SESSION 4

KINETICS

ABSTRACT

The dynamics of laminar spherical flame kernels in hydrogen/air mixtures diluted with steam are studied numerically for hydrogen percentage ranging from lean to rich flammability limits at atmospheric pressure and temperature of 373 K. Ignition is simulated by means of a one-dimensional time dependent computational model (conservation of mass, energy, momentum) taking into account detailed chemistry, multicomponent transport properties, and thermal radiation of water molecules. The analysis is focused on the transient processes leading to the development or extinction of lean, stoichiometric, and rich flame kernels initiated by an ignition source with variable energy. The formation of a propagating flame kernel is used as a criterion for mixture flammability. The extinction turning-point behavior is demonstrated for lean hydrogen-air mixtures. The turning point corresponds to about 3.5% hydrogen at 373 K. Both dynamics and critical size of a flame kernel are strongly affected by hydrogen diffusion and thermal radiation. The specific effect of hydrogen diffusion is important for a range of steam diluted lean mixtures extending to the inerting point. The amount of steam required for quenching is 56.5% by volume. The characteristic modes of behavior of developing and extinguishing flame kernels are demonstrated and analyzed for near-limit lean hydrogen/air/steam mixtures confined in a spherical laboratory-scale explosion vessel. The flammability limits obtained by numerical simulations are in reasonable agreement with available experimental data.

ABSTRACT

An exhaustive experimental investigation has been performed on the structures of five low pressure premixed C₂H₄/O₂/Ar flames at equivalence ratios from 1.00 to 2.00. Stable species, atoms and radicals have been monitored by using molecular beam mass spectrometry (MBMS). This study has shown that some maximum concentrations of C_2^-, C_3^-, C_4^-, C_5^- and C_6^- species increase drastically with equivalence ratio. Some of these species like C₂H₂, C₃H₄ and C₄H₂ remain in non negligible amounts in burnt gases of the richest flames .(f = 1.75 and 2.00). We can consider they are soot precursors. It has been noticed for some C_3^- (C₃H₃ and C₃H₄) and C_4^- (C₄H₂, C₄H₄, C₄H₆, and C₄H₈) species a dependence in the form (X_i)_max = a_i fⁿⁱ in the equivalence ratio (f) range investigated. The exponent n_i of f depends on the species considered and decreases along the following sequence:

By using the net reaction rate of C₂H₄ , the rate coefficient of the reaction " C₂H₄ + O ->Products " has been deduced (k = 1.93 10¹³ cm³mol^-1s^-1 at 1400K). It is the main process leading to the formation of the first C_1^- compound in these flames, CH₃ or less likely CH₂. Moreover, it has been established that the building-up of C₃H₄ must proceed along the formation of the propenyl radical (C₃H₅) by the reaction between C₂H₂ and CH₃ . The deduced experimental rate coefficient of this reaction is 3.15*10¹¹exp(-1422/T) cm³mol^-ls^-1 from 1000K to 1670K.

In addition, the rate coefficient of reaction:

ABSTRACT

A detailed H/N/O reaction mechanism has been validated with respect to the experimental species profiles in the hydrogen oxidation by nitrous oxide (Allen, M.T. et al., Combust. Flame 112: 302311 (1998)). It is shown that incorporation of the reactions between NH and NH₂ radicals and N₂O with currently adopted rate constants prohibits correct modeling of the minor species, NH₃ and NO. A sensitivity analysis is used to reveal which reactions are critical for the quality of the modeling. The choice of the rate constants for these reactions is discussed. It has been concluded that only significant decrease in the rate constant of one or both reactions mentioned can improve the agreement between the modeling and experiment. The upper limit rate constant of the reaction

Ignition delays in the H₂ - N₂O - Ar mixtures at temperatures below 1400 K are calculated. The modeling is in agreement with the high temperature data, however at temperatures lower than 1200 K the difference is almost 10-fold. It is concluded that the ignition measurements in a static by-pass apparatus are incompatible with the flow reactor data.

ABSTRACT

In this work we analyse and study several experimental data on pyrolysis, partial oxidation and combustion of C₂ and C₃ acetylenics as well as allene and ketene. These data, already presented in the literature, refer to turbulent flow reactors, jet stirred reactors and shock tube experiments and span wide ranges of concentration, pressure and temperature conditions. In order to stress the importance of condensation reactions of unsaturated species some data of ethylene and propylene pyrolysis are also discussed.

C₂ and C₃ unsaturated species are always present as important intermediates in combustion processes and their reactions play a crucial role in explaining the formation of aromatic and polyaromatic species in sooting flames.

A detailed kinetic scheme able to describe the partial oxidation and the combustion of hydrocarbon mixtures up to commercial gasolines and liquid fuels is extended and more carefully validated on these sets of experimental data. The key reactions are discussed and compared with literature kinetic data. Moreover, this kinetic analysis is very useful for the study of 'mild combustion' in the intermediate temperature region where the 'flameless combustion' seems to offer the great advantage of a reduced formation of toxic components such as nitric oxides, polyaromatics and soot.

ABSTRACT

The optimisation of numerical methods applied to the description of char combustion behaviour is required in order to achieve a good prediction of the amount of unburned carbon in fly ash from pulverised coal fired boilers. A question arising is whether it is necessary to take into account the distribution of the coal properties and reactivity and how to describe them in. such a simplified way in order to be handled in a simulation code.

The reliability of a simple model for char heterogeneous oxidation describing the burning of single char particles has been assessed through the use of available experimental data for a high volatile bituminous coal in a pilot plant, under single stage and staged combustion conditions. Experiments in an electrodynamic chamber effected on the char particles collected in the pilot plant, showed the heterogeneity of particles properties, evaluated by fitting the temperature-time histories of single burning particles.

The model was later used to predict kinetics data at high heating rate to be used in comprehensive code: the results of the prediction provide a reactivity distribution expressed in the form of a pre-exponential factor as a function of particle size.