S.B. Dorofeev

A model for detonation cell size prediction from chemical kinetics

A correlation between characteristic reaction zone widths, calculated from detailed chemical kinetic models, and experimentally measured or numerically simulated detonation cell sizes is analyzed. An approach is proposed to generalize such a correlation, taking into account the multidimensional structure of real detonations. It is based on the characteristic reaction zone width, δ, calculated at initial conditions that are representative for a multidimensional detonation wave. The ratio, A, of the detonation cell width, λ, to the characteristic reaction zone width δ is considered to be a function of two stability parameters (dimensionless effective activation energy and a parameter describing the relation between chemical energy and initial thermal energy of the combustible mixture). This approach is evaluated against experimental data and results of multidimensional calculations. The resulting semi-empirical correlation is suggested to describe the dependence of the λ/δ-ratio on the stability parameters. This gives a basis for the prediction of detonation cell widths from detailed chemical kinetic calculations over a wide range of mixture compositions and initial conditions.

Evaluation of limits for effective flame acceleration in hydrogen mixtures

Abstract Results of experiments and data analysis on turbulent flame propagation in obstructed channels are presented. The data cover a wide range of mixtures: H 2 /air, H 2 /air/steam (from lean to rich) at normal and elevated initial temperatures (from 298 to 650 K) and pressures (from 1 to 3 bar); and stoichiometric H 2 /O 2 mixtures diluted with N 2 , Ar, He and CO 2 at normal initial conditions. The dataset chosen also covers a wide range of scales exceeding two orders of magnitude. It is shown that basic flame parameters, such as mixture expansion ratio σ , Zeldovich number β and Lewis number Le , can be used to estimate a priori a potential for effective flame acceleration for a given mixture. Critical conditions for effective flame acceleration are suggested in the form of correlations of critical expansion ratio σ ∗ versus dimensionless effective activation energy. On this basis, limits for effective flame acceleration for hydrogen combustibles can be estimated. Uncertainties in determination of critical σ ∗ values are discussed.

Deflagration to detonation transition in large confined volume of lean hydrogen-air mixtures☆

The results of large-scale experiments on turbulent flame propagation and transition to detonation in a confined volume of lean hydrogen-air mixtures are presented. The experiments were in a strong concrete enclosure of 480 m3, and 69.9 m length. The experimental volume consists first of a channel (34.6 m length, 2.3 m height, 2.5 m width) with or without obstacles, a canyon (10.55∗6.3∗2.5 m), and a final channel. Ignition was with a weak electric spark at the beginning of the first channel. The effect of hydrogen concentration (9.8%–14% vol.) on turbulent flame propagation and transition to detonation was studied. The obstacle configuration in the first channel (blockage ratio 0.3, 0.6, and no obstacles), exit cross section to the canyon (1.4, 2, and 5.6 m2), and vent area at the end (0, 2.5, and 4 m2) were varied in the tests. Details of turbulent flame propagation, of pressure field, and of detonation onset are presented. A minimum of 12.5% of hydrogen was found to be necessary for transition to detonation. This is a much less sensitive mixture than those in which the onset of spontaneous detonation has previously been observed (minimum of 15% of hydrogen in air). The effect of scale on the onset conditions for spontaneous detonation is discussed. The characteristic geometrical size of the mixture for transition to detonation is shown to be strongly related to the mixture sensitivity.

Slow and fast deflagrations in hydrocarbon-air mixtures

Results of experiments are presented on the behavior of turbulent flames in mixtures of methane, ethane, and propane with air. Tests were performed in two explosion tubes, one with an inner-diameter of 174 mm and the other of 520 mm. It was found that (similar to hydrogen combustibles) the flame acceleration can be weak or strong resulting either in slow subsonic flames, or in fast supersonic combustion regimes. Critical mixture compositions for the threshold between slow and fast combustion regimes were determined in the tests. In lean mixtures of hydrocarbon fuels, the critical compositions were not dependent on the tube size. On the rich side, a significant scale effect was observed. Effect of the basic flame properties, such as mixture expansion ratio, laminar flame thickness, and effective, activation energy on the critical conditions for strong flame acceleration is discussed.

CREBCOM code system for description of gaseous combustion

Abstract An approach is presented to combine a system of criteria and combustion models into a computer tool for conservative estimates of possible pressure loads resulting from combustion of fuel–air mixtures. This approach is based on identification of characteristic combustion regimes by their relative severity, application of criteria to identify the strongest possible regime for particular initial conditions and geometry, and on development of appropriate three-dimensional models to describe flame propagation and loads for selected combustion regimes. A computer code (CREBCOM) was developed using this approach. Short descriptions of the code, criteria and models are given. The computer code was applied to simulate turbulent deflagration and detonation experiments at different scale and geometrical configurations. These experiments included tests with flame propagation in explosion channels (tubes) and large-scale tests in the RUT facility. Numerical simulations showed that such a code is able to give reliable estimations of overpressure for a wide range of combustion regimes, compositions and geometry.

Experimental study of flame acceleration and the deflagration-to-detonation transition under conditions of transverse venting

Abstract Results of experiments on critical conditions for flame acceleration and the deflagration-to-detonation transition in tubes with transverse venting are presented. Tests were made with hydrogen mixtures in two tubes (inner diameter of 46 and 92 mm) with obstacles. Ratios of vent area to total tube area were 0.2 and 0.4. Venting was shown to influence flame acceleration significantly. The greater the vent ratio, the more reactive the mixture necessary for development of fast flames. Critical conditions for flame acceleration in tubes with venting, expressed through a critical mixture expansion ratio σ cr , were found to be σ cr / σ 0 ∼1+2 α , where σ 0 is the critical value for a closed tube. Critical conditions for detonation onset in a vented tube were found to be very close to those in a closed tube with similar configuration of obstacles.

Detonation propagation, decay, and reinitiation in nonuniform gaseous mixtures

Experiments on the behavior of detonation waves in nonuniform mixtures are presented. The situation studied was the propagation of a detonation wave from a driver mixture of variable length through a concentration gradient of variable width into a less reactive acceptor mixture. The effect of the gradient on the transmission process were studied. A detonation tube of 174 mm id. was used. The tube was initially divided by a fast-opening, stretched rubber diaphragm. Stoichiometric hydrogen/air mixture was used in the driver section. Hydrogen/air mixtures (14.0–19.0% H 2 ) were used as acceptor mixtures. Natural diffusion was used to create a concentration gradient between two mixtures. It was shown that the behavior of detonations at concentration gradients depends significantly on the sharpness of the gradient. For relatively sharp gradients a detonation always decays in the nonuniform region. It can be reinitiated downstream in the acceptor mixture, if the driver length is large enough for a particular acceptor mixture. For relatively smooth gradients, detonation is able to propagate through without decay. The boundary between these cases is defined only by the value of sensitivity gradient for a particular pair of driver and acceptor mixtures. The critical value of the gradient depends strongly on the difference in energy content of driver and acceptor mixtures. The more overdriven is the detonation in the driver mixture compared to that in the acceptor, the sharper gradient is necessary for detonation decay. The order of magnitude of critical values of the gradient shows that evolution of the cellular structure may play a role effecting conditions for detonation decay at concentration gradients.

Turbulent jet initiation of detonation in hydrogen-air mixtures

Large scale experiments (50 m3) have been carried out on the initiation of detonation by means of a jet of hot combustion products. The effects of hydrogen concentration (18–30% vol.), jet orifice diameter (100–400 mm), and the mixture composition in constant volume explosion chamber (25–50%) were investigated. Both high enough hydrogen concentration and large enough jet size are necessary for detonation initiation. The minimum values are within the ranges of 20 to 25% vol. H2, and of 100 to 200 mm correspondingly. A minimum ratio of jet size and mixture cell width 12–13 is required for detonation initiation.

Air blast and heat radiation from fuel-rich mixture detonations

Large scale experiments were carried out to study the effect fuel concentration on air blast parameters and heart radiation from gaseous detonations. Hemispheric plastic envelope (4 meters in radius) was used with propane-air mixtures containing from 4 to 7 vol. % of fuel. The expressions for overpressures and impulses were determined in Sachs variables. The effect of fuel concentration on blast parameters is shown to be insignificant for the same amount of oxygen in the mixture volume. Thus the blast wave parameters can be described as for stoichiometric mixtures using additional scaling for the explosion energy according to oxygen content (cloud volume). The results of large scale experiments with fuel spray clouds containing 0.16–100 tons of fuel with mean concentration from stoichiometric (C0) up to 3C0 are reconsidered. These results confirm the proposed scaling of air blast parameters for a wide range of fuel types, cloud volumes and fuel concentrations. Detonations of fuel rich gaseous mixtures result in a strong heat radiation. Heat radiation energy, time and size of the fireball formed are studied as a function of fuel concentration.

Direct initiation of detonations in unconfined gasoline sprays

Large scale experiments on detonation initiation in gasoline-air by two different sources were carried out at stoichiometric conditions. Unconfined clouds of 1100 m3 volume generated by a special facility had a shape of semicylinder of 15–17 m in length and 6–8 m in radius. Both the charge of condensed HE and the charge of stoichiometric propane-air were used to initiate detonation in the mixture. In case of initiation by a propane-air charge the critical initiation energy was up to 7 times as large as that for HE initiation. The detonation cell size for gasoline-air was determined as 0.04–0.05 m. It was shown, that the well-known correlation between the critical energy of point blast initiation and the cell size failed for this system. The cell size obtained is close to one of propane-air, but no direct transfer of detonation from one mixture to another was observed.