Statistical-Spectral analysis, has been performed based on the experimentally acquired OH* Chemiluminescence and Acoustic pressure time-series signals. Subsequently, a stacking ensemble-based machine learning framework has been implemented for mesoscale flame regime classification based on the features extracted from the two aforementioned analyses. In addition, Continuous wavelet transform (CWT) scalograms and three-dimensional phase plots have been graphed to visually elucidate the evolution of system dynamics and the complex interaction of competing time scales in these flame regimes.

physics.flu-dyn↗

arXiv2025

Blast wave induced unsteady flow at the shock tube opening

Shock tubes have been a crucial device, facilitating studies across a wide range of practical applications. An open-ended shock tube employing the wire-explosion technique with a rectangular cross section is used in the present study to generate blast waves over a Mach number range of 1.2-1.8, enabling detailed investigation of unsteady compressible flow at the tube opening. The blast wave produces a complex flow field comprising a compressible vortex ring with a trailing jet, and several transient structures, including embedded shocks, inward-moving shock or reverse shocks, shear layers, and Prandtl-Meyer expansion fans. An approximate model based on a power-law density profile describes the blast evolution inside and outside the tube, with the equivalent source deduced from measured shock trajectories. The blast wave-tube exit interaction is analyzed using the method of characteristics with alternate exit boundary conditions. A steady-pressure outlet best reproduces experimental observations, predicting supersonic efflux, embedded shocks, expansion waves, and circulation production. Several previously unreported unsteady features, including reverse shock or "reshock" formation and embedded shock shedding, are documented. The findings highlight the intricate dynamics of various features associated with such highly transient, blast-driven flows emanating from an open-ended shock tube.

physics.flu-dyn↗

arXiv2024

Effect of the blast wave interaction on the flame heat release and droplet dynamics

The study comprehensively investigates the response of a combusting droplet during its interaction with high-speed transient flow imposed by a coaxially propagating blast wave. The blast wave is generated using a miniature shock generator which facilitates wide Mach number range ($1.01 1.06$. The timescale of the flame extinction is faster (interaction with $\rm v_s$) for $M_s>1.1$. The study investigates the effect on droplet regression, flame heat release rate and flame topological evolution during the interaction. The droplet regression rate gets enhanced after the interaction with blast wave for $M_s < 1.06$, while it slowed down due to complete extinction for $M_s > 1.06$. A momentary flame heat release rate (HRR) enhancement occurs during the interaction with shock flow, and this HRR enhancement is found to be more than 8 times the nominal unforced flame HRR for $M_s > 1.1$, where rapid flame extinction occurs due to faster interaction with $\rm v_s$ ($\sim O(10^{-1})ms$). HRR enhancement has been attributed to the fuel vapor accumulation during the interaction. Furthermore, for $M_s > 1.1$), compressible vortex interaction occurs with the droplet resulting in droplet atomization. The droplet shows a wide range of atomization response modes ranging for different shock strengths. No significant effect of nanoparticle (NP) addition has been found on the flame dynamics due to the faster timescales. However, minimial effects of NP addition are observed during droplet breakup due to fluid property variation.

physics.flu-dyn↗

arXiv2024

Dynamics of unsteady premixed flames in meso scale channels and the effects of varying the wall heating conditions

Understanding the dynamics of flames at small scales opens up opportunities to enhance the performance of small-scale power generation devices, micro-reactors, fire safety devices, and numerous other systems that confine combustion to micro/meso scales. The current study delves into the dynamics of laminar premixed methane-air flames in mesoscale channels, subject to different wall heating conditions. Two external heaters, positioned at adjustable distances, are employed to create a bimodal wall heating profile on the combustor walls. The separation distance (d) between the heaters was varied, and the resulting flame dynamics were examined across a wide range of equivalence ratios and Reynolds numbers (Re). The observed dynamics were also compared against a baseline configuration that utilises a single heater to establish an axial unimodal wall temperature profile. Apart from the previously documented observations of unsteady flames with repetitive extinction and ignition (FREI) characteristics, this study identifies an additional unsteady propagating flame (PF) regime. While FREI appeared at stoichiometric and fuel-rich conditions, propagating flames were observed at the equivalence ratio of 0.8. Unlike the FREI regime where the flame extinguishes after a characteristic travel distance, propagating flames continue to travel till they reach the upstream end of the combustor tube, where they extinguish upon encountering a meshed constriction. These flames are associated with a characteristic heat-release-rate oscillation that couples with the pressure fluctuations at frequencies close to the natural harmonic of the combustor tube. These dynamics are discussed in detail with appropriate theoretical arguments to justify the observed trends.

physics.flu-dyn↗

arXiv2024

Response of premixed Jet Flames to Blast Waves

The study investigates the response dynamics of premixed jet flames when incident with blast waves along the jet axis. In the present work, blast waves are generated using the wire-explosion technique. The generated blast wave interacts with premixed jet flames that are stabilised over a thin fuel-air jet nozzle. The study is performed over a wide range of parametric space, varying the Reynolds number ($Re$) and normalised equivalence ratio ($Φ$) of the premixed jet and the strength of the generated blast fronts ($M_{s,r}$). The blast wave imposes a decaying flow field profile characterised by a sharp discontinuity at the blast front. This is accompanied by a subsequent induced flow arising due to entertainment from the surroundings as the blast-imposed flow fields decay to sub-ambient levels. While the jet flame is observed to respond to the blast front with a jittery motion, it is found to lift off following the interaction with the induced flow. Depending on the operating conditions ($Re$, $Φ$, and $M_{s,r}$), the flame lift-off is followed by an extinction or a re-attachment event. The flame response is further classified into two re-attachment and three extinction sub-regimes based on the response dynamics of the flame base and flame tip following the interaction process. The flame response entails flame base lift-off and flame tip distortion, potentially leading to a flame pinch-off event contingent on the operating conditions. A mathematical model was developed to explain the flame base response dynamics, yielding a scaling law for flame base lift-off height. Flame tip response trends were elucidated by extending the vorticity transport equation to estimate the vortex roll-up rate in the shear boundary surrounding the flame. Flame pinch-off was observed when shear layer vortices reached critical circulation limits and shed at length scales lower than the flame height.

physics.flu-dyn↗

arXiv2024

Insights into Spatio-temporal dynamics during shock -- droplet flame interaction

The study comprehensively investigates the response of a combusting droplet during its interaction with a high-speed transient flow that is imposed by a coaxially propagating blast wave. The blast wave is generated using a specially designed unique miniature shock generation apparatus that generates blast waves using the wire-explosion technique which facilitates a wide range of shock Mach number (1.03 < Ms < 1.8). The experiments are performed in two configurations: Open field blast wave and focused blast wave. The charging voltage and the configuration determine the shock Mach number (Ms) and flow characteristics. The flame is found to exhibit two major response patterns: partial extinction followed by re-ignition and full extinction. Simultaneously, the droplet also interacts with the flow imposed by the blast wave exhibiting different modes of response ranging from pure deformation, Rayleigh-Taylor piercing bag breakup, and shear-induced stripping. The KH instability is exhibited along the windward side interface of the droplet during the interaction with the blast wave decay profile which gets aggravated when the induced flow interaction ensues. Increasing the Mach number (Ms > 1.1) makes the droplet flame more vulnerable to extinction. However, the flame exhibits stretching and shedding, followed by re-ignition at lower Mach numbers (Ms < 1.06). In all cases, the flame base lifts off in response to the imposed flow, and the advection of the flame base interacting with the flame tip results in flame extinction. The entire interaction occurs in two stages: 1) interaction with the blast wave and the decaying velocity profile associated with it, and 2) interaction with the induced flow behind the blast wave as a result of the entrainment (delayed response). The criteria for partial and complete extinction of flame have been postulated which is in good agreement with the experiments.

physics.flu-dyn↗

arXiv2023

Response of Non-premixed Jet Flames to Blast Waves

The work investigates the response dynamics of non-premixed jet flame to blast waves that are incident along the jet axis. In the present study, blast waves, generated using the wire-explosion technique, are forced to sweep across a non-premixed jet flame that is stabilised over a nozzle rim positioned at a distance of 264 mm from the source of blast generation. The work spans a wide range of fuel jet Reynolds numbers ($Re$) and incident blast wave Mach numbers ($M_{s,r}$). The interaction imposes a characteristic flow field over the jet flame, marked by a sharp discontinuity followed by a decaying profile and a delayed second spike. The second spike in the flow field profile corresponds to the induced flow that follows the blast front. While the response of the flame to the blast front was minimal, it was found to detach from the nozzle rim and lift off following the interaction with the induced flow. Subsequently, the lifted flame was found to re-attach back at the nozzle or extinguish, contingent on the operating $Re$ and $M_{s,r}$. Alongside flame lift-off, flame tip flickering was aggravated under the influence of the induced flow. A simplified theoretical model extending the vorticity transport equation was developed to estimate the change in flickering timescales and length scales owing to the interaction with the induced flow. The observed experimental trends were further compared against theoretical predictions from the model.

physics.flu-dyn↗

arXiv2021

Leading Edge Vortex Dynamics of Airfoils, Pitching Continuously at High Amplitudes

Airfoils pitching in the stalled regime have been of keen interest in recent years due to their desirable aerodynamic force characteristics. In this study, we are numerically investigating the unsteady flow past a NACA 0012 airfoil under sinusoidally pitching motion, using a finite volume based sharp interface immersed boundary solver. The flow is investigated in the low Reynolds number regime (Re=3000) for reduced frequencies of 0.1 and 0.5, at three different pivot locations (c/3, c/2 and 2c/3 from the leading edge). The airfoil is subjected to sinusoidal oscillations with its incidence angle varying from 15o to 45o. Leading edge vortices (LEVs) that are formed during the pitching motion dictate the transient aerodynamic characteristics. The flow field data is used to identify individual LEVs in the flow field. The spatio-temporal evolution of LEVs in terms of their strengths are traced throughout the pitching cycle to obtain a quantitative estimate of its evolution. The effect of pivot location and pitching frequency on the vortex evolution and aerodynamic forces is investigated. Change in pitching frequency is found to have a drastic effect on the vortex dynamics. LEV formation is delayed to higher fractional times and LEVs are found to interact and merge into each other at higher frequencies. The maximum lift and drag are also found to increase with pitching frequency. Pivot location is found to have a higher influence on the magnitude and phase of aerodynamic coefficients at higher frequencies. However, at both frequencies, moving the pivot axis aftward is found to delay LEV evolution and cause a phase lag on the aerodynamic forces.

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