The work investigates the response dynamics of non-premixed jet flames 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 the blast waves. The work spans a wide range of fuel-jet Reynolds numbers (; ranging from 267 to 800) and incident blast-wave Mach numbers (; ranging from 1.025 to 1.075). 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 reattach back at the nozzle or extinguish, contingent on the operating and. 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 time scales and length scales owing to the interaction with the induced flow. The observed experimental trends were further compared against theoretical predictions from the model.

This study investigates the shock-induced atomisation dynamics of an acoustically levitated TM-10 nanofluid droplet subjected to a coaxially propagating blast wave and subsequent compressible vortex ring, generated using a compact wire-explosion shock source. The blast wave imposes a sharp velocity discontinuity, followed by a decaying flow field and a vortex-dominated interaction that drives droplet disintegration. Laser-induced heating promotes evaporation, increasing nanoparticle concentration, viscosity, and agglomeration within the droplet. Progressive evaporation leads to interfacial nanoparticle accumulation, initiating a sol-gel transition when the local volume fraction exceeds the gelation threshold, and ultimately forming a solid outer shell as the maximum packing limit is approached. Shock interactions are systematically examined across three evaporation stages: (i) steady liquid phase, (ii) gel-shell phase, and (iii) solid-shell phase. Each regime exhibits distinct atomisation responses due to the evolving interfacial morphology. In the gel-shell regime, deformation is resisted by the viscous shell, producing a bag-on-sheet mode, followed by puncture, jetting, and eventual shell delamination. In the solid-shell regime, interactions intensify, resulting in brittle fragmentation and catastrophic fracture. The findings reveal how evaporation-driven interfacial transitions fundamentally alter breakup mechanisms under transient shock loading. By linking nanoparticle transport, shell formation, and flow droplet interaction across multiple timescales, this work establishes new physical insights into the atomisation of complex, multicomponent, multiphase, and transiently evolving droplets under extreme aerodynamic 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 investigates the dynamics of laminar premixed methane-air flames in meso-scale channels. A cylindrical quartz tube, functioning as an optically accessible meso-scale combustor, is externally heated by a primary heater to facilitate the auto-ignition of the reactant mixture flowing through the tube. Experiments were conducted over a wide range of Reynolds numbers and equivalence ratios . 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, PFs were observed at the equivalence ratio of. Unlike the FREI regime, where the flame extinguishes after a characteristic travel distance, PFs 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. The study further investigates how variations in the wall temperature profile affect the dynamics of FREI and PF regimes. To achieve this, a secondary heater is introduced at varying distances from the primary heater, effectively imposing distinct bimodal wall heating profiles over the combustor tube. The observations and trends from the study were justified using simplified theoretical arguments based on the estimate of the mean flow temperature of the reactant mixture and a flame propagation model that accounts for wall heat losses. The novel findings from this work provide valuable insights that can significantly impact the design and development of advanced micro/meso-scale combustion systems.

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 the premixed jet flame stabilized over a thin fuel-air jet nozzle. The study is performed over a wide range of parametric space, varying the Reynolds number (Re) and normalized equivalence ratio (Φ) of the premixed jet and the strength of the generated blast wave fronts (characterized by M _s,r). The blast wave imposes a characteristic flow field, marked by a sharp discontinuity at the blast front, followed by an exponential decay. After this decay phase, a subsonic bulk flow, referred to as the induced flow, follows the blast front. 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. Contingent on the operating conditions (Re, Φ, and M _s,r), an extinction or a reattachment event follows the observed flame lift-off. Additionally, the flame tip is found to exhibit stretching/distortion events, causing the tip to pinch off in a characteristic operating space of Re, M _s,r and Φ. The reattachment and extinction regimes were thus classified into sub-regimes based on the response dynamics of the flame base and the flame tip. A simplified 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 tip pinch-off was observed when shear layer vortices reached critical circulation limits and shed at length scales lower than the flame height owing to the interaction with the induced flow. Novelty and significance statement This work is the first of its kind to examine the response of premixed jet flames to blast waves that are incident along the direction of the jet axis. The study identifies qualitative and quantitative changes induced in the premixed jet flame following the interaction process and proposes a regime map classifying the flame response behaviour across the parametric space of premixed jet Reynolds number, equivalence ratio, and strength of the incident blast wave. The axial blast-flame interaction setting explored in the current work can provide valuable insights into comprehending and improving practical high-speed propulsion systems such as scram jets and RDEs, wherein flame interactions with non-linear pressure waves, such as decaying shocks and blast waves, are common.

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<M _s<1.6). The interaction of the shock flow occurs in two stages: (1) interaction of the temporally decaying velocity (v _s) imposed by the blast wave and (2) interaction with the induced flow (v _ind). The flame base lifts off due to the imposed flow and the advection of flame base towards flame tip results in flame extinction for M _s>1.06. The timescale of flame extinction is faster (interaction with 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 v _s (∼O(10 ⁻¹)ms). The 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 from pure deformation, Rayleigh–Taylor piercing bag breakup, and shear-induced stripping. No significant effect of nanoparticle (NP) addition has been found on the flame dynamics due to the faster timescales. However, minimal effects of NP addition are observed during droplet breakup due to fluid property variation.

This study comprehensively investigates the response of a combusting droplet during its interaction with a high-speed transient flow imposed by a coaxially propagating blast wave. The blast wave is generated using a specially designed miniature shock generator that produces blast waves using the wire-explosion technique, facilitating a wide range of Mach numbers (1.03 < M _s < 1.8). The experiments are performed in two configurations: open field and focused blast wave. The charging voltage and the configuration determine the Mach number (M _s) and flow characteristics. The flame is found to exhibit two major response patterns: partial extinction followed by reignition and full extinction. Increasing the Mach number (M _s > 1.1) makes the droplet flame more vulnerable to extinction. Additionally, the flame exhibits stretching and shedding, followed by reignition at lower Mach numbers (M _s < 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: (i) interaction with the blast wave and the decaying velocity profile associated with it, and (ii) interaction with the induced flow behind the blast wave as a result of the entrainment (delayed response). Alongside the flame’s response, the droplet also interacts with the flow imposed by the blast wave, exhibiting different response modes including pure deformation, Rayleigh–Taylor piercing bag breakup and shear-induced stripping.

The present study investigates the flame dynamics of a contactless burning fuel droplet under free fall subjected to a co-flow. The dynamic external relative flow established due to co-flow and droplet acceleration results in a series of droplet flame transitions. Different flame structures were observed, including a wake flame, reversed wake flame and enveloped flame. Following ignition, the droplet is allowed to fall through the central tube of a co-flow arrangement, and, at its exit, the droplet flame encounters the co-flow. The wake flame, which was established based on the droplet's instantaneous velocity of descent, encounters the abrupt relative velocity jump due to the co-flow. This causes the droplet flame to go through various transitions as it approaches equilibrium with the surrounding flow. Once it equilibrates, the droplet flame evolves in response to the instantaneous relative flow velocity. The droplet flame evolves by altering both its shape and the stabilization mechanism. Two stabilization mechanisms were identified for the droplet wake flame: edge-flame stabilization and bluff-body stabilization. The stabilization mechanism for different flame structures and the transition events have been theoretically analysed, and the relation between flame shape evolution and flow velocity has been determined based on the flow-field characteristics at the corresponding Re (Reynolds number) range. Furthermore, these correlations are employed in a mathematical formulation based on the spring–mass analogy, which predicts the droplet flame evolution after encountering the co-flow, including all the transition events.

The advent of the COVID-19 pandemic has necessitated the use of face masks, making them an integral part of the daily routine. Face masks occlude the infectious droplets during any respiratory event contributing to source control. In the current study, spray impingement experiments were conducted on porous surfaces like masks having a different porosity, pore size, and thickness. The spray mimics actual cough or a mild sneeze with respect to the droplet size distribution (20-500 μ m) and velocity scale (0-14 m/s), which makes the experimental findings physiologically realistic. The penetration dynamics through the mask showed that droplets of all sizes beyond a critical velocity penetrate through the mask fabric and atomize into daughter droplets in the aerosolization range, leading to harmful effects due to the extended airborne lifetime of aerosols. By incorporating spray characteristics along with surface tension and viscous dissipation of the fluid passing through the mask, multi-step penetration criteria have been formulated. The daughter droplet size and velocity distribution after atomizing through multi-layered masks and its effects have been discussed. Moreover, the virus-emulating particle-laden surrogate respiratory droplets are used in impingement experiments to study the filtration and entrapment of virus-like nanoparticles in the mask. Furthermore, the efficacy of the mask from the perspective of a susceptible person has been investigated.

The interaction of incident vortex with buoyant diffusion flame of a pendant fuel droplet (n-dodecane) is investigated in the present study. The flame stretching/shedding events occur depending on vortex strengths, which are characterized using vortex Reynolds number (Rev). Different flame response regimes have been identified for different vortex strengths. The velocity scale associated with the vortices is higher when compared to natural convection velocity leading to forward extinction, followed by flame lift-off. The flame lift-off increases with Rev, and partial extinction occurs at very high Rev. In all the cases, the flame oscillates/sheds during the vortex interaction, which causes a momentary increase in the flame heat release rate. As the vortex moves past the flame, the oscillations dampen gradually, and the flame regains the natural buoyant behavior. The high-velocity scales imposed during vortex interaction make the flow momentum-dominant, resulting in a low Richardson number (Ri). Since, the flickering frequency scaling valid for Ri>>1 is not obeyed, a new scaling has been proposed to incorporate the vortex momentum effect by using vortex circulation instead of gravity. Only a portion of the incident vortex circulation contributes to the shedding as some of it is lost to forward extinction and lift-off events. Thus, the reduced vortex velocity scale is used to estimate the critical circulation. The flame shedding height calculated using new scaling in the critical circulation equation agrees with experimental observations.

We report the mechanism of acoustic-flame coupling for buoyant diffusion flame in droplets. The flame under acoustic excitation exhibits differential varicose and sinuous modes including partial extinction and pinch offs at certain shedding heights. Contrary to traditional buoyant flames, the diameter (d) regression of a droplet results in self-tuning of the instability modes with progression in burning times. In essence, we show that a droplet exhibits multi frequency response with advent of new oscillations modes at different points of the burning lifetime. The acoustic perturbations modulate the flame surface or heat release only at lower frequency bands by feeding energy into the natural vortical stability modes of a buoyant plume. Interestingly, we unearth that a flame is naturally unstable at all frequencies lower than g/d. Acoustic pumping at any such frequency (<g/d) corresponds to a unique convective length scale h such that f _flame =g/h. We therefore establish that the flame exhibits multiple convective length scales throughout the burning cycle. We subsequently established that the shedding length scale of the droplet flame shortens by more than 25% with increase in acoustic pressure. We theoretically proved that using critical circulation argument, one can uniquely determine the shedding length scale at any acoustic loading. However, it is interesting to note that while the acoustic pressure amplitude is crucial for determining the convective length at which a flame rolls up, the frequency of excitation has no role to play. In other words, the velocity field introduced by the acoustic pressure is responsible for the shedding of the flame. The acoustic perturbations also lead to bulk oscillation of the flame surface near the forward stagnation zone. This is attributed to the acoustic induced velocity that leads to transient shifting of the stoichiometric surface.

Ever since the emergence of the ongoing COVID-19 pandemic, the usage of makeshift facemasks is generally advised by policymakers as a possible substitute for commercially available surgical or N95 face masks. Although such endorsements could be economical and easily accessible in various low per-capita countries, the experimental evidence on the effectiveness of such recommendations is still lacking. In this regard, we carried out a detailed experimental investigation to study the fate of a large-sized surrogate cough droplet impingement at different velocities (corresponding to mild to severe coughs) on various locally procured cloth fabrics. Observation shows that larger ejected droplets (droplets that would normally settle as fomites in general) during a coughing event have enough momentum to penetrate single-layer cloth masks; the penetrated volume atomize into smaller daughter droplets that fall within aerosol range, thereby increasing infection potential. Theoretically, two essential criteria based on the balances of viscous dissipation-kinetic energy and surface tension-kinetic energy effects have been suggested for the droplet penetration through mask layers. Furthermore, a new parameter called η (the number density of pores for a fabric) is developed to characterize the volume penetration potential and subsequent daughter droplet size. Finally, the effect of mask washing frequency is analyzed. The outcomes from the current study can be used as a guide in selecting cloth fabrics for stitching multi-layered.

The combustion of a freely falling dodecane droplet has been studied experimentally in a droptower-like facility under ambient conditions. A unique ignition mechanism is used by igniting the droplet in pendant mode and releasing it to fall freely. This unveils a different type of droplet wake flame behavior which is explored in this study. Initially, the droplet flame transitions from fully enveloped to a wake flame configuration due to forward extinction. The wake flame has similar characteristics as a laminar lifted triple-flame. As the droplet accelerates, the flame stand-off increases continuously. The change in wake flame topology and intensity occurs in two different regimes corresponding to different droplet diameters. A new non-dimensional parameter has been derived to account for the local balance between buoyancy and momentum diffusion that alters the fuel availability. To explain the flame topological evolutions and transitions for different droplet diameters and Reynolds numbers, a theoretical formulation has been proposed based on the momentum diffusion from surrounding due to relative motion. Further, at very high Reynolds number, flame stretching or shedding regime occurs, causing momentary spikes in flame intensity due to the interaction with asymmetric vortex shedding induced by the Bernard–Von Karman instability. Interestingly, the flame shedding height follows the buoyant flickering scaling, even for the momentum-dominant droplet wake flame. Additionally, the circulation build-up mechanisms are shown to be responsible for the flame shedding events for droplet wake flame at high Reynolds number.

In the present study, flame dynamics of free-falling contactless burning fuel droplets (pure and nanofuel) is investigated. A droplet under free fall undergoes acceleration resulting in progressive increase of its velocity (increase in Reynolds Number, Re) along the path. The consequent dynamic external relative flow induced along the drop trajectory allows self-tuning of the flame through a series of transitions. At low Re, the droplet flame transitions from an initial fully enveloped buoyant diffusion flame to a premixed wake flame structure. It is shown that this transition occurs due to flame extinction at the droplet forward stagnation point when critical strain rate is exceeded due to the external flow. Subsequently the wake flame topology undergoes significant variations with further increase in Reynolds number. The flow conditions necessary for wake-flame stabilization are characterized. Using a round-jet analogy, a linear relationship between the flame height and Reynolds number is established which shows that topological transitions are predominantly hydrodynamic in nature. Droplet burning rate for all functional droplets exhibits minimal change due to reduced energy input from the wake flame. However, for nanofuel droplets, the global heat release is lowered due to the reduction in gasification rate brought about by the in-situ formation of porous structure by the agglomeration of nanoparticles. Furthermore, for particle laden droplets, absence of sufficient energy input from the flame suppresses internal heterogeneous boiling as found in most pendant droplet experiments.