Three-stage heat release in n-heptane auto-ignition

Abstract

Colloquium: REACTION KINETICS / INTERNAL COMBUSTION ENGINES Word Count (Method 1): The total word count (exclusive of title page, abstract) is: 5793 words Word Count (Performed from automatic counting function in MS Word plus References/Tables/Equations/ Figures) Abstract: 235 words, not included in word count Equations: 0 words (0 equations, single column) Figures: 1030 words (4 figures with captions) 3 single 50(+10) 132 4 double 52(+10) 273 Captions 116 Total: 6055 words Supplemental Materials: One supplemental material is available. AbstractMulti-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 autoignition 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 threestage 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 autoignition 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.

Algorithmic Analysis of Chemical Dynamics of the Autoignition of NH3–H2O2/Air Mixtures

“…On the other hand, when the temperature is pointed strongly, then the system undergoes the thermal runaway, which usually develops in the last part of the explosive stage [22,29,42]. In previous studies relating to the autoignition of H 2 /air, CH 4 /air, n-hexane/air, n-heptane/air, DME/air, and ethanol/air mixtures, it was shown that chemical runaway regime practically occupies all the explosive stage, while the thermal runaway regime only appears at the very final portion of the explosive stage [22,34,37,38,41,43,44,46,[52][53][54][55][56]. The results diaplayed in Table 2 suggest that autoignition of ammonia is qualitatively different than this of hydrocarbons in that the chemical runaway process is very short and the process develops as a thermal explosion.…”

“…Note that the point where the two time scales meet is right before the point where the temperature completes its steep increase. This feature appears also in the autoignition of H 2 /air, CH 4 /air, n-hexane/air, n-heptane/air, DME/air, and ethanol/air mixtures [22,34,37,38,41,43,44,46,[52][53][54][55][56].…”

Computational Singular Perturbation Method and Tangential Stretching Rate Analysis of Large Scale Simulations of Reactive Flows: Feature Tracking, Time Scale Characterization, and Cause/Effect Identification. Part 2, Analyses of Ignition Systems, Laminar and Turbulent Flames

“…As indicated in the figure, almost all timescales at any given instance are dissipative (indicated by solid curves) and very few are explosive (indicated in dotted curves). This is a common feature of the homogeneous autoignition process that has been demonstrated extensively; e.g., [41,42,46,47,[59][60][61][62][63][64][65][66][67][68]. The few explosive timescales appear from the start of the process until the point where the temperature undergoes a steep increase.…”

Computational investigation of rod-stabilized laminar premixed hydrogen–methane–air flames

“…The system of the M algebraic equations ( f m ≈ 0) defines a low dimensional surface, known as slow invariant manifold (SIM), on which the solution evolves, while the system of ordinary differential equations (ODEs) governs the slow evolution of the solution on the SIM and its dynamics is characterized by the fastest of the slow time scales (M + 1), when the solution evolves sufficiently far from the boundaries of the SIM [29]. The identification of the slow characteristic CSP mode (i.e., the M + 1 mode) can be achieved only in the context of a reduced model, where the fast and slow modes can be well-defined [30][31][32][33][34]. The sign of the real part of the eigenvalue associated with each mode determines the mode's nature: a positive or negative real part indicates an explosive or dissipative mode, respectively.…”