CH and C₂ Measurements Imply a Radical Pool within a Pool in Acetylene Flames

Abstract

Measured CH and C2 profiles show a striking resemblance as a function of time in a series of seven well-characterized fuel-rich (phi=1.2-2.0) non-sooting acetylene flames. This implied commonality and interrelationship are unexpected as these radicals have dissimilar chemical kinetic natures. As a result, a rigorous examination was undertaken of the behavior of each of the hydrocarbon species known to be present, C, CH, CH2, CH3, CH4, CHO, CHOH, CH2O, CH2OH, CH3O, CH3OH, C2, C2H, C2H2, CHCO, CH2CO, and C2O. This emphasized the main region where CH and C2 are observed (50-600 micros) and reduced the kinetic reactions to only those that operate efficiently and are dominant. It was immediately apparent that this region of the flame reflects the nature of a hydrogen flame heavily doped with CO and CO2 and containing traces of hydrocarbons. The radical species, H, OH, O, along with H2, H2O, and O2, form an important controlling radical pool that is in partial equilibrium, and the concentrations of each of the hydrocarbon radicals are minor to this, playing secondary roles. As a result, the dominant fast reactions are those between the hydrocarbons and the basic hydrogen/oxygen radicals. Hydrocarbon-hydrocarbon reactions are unimportant here at these equivalence ratios. CH and C2 are formed and destroyed on a sub-microsecond time scale so that their flame profiles are the reflection of a complex kinetically dynamic system. This is found to be the case for all of the hydrocarbon species examined. As might be expected, these rapidly form steady-state distributions. However, with the exceptions of C, CHO, CHOH, and CH2O, which are irreversibly being oxidized, the others all form an interconnected hydrocarbon pool that is under the control of the larger hydrogen radical pool. The hydrocarbon pool can rapidly adjust, and the CH and C2 decay together as the pool is drained. This is either by continuing oxidation in less rich mixtures, or in richer flames where this is negligible by the onset of hydrocarbon-hydrocarbon reactions. The implications of such a hydrocarbon pool are significant. It introduces a buffering effect on their distribution and provides the indirect connection between CH and C2. Moreover, because they are members of this radical pool, flame studies alone cannot answer questions concerning their specific importance in combustion other than their contributing role to this pool. The presence of such a pool modifies the exactness that is needed for kinetic mechanisms, and knowledge of every species in the system no longer is necessary. Furthermore, as rate constants become refined, it will allow for the calculation of the relative concentrations of the hydrocarbon species and facilitate reduced kinetic mechanisms. It provides an explanation for previous isotopically labeled experiments and illustrates the difficulty of exactly identifying in flames the role of individual species. It resolves the fact that differing kinetic models can show similar levels of accuracy and has implications for sensitivity analyses...

A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines

“…Since it has also been observed that the production of CH and NO increases with increasing hydrocarbon-chain length for small hydrocarbons [146,208,213,214], with the level of CH appearing to level off for the longest chains studied so far [214], it can be inferred that alcohol fuels, especially the short-chain alcohols, produce fundamentally lower NO x emissions than commercial gasoline or diesel hydrocarbon fuels under similar fuel-air stoichiometric ratios. The concentration of CH radicals increases with the carbon content of the fuel-air mixture [151], indicating that mixtures of the same stoichiometry, and similar H/C ratios in the fuel, should show similar CH levels. Recent experiments have shown that the predictions of CH and NO profiles for alkane and alcohol fuels, from methane and methanol to butane and butanol, in wellcharacterized laminar stagnation flames varies greatly from model to model [207,208,212,214], with relatively high uncertainties evident in all NO x formation routes [215,207].…”

“…Modeling results are not sensitive to the details of the various combustion chemistry models that have been proposed [136,146], since the intermediate species are inter-connected through complex reaction networks with very fast reaction rates between many common reaction partners. This inter-connectivity creates a local equilibrium that partitions the available carbon amongst these various possible intermediate species in a relative balance with each other [151]. The overall radical pool, therefore, evolves in unison, over space or time, as various species are added, by fuel breakdown reactions, or removed, by conversion of intermediates to stable combustion products, from the radical pool [146,151].…”

“…This inter-connectivity creates a local equilibrium that partitions the available carbon amongst these various possible intermediate species in a relative balance with each other [151]. The overall radical pool, therefore, evolves in unison, over space or time, as various species are added, by fuel breakdown reactions, or removed, by conversion of intermediates to stable combustion products, from the radical pool [146,151]. Therefore, even though the combustion chemical-kinetic mechanisms discussed above involve hundreds to thousands of different species, it is possible to reduce the mechanism size to around 50-100 species per surrogate fuel component while maintaining reasonable simulation accuracy of major combustion properties [136,138,145,[152][153][154][155].…”

“…Therefore, even though the combustion chemical-kinetic mechanisms discussed above involve hundreds to thousands of different species, it is possible to reduce the mechanism size to around 50-100 species per surrogate fuel component while maintaining reasonable simulation accuracy of major combustion properties [136,138,145,[152][153][154][155]. The number of additional intermediate species that must be added to the model, to maintain consistent accuracy, per surrogate-fuel component decreases as the total number of surrogate-fuel components increases [156,157], because many of the intermediate chemical species are shared between all hydrocarbon fuels [146][147][148][149][150][151][152]. Simplified 2-step models that use empirical fitting to match high-and lowtemperature chemical behavior have also been proposed, e.g.…”

Experiments and simulations of NOx formation in the combustion of hydroxylated fuels

“…Oxidation then proceeds primarily through acetaldehyde, as opposed to formaldehyde for the glycerol/ethylene glycol/methanol group. By proceeding through acetaldehyde, a greater pool of CH, CH 2 , and CH 3 radicals is formed than through formaldehyde which is irreversibly oxidized [44]. These CH radicals are ultimately needed to fix nitrogen to form NO x through non-thermal channels.…”