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This work was supported by the Clean Combustion ResearchCenter at the King Abdullah University of Science and Technology (KAUST).

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...

Multi-stage heat release is an important feature of hydrocarbon auto-ignition that influences engine operation. This work presents findings of previously unreported three-stage heat release in the auto-ignition of n-heptane/air mixtures at lean equivalence ratios and high pressures. Detailed homogenous gas-phase chemical kinetic simulations were utilized to identify conditions where two-stage and three-stage heat release exist. Temperature and heat release profiles of lean n-heptane/air auto-ignition display three distinct stages of heat release, which is notably different than two-stage heat release typically reported for stoichiometric fuel/air mixtures. Concentration profiles of key radicals (HO2 and OH) and intermediate/product species (CO and CO2) also display unique behavior in the lean auto-ignition case. Rapid compression machine measurements were performed at a lean equivalence ratio to confirm the existence of three-stage heat release in experiments. Laser diagnostic measurements of CO concentrations in the RCM indicate similar concentration-time profiles as those predicted by kinetic modeling. Computational singular perturbation was then used to identify key reactions and species contributing to explosive time scales at various points of the three-stage ignition process. Comparisons with two-stage ignition at stoichiometric conditions indicate that thermal runaway at the second stage of heat release is inhibited under lean conditions. H + O2 chain branching and CO oxidation reactions drive high-temperature heat release under stoichiometric conditions, but these reactions are suppressed by H, OH, and HO2 radical termination reactions at lean conditions, leading to a distinct third stage of heat release.

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.

A model reduction methodology, based on the quasi steady-state approximation (QSSA), is employed for the construction of reduced mechanisms in the case of an n-heptane/air premixed flame. Several issues related to the construction of these reduced mechanisms are discussed; such as the influence of the size of the starting skeletal mechanism, the stiffness reduction, and the truncation/simplification of (i) the expressions of the global rates and (ii) the steady-state relations. The starting point for the reduction is two skeletal mechanisms that involve 177/768 and 66/326 species/reactions, respectively [J. Prager, H.N. Najm, M. Valorani, and D.A. Goussis, Skeletal mechanism generation with CSP and validation for premixed n-heptane flames, Proc. Combust. Inst. 32 (2009), pp. 509–517] and which were derived from the detailed mechanism of Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, and C.K. Westbrook, A comprehensive modeling study of iso-octane oxidation, Combust. Flame 129 (2002), pp. 253–280], whi...

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 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.

The autoignition kinetics of DME/air and EtOH/air stoichiometric mixtures are compared with the use of algorithmic tools from the CSP method at a range of initial conditions that refers to the operation of reciprocating engines. DME and EtOH are two isomer fuels, with the potential for production from renewable sources, that have virtually identical thermochemistry; i.e. very closely equal heat of combustion and adiabatic flame temperature. These isomer fuels have drastically different ignition delays because of their different kinetics. In particular, the first and largest part of the ignition delay in the DME and EtOH cases is dominated by two different sets of components of carbon chemistry, while the last and shortest part is dominated by the same hydrogen chemistry. Considering sufficiently large initial temperatures, in the DME case the time scale that characterizes autoignition in the first part is promoted by single-carbon chemistry and is opposed mainly by recombination of CH 3 radicals. On the contrary, in the EtOH case the two-carbon chain retains its bond in that part. Therefore, the hydrogen chemistry plays an important role in promoting the generation of the time scale that characterizes autoignition from the start of the process, while the reactions that oppose the generation of this time scale involve HO 2 and H 2 O 2 and they are not as effective as the reactions opposing ignition for DME. These features generate a substantially shorter ignition delay for EtOH. This situation is reversed for sufficiently low initial temperatures due to the shift in relative importance between internal and external H-abstraction that occurs as temperature increases.

Lactate in the brain is considered an important fuel and signalling molecule for neuronal activity, especially during neuronal activation. Whether lactate is shuttled from astrocytes to neurons or from neurons to astrocytes leads to the contradictory Astrocyte to Neuron Lactate Shuttle (ANLS) or Neuron to Astrocyte Lactate Shuttle (NALS) hypotheses, both of which are supported by extensive, but indirect, experimental evidence. This work explores the conditions favouring development of ANLS or NALS phenomenon on the basis of a model that can simulate both by employing the two parameter sets proposed by Simpson et al. (J Cereb. Blood Flow Metab., 27:1766, 2007) and Mangia et al. (J of Neurochemistry, 109:55, 2009). As most mathematical models governing brain metabolism processes, this model is multi-scale in character due to the wide range of time scales characterizing its dynamics. Therefore, we utilize the Computational Singular Perturbation (CSP) algorithm, which has been used extensively in multi-scale systems of reactive flows and biological systems, to identify components of the system that (i) generate the characteristic time scale and the fast/slow dynamics, (ii) participate to the expressions that approximate the surfaces of equilibria that develop in phase space and (iii) control the evolution of the process within the established surfaces of equilibria. It is shown that a decisive factor on whether the ANLS or NALS configuration will develop during neuronal activation is whether the lactate transport between astrocytes and interstitium contributes to the fast dynamics or not. When it does, lactate is mainly generated in astrocytes and the ANLS hypothesis is realised, while when it doesn't, lactate is mainly generated in neurons and the NALS hypothesis is realised. This scenario was tested in exercise conditions.