Makihito Nishioka

A flame-controlling continuation method for generating S-curve responses with detailed chemistry

Abstract A flame-controlling continuation method is formulated for the generation of the ignition-extinction S-curve characteristic of quasi-one-dimensional flames as well as the investigation of the associated flame structure and response, especially for states near the turning points. Using the counterflow premixed and diffusion flames as examples, the method capitalizes on the distinct nature of the profile and location of the scalars of the flame properties, such as the temperature and species concentrations, in response to changes in the flow strain rate. Thus instead of using the strain rate as an imposed parameter and the scalars as the flame responses, the value of a flame scalar at a given location y ∗ is used as an internal boundary condition while the strain rate becomes the flame response. Consequently, by fixing y ∗ and incrementing the value of the flame scalar, continuous mapping of the relation between the flame response and strain rate is accomplished. Sample calculations were performed for the premixed twin flame and for diffusion flames with equal and unequal exit velocities from the opposing nozzles. Continuations using one-point temperature controlling, two-point temperature controlling, and one-point hydrogen radical concentration controlling were demonstrated. The method appears to be fairly expedient in implementation.

NO emission characteristics of methane-air double flame

Abstract NO emission characteristics of methane-air Bunsen-type burner flames were studied numerically in terms of counterflow flame. The flames have the well-known double flame structure: the rich premixed flame to produce CO and H 2 as the main intermediate products and the diffusion flame where the intermediate products burn with surrounding air, and the structure can be simulated by using rich counterflow flame with air. The similarity solution was adopted to describe the flow, temperature, and concentration fields and the detailed kinetics calculation was made by using C 2 chemistry with the all mechanisms leading to NO formation, including thermal and prompt NO mechanisms. The calculation was made as well for thermal mechanism alone, so as to distinguish contribution of the respective NO formation mechanisms in total NO production. The emission characteristics were evaluated quantitatively in terms of the emission index, as compared to the normal premixed and pure diffusion flames. The effects of equivalence ratio and the velocity gradient on the emission index of these flames were studied.

Species conservation and emission indices for flames described by similarity solutions

The objective of the present study is to derive the species conservation relation in the counterflow flames and the tubular flames described by similarity solutions and to make it possible to evaluate NO x emission indices

The role of kinetic versus thermal feedback in nonpremixed ignition of hydrogen versus heated air

Abstract System response S-curves for a hydrogen-air diffusion flame have been simulated numerically using detailed chemistry and transport. In particular, the globally nonpremixed ignition state has been studied in three distinct ignition regimes at pressures of 0.1, 1, and 10 atm. The role of heat release in providing “thermal feedback” at the ignition turning point is examined in detail for all three regimes. Contrary to classical notions based upon one-step overall chemistry, thermal feedback is shown to play essentially no or minimal role in the steady-state solution at the ignition turning point—either in its character or parametric dependence. In the majority of cases studied, turning point and S-curve behavior are found to exist in the complete absence of heat release, driven solely by “kinetic” feedback provided by nonlinearities in the coupled chemical kinetics. As a result, the location of the ignition turning point, which depends parametrically upon global variables such as air temperature, strain rate, pressure, and fuel concentration, is essentially governed by the kinetics of gain versus loss of key radicals in the ignition kernel. One cause of this phenomenon is the extremely small size of the radical pool at the ignition turning point, which necessarily limits the degree of localized heat release and temperature perturbation. The small radical pool is also found to decouple the problem such that, on the lower branch and around the ignition turning point, the temperature and possibly major species profiles may be solved independently of the complex chemistry involving the minor species. Furthermore, it is also suggested that when heat release is not significant at the ignition turning point, the transient ignition process (from the turning point to a diffusion flame) must begin with an induction period wherein the radical pool increases via essentially isothermal chemical kinetics before thermal feedback can ensue.

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 of counterflow diffusion flames under velocity oscillations

The effects of sinusoidal velocity oscillation on counterflow diffusion flames of diluted methane against air were experimentally and computationally investigated. Experimentally, the unsteady flow characteristics as well as the local extinction strain rates were measured over extensive ranges in frequency and amplitude of the flow oscillation by using laser Doppler velocimetry (LDV). Computationally, the phenomena of interest were simulated with detailed descriptions of chemistry and transport. Results show that the hydrodynamic flow field does not respond instantaneously upon imposition of the velocity perturbation at the nozzle exits of the counterflow burners in that, with increasing frequency or amplitude, a noticeable lag develops between the imposed perturbation and the response of the flow further downstream. It is also demonstrated that extinction basically behaves quasi-steadily either for low-frequency oscillations, or for high-frequency oscillations imposed on weakly burning flames, in that the (maximum) extinction strain rate is largely independent of the mean strain rate, being only slightly larger than the steady-state extinction value. However, for strongly burning flames subjected to high-frequency oscillations, increasingly larger amplitudes are needed to effect extinction as the frequency increases. The present results therefore further substantiate and quantify the concept that since extinction is a transient process, for sufficiently rapid oscillation the flame may not have enough time to extinguish before the flow condition again becomes favorable for burning, and as such, with increasing frequency a flame can persist beyond the strain rate regime in which steady-state flames do not exist.

A numerical study of ignition in the supersonic hydrogen/air laminar mixing layer

Abstract The ignition evolution in the supersonic nonpremixed hydrogen/air laminar mixing layer, consisting of a relatively hot, fast air stream next to a cold, slower hydrogen stream, was computationally simulated using detailed transport and chemical reaction mechanisms and compared with results from asymptotic analysis with reduced mechanisms. The study emphasizes identifying the controlling chemical mechanisms in effecting ignition, on the relative importance of external versus viscous heating as the dominant ignition source, on the roles of thermal versus kinetic-induced ignition in which heat release and hence nonlinear thermal feedback are not needed in initiating system runaway, and on the consequences of imposing the conventional constant property assumptions in analytical studies. Results show that the state of the hydrogen/oxygen second explosion limit has the dominant influence in the system response in that, for all practical purposes, ignition is not possible when the air-stream temperature is lower than the crossover temperature, even allowing for viscous heating. On the other hand, when the air-stream temperature is higher than the crossover temperature, the predicted ignition distance indicates that ignition is feasible within practical supersonic combustion engines. Furthermore, for the latter situations, the ignition event is initiated by radical proliferation and hence runaway instead of thermal runaway. Finally, it is shown that, while the present computed results qualitatively agree well with those from the asymptotic analysis with reduced mechanisms, the analytically predicted ignition distances are much shorter than the computed values because the analysis has overemphasized the viscous effect through the constant Chapman-Rubesin parameter ϱμ and unity Prandtl number assumptions.

A theoretical study of extinction of a tubular flame

Abstract A theoretical study of extinction of a tubular flame was made by means of asymptotic analysis based on the flame surface model. The numerical calculation was made as well so as to make a comparison with and assure the validity of the analysis. The flame temperature at extinction was found to increase over the adiabatic flame temperature when Lewis number Le 1. The extinction Reynolds number increases, while the flame radius at extinction decreases, with a decrease in Le. It was found that the general extinction behavior predicted by the analysis agrees with that of the numerical solution. The discrepancy in the details could be explained in terms of the overestimated reaction rate and of the zero reaction zone width of the flame surface model. The relevant parameter which describes the extinction behavior was found to be the flame stretch, and the flame is extinguished when it exceeds the critical value. The value was a function of a single parameter, which contains kinetic, transport, and thermodynamic properties of the mixture simultaneously in a very simple form. The extinction behavior of the axisymmetric tubular flame was found to be different from that of the plane flame in stagnation flows due to the effect of curvature, which provides the increased cooling.

EFFECTS OF VITIATED AIR ON HYDROGEN IGNITION IN A HIGH-SPEED LAMINAR MIXING LAYER

One associated complication for the use of vitiated air in laboratory supersonic combustion studies to simulate flight enthalpy is that the test media are contaminated by species that are not representative of the actual atmosphere. When burning hydrogen in oxygen-enriched air is employed to produce vitiated air in experimentation, the resulting high-enthalpy airflow contains substantial amounts of, for example, H2O, H, OH, O, and NO. Therefore, the primary objective of the present numerical study is to assess the effects of vitiated air on the ignition characteristics in supersonic experiments. Specifically, the ignition evolution in the high-speed hydrogen/air laminar mixing layer is computationally simulated using detailed chemistry and transport properties. Individual and combined influences of important contaminants on ignition are systematically examined over a range of pressure, air temperature, and freestream velocity variations. Vitiations of the active radicals, like H, O, and OH, are found to enhance ignition, as expected. Results also show that contamination of H2O and NO can inhibit and promote supersonic ignition, respectively. The computed results on the net effect of simultaneous contaminations of H, O, OH, H2O, and NO on the ignition response further provide insight into the interpretation of the experimental data using the vitiated air facilities.

Effects of pressure on structure and extinction of tubular flame

Abstract A numerical study on effects of pressure on structure and extinction of a tubular flame was made. The adopted approach was the detailed kinetics theory combined with the exact similarity solution for the flow field. The calculation was made for stoichiometric methane-air mixture, and the structure and the extinction limits were determined for pressures ranging from 1 to 8 atm by using the so-called C1 chemistry. It was found that at elevated pressures the flame radius is decreased because of the decrease in the burning velocity, while the reaction zone width is decreased due to the decelerated transport properties. The flame temperature is increased, approaching the equilibrium flame temperature due to the accelerated reaction rates. The extinction flame temperature increases, whereas the extinction flame radius decreases with pressure, with a minimum radius of around 0.2 mm at the highest pressure of 8 atm. The critical strain rate at extinction limit increases with pressure initially, reaching a maximum at around 2.7 atm and then decreasing for higher pressures. It appears that this behavior is closely related to the effects of curvature on extinction, which can be explained in terms of the simplified asymptotic analysis.