Symposium (International) on Combustion最新文献

A turbulent diffusion flame at a convective Mach number 1.2 was investigated using direct numerical simulation (DNS). The DNS employed the full time-dependent compressible Navier-Stokes equations coupled with a one-step chemical reaction governed by the Arrhenius kinetics. Detailed study of combustion-induced pressure effects on turbulence generation, conserved scalar, and stagnation enthalpy transport was conducted. Local countergradient diffusion (CGD) of a conserved scalar flux was observed for the first time in a diffusion flame where heat release was strong enough while gradient diffusion prevailed when heat release was zero or weak. The CGD occurred in spite of the absence of an externally imposed mean pressure gradient and was attributed to combustion-induced pressure fluctuations. The balance of the turbulent kinetic energy budget was strongly influenced by the pressure dilatation and the (combustion-induced) mean pressure work when heat release was strong. Both terms can be a source or a sink of turbulence, depending on the intricate interactions between turbulence and combustion. However, the temporal change in pressure ϱp/ϱt had an insignificant influence on the stagnation enthalpy transport. A linear relation between the stagnation enthalpy and the mixture fraction was confirmed, which could lead to considerable simplification in modeling high-speed turbulent combustion.

Laser diagnostic imaging techniques were used to obtain detailed in-cylinder data from a commercial gasoline engine. Mean flowfields and turbulence intensities were acquired using particle imaging veloci-metry (PIV). Instantaneous quantitative NO-concentration fields were measured using planar laser-induced fluorescence (LIF). From experimental images of NO, also the flame propagation could be deduced. Combustion and NO formation were simulated with a 3-D computer code SPEEDSTAR. The overall agreement between experimental data and computational results is encouraging in general, with remaining issues to be solved. The mean flow is well predicted, whereas the prediction of turbulence quantities is less satisfactory. Calculated results for flame propagation are in good agreement with measurements. NO concentrations resulting from calculations are close to those measured, both in respect to their spatial distribution and absolute number densities. As could be expected, the highest NO concentrations are found in regions where combustion started earliest. Local concentrations of NO are found be up to 4 times higher than those in the exhaust. The comparison of experimental results with calculations clearly shows that, although the 3-D computer model can predict major features of the in-cylinder processes in agreement with measurements, details such as the exact flow pattern and flame development are difficult to capture and depend critically on some of the models parameters used.

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 CH₄+N₂ versus air flames can be extended well beyond steady-state extinction limits. Hydroxyl radical concentrations are measured in a CH₄+N₂ 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.

Experimental and numerical studies are made of transient H₂/N₂-air counterflow diffusion flames unsteadily strained by an impinging micro jet. Two-dimensional temperature measurements by the laser Rayleigh scattering method and numerical computations taking into account detailed chemical kinetics are conducted, paying attention to transient local extinction and reginition in relation to the unsteadiness, flame curvature, and preferential diffusion effects. The results are as follows: (1) Transient local flame extinction is observed where the micro jet impinges. However, the transient flame can survive instantaneously in squite of quite high stretch rate where the steady flame cannot exist. (2) Reignition is observed after the local extinction due to the micro air jet impingement. the temperature after reignition becomes significantly higher than that of the original flame. This high temperature is induced by the concentration of H₂ species due to the preferential diffusion in relation to the concave curvature. The predicted behaviors of the local transient extinction and reignition are well confirmed by the experiments. (3) The reignition is induced after the formation of combustible premixed gas mixture and the consequent flame propagation. (4) The reignition is hardly observed after the extinction by micro fuel jet impingement. This is due to the dilution of H₂ species induced by the preferential diffusion in relation to the convex curvature. (5) The maximum flame temperature cannot be rationalized by the stretch rate but changes widely, depending on the unsteadiness and the flame curvature in relation with preferential diffusion.

Detonation behavior associated with the propagation of a detonation wave through an orifice plate located within a circular tube is investigated. The tube and orifice diameter used in the study are 27.3 cm and 10 cm, respectively. The test gas used is hydrogen-air at 1 atmosphere and at various initial temperatures up to 650 K. Immediately after the orifice, the detonation wave decouples and either fails or reinitiates. The reinitiation process is characterized by either spontaneous initiation, initiation due to shock reflection, or deflagration-to-detonation transition (DDT). In the case of DDT, transition is preceded by the degeneration of the decoupled detonation wave to a velocity consistent with a CJ deflagration. Delineation between these various propagation regimes could not be correlated with the detonation cell size, λ, and orifice diameter, d. The data, although limited, demonstrate for the first time that the d_c/λ=13 critical tube criterion obtained at room temperature may not apply at elevated temperature conditions. The evidence for this is data obtained at 500 K that shows no detonation transmission for 30% hydrogen in air that corresponds to d/λ=16.7. The tests also indicate that a simple d/λ correlation cannot be used to determine when reinitiation due to shock reflection is possible. For example, at 650 K detonation wave failure was observed for d/λ<7.4, and at 300 K failure was observed for d/λ<11.

An extended Landau-Levich model of liquid-propellant combustion, one that allows for a local dependence of the burning rate on the (gas) pressure at the liquid-gas interface, exhibits not only the classival hydrodynamic cellular instability attributed to Landau but also a pulsating hydrodynamic instability associated with sufficiently negative pressure sensitivities. Exploiting the realistic limit of small values of the gas-to-liquid density ratio p, analytical formulas for both neutral sability boundaries may be obtained by expanding all quatities in appropriate powers of p in each of three distinguished wave-number regimes. In particular, composite analytical expressions are derived for the neutral stability boundaries A_p(k), where A_p is the pressure sensitivity of the burning rate and k is the wave number of the disturbance. For the cellular boundary, the results demonstrate explicitly the stabilizing effect of gravity on long-wave disturbances, the stabilizing effect of viscosity (both liquid and gas) and surface tension on short-wave perturbations, and the instability associated with intermediate wave numbers for negative values of A_p, which is characteristic of many hydroxylammoninum nitrate-based liquid propellants over certain pressure ranges. In contrast, the pulsating hydrodynamic stability boundary is insensitive to gravitational and surface-tension effects but is more sensitive to the effects of liquid viscosity because, for typical nonzero values of the latter, the pulsating boundary decreases to larger negative values of A_p as k increases through O(1) values. Thus, liquid-propellant combustion is predicted to be stable (that is, stealy and planar) only for a range of negative pressure sensitivities that lie below the cellular boundary that exists for sufficiently small negative values of A_p and above the pulsating boundary that exists for larger negative values of this parameter.

Numerical analysis was conducted to simulate the ignition phenomena of a fuel droplet array in hot stagnant air. Previous experimental results showed that ignition times of an n-heptane droplet array quickly put into quiescent hot air were less than the ignition time of a single droplet. The objective of the present study was to clarify this interesting behavior of ignition time by numerical analysis. We assumed that a heptane droplet array with a droplet diameter of 0.75 to 1.25 mm and spacing of 4 to 20 mm was immersed in hot air with a temperature of 1123 K at time zero. The unsteady equation set for the array system was solved numerically by means of the finite-difference method. The results showed that ignition times became shorter than that of a single droplet as the droplet spacing decreased and that ignition times increased rapidly when the spacing further decreased. These ignition time behaviors were consistent with experimental results. Time-dependent temperature distributions indicated that the first ignition position(s) was located between droplets when the ignition time was less than that of a single droplet. When the spacing was smaller, an intense reaction region surrounded the array as a cylindrical tube. The basic mechanism of the shorter ignition time of a droplet array is a slight decrease of the vaporized fuel mass flux due to the suppression of the increase in droplet surface temperature in the array.

A model is presented and discussed treating the complex interactions between the fluid dynamics, the size, space, and time distributions of droplets as well as the combustion chemistry in a Diesel spray. The three-dimensional droplet characteristics are modeled with an overset gridding scheme, CHIMERA, and time dependent solutions of the Navier-Stokes equations have been carried out with the digital computer. The chemistry is treated with global and detailed reaction kinetics in a two-stage model. The global kinetics were used with the detailed Navier-Stokes equations, and the detailed kinetics were employed with a time dependent one-dimensional model that used information from the detailed model. The typical thermodynamic conditions of a modern truck engine have been used to provide insight into the details of ignition, combustion, and pollutant formation in spray commbustion. The study has considered droplet groups that have both uniform and different size droplets, and different groupings of the droplets have been investigated. The present model allows the droplets to move and vaporize in time, and the time development of ignition has been studied. The results indicate that group combustion effects have a dominant influence on all aspects of spray combustion.

Using a computer algorithm for automatic generation of reduced chemistry, an augmented reduced mechanism, consisting of 16 species and 12 lumped reaction steps, has been developed for methane oxidation from GRI-Mech 1.2. Because the present mechanism consists of a larger number of non-steady-state intermediates than the conventional four- or five-step reduced mechanisms, it exhibits good to excellent performance in predicting a wide range of combustion phenomena under extensive thermodynamical parametric variations. Specifically, the phenomena tested include perfectly stirred reactor responses, autoignition and shock-tube ignition delay times, laminar flame propagation speeds, and ignition-extinction limits of counterflowing systems, whereas the thermodynamical parametric variations include those of temperature, pressure, and composition. It is recognized that, with the anticipated increase in computing capability in the foreseeable future, use of the present four- to five-step mechanisms will be unnecessarily limiting. Consequently, it is suggested that efforts should be expended toward development of augmented reduced mechanisms for more comprehensive description of combustion phenomena and for their potential implementation in the computational simulation of complex flows and systems.

The reaction of C₂ radicals with atomic and molecular oxygen were studied behind reflected shock waves in the temperature range 2750 K≤T≤3950 K by using ring-dye-laser absorption spectroscopy. The shock-induced pyrolysis of acetylene was used as a well-defined C₂ source, which was perturbated by the addition of N₂O (O-atom source) or O₂. The addition of these species results in specific changes in the C₂ concentration compared to the unperturbated pyrolysis, mainly caused by the following reactions: {fx1} for which the rate coefficients {fx2} were obtained.