V. Ya. Basevich

Mechanisms of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C1–C10 to C11–C16

Recently, detailed kinetic mechanisms of the oxidation and combustion of higher hydrocarbons, composed of hundreds of components and thousands of elementary reactions, have been proposed. Despite the undoubtful advantages of such detailed mechanisms, their application to simulations of turbulent combustion and gas dynamic phenomena is difficult because of their complexity. At the same time, to some extent limited, they cannot be considered exhaustive. This work applies previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons, which takes into account the main processes determining the reaction rate and the formation of key intermediates and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: (1) it does not include reactions of so-called double oxygen addition (first to the peroxide radical, and then to its isomeric form), i.e., the first addition turns out to be sufficient; (2) it does not include isomeric compounds and their derivatives as intermediates, since this oxidation pathway is slower than the oxidation of molecules and radicals with normal structure. Application of the algorithm makes it possible to compile a compact mechanism, which is important for modeling chemical processes involving paraffin hydrocarbons Cn with large n. Previously, based on this algorithm, compact mechanisms of the oxidation and combustion of propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane have been constructed. In this work, we constructed a nonempirical detailed mechanism of the oxidation and combustion of hydrocarbons from n-undecane to n-hexadecane. The most important feature of the new mechanism is its staged nature, which manifests itself through the emergence of cool and blue flames during low-temperature autoignition. The calculation results are compared with experimental data.

Mechanisms of the oxidation and combustion of normal alkanes: Passage from C1-C4 to C2H5

A previously proposed algorithm was applied to constructing an optimal n-pentane oxidation mechanism capable of predicting the reaction rate and the formation of the main intermediate and final products. The mechanism can be considered a nonempirical detailed mechanism, since the constituent elementary reactions are kinetically validated. The mechanism is based on two assumptions: it ignores reactions involving double addition of oxygen (first, to a peroxy radical and then to its isomeric form) and isomer compounds and derivatives thereof. For low-temperature autoignition, the mechanism reproduces the stage character of the oxidation of n-pentane, more specifically, the emergence of cool and blue flames. The calculation results were compared with the published experimental data.

Simulation of the autoignition and combustion of n-heptane droplets using a detailed kinetic mechanism

The autoignition and combustion of n-heptane droplets are simulated using a detailed kinetic mechanism. A mathematical model, based on first principles, contains no adjustable parameters. The burning rate constants for the combustion of droplets are calculated over a wide range of pressures, temperature, fuel-to-oxidizer equivalence ratios of the gas-droplet suspension, and droplet diameters. The calculated and measured delay times of autoignition of droplets are compared. The calculation results agree well with the available experimental data. The detonability of gas-droplet suspensions with partial pre-evaporation of fuel is estimated.

Mechanisms of the oxidation and combustion of normal alkanes: Transition from C1-C5 to C6H14

A previously proposed algorithm of constructing optimal mechanisms of the low- and high-temperature oxidation and combustion of normal alkanes was applied to n-hexane. The proposed mechanism can be considered a nonempirical detailed mechanism, since all the constituent reactions have a solid kinetic substantiation. The mechanism features two main peculiarities: it contains no reactions of double oxygen addition (first to the peroxide radical and then to its isomerized form) and (2) involves no isomeric compounds and derivatives thereof. Application of the algorithm to n-hexane made it possible to create a new compact kinetic mechanism. The mechanism was demonstrated to correctly describe the multistage character of low-temperature self-ignition: the appearance of a cool and then a blue flame.

The mechanisms of oxidation and combustion of normal alkane hydrocarbons: The transition from C1–C3 to C4H10

The known detailed mechanisms of oxidation of the higher hydrocarbons include hundreds of particles and thousands of reactions. In spite of their merits, the use of such mechanisms for solving applied problems of the gas dynamics of combustion is impeded at present because of great computational expenditures. We suggest a compact kinetic mechanism of the oxidation of n-butane including the main processes and intermediate and final reaction products. The mechanism can be classified as a nonempirical detailed mechanism, because all its elementary reactions are kinetically substantiated. The mechanism does not contain reactions of the double addition of oxygen and intermediate species in the form of isomeric compounds and their derivatives. The calculation results are compared with the experimental data on the oxidation, self-ignition, and combustion of n-butane.

Autoignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous ternary mixtures

A numerical simulation of the ignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous (gas-drop) ternary mixtures for three hydrocarbon fuels (n-heptane, n-decane, and n-dodecane) is for the first time performed. The simulation is carried out based on a fully validated detailed kinetic mechanism of the oxidation of n-dodecane, which includes the mechanisms of the oxidation of n-decane, n-heptane, and hydrogen as constituent parts. It is demonstrated that the addition of hydrogen to a homogeneous or heterogeneous hydrocarbon-air mixture increases the total ignition delay time at temperatures below 1050 K, i.e., hydrogen acts as an ignition inhibitor. At low temperatures, even ternary mixtures with a very high hydrogen concentration show multistage ignition, with the temperature dependence of the ignition delay time exhibiting a negative temperature coefficient region. Conversely, the addition of hydrogen to homogeneous and heterogeneous hydrocarbon-air mixtures at temperatures above 1050 K reduces the total ignition delay time, i.e., hydrogen acts as an autoignition promoter. These effects should be kept in mind when discussing the prospects for the practical use of hydrogen-containing fuel mixtures, as well as in solving the problems of fire and explosion safety.

Detailed kinetic mechanism of the multistage oxidation and combustion of isobutane

The aim of the study is to construct a detailed kinetic mechanism of the oxidation and combustion of isobutane, capable of describing both the high-temperature process and the multistep process at low temperatures. Isobutane was chosen because it is the first member of the homologous series of isomerized alkanes, with isooctane, a higher member of the series, exhibiting a multistage autoignition in experiment. It is shown that, under certain conditions, the autoignition of isobutane occurs in three stages, typical of normal alkanes and isooctane: cool and blue flames and a hot explosion. The proposed detailed kinetic mechanism is used to calculate the ignition delay time and laminar flame speed, with the simulation results being compared with the available experimental data. A satisfactory qualitative and quantitative agreement is observed. Autoignition during compression and an increased knock resistance of isobutane with respect to autoignition in internal combustion engines is considered. The anti-knock properties of isobutane are demonstrated to be better than those of normal butane.

Oxidation and combustion mechanisms of paraffin hydrocarbons: Transfer from C1-C7 to C8H18, C9H20, and C10H22

At the present time, detailed kinetic mechanisms (DKMs) for higher hydrocarbons, which include hundreds of particles and thousands of reactions, are proposed. These DKMs have a number of undoubted advantages because they aspire to description of a wide class of phenomena. However, their application, e.g., for modeling of turbulent combustion, is difficult due to their extreme inconvenience. In addition, they are limited to a certain degree and cannot be considered to be comprehensive. As an alternative to such DKMs, we construct no maximum mechanism in this work, but an optimum mechanism of high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons. This mechanism, in accordance with the previously proposed algorithm, contains only general processes that govern the reaction rate and the formation of basic intermediate and final products. A such mechanism has the status of an nonempirical DKM because all parts, including elementary reactions, have kinetic substantiation. The mechanism itself has two features: (i) Reactions of so-called double addition of oxygen (first, to the alkyl radical, then to the isomerized form of the formed peroxide radical) are lacking because the first addition is considered to be sufficient; (ii) Isomeric compounds and their derivative substances as intermediate particles are not considered, because this means of oxidation is slower than through molecules and radicals of the normal structure. The application of the algorithm results in sufficiently compact mechanisms that are important for modeling chemical processes with the participation of paraffin hydrocarbons Cn with large n. Previously, this was done for propane, n-butane, n-pentane, n-hexane, and n-heptane; in the present article, it was done for n-octane, n-nonane, and n-decane. The major feature of all the mechanisms is the appearance of stages, viz., cold and blue flames during low-temperature spontaneous ignition. The direct comparison of the calculation and experiment results is carried out.

Correlation between Drop Vaporization and Self-Ignition

A parametric analysis of numerical solutions to problems of vaporization and self-ignition of liquid hydrocarbon drops was performed, and a new criterion determining the conditions of drop self-ignition was suggested. According to this criterion, self-ignition at a given reduced distance from the drop begins when the required reduced gas temperature and equivalence ratio are reached. A new model of heating and vaporization of drops in dense gas suspensions was suggested. The model was verified in multidimensional calculations of self-ignition and combustion of drop clouds. Calculations showed that the model correctly described the phenomenology of local formation and anisotropic propagation of self-ignition waves in suspensions of drops in gases.

Modeling of Low-temperature oxidation and combustion of droplets

Key features of radiation extinguishing of spherical hot flame around a single droplet with its subsequent low-temperature oxidation and combustion under microgravity conditions—a phenomenon discovered in experiments onboard the International Space Station—have been reproduced using the mathematical model of droplet combustion and detailed kinetic mechanism of n-heptane oxidation and combustion. It has been demonstrated that experimentally observed repeated temperature flashes were blue flame flashes, and their duration was determined by the hydrogen peroxide decomposition time. In addition to this phenomenon, calculations predict the existence of new modes of low-temperature oxidation and combustion of droplets without the hot flame stage. In such modes, the basic reaction is concentrated very close to the droplet surface, and fuel vapor reacts in it only partially.