C.J. Sung

Structural response of counterflow diffusion flames to strain rate variations

The structural response of counterflowing methane/oxygen/nitrogen diffusion flames to aerodynamic straining was experimentally and computationally investigated. The temperature and major species concentration profiles were experimentally determined as functions of the applied strain rate by using spontaneous Raman scattering. The experimental situations were further computationally simulated with detailed reaction mechanisms and transport properties. The computed results were found to be in close quantitative agreement with the experimental data. Results demonstrate that, in contrast to counterflow premixed flames, a strained, counterflow diffusion flame has less flexibility to freely adjust its location in response to strain rate variations such that its structure in the direction normal to the flame surface is quite sensitive to variations in the strain rate. Specifically, the counterflow diffusion flame becomes thinner with increasing strain rate a, with its thickness varying inversely with √a. This leads to increased amount of reactant leakage, progressive reduction in the flame temperature, and eventually extinction of the flame. Computational results further show that while the heat release rate of premixed flames is characterized by a single sharp maximum, for diffusion flames a secondary maximum described by a distinctively different reaction submechanism exists on the oxidizer side of the primary heat release zone. The nature and extent of the negative heat release rate on the fuel side of the flame, as well as its relation to the presence of a dent around the same location on the temperature profile as observed in previous experiments, were also discussed.

Thermophoretic Effects on Seeding Particles in LDV Measurements of Flames

Abstract The motion of LDV seeding particles under the influence of viscous and thermophoretic forces in the rapidly-accelerating, high-temperature-gradient flame environment was studied via the counterfiow premixed twin-flame configuration. Results demonstrate that thermophoretic force can induce significant lag between the fluid and,particle velocities in the active preheat zone of a flame, and suggest that caution should be exercised when interpreting LDV data obtained in this region. A thermophoretic velocity correction to the LDV-determined velocity, with known experimental or computational temperature profile, is proposed. Additional considerations of LDV diagnoslics and its determination of laminar flame speed are also presented.

On the structural sensitivity of purely strained planar premixed flames to strain rate variations

Abstract The effects of aerodynamic straining on the structure and response of adiabatic, unrestrained, equidiffusive, planar premixed flames were experimentally and computationally studied via the counterflow, twin-flame configuration formed by oppositely directed identical jets of nitrogen-diluted, near-stoichiometric methane/air mixtures. Experimentally, the velocity, temperature and major species concentration profiles were determined as functions of the applied strain rate by using LDV and spontaneous Raman scattering. Computationally, the experimental situation was simulated with detailed reaction mechanisms and transport properties. Both the experimental and computational results show that the temperature and species structure of the flame in the direction normal to the flame surface remains largely similar in response to variations in strain rate as long as the flame is sufficiently far away from the stagnation surface so that incomplete reaction is minimal. These results substantiate the concepts that the scalar structure of the flame, and thereby the flame thickness, are insensitive to strain rate variations for these purely strained flames, and that these flames cannot be extinguished by straining alone. The computed results are further shown to agree quantitatively with the experimental data, hence supporting the usefulness of the computational model for the simulation of strained flames. Implications of present findings on the concept of the local flow time, the extinction of strained flames, the modelling of turbulent flames through the concept of laminar flamelets, and flame stabilization and blowoff, are discussed.

Further studies on effects of thermophoresis on seeding particles in LDV measurements of strained flames

The axial velocity profiles for counterflow premixed and diffusion flames were experimentally measured by laser-doppler velocimetry (LDV) and computationally simulated with detailed reaction mechanism and transport properties. The LDV measurements were found to agree well with the computed values in the cooler, decelerating part of the flow upstream of the flame, but to significantly deviate from the calculated values in the rapidly-accelerating preheat region of the flame in which substantial thermal expansion occurs over a very short distance. An analysis of the motion of the LDV seeding particles under the influence of viscous drag and thermophoresis in these well-characterized counterflow flame environments demonstrates that such deviations are consequences of thermophoresis. Furthermore, since the thermophoretic force is in the direction opposite to that of the temperature gradient, and its influence on the motion of the particle depends upon the local flow velocity, a rich variety of LDV velocity profiles were observed for flames with different temperature profiles and distances to the stagnation surface. The stoichiometric mixture fraction was found to be a useful parameter to characterize the velocity profile variation. The study emphasizes the importance of accounting for the effects of thermophoresis in interpreting LDV as well as PIV (particle image velocimetry) data in flames, both laminar and turbulent. An approach to closely simulate experimental counterflow flames is also presented.

Extinction mechanisms of near-limit premixed flames and extended limits of flammability

The response of near-flammability-limit, weakly burning, counterflow premixed flames as a function of stretch rate has been studied computationally with detailed chemistry and transport properties. The limit mechanisms and extinction phenomena are found to be strongly influenced by the combined effects of flame stretch, mixture non-equidiffusion, and radiative loss. For subunity Lewis number flames such as those of lean methane/air, the combined effects of mixture non-equidiffusion and positive stretch elevate the combustion intensity such that steady burning persists beyond the fundamental flammability limit defined for the one-dimensional planar flame. Furthermore, the flame response to stretch rate variations exhibits a dual-extinction, turning-point behavior in that flame extinction occurs not only for sufficiently large stretch rates and minimal radiative heat loss, but also for sufficiently small stretch rates and relatively substantial heat loss. Consequently, for a given mixture strength, steady combustion is possible only within a finite range of the stretch rate. This range steadily diminishes with decreasing mixture strength such that there exists a critical equivalence ratio, the extended flammability limit, beyond which steady burning for the stretch-enhanced flame also ceases to be possible. For lean propane/air flames, however, the mixture Lewis numbers are greater than unity such that the combined stretch and non-equidiffusion effects always diminish the burning intensity. Consequently, the transition from stretch-dominated extinction to loss-dominated extinction is monotonic in terms of the equivalence ratio, and the fundamental flammability limit is the proper flammability limit. The present results agree well with the recent experimental observations of near-limit, lean methane/air and propane/air flames, obtained under microgravity conditions needed to eliminate the influence of buoyancy, which could severely affect the response of these weakly burning flames. Implications of the present understanding on the experimental determination of flammability limits using the counterflow flame technique are also discussed.

Structure and sooting limits in counterflow methane/air and propane/air diffusion flames from 1 to 5 atmospheres

The structure of counterflow methane/nitrogen and propane/nitrogen diffusion flames for pressures from 1 to 5 atm was investigated experimentally and computationally. The temperature and major species concentration profiles were measured with spontaneous Raman scattering and computationally simulated with detailed kinetics and transport. Good agreement was found between the experimental data and the computational simulation. It was further shown that the previously developed global and local sooting limit correlations are again applicable, respectively relating the density-weighted strain rate at the sooting limit with the global parameters of the system pressure and the fuel mole fraction in the fuel stream, and with the local flame parameter of the peak acetylene partial pressure. In addition, the local correlations for the propane and ethylene flames collapse into a single relation. An interpretation of these correlations is provided, and their fundamental importance is emphasized.

Oscillatory stretch effects on the structure and extinction of counterflow diffusion flames

The effects of oscillatory stretch on atmospheric laminar counterflow diffusion flames are investigated both numerically and experimentally. Measurements indicate that, at high excitation frequencies, the peak extinction strain rates of oscillating CH 4 +N 2 versus air flames can be extended well beyond steady-state extinction limits. Hydroxyl radical concentrations are measured in a CH 4 +N 2 versus air flame excited at moderate frequencies and are compared to numerical simulations including complex chemistry and detailed transport. Measurements and simulations of OH concentration oscillations show similar phase delays. However, the measurements indicate a larger variation of OH concentration. AT moderate frequencies, the time-dependent OH variation is quasi-steady where the time-dependent flame can be described by a series of steady-state flames. However, the time-dependent flame does not quite recover to its steady-state structure at the low-strain-rate extreme.

Detailed oxidation kinetics and flame inhibition effects of chloromethane

A comprehensive experimental and numerical study has been performed on the detailed oxidation kinetics and the flame inhibition effects of chloromethane, with an emphasis on the isolation of the temperature and chemical effects caused by substitution of methane in the fuel by chloromethane. The experimental efforts involved the determination of laminar burning velocities for a series of fuel mixtures of different ratios of chloromethane to methane, but with a fixed ratio of total fuel to oxygen (and air). The thermal and chemical effects were isolated by comparing the laminar burning velocities obtained with the adiabatic flame temperature uncompensated with the substitution of methane by chloromethane, versus those obtained with fixed adiabatic flame temperature achieved by replacing nitrogen in air with an equal amount of argon. The experimental results indicate that temperature reduction due to increased chloromethane substitution is a significant factor for the reduction in the laminar burning velocity. Furthermore, when the results at a fixed flame temperature were examined on the basis of the mass burning rate, which is the eigenvalue for laminar flame propagation, the response was found to be insensitive to the chloromethane concentration in the mixture. This implies the possibility of a corresponding insensitivity to the chlorine flame chemistry. Concurrently, a detailed reaction mechanism of chloromethane/methane oxidation was compiled and validated against literature data from shock tube to flow reactor studies. Numerical simulation of the present experimental situation was then performed. The numerical results were found to be in close agreement with the current experimental findings.

Mild oxidation regimes and multiple criticality in nonpremixed hydrogen-air counterflow

Abstract This study investigates experimentally and computationally the existence of mild oxidation regimes and multiple ignition and extinction states in a system of nonpremixed, counterflowing hydrogen against heated air. Spontaneous Raman spectroscopy measurements of the water concentration show that up to three stable stationary states can be achieved for identical boundary conditions. Computationally, up to five steady-state solutions can be found, although only three are likely to be stable. This multiplicity is the result of combined thermokinetic and transport effects on the behavior of critical ignition and extinction states. To understand these effects, the system response was simulated using detailed kinetics and transport properties, and S-curve sensitivity was employed to identify the dominant chemistry near the critical states and to simplify the kinetic mechanism. The response to changes in the fuel concentration and system pressure was investigated experimentally by measuring the air temperatures corresponding to ignition and extinction, for fuel concentrations in the range of 6–38% H 2 in N 2 by volume, and pressures between 0.3 and 8 atm, at a constant pressure-weighted strain rate of 300 s −1 . The experimental results were found to agree well with the computational results. The experimental triple-solution multiplicity disappears for fuel concentrations in excess of ∼25% or below ∼7% H 2 in N 2 , and was only found in the pressure range between ∼1.5 and 7 atm, at 9% H 2 in N 2 and a pressure-weighted strain rate of 300 s −1 . In addition, the response to changes in the strain rate was studied computationally, for strain rates between 10 and 40,000 s −1 and for air boundary temperatures ranging between 950 and 1100 K. The same features of up to five steady-state multiplicities and up to two ignition and extinction states can be obtained by changing the flow strain rate. In the strain rate space, the computational quintiple-solution multiplicity extends from ∼ 100–10,000 s −1 , at 9% H 2 in N 2 and 4 atm.

Response of counterflow premixed and diffusion flames to strain rate variations at reduced and elevated pressures

The thermal structure of counterflow premixed and diffusion flames was experimentally and computationally studied to examine the response of flame structure to strain rate and pressure variations. The temperature profiles were experimentally measured as a function of strain rate at reduced and elevated pressures by using spontaneous Raman scattering, and were found to agree well with the independently computed profiles using detailed reaction mechanisms and transport properties. For both near-equidiffusive and non-equidiffusive premixed flames, results show that variation of the thermal structures is much smaller than that of the strain rate, and the structural sensitivity decreases with increasing pressure due to the reduced flame thickness. For diffusion flames, results show that the flame structure at different pressures largely scales with the density-weighted strain rate instead of the strain rate alone. Further analysis was conducted to extract the global flame parameters from the thicknesses of stretched non-equidiffusive flames in response to strain rate and pressure variations. It is demonstrated that the effects of stretch vary linearly with the imposed strain rate for the flames studied herein, and that the stretch effects can be predicted with good accuracy by knowing the laminar flame speeds of the one-dimensional planar flame as a function of the system pressure and equivalence ratio and by knowing the mixture Lewis number as a function of the equivalence ratio.