Oscillating Strain Rates Research Articles

Effects of Ammonia Addition on the Sooting Characteristics of Ethylene Counterflow Diffusion Flames with Oscillating Strain Rates

Effects of Ammonia Addition on the Sooting Characteristics of Ethylene Counterflow Diffusion Flames with Oscillating Strain Rates

Effects of oxygen partial premixing on soot formation in ethylene counterflow flames with oscillating strain rates

The sooting characteristics of counterflow partially-premixed flames subject to monochromatic oscillation of strain rates were investigated. Strain rate oscillations with well controlled frequencies and amplitudes were imposed on a series of ethylene counterflow partially-premixed flames. Flow fields and soot volume fractions were measured non-intrusively in a spatially and temporally resolved manner. Numerical simulations were also performed to support the experimental observations. The results showed that oxygen partial premixing in the ethylene fuel stream notably increased soot production due to the enhanced temperature and oxidative pyrolysis that led to increased formation of benzene precursors. More importantly, despite these enhancing effects on soot loading, we demonstrated for the first time that partial premixing does not alter the sensitivity of soot formation to strain rate in steady counterflow flames. In addition, when subject to oscillating strain rates, the unsteady responses of partially-premixed flames were still seen to be independent of partial premixing. This indicates that our previous conclusion—the sensitivity of soot formation to strain rate in steady counterflow diffusion non-premixed flames determines its response under unsteady condition—still holds for partially-premixed flames. The results could be useful for the development of advanced flamelet model to quantitatively account for unsteady effects on soot formation in practical turbulent flames that frequently encounters partial premixing.

Sensitivity of soot formation to strain rate in steady counterflow flames determines its response under unsteady conditions

This work reports an original experimental investigation on the sooting characteristics of counterflow diffusion flames (CDFs) subject to sinusoidal oscillations of strain rates. The study was motivated by the necessity to consider transient responses of soot formation when using the laminar flamelet model to simulate soot formation in turbulent flames. Acoustic forcing was applied through loudspeakers installed at the inlet end of both the oxidizer and fuel nozzles of the counterflow burner to generate strain rate oscillations at the desired frequency and amplitudes. Particle image velocity and planar laser induced incandescence were used respectively to measure flow field and the soot volume fraction in a time-resolved manner. Counterflow flames of C2H4 and C3H8 fuels were tested with oxidizer streams of various oxygen mole fractions (XO). The experimental data revealed notable differences in unsteady soot responses between CDFs of C2H4 and C3H8 at the same XO. However, after matching the soot loadings of the two CDFs to have the same sensitivity to strain rate at steady states, their responses to unsteady strain oscillation became almost identical. We experimentally demonstrated that the unsteady response of soot formation to oscillating strain rates, as characterized by phase lag and amplitude attenuations, are strongly correlated with the sensitivity of the soot loading to strain rate under steady conditions. The results may have important implications for the development of transient flamelet models that incorporate unsteady effects on soot formation.

Extinction of non-equidiffusive premixed flames with oscillating strain rates

Extinction of premixed flames under non-uniform, unsteady strain is a phenomenon commonly observed in turbulent combustors. To assess the role of inequity in thermal and mass diffusion, represented by a global Lewis number (Le) - defined as the ratio of the mixture’s thermal diffusivity to the mass diffusivity of the deficient species, on such extinctions, we present a study of counterflow twin-flames with various Le under oscillating strain rates. Experimental results confirm that for low mean strain rates, the amplitude of strain rate oscillation required for extinction is so large that the flow temporarily alters its direction, leading to distortion of the counterflow flow-field, destabilization, and eventual extinction of the flame irrespective of Lewis number. However, for relatively large mean strain rates, extinction results from flame-response to the peak instantaneous strain rate. For Le>1 mixtures, the maximum strain rate that the flames can sustain is greater than the steady state extinction values, while Le≤1 flames extinguish at a maximum strain rate, approximately equal to the steady state strain condition. This distinctively disparate behaviors of extinctions depending on the Lewis number are analyzed and delineated using numerical simulations of unsteady flames.

REDIM reduced chemistry for the simulation of counterflow diffusion flames with oscillating strain rates

The dynamic behaviour and structures of laminar counterflow diffusion methane flames under oscillatory strain rates are investigated based on both detailed and reduced chemistry. It is known both from experiment and numerical studies that flame can deviate from its quasi-steady manner for high frequencies, which makes the well-known steady laminar flamelet model questionable. In this work, the Reaction-diffusion manifolds (REDIM) concept for simplified chemistry is applied to study the laminar counterflow diffusion flames with oscillating strain rates. In the construction of REDIM, one needs little knowledge of the studied system, and the REDIM reduced chemistry can cover both stable and unstable (extinction) regimes of the studied system. It is shown that the REDIM can capture both the steady and unsteady behaviours of the flame structures even for high frequencies, where the standard laminar steady flamelet approach is not appropriate.

An equivalent dissipation rate model for capturing history effects in non-premixed flames

The effects of strain rate history on turbulent flames have been studied in the past decades with 1D counter flow diffusion flame (CFDF) configurations subjected to oscillating strain rates. In this work, these unsteady effects are studied for complex hydrocarbon fuel surrogates at engine relevant conditions with unsteady strain rates experienced by flamelets in a typical spray flame. Tabulated combustion models are based on a steady scalar dissipation rate (SDR) assumption and hence cannot capture these unsteady strain effects; even though they can capture the unsteady chemistry. In this work, 1D CFDF with varying strain rates are simulated using two different modeling approaches: steady SDR assumption and unsteady flamelet model. Comparative studies show that the history effects due to unsteady SDR are directly proportional to the temporal gradient of the SDR. A new equivalent SDR model based on the history of a flamelet is proposed. An averaging procedure is constructed such that the most recent histories are given higher weights. This equivalent SDR is then used with the steady SDR assumption in 1D flamelets. Results show a good agreement between tabulated flamelet solution and the unsteady flamelet results. This equivalent SDR concept is further implemented and compared against 3D spray flames (Engine Combustion Network Spray A). Tabulated models based on steady SDR assumption under-predict autoignition and flame lift-off when compared with an unsteady Representative Interactive Flamelet (RIF) model. However, equivalent SDR model coupled with the tabulated model predicted autoignition and flame lift-off very close to those reported by the RIF model. This model is further validated for a range of injection pressures for Spray A flames. The new modeling framework now enables tabulated models with significantly lower computational cost to account for unsteady history effects.

Unsteady dynamics of PAH and soot particles in laminar counterflow diffusion flames

Due to their low chemical time scales, the production of soot particles in turbulent diffusion flames is highly impacted by large range of local strain rate fluctuations.In order to understand the response of soot production to strain rate fluctuations, unsteady laminar counterflow diffusion flames with an imposed oscillating strain rate are investigated both analytically and numerically. First an analytical linearized model is developed to predict the unsteady response of a flame quantity of interest from information on laminar steady flames. Three critical parameters governing flame response are identified: the Stokes number which compares the characteristic time associated to the mean imposed strain rate to the oscillation frequency, the Damköhler number associated to the quantity of interest, and a third one characterizing the response of this quantity to an imposed steady strain rate. This model is then applied to soot predictions. Parallely, the response of soot production in propane-air counterflow diffusion flames to unsteady strain harmonic oscillations is studied numerically using a detailed sectional soot model. A wide range of frequencies and amplitudes are considered. A specific trend is highlighted for soot precursors and particles production according to their respective chemical time scales: the bigger the PAH or soot particle, the higher its chemical time scale, resulting in a more damped and phase-lagged response. The particle size distribution evolves accordingly during the considered oscillations, so that the quasi-steady state behavior is not verified for high frequencies. The numerical results are compared to those obtained by the analytical approach and a very good agreement is obtained at low amplitudes. Non-linear response of soot precursors and soot particles production to strain oscillations are finally discussed in case of high oscillation amplitudes and the limits of the proposed analytical model are identified.

Effect of strain rate cycling on microstructures and crystallographic preferred orientation during high-temperature creep

Strain rate histories and strain magnitude are two crucial factors governing the evolution of dynamic recrystallized grain size and crystallographic preferred orientation (CPO) in rocks and ice masses. To understand the effect of cyclic variations in strain rate or non-steady-state deformation, we conducted two-dimensional, coaxial plane strain experiments with time-lapse observations from a fabric analyzer. There is a continuous reequilibration of microstructure and CPO development associated with constant and oscillating strain rate cycles. These can be correlated with c -axis small circle distributions, diagnostic of dynamic recrystallization involving new grain nucleation and grain boundary migration (GBM) as observed in the high-temperature deformation of ice and quartz. Inhomogeneous stress distribution can lead to grain size reduction for relatively slower strain rates and GBM for relatively faster rates within a long-interval cycling event, a behavior that contradicts the classic view for dynamic recrystallization processes. Where there is a rapid short-term cycling of strain rate, GBM is hampered and nucleation dominates, accompanied by a marked reduction of grain size and no new CPO development. During such short-term cycling, GBM and crystallographic change is impeded not by impurities, but by an inability of newly nucleated grains to grow.

Effect of flame thickness variation on the mass burning rate of premixed counterflow flames with an oscillating strain rate

The mass-based stretch rate is used to study the response of premixed axisymmetric counterflow flames subject to an oscillating strain rate. Integral analysis is used to estimate the mass burning rate of the oscillating counterflow flames. From this study it can be concluded that the flame responds in a nonlinear manner. With an increase of the applied strain frequencies, it is found that unsteady stretch effects arising due to flame thickness variations become significant and the mass-based stretch rate is able to capture these nonlinear effects. The inclusion of these unsteady stretch effects in the mass-based stretch helps the integral analysis to predict the mass-burning rate of oscillating flames more accurately.

RELATIVE POLYCYCLIC AROMATIC HYDROCARBON CONCENTRATIONS IN UNSTEADY COUNTERFLOW DIFFUSION FLAMES

ABSTRACT Qualitative measurements of polycyclic aromatic hydrocarbon (PAH) concentrations made in counterflow diffusion flames subjected to both steady and oscillating strain rates are presented. Planar laser-induced fluorescences was used to make spatially and temporally resolved measurements of the relative concentrations of PAH. The measurements for each PAH size class were made as a function of strain rate, forcing frequency, and fuel type. The peak laser-induced fluorescence spectra from PAHs is known to be a function of the number of benzene rings that make up the particular PAH. It was found that, with increasing strain rate, the intensity of PAH fluorescence decreases dramatically, though unequally, among the three size classes measured. With an unsteady strain rate, the PAH intensity oscillates in phase for frequencies below 50 Hz. When nondimensionalized relative concentration is plotted, it is shown that the smaller PAHs continue to respond to the instantaneous strain rate at a higher frequency than larger PAHs.

Resonant formation and control of 2D symmetric vortex waves.

It is shown that m-fold symmetric vortex waves in two dimensions ( V states) preserve their functional form in a weak straining flow having appropriate symmetry, but arbitrary time dependence. This phenomenon is used in driving the V states into a highly nonlinear excitation by subjecting a circular vortex patch to rotation and strain with oscillating strain rate and varying the rotation angular velocity. The effect is due to autoresonance in the system as the excited vortex state boundary self-adjusts its aspect ratio to synchronize with the external flow.

Control of Kirchhoff vortices by a resonant strain

It is shown that placing a Kirchhoff vortex in a weak resonant strain having uniform, but oscillating strain rate with chirped oscillation frequency, allows adiabatic control of the axis ratio $\ensuremath{\rho}=a/b$ and rotation phase of the vortex. The phenomenon has a threshold on the amplitude of oscillations of the strain rate and is due to the persisting nonlinear phase locking (autoresonance) between the elliptic vortex and the adiabatically varying straining flow.

Weakly stretched premixed flames in oscillating flows

The response of a premixed flame in stagnation-point flow with an imposed oscillating strain rate has been examined. This configuration is of fundamental interest and has potential application to turbulent combustion modelling. Of interest are flames which stand well clear of the front stagnation point of a bluff body. Under these conditions the flame can be treated as a surface of density discontinuity. A detailed solution is constructed in the burned and unburned gas regions and includes the flame response to the imposed fluctuations as well as the resulting displacement of the incident flow. Our analysis accounts for the full coupling between the flame and the underlying flow field and, unlike most previous studies, is not restricted to small-amplitude oscillations.

96/05300 Response of counterflow premixed flames to oscillating strain rates

96/05300 Response of counterflow premixed flames to oscillating strain rates

Response of counterflow premixed flames to oscillating strain rates

We study the response of premixed counterflow flames to an imposed oscillating strain rate which has potential application to both turbulent combustion and acoustic instabilities. We exploit the limit of large activation energy to resolve the reaction zone structure, and conduct a linear perturbation analysis for small amplitude of oscillation. Reaction-sheet fluctuations and the net heat release rate are investigated for both the symmetric twin-flame configuration and a single flame residing in a counterflow system consisting of a fresh-mixture stream impinging on a cold inert stream. The Lewis number is found to play an important role in the flame response, especially for flames near extinction. For those flames that exhibit turning point behavior, near extinction the effect of finite-rate chemistry overtakes fluid-dynamic effects such that increasing strain rate can lead to a phase reversal of the heat release response with the imposed flow oscillations. Results of the present study suggest the possibility of a wide spectrum of unsteady flame characteristics depending on Lewis number. Our results also demonstrate that extinction can be delayed when the strain rate oscillates about the static extinction point. Thus the laminar flamelet regime of turbulent combustion may be broader than predicted by steady analyses.

Response of counterflow diffusion flames to oscillating strain rates

Response of counterflow diffusion flames to oscillating strain rates

Response of counterflow diffusion flames to oscillating strain rates

The response of counterflow flames to oscillating strain rates is analyzed by using large activation energy asymptotics, as a potential application to turbulent combustion and acoustic instability of rocket engines. The characteristic oscillation time of practical interest is found to be of the same order as the characteristic diffusion time of the flame, so that the flame structure consists of a quasi-steady reactive-diffusive layer embedded in the outer unsteady-diffusive-convective zone. A linear analysis is conducted by assuming that the amplitude of the strain rate oscillation is small relative to the mean strain rate. Results show that the flame response is controlled mainly by two effects: (a) the response of the convective mass flux into the reaction sheet, which is directly related to the flow-field variation applied at the boundary, and (b) the response of the reaction sheet to adjust the reduced residence time due to finite-rate chemistry. For flames near equilibrium, the former effect tends to be dominant, so that the response of the net heat release is in phase with the strain rate oscillation. For flames near extinction, however, the finite-rate chemistry effect overtakes the fluid-dynamic effect such that increasing strain rate leads to a reduction of the reactivity of the flame during the oscillatory cycle. As such, the net heat release response of the near-extinction flame becomes out of phase with the strain rate oscillation in the sense of the Rayleigh's criterion. Results of the present study suggest the possibility that the unsteady characteristics of the near-extinction diffusion flame can be significantly different from those in the Burke-Schumann limit.