Hong G. Im

Characteristic boundary conditions for direct simulations of turbulent counterflow flames

Improved Navier–Stokes characteristic boundary conditions (NSCBC) are formulated for the direct numerical simulations (DNS) of laminar and turbulent counterflow flame configurations with a compressible flow formulation. The new boundary scheme properly accounts for multi-dimensional flow effects and provides nonreflecting inflow and outflow conditions that maintain the mean imposed velocity and pressure, while substantially eliminating spurious acoustic wave reflections. Applications to various counterflow configurations demonstrate that the proposed boundary conditions yield accurate and robust solutions over a wide range of flow and scalar variables, allowing high fidelity in detailed numerical studies of turbulent counterflow flames.

Preferential diffusion effects on the burning rate of interacting turbulent premixed hydrogen-air flames

Abstract The upstream interaction of twin premixed hydrogen-air flames in 2-D turbulence is studied using direct numerical simulations with detailed chemistry. The primary objective is to determine the effect of flame stretch on the overall burning rate during various stages of the interaction. Preferential diffusion effects are accounted for by varying the equivalence ratio from symmetric rich-rich to lean-lean interactions. The results show that the local flame front response to turbulence is consistent with previous understanding of laminar premixed flames, in that rich premixed flames become intensified in regions of negative strain or curvature, while the opposite response is found for lean premixed flames. The overall burning rate history with respect to the surface density variation is found to depend on the mixture condition; the consumption rate enhancement advances (follows) the surface enhancement for the rich-rich (lean-lean) case. For the lean-lean case, a self-turbulization mechanism results in a large positive skewness in the area-weighted mean tangential strain statistics. Because of the statistical dominance of positive stretch on the flame surface, the lean-lean case results in a significantly larger burning enhancement (over a twofold increase) in addition to the surface density production. For the case of rich-rich interaction, the abundance in hydrogen species results in an instantaneous overshoot of the radical pool in the post-flame region, resulting in an additional “burst” in the reactant consumption rate history, suggesting its potential impact on the pollutant formation process.

Effects of flow strain on triple flame propagation

Abstract The primary objective of this study is to determine the effect of strain rate and scalar dissipation rate on the instantaneous local displacement speed at the triple flame edge. This is accomplished by performing direct numerical simulations of a hydrogen-air triple flame subjected to an unsteady strain field induced by a pair of counter-rotating vortices. It is observed that the triple flames maintain a positive displacement speed when the vortex strength is weak, such that they penetrate into the channel between the vortices. For the stronger vortex cases, the intense compressive strain field induced by the vortex pair yields a negative displacement speed and partial quenching of the leading edge of the flame in an extreme case. The displacement speed variations are analyzed in terms of curvature, and effective Karlovitz and Damkohler numbers. It is found that the triple flame tip speed is predominantly governed by the curvature-induced compressive strain rather than by scalar dissipation rate. As a result, the displacement speed measured at the triple flame tip exhibits a strong correlation with flame stretch and curvature, and not with scalar dissipation rate. The correlation with flame stretch is similar to results found in earlier studies of turbulent premixed flames, suggesting that the propagation aspects of triple flames are the same as for a premixed flame. The trailing diffusion flame essentially has minimal impact on the propagation of the leading edge. A secondary observation is that for real chemical systems, ambiguity in the definition of the “leading edge” can lead to significant differences in the propagation response to strain. For instance, the displacement speed measured at the maximum heat release location rather than at the leading edge remains positive throughout the entire duration of interaction. This suggests that care should be taken in identifying the triple flame speed subjected to a large strain field.

Dynamic flammability limits of methane/air premixed flames with mixture composition fluctuations

As a fundamental study in the application to direct-injection spark-ignition engines or gas turbines, in which mixture stratification and partial quenching are of serious concerns, unsteady premixed methane/ air flames subjected to time-varying composition fluctuations are investigated computationally. The code OPUS employs an unsteady opposed-flow combustion configuration, including detailed chemical kinetics, transport, and radiation models, using an adaptive time integration method for a stiff system of differential-algebraic equations with a high index. The primary issue of the study is to establish the concept of the dynamic flammability limit , defined as the minimum equivalence ratio above which the unsteady flame can sustain combustion. For the weak and strong strain rate cases studied, it is observed that the dynamic flammability limit depends on the mean and frequency of the composition fluctuation. The parametric study demonstrated that the flammability limit of an unsteady premixed flame is further extended to a leaner condition as the frequency or mean equivalence ratio fluctuation increases. It is also found that, under all conditions, the mean equivalence ratio and the minimum flame temperature must be higher than those at the steady flammability limit to sustain combustion. It is further shown that the dynamic flammability limit is primarily determined by the instantaneous, branching-termination balance at the reaction zone. The behavior of the flame response attenuation with increasing frequency is found to scale properly using the normalized frequency based on the imposed flow strain rate, which represents the characteristic time scale of the transport process within the flame.

EFFECTS OF HYDROGEN ADDITION ON THE MARKSTEIN LENGTH AND FLAMMABILITY LIMIT OF STRETCHED METHANE/AIR PREMIXED FLAMES

A computational study is performed to investigate the effects of hydrogen addition on the fundamental characteristics of steady and unsteady stretched methane/air premixed flame in an opposed flow configuration. The problem is of interest as a potential application to gas turbines and sparkignition engines, where the addition of a small amount of hydrogen allows combustion at leaner conditions to achieve lower NOx emission. The flame response is first studied under steady conditions with different levels of hydrogen addition. The effective Markstein length is found to exhibit a nonmonotonic function of the level of blending due to the competing effects between the Zeldovich and Lewis number variations. The results also show that the lean flammability limit is significantly extended due to the presence of hydrogen in the mixture, consistent with previous studies. On the other hand, the consumption speed and time scale of the flame at the extinction condition are found to be rather insensitive to the extent of blending. Unsteady flame response is subsequently studied by imposing oscillatory equivalence ratio at the boundary, as a means to characterize the effects of mixture stratification at various time scales. Consistent with the steady results, the attenuation of the dynamic flammability limits in a normalized scale collapse very well for various levels of hydrogen blending, implying that the unsteady flame response depends strongly on the characteristic chemical time scale irrespective of the amount of fuel blending. A simple analytical solution is derived to predict consistent qualitative behavior. The net effects of unsteady composition fluctuation on the NOx formation are also discussed.

A Regime Diagram for Autoignition of Homogeneous Reactant Mixtures with Turbulent Velocity and Temperature Fluctuations

A theoretical scaling analysis is conducted to propose nondimensional criteria to predict weak and strong ignition regimes for a compositionally homogeneous reactant mixture with turbulent velocity and temperature fluctuations. This leads to a regime diagram that provides guidance on expected ignition behavior based on the thermo-chemical properties of the mixture and the flow/scalar field conditions. The analysis extends the original Zeldovich’s theory by combining the turbulent flow and scalar characteristics in terms of the characteristic Damköhler and Reynolds numbers of the system, thereby providing unified and comprehensive understanding of the physical and chemical mechanisms controlling autoignition. Estimated parameters for existing experimental measurements in a rapid compression facility show that the regime diagram predicts the observed ignition characteristics with good fidelity.

Characteristics of auto-ignition in a stratified iso-octane mixture with exhaust gases under homogeneous charge compression ignition conditions

Ignition and propagation of a reaction front in a counterflow system of an iso-octane/air stream mixing with an exhaust gas stream is computationally investigated to understand the fundamental characteristics of homogeneous charge compression ignition (HCCI) auto-ignition. Various mixing rates are imposed on the system and the effects of dissipation rates on auto-ignition are studied. Ignition delay and front propagation speed across the mixing layer are determined as a function of a local mixture fraction variable. The results show that mixture inhomogeneity and dissipation rate have a significant influence on ignition. Diffusive transport is found to either hamper or advance ignition depending on the initial reactivity of the mixture. Based on the relative importance of diffusion on ignition front propagation, two distinct ignition regimes are identified: the spontaneous ignition regime and the diffusion-controlled regime. The transition between these two regimes is identified using a criterion based on the ratio of the timescales of auto-ignition and diffusion. The results show that ignition in the spontaneous regime is more likely under typical HCCI operating conditions with iso-octane due to its high reactivity. The present analysis provides a means to develop an improved modelling strategy for large-scale engine simulations.

Numerical Simulations of Hollow-Cone Injection and Gasoline Compression Ignition Combustion With Naphtha Fuels

We acknowledge the help and support from Saurav Mitra and Sarangarajan Vijayraghavan from Convergent Science, Inc. (CSI). This work was sponsored by the Fuel Technology Division at Saudi Aramco R&DC. The work at King Abdullah University of Science and Technology (KAUST) was funded by KAUST and Saudi Aramco under the FUELCOM program.

Effects of unsteady scalar dissipation rate on ignition of non-premixed hydrogen/air mixtures in counterflow

As a step toward understanding the effects of a turbulent environment on ignition delay, the effect of impulsive strain forcing on the autoignition of non-premixed hydrogen/air mixtures is studied numerically using a one-dimensional unsteady opposed-flow code with detailed chemistry. The sensitivity of ignition kernel growth to changes in the scalar dissipation rate during the ignition process is studied at conditions in the second and third ignition limits. The sensitivity of the kernel growth is quantified by examining the time evolution of key radical species as well as their reaction and flow flux balances over a range of impulse amplitudes and times. Results show that transient ignition in both the second and the third limits is sensitive to changes in scalar dissipation rate. Increases in ignition delay of as much as five times are observed, depending upon the impulsive forcing amplitude and timing. For a given impulse amplitude, kernels that have accumulated more radicals at a given time during induction are found to ignite much sooner, indicating that the time history of the kernel radical pool relative to the impulse time is important. Furthermore, kernels are found to be able to survive excursions in the scalar dissipation rate to values that far exceed the steady ignition state. The increase in ignition delay in both limits is attributed to a shorter residence time of radicals in the kernel as measured by an instantaneous Damkohler number. A new ignition criterion based on the instantaneous Damkohler is found to be an accurate measure of predicting the ignitability under highly transient conditions.

Assessment of flamelet versus multi-zone combustion modeling approaches for stratified-charge compression ignition engines

The spray-interactive flamelet and extended multi-zone combustion models coupled with multi-dimensional computational fluid dynamics are applied to investigate the effects of charge stratification in a direct-injection compression ignition engine under low load conditions. A parametric study was carried out in order to compare the two approaches for early and late fuel injection timings. Comparison of numerical results with available experimental data shows that for early fuel injection, both models predict the auto-ignition and combustion characteristics with comparable fidelity. As the fuel injection timing is delayed, however, the spray-interactive flamelet model is found to capture the onset of combustion and subsequent heat release with greater accuracy. Further investigation reveals that the better performance of the spray-interactive flamelet model over a wider range of mixture-stratified conditions is mainly attributed to its ability to capture the diffusive transport resulting from small-scale mixing and turbulence–chemistry interaction, which becomes more important when significant mixture inhomogeneities exist in the engine cylinder.