Search

In the current work, the effect of H2O2 addition on the flame structure, laminar flame speed and NOx emissions is investigated in the context of 1D laminar premixed H2/air flames at Tu = 300 and 600 K, p = 1 and 30 atm, ϕ = 0.5. Mathematical tools from the computational singular perturbation approach are used in order to identify the key chemical and transport mechanisms. The H2O2 addition causes a significant increase to the laminar flame speed (sL), heat release (Q) and NOx emissions. Indicatively, 10% H2O2 addition (per fuel volume) at Tu = 300 K, p = 1 atm results in 72% increase of sL, 100% increase of Q, and 140% increase of the mass fraction of NO. Depending the conditions the flame structure is altered through the chain carrying reaction 10f (H2O2 + H → H2O + OH) or the chain branching 9f (H2O2 (+M) → 2OH (+M)); the first is favored at low temperatures/pressures while the latter is favored at sufficiently high temperatures/pressures. Both reactions boost the radical pool generation, therefore contributing to the broadening of the reaction zone. The reaction with the largest contribution to Q that is mostly affected (decreased) by the addition of H2O2 is reaction 21 (H + O2 (+M) ↔ HO2 (+M)). Moreover, the H2O2 addition enhances the stability of the flame. Finally, the increased production of NO is mainly associated with the increased temperature that is reached with the addition of H2O2.

In the current work, the use of hydrogen peroxide as an additive to hydrogen/air mixtures is proposed and explored computationally, in conditions relevant to compression ignition engines. The hydrogen/hydrogen peroxide blends are supplemented with steam for NOx emissions reduction purposes. The objective of the current work is to explore fundamental aspects of the proposed technology, with an emphasis on identifying the key chemical pathways that control the ignition delay time and NOx emissions, using mathematical tools from the computational singular perturbation (CSP) approach. The proposed technology demonstrates a noteworthy potential for use in CI engines, since a 10% (per fuel volume) addition of hydrogen peroxide decreases the ignition delay time to 1 ms, while the mass fraction of NO in equilibrium drops by 100%. Reactions H + O2 → OH + O and H + O2 (+M) → HO2 (+M) play key roles in the acceleration of the ignition delay time, while the thermal and the NNH mechanisms are identified as the dominant pathways for the production of NO. A further 12% addition of steam (per mixture’s volume) induces a two orders of magnitude drop to NO emissions and slightly increases the ignition delay time by 8%. Finally, at sufficiently high steam addition conditions (in the region of 30% and above by mixture’s volume), the system exhibits two stage ignition (mainly owed to reaction HO2 + OH → H2O + O2), a phenomenon that is unique, considering that the initial mixture includes solely hydrogen-based chemical species.

Dominant physical processes that characterize the combustion of a lean methane/air mixture, diluted with exhaust gas recirculation (EGR), under turbulent MILD premixed conditions are identified using the combined approach of Computational Singular Perturbation (CSP) and Tangential Stretching Rate (TSR). TSR is a measure to combine the time scale and amplitude of all active modes and serves as a rational metric for the true dynamical characteristics of the system, especially in turbulent reacting flows in which reaction and turbulent transport processes compete. Applied to the MILD conditions where the flame structures exhibit nearly distributed combustion modes, the TSR metric was found to be an excellent diagnostic tool to depict the regions of important activities. In particular, the analysis of turbulent DNS data revealed that the system’s dynamics is mostly dissipative in nature, as the chemically explosive modes are largely suppressed by the dissipative action of transport. On the other hand, the convective transport associated with turbulent eddies play a key role in bringing the explosive nature into the system. In the turbulent MILD conditions under study, the flame structure appears nearly in the distributed combustion regime, such that the conventional statistics conditioned over the progress variable becomes inappropriate, but TSR serves as an automated and systematic way to depict the topology of such complex flames. In addition, further analysis of the CSP modes revealed a strong competition between explosive and dissipative modes, the former favored by hydrogen-related reactions and the convection of CH4, and the latter by carbon-related processes. This competition results in a much smaller region of explosive dynamics in contrast to the widespread existence of explosive modes.

Synergistic octane blending behavior of ethanol with gasoline and its surrogates has been observed by many researchers. The nonlinear octane boosting tendency is observed at mid and high molar blen...

The autoignition dynamics of a three-component surrogate jet fuel (66.2% n-dodecane, 15.8% n-proplylbenzene, 18.0% 1,3,5-trimethylcyclohexane) suitable for usage as Jet A-1 and RP-3 aviation fuels are analyzed, using the detailed mechanism of Liu et al. (2019). The conditions considered are relevant to the operation of gas turbines and the analysis is performed using mathematical tools of the computational singular perturbation (CSP) method. The key chemical pathways and species are identified in the analysis of a homogeneous adiabatic and constant pressure ignition system for a wide range of initial conditions. In particular, the key role of hydrogen and CO-related chemistry is highlighted, with an increasing importance as the initial temperature increases. The C2H4→C2H3→CH2CHO pathway is also identified as playing a secondary but nonnegligible role with an importance increasing with initial temperature, favoring the system’s explosive dynamics and, thus, promoting ignition. Finally, C2H4 is identified as being a species with a key (secondary) role to the system’s explosive dynamics, but its role is replaced by C3H6 and, eventually, by O2 as the initial temperature increases. In the second part of the current work, a 58-species skeletal mechanism is generated using a previously developed algorithmic process based on CSP. The developed skeletal mechanism was tested in a wide range of initial conditions, including both ignition delay time and laminar flame speed calculations. For the conditions that were of interest in the current work, the skeletal mechanism approximated the detailed mechanism with very small error. The 58-species skeletal mechanism is shown to be ideal for use in computational fluid dynamics applications not only because of its small size but also because of its sufficiently slow associated fast timescale.

To provide fundamental insights into the ignition enhancement of methanol (MeOH) by the addition of the more reactive dimethyl ether (DME), computational parametric studies were conducted in a one-dimensional counterflow fuel versus air mixing layer configuration with the incorporation of detailed chemistry and transport. Various computational analysis tools based on the computational singular perturbation (CSP) framework were employed for detailed identifications of complex chemical pathways. CSP tools were also used to develop a 43-species skeletal mechanism for efficient computation of ignition of methanol-DME blends at engine conditions. The overarching practical question was the extent to which the addition of DME improves the ignitability of the methanol. As a baseline analysis, the results of a uniform temperature condition at 850 K showed that the low temperature chemistry associated with the DME fuel was highly effective in promoting autoignition. The increase in the oxidizer side temperature was found to diminish the ignition enhancement by DME blending, as the overall reactivity increases and the dominant chemical pathways become shifted towards the high temperature reactions. Finally, the strain rate effect on the ignition delay time was found to be significant for the pure methanol case, and then the effect diminishes as the amount of DME addition increases. This behavior was explained by examining the spatial locations of the ignition kernels and the Damkohler number history for different strain rate conditions.

Pre-ignition engine cycles leading to super-knock were simulated with a 48 species skeletal iso-octane mechanism to identify the dominant reaction pathways that are present in super-knock. To mimic pre-ignition, a deflagration front was generated via a hot spot that is placed over the piston at close proximity to the end-wall. Computational singular perturbation (CSP) was used to analyze the chemical dynamics at various in-cylinder locations: a point at the center of the cylinder where the deflagration front consumes the air/fuel mixture and two points located at 3 mm from the end-wall where super-knock and mild knock occur. The CSP analysis of the point at the center of the cylinder reveals weak two-stage ignition-like dynamics with a short second stage. At the other points, a pronounced two-stage ignition is displayed with a long second stage. A distinct contribution of formaldehyde (CH2O) at the second stage of ignition that adds to fast explosive modes in the super-knock points is not observed in the point at the center. A comparison between knock and super-knock analysis indicates that a similar set of reactions is responsible for the abnormal behavior but the fast explosive time scales are comparatively slower for knock, indicating lower reactivity, which results in the reduced intensity of knock. The analyzed results decoded important reactions responsible for the occurrence of super-knock.

Reactions contributing to the generation of the explosive time scale that characterise autoignition of homogeneous stoichiometric CH4/air mixture are identified using two different chemical kinetics models; the well known GRI-3.0 mechanism (53/325 species/reactions with N-chemistry) and the AramcoMech mechanism from NUI Galway (113/710 species/reactions without N-chemistry; Combustion and Flame 162:315-330, 2015). Although the two mechanisms provide qualitatively similar results (regarding ignition delay and profiles of temperature, of mass fractions and of explosive time scale), the 113/710 mechanism was shown to reproduce the experimental data with higher accuracy than the 53/325 mechanism. The present analysis explores the origin of the improved accuracy provided by the more complex kinetics mechanism. It is shown that the reactions responsible for the generation of the explosive time scale differ significantly. This is reflected to differences in the length of the chemical and thermal runaways and in the set of the most influential species.

Normal alkanes constitute a significant fraction of transportation fuels, and are the primary drivers of ignition processes in gasoline and diesel fuels. Low temperature ignition of n-alkanes is driven by a complex sequence of oxidation reactions, for which detailed mechanisms are still being developed. The current study explores the dynamics of low-temperature ignition of n-hexane/air mixtures, and identifies chemical pathways that characterize the combustion process. Two chemical kinetic mechanisms were selected as a comparative study in order to better understand the role of specific reaction sequences in ignition dynamics: one mechanism including a new third sequential O 2 addition reaction pathways (recently proposed by Wang et al. 2017), while the other without (Zhang et al. 2015). The analysis is conducted by applying tools generated from the computational singular perturbation (CSP) approach to two distinct ignition phenomena: constant volume and compression ignition. In both cases, the role of the third sequential O 2 addition reactions proves to be significant, although it is found to be much more pronounced in the constant volume cases compared to the HCCI. In particular, in the constant volume ignition case, reactions present in the third sequential O 2 addition reaction pathways (e.g., KDHP → products + OH) contribute significantly to the explosivity of the mixture; when accounted for along with reactions P(OOH) 2 + O 2 → OOP(OOH) 2 and OOP(OOH) 2 → KDHP + OH, they decrease ignition delay time of the mixture by up to 40%. Under HCCI conditions, in the first-stage ignition, the third-O 2 addition reactions contribute to the process, although their role decays with time and becomes negligible at the end of the first stage. The second ignition stage is dominated almost exclusively by hydrogen-related chemistry.

The effect of CO 2 dilution on the adiabatic and isochoric autoignition of CH 4 /air mixtures is analyzed with Computational Singular Perturbation (CSP) algorithmic tools, with a particular emphasis on the determination of the features of the chemical dynamics that control ignition delay and emission formation. Increasing CO 2 dilution causes longer ignition delays, lower final temperatures and decreased formation of NO and CO. These effects of CO 2 dilution are shown to be entirely thermal, contrary to what happens with dilution with H 2 O, which also has chemical activity and can reduce ignition delay. For the same initial mole fraction of the diluent, the decrease in final temperature and in NO concentration is larger in the CO 2 case whereas the decrease in CO is larger in the H 2 O case. The thermal effect of CO 2 is entirely analogous with those of dilution with the chemically inert Ar, only stronger for the same percentage of initial dilution, because of the larger specific heat of CO 2 . The reactions that have the largest contribution to the characteristic explosive time scale of the system during ignition delay (H 2 O 2 (+M) → OH + OH(+M), CH 3 O 2 + CH 2 O → CH 3 O 2 H + HCO, CH 4 + CH 3 O 2 → CH 3 + CH 3 O 2 H, H + O 2 → O + OH, etc.) are not substantially affected by CO 2 dilution, neither are the species that are pointed by CSP (CH 3 O 2 , H 2 O 2 , CH 2 O, etc.) as having the largest impact on the this timescale. The same holds for the modes that control CO and NO formation. The results point to the possibility of cold exhaust gas recirculation being used in order to produce mixtures with longer ignition delays and therefore substantial resistance to uncontrolled ignition.

The dynamics of a homogeneous adiabatic autoignition of an ammonia/air mixture at constant volume was studied, using the algorithmic tools of Computational Singular Perturbation. Since ammonia combustion is characterized by both unrealistically long ignition delays and elevated NO x emissions, the time frame of action of the modes that are responsible for ignition was analyzed by calculating the developing time scales throughout the process and by studying their possible relation to NO x emissions. The reactions that support or oppose the explosive time scale were identified, along with the variables that are related the most to the dynamics that drive the system to an explosion. It is shown that reaction H 2 O 2 (+M) → OH + OH (+M) is the one contributing the most to the time scale that characterizes ignition and that its reactant H 2 O 2 is the species related the most to this time scale. These findings suggested that addition of H 2 O 2 in the initial mixture will influence strongly the evolution of the process. It was shown that ignition of pure ammonia advanced as a slow thermal explosion with very limited chemical runaway. The ignition delay could be reduced by more than two orders of magnitude through H 2 O 2 addition, which causes only a minor increase in NO x emissions.

This paper examines the influence of wall reactions on the generation of the explosive time scale that characterizes ignition delay around the third explosion limit of a stoichiometric H2/a...

The Computational Singular Perturbation (CSP) algorithm is employed in order to determine how H 2 O -dilution influences ignition delay and chemical paths that generate NO during isochoric homogenous lean CH 4 /air autoignition. Regarding the ignition delay, it is shown that H 2 O -dilution enhances reactivity, mainly due to the increased OH production throughout the explosive stage via reaction H 2 O 2 ( + H 2 O ) → OH + OH ( + H 2 O ) . With regard to NO generation, the relative importance of thermal and chemical effects are examined and it is concluded that both are important. The thermal effects result in a lower temperature at the end of the explosive stage, while the most notable chemical effect is the lower level of O after this stage, mainly due to the effect of H 2 O -dilution on the equilibrium of the reaction O + H 2 O ↔ OH + OH . The depletion of O, together with the thermal effect, causes a substantial decrease in final NO generation.

The effect of selected additives on the ignition delay of ethanol (EtOH)/air and dimethylether (DME)/air mixture is investigated. Computational Singular Perturbation (CSP) tools are utilized in an effort to determine algorithmically which species to select as additives and it is established that CSP can identify species whose addition to the mixture can affect ignition delay. However, this is not a necessary condition for additives to be effective. Additives that are not identified by CSP can have a substantial effect on ignition delay, provided that they drastically alter the prevailing chemistry, by altering the instant in time when the thermal runaway regime develops. Some of the additives that were studied computationally are unstable radicals whose injection in practical mixtures is unrealistic. However, chemically stable, relatively light species were also determined that can drastically affect ignition delay, such as hydrogen peroxide, formaldehyde and acetaldehyde.

When the fast dissipative time scales become exhausted, the evolution of reacting processes is characterized by slower time scales. Here the case where these slower time scales are of explosive character is considered. This feature allows for the acquisition of significant physical understanding; among others, the identification of intermediates in the reacting process that can be used as additives for the control of the ignition delay. The case of the homogeneous autoignition of CH 4 /air mixtures is analyzed here and the effects of adding the stable intermediates CH 2 O and H 2 O 2 to the fuel. These two species are identified as those relating the most to the explosive mode that causes autoignition, throughout the largest part of the ignition delay. Small quantities of these species in the initial mixture decrease considerably the ignition delay, by expediting the development of the thermal runaway.