arXiv subjects

Prabakaran Rajamanickam

Publications and source records attributed to Prabakaran Rajamanickam.

35 records · Page 2Linked to original sources

arXiv2024

Effect of a shear flow on the Darrieus-Landau instability in a Hele-Shaw channel

The Darrieus--Landau instability of premixed flames propagating in a narrow Hele-Shaw channel in the presence of a strong shear flow is investigated, incorporating also the Rayleigh--Taylor and diffusive-thermal instabilities. The flow induces shear-enhanced diffusion (Taylor dispersion) in the streamwise direction, but not in the spanwise direction and this leads to anisotropic diffusion and flame propagation. To understand how such anisotropies affect flame stability, two important cases are considered. These correspond to initial unperturbed conditions pertaining to a planar flame propagating in the streamwise or spanwise directions. The analysis is based on a two-dimensional model derived by asymptotic methods and solved numerically. These address the influence of the shear-flow strength (or Peclet number $Pe$), preferential diffusion (or Lewis number $Le$) and gravity (or Rayleigh number $Ra$). Dispersion curves characterizing the perturbation growth rate are computed for selected values of $Pe$, $Le$ and $Ra$. Taylor dispersion induced by strong shear flows is found to suppress the Darrieus--Landau instability and to weaken the flame wrinkling when the flame propagates in the streamwise direction. In contrast, when the flame propagates in the spanwise direction, the flame is stabilized in $Le<1$ mixtures, but destabilized in $Le>1$ mixtures. In the latter case, Taylor dispersion coupled with gas expansion facilitates flame wrinkling in an unusual manner. Specifically, stagnation points and counter-rotating vortices are encountered in the flame close to the unburnt gas side. More generally, an original finding is the demonstration that vorticity can be produced by a curved flame in a Hele-Shaw channel even in the absence of gravity, whenever $Pe \neq 0$, and that the vorticity remains confined to the flame preheat and reaction zones.

physics.flu-dyn↗

arXiv2024

Tricritical point as a crossover between type-I$_s$ and type-II$_s$ bifurcations

A tricritical point as a crossover between (stationary finite-wavelength) type-I$_s$ and (stationary longwave) type-II$_s$ bifurcations is identified in the study of diffusive-thermal (Turing) instability of flames propagating in a Hele-Shaw channel in a direction transverse to a shear flow. Three regimes exhibiting different scaling laws are identified in the neighbourhood of the tricritical point. For these three regimes, sixth-order partial differential equations are obtained governing the weakly nonlinear evolution of unstable solutions near the onset of instability. These sixth-order PDES may be regarded as the substitute for the classical fourth-order Kuramoto--Sivashinsky equation which is not applicable near the tricritical point.

nlin.PS↗

arXiv2024

Three-dimensional diffusive-thermal instability of flames propagating in a plane Poiseuille flow

The three-dimensional diffusive-thermal stability of a two-dimensional flame propagating in a Poiseuille flow is examined. The study explores the effect of three non-dimensional parameters, namely the Lewis number $Le$, the Damköhler number $Da$, and the flow Peclet number $Pe$. Wide ranges of the Lewis number and the flow amplitude are covered, as well as conditions corresponding to small-scale narrow ($Da \ll 1$) to large-scale wide ($Da \gg 1$) channels. The instability experienced by the flame appears as a combination of the traditional diffusive-thermal instability of planar flames and the recently identified instability corresponding to a transition from symmetric to asymmetric flame. The instability regions are identified in the $Le$-$Pe$ plane for selected values of $Da$ by computing the eigenvalues of a linear stability problem. These are complemented by two- and three-dimensional time-dependent simulations describing the full evolution of unstable flames into the non-linear regime. In narrow channels, flames are found to be always symmetric about the mid-plane of the channel. Additionally, in these situations, shear flow-induced Taylor dispersion enhances the cellular instability in $Le<1$ mixtures and suppresses the oscillatory instability in $Le>1$ mixtures. In large-scale channels, however, both the cellular and the oscillatory instabilities are expected to persist. Here, the flame has a stronger propensity to become asymmetric when the mean flow opposes its propagation and when $Le<1$; if the mean flow facilitates the flame propagation, then the flame is likely to remain symmetric about the channel mid-plane. For $Le>1$, both symmetric and asymmetric flames are encountered and are accompanied by temporal oscillations.

physics.flu-dyn↗

arXiv2024

Near-limit H$_2$-O$_2$-N$_2$ combustion in nonpremixed counterflow mixing layers

Numerical computations employing detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition.

physics.flu-dyn↗

arXiv2024

Stability of diffusion flames under shear flow: Taylor dispersion and the formation of flame streets

Diffusion flame streets, observed in non-premixed micro-combustion devices, align parallel to a shear flow. They are observed to occur in mixtures with high Lewis number ($Le$) fuels, provided that the flow Reynolds number, or the Peclet number $Pe$, exceeds a critical value. The underlying mechanisms behind these observations have not yet been fully understood. In the present paper, we identify the coupling between diffusive-thermal instabilities and Taylor dispersion as a mechanism which is able to explain the experimental observations above. The explanation is largely based on the fact that Taylor dispersion enhances all diffusion processes in the flow direction, leading effectively to anisotropic diffusion with an effective (flow-dependent) Lewis number in the flow direction which is proportional to $1/Le$ for $Pe\gg 1$. Validation of the identified mechanism is demonstrated within a simple model by investigating the stability of a planar diffusion flame established parallel to a plane Poiseuille flow in a narrow channel. A linear stability analysis, leading to an eigenvalue problem solved numerically, shows that cellular (or finite wavelength) instabilities emerge for high Lewis number fuels when the Peclet number exceeds a critical value. Furthermore, for Peclet numbers below this critical value, longwave instabilities with or without time oscillations are obtained. Stability regime diagrams are presented for illustrative cases in a $Le$-$Pe$ plane where various instability domains are identified. Finally, the linear analysis is supported and complemented by time dependent numerical simulations, describing the evolution of unstable diffusion flames. The simulations demonstrate the existence of stable cellular structures and show that the longwave instabilities are conducive to flame extinction.

physics.flu-dyn↗

arXiv2024

Effective Lewis number and burning speed for flames propagating in small-scale spatio-temporal periodic flows

Propagation of premixed flames having thick reaction zones in rapidly-varying, small-scale, zero-mean, spatio-temporal periodic flows is considered. Techniques of large activation energy asymptotics and homogenization theory are used to determine the effective Lewis number $Le_{\mathrm{eff}}$ and the effective burning speed ratio $S_T/S_L$, which are influenced by the flow through flow-enhanced diffusion. As the flow Peclet number $Pe$ becomes large, the effective fuel and thermal diffusivities behave respectively like $(PeLe)^σ$ and $Pe^σ$, where $Le$ is the Lewis number and $σ\leq 2$ is a constant that depends on the flow and the flame-propagation direction. The maximal value $σ=2$ is achieved for steady, unidirectional, spatially periodic shear flows, while for steady 2D square vortices, we have $σ=1/2$. In general, the constant $σ$ is determined by solving a linear partial differential equation. The scaling laws for the diffusion coefficients lead to corresponding scaling laws for the effective Lewis number and the effective burning speed ratio of the form $Le_{\mathrm{eff}}\simeq Le^{1-σ}$ and $S_T/S_L\sim (Pe/Le)^{σ/2}$. Effects of thermal expansion and volumetric heat loss on the flame are also briefly discussed. In particular, it is shown that the quenching limit is enlarged by a factor $1/Le^σ$ for $Le<1$ and diminished by the same factor for $Le>1$, due to the flow-enhanced diffusion. Potential implications of the results for turbulent combustion are discussed. A special emphasis is placed on the dependence of the flame on $Le$ in high-intensity, small-scale flows. This dependence is intimately linked to flow-induced diffusion, rather than to the traditional molecular diffusion coupled with curvature effects. The effective Lewis number may also explain why turbulence aids ignition in $Le>1$ mixtures but hinders it in $Le<1$ mixtures.

physics.flu-dyn↗

arXiv2024

Non-Boussinesq stability analysis of natural-convection gaseous flow on inclined hot plates

The buoyancy-driven boundary-layer flow that develops over a semi-infinite inclined hot plate is known to become unstable at a finite distance from the leading edge, characterized by a critical value of the Grashof number Gr based on the local boundary-layer thickness. The nature of the resulting instability depends on the inclination angle $ϕ$, measured from the vertical direction. For values of $ϕ$ below a critical value $ϕ_c$ the instability is characterized by the appearance of spanwise traveling waves, whereas for $ϕ>ϕ_c$ the bifurcated flow displays Görtler-like streamwise vortices. The Boussinesq approximation, employed in previous linear stability analyses, ceases to be valid for gaseous flow when the wall-to-ambient temperature ratio $Θ_w$ is not close to unity. The corresponding non-Boussinesq analysis is presented here, accounting also for the variation with temperature of the different transport properties. A temporal stability analysis including nonparallel effects of the base flow is used to determine curves of neutral stability, which are then employed to delineate the dependences of the critical Grashof number and of its associated wave length on the inclination angle $ϕ$ and on the temperature ratio $Θ_w$ for the two instability modes, giving quantitative information of interest for configurations with $Θ_w-1\sim 1$. The analysis provides in particular the predicted dependence of the crossover inclination angle $ϕ_c$ on $Θ_w$, indicating that for gaseous flow with $Θ_w-1\sim 1$ spanwise traveling waves are predominant over a range of inclination angles $0 \le ϕ\le ϕ_c$ that is significantly wider than that predicted in the Boussinesq approximation.

physics.flu-dyn↗

arXiv2024

Influences of stoichiometry on steadily propagating triple flames in counterflows

Most studies of triple flames in counterflowing streams of fuel and oxidizer have been focused on the symmetric problem in which the stoichiometric mixture fraction is $1/2$. There then exist lean and rich premixed flames of roughly equal strengths, with a diffusion flame trailing behind from the stoichiometric point at which they meet. In the majority of realistic situations, however, the stoichiometric mixture fraction departs appreciably from unity, typically being quite small. With the objective of clarifying the influences of stoichiometry, attention is focused on one of the simplest possible models, addressed here mainly by numerical integration. When the stoichiometric mixture fraction departs appreciably from $1/2$, one of the premixed wings is found to be dominant to such an extent that the diffusion flame and the other premixed flame are very weak by comparison. These curved, partially premixed flames are expected to be relevant in realistic configurations. In addition, a simple kinematic balance is shown to predict the shape of the front and the propagation velocity reasonably well in the limit of low stretch and low curvature.

physics.flu-dyn↗

arXiv2024

A model for the constant-density boundary layer surrounding fire whirls

This paper investigates the steady axisymmetric structure of the cold boundary-layer flow surrounding fire whirls developing over localized fuel sources lying on a horizontal surface. The inviscid swirling motion found outside the boundary layer, driven by the entrainment of the buoyant turbulent plume of hot combustion products that develops above the fire, is described by an irrotational solution, obtained by combining Taylor's self-similar solution for the motion in the axial plane with the azimuthal motion induced by a line vortex of circulation $2 πΓ$. The development of the boundary layer from a prescribed radial location is determined by numerical integration for different swirl levels, measured by the value of the radial-to-azimuthal velocity ratio $σ$ at the initial radial location. As in the case $σ=0$, treated in the seminal boundary-layer analysis of Burggraf et al. (Phys. Fluids, 1971), the pressure gradient associated with the centripetal acceleration of the inviscid flow is seen to generate a pronounced radial inflow. Specific attention is given to the terminal shape of the boundary-layer velocity near the axis, which displays a three-layered structure that is described by matched asymptotic expansions. The resulting composite expansion, dependent on the level of ambient swirl through the parameter $σ$, is employed as boundary condition to describe the deflection of the boundary-layer flow near the axis to form a vertical swirl jet. Numerical solutions of the resulting non-slender collision region for different values of $σ$ are presented both for inviscid flow and for viscous flow with moderately large values of the controlling Reynolds number $Γ/ν$. The velocity description provided is useful in mathematical formulations of localized fire-whirl flows, providing consistent boundary conditions accounting for the ambient swirl level.

physics.flu-dyn↗

arXiv2024

Dispersion of solute in straining flows and boundary layers

Solute dispersion due to an instantaneously released source in steady, laminar, axisymmetric flows with an axial inflow and radial outflow is investigated analytically. Attention is given to large-time characteristics of dispersion, where the concentration reduces in proportion to $e^{-2λτ}$, where $λ$ is an eigenvalue that depends on the axial inflow and $τ$ is the time measured in units of axial-diffusion times. Prospects of some other flows are also considered.

physics.flu-dyn↗

arXiv2024

On the two-reactant one-step activation-energy asymptotics for steady, adiabatic, planar flames with Lewis numbers of unity

Aspects of predictions of activation-energy asymptotics concerning the dependence of the burning velocity on the equivalence ratio are examined here through both asymptotic analyses and numerical computation. In typical hydrocarbon-air flames, the burning velocity achieves its maximum value for fuel-rich mixture, the cause being generally attributed to the effects of detailed chemical kinetics and unequal diffusivities of the reactants. The present results demonstrate the possibility of this attribute of the burning velocity occurring even when these two effects are absent. This is accomplished by parametrically studying the burning-velocity formula valid for all equivalence ratios under the conditions specified in the title of this article, with special attention paid to implications for hydrocarbon-air flames.

physics.flu-dyn↗

arXiv2024

Effects of thermal expansion on Taylor dispersion-controlled diffusion flames

A theoretical analysis is developed to investigate the effects of gas expansion due to heat release on unsteady diffusion flames evolving in a pipe flow in which the mixing of reactants is controlled by Taylor's dispersion processes thereby extending a previously developed theory based on the thermo-diffusive model. It is first shown that at times larger than radial diffusion times, the pressure gradient induced by the gas expansion is, in the first approximation, small in comparison with the prevailing pressure gradient driving the flow, indicating that corrections to the background velocity profile are small. The corrections to the velocity components along with the leading-order mixing variables such as the concentrations, temperature and density are solved for a Burke-Schumann flame. Due to the dependence of the effective Taylor diffusion coefficients on the gas density, quantitative and sometimes qualitative departures in predictions from the thermo-diffusive model are observed.

physics.flu-dyn↗

arXiv2024

A thick reaction zone model for premixed flames in two-dimensional channels

Direct interactions between the flow field and the chemical reaction in premixed flames occur when the reaction zone thickness is comparable to, or greater than flow length scales. To study such interactions, a laminar model is considered that has direct bearings to steadily propagating deflagrations in a Hele-Shaw channel with a background plane Poiseuille flow. The study employs asymptotic analyses, pertaining to large activation energy and lubrication theories and considers a distinguished limit where the channel width is comparable to the reaction zone thickness, with account being taken of thermal-expansion and heat-loss effects. The reaction zone structure and burning rates depend on three parameters, namely, the Peclet number, $\mathcal{P}$, the Lewis number, $Le$ and the ratio of channel half-width to reaction zone thickness, $λ_*$. When the parameter $λ_*$ is small, transport processes are controlled by Taylor's dispersion mechanism and an explicit formula for the effective burning speed $S_T$ is obtained. The formula indicates that $S_T/S_L \propto 1/Le$ for $\mathcal{P}\gg 1$, which interestingly coincides with a recent experimental prediction of the flame speed in a highly turbulent jet flame. The results suggest that the role played by differential diffusion effects is significant both in laminar and turbulent cases. The reason for the peculiar $1/Le$ dependence can be attributed, in our laminar model, to Taylor dispersion. Presumably, this dependence may be attributed to a similar but more general mechanism in the turbulent case, rather than to diffusive-thermal curvature effects. The latter effects play however an important role in determining the flame speed when $λ_*$ is large. The magnitude of heat losses at extinction, is multiplied by a factor $1/Le^2$ in comparison with those corresponding to the no-flow case in narrow channels.

physics.flu-dyn↗

arXiv2024

Diffusive-thermal instabilities of a planar premixed flame aligned with a shear flow

The stability of a thick planar premixed flame, propagating steadily in a direction transverse to that of unidirectional shear flow, is studied. A linear stability analysis is carried out in the asymptotic limit of infinitely large activation energy, yielding a dispersion relation. The relation characterises the coupling between Taylor dispersion (or shear-enhanced diffusion) and the flame thermo-diffusive instabilities, in terms of two main parameters, namely, the reactant Lewis number $Le$ and the flow Peclet number $Pe$. The implications of the dispersion relation are discussed and various flame instabilities are identified and classified in the $Le$-$Pe$ plane. An important original finding is the demonstration that for values of the Peclet number exceeding a critical value, the classical cellular instability, commonly found for $Le<1$, exists now for $Le>1$ but is absent when $Le<1$. In fact, the cellular instability identified for $Le>1$ is shown to occur either through a finite-wavelength stationary bifurcation (also known as type-I$_s$) or through a longwave stationary bifurcation (also known as type-II$_s$). The latter type-II$_s$ bifurcation leads in the weakly nonlinear regime to a Kuramoto--Sivashinsky equation, which is determined. As for the oscillatory instability, usually encountered in the absence of Taylor dispersion in $Le>1$ mixtures, it is found to be absent if the Peclet number is large enough. The stability findings, which follow from the dispersion relation derived analytically, are complemented and examined numerically for a finite value of the Zeldovich number. The numerical study involves both computations of the eigenvalues of a linear stability boundary-value problem and numerical simulations of the time-dependent governing partial differential equations. The computations are found to be in good qualitative agreement with the analytical predictions.

physics.flu-dyn↗

arXiv2024

Steady axisymmetric vortices in radial stagnation flows

A class of axisymmetric vortex solutions superposed upon radial stagnation flows is described. The new vortex solutions generalize the classical Burgers' vortex and Sullivan's vortex solutions in the presence of a volumetric line source at the symmetry axis, the former approaching the Burgers' vortex sheet when the source strength becomes very large. The stability of the generalized Burgers' vortex is studied. In a different manner from the classical solution, the generalized Burgers' vortices are found to be unstable for two-dimensional disturbances when the vortex Reynolds number is increased above a critical value, for a fixed strength of the volumetric source.

physics.flu-dyn↗

arXiv2024

A note on viscous flow induced by half-line sources bounded by conical surfaces

In this paper axisymmetric solutions of the Navier-Stokes equations governing the flow induced by a half-line source when the fluid domain is bounded by a conical wall are discussed. Two types of boundary conditions are identified; one in which the radial velocity along the axis is prescribed, and the other in which the radial velocity along the axis is obtained as an eigenvalue of the problem. The existence of these solutions are limited to a range of Reynolds numbers and the transition from one case to the other are discussed in detail.

physics.flu-dyn↗

arXiv2024

Thermal explosion characteristics of a gelled hypergolic droplet

When a sphere of one reactant is placed in the medium of another reactant with which it is hypergolic, a chemical reaction (modeled here as a zeroth-order one-step irreversible Arrhenius reaction) occurs at the common interface of the two reactants, and the heat generated at the interface then is transmitted away from it by thermal conduction. Depending on the nature of the problem, the system may approach an explosive mode, or it may settle into a steady-state mode. The critical condition defining the transition between these two states is determined analytically. In particular, explicit formulas for the ignition delay time for the explosive mode are provided for two limits, one in which an appropriately defined Damköhler number is large and the other in which it is closer to the critical conditions. This is accomplished here by deriving and solving an integral equation for the time evolution of the interface temperature, employing activation-energy asymptotics.

physics.flu-dyn↗