Steady-state Combustion Regime Research Articles

Numerical Simulations of a Mechanically-Ventilated Multi- Compartment Fire

The objective of this work was to evaluate the capabilities of a widely used Computational Fluid Dynamics (CFD) code in the fire community, namely the Fire Dynamics Simulator (FDS 5.5.3), in the simulation of a large-scale, well-confined and mechanically ventilated multi-room fire scenario. The CFD analysis focuses on the effect of pressure build-up induced by the fire on the ventilation network. The measured heat release rate (HRR) was therefore prescribed as input in the simulations. Computational results were compared to measurements obtained for one of several experimental scenarios performed at the French Institut de Radioprotection et de Surete Nucleaire (IRSN). The overall trend was well reproduced by FDS. Quantitative comparisons for respectively the total relative pressure, ventilation flow rates and gas temperature (in the fire room) at the steady-state combustion regime have shown underestimations of 18 to 22 %.

Droplet lasing spectroscopy applied to droplet stream flames

Droplet lasing spectroscopy (DLS) has been applied to the measurement of droplet size and vaporization rates in both reacting and non-reacting rectilinear droplet streams. A Berglund-Liu droplet generator was used to generate a stream of droplets, approximately 63 microns in diameter and 6.5 droplet diameters apart. Ethanol, methanol, and a pentane/ethanol mixture were doped with Rhodamine 6G. Lasing spectra were examined in the steady-state combustion regime. In the pentane/ethanol case the measurements were carried out in a sooting region of the flame. In some cases, vaporization rates were high enough to measure the rate from consecutive droplets, yielding a quasi-instantaneous measurement. In all cases, the D 2 law of droplet vaporization was evident. In addition, photographs of the flames yielded measurements of flame height and thickness.

Frontal regime of exothermal conversion of refractory condensed compounds

We consider a mathematical model of propagation of the combustion front in heterogeneous condensed compounds. For these compounds, the adiabatic combustion temperature is lower than the melting point of the initial reagents and condensed products; it is also lower than the minimum temperature of their eutectics. The chemical interaction in the combustion front is accomplished by a gas transport mechanism due to the gasification of additive oxides. Within the framework of the investigated model, values are obtained for the front velocity, which are of the same order as those observed experimentally. Periodic unstable combustion regimes (fluctuating and spinning) arise when the macrokinetics of the chemical transformation is strongly activated. It is shown that the activation can depend on the gasification energy and, consequently, the evaporation of additive oxides influences the stability of a steady-state combustion regime.

Instability of a combustion model with surface vaporization and overheat in the condensed phase

Using numerical modeling, we have revealed the instability of a steady-state combustion regime which was previously obtained using analytical methods in the substance-combustion model with surface vaporization and with exothermic reactions in the condensed phase which are intense enough to form a maximum of temperature underneath the surface. The instability has been studied analytically using the method of small perturbations to eliminate the version of its nonphysical (numerical) character. Steady-state combustion regimes with maxima on the condensed-phase temperature profile are shown to be actually unsteady. It is suggested that convection in a liquid-subsurface layer owing to bubble motion caused by the Marangoni effect should be taken into account to describe correctly the experimentally observed steady-state regimes with a leading role of the condensed phase.

Experimental study of different regimes of low-gas compound combustion in argon at atmospheric pressure

To study the possibilities of occurrence of a low-temperature steady-state combustion regime and to determine the existence regions of other regimes in more detail, we performed experiments on combustion of low-gas compounds in argon at atmospheric pressure. As a model system, we used a diluted ferro—zirconium thermite with a gas-forming admixture of molybdenum trioxide. The absence of a low-temperature steady-state combustion regime and a change in the existence regions of the other combustion regimes for low-gas systems compared with those observed for combustion in air were revealed.

Seepage gas combustion

Seepage gas combustion is defined as the propagation of a region of gaseous exothermic reaction in an inert porous medium, as the gaseous reactants seep into the region of chemical transformation. Such combustion is heterogeneous because of the active participation of two phases (a solid porous medium and a reacting gas) in the propagation of the combustion waves. The study of these waves revealed the existence of two subsonic steady-state combustion regimes, one with low propagation velocity and one with high propagation velocity. In this paper the theoretical study of seepage gas combustion is based on a system of one-dimensional equations describing the propagation of heat in the inert medium and the gas, the mass transfer of the deficient component, and the conservation of the total amount of matter, together with the equation of state of an ideal gas. The parameters of seepage combustion, including the stationary and velocity of the combustion wave, the wave structure, and the wave propagation mechanism and limits, were derived for both velocity regimes. Supersonic regimes in porous media were also analyzed.

Effect of oxidizer-particle size on the unsteady-state combustion time of a composite solid fuel during a pressure drop

A rapid variation of pressure during the combustion of a composite solid fuel alters considerable the composition and temperature of the combustion products during the unsteady-state-combustion period. The time required to reach a new steady-state combustion regime following a sharp rise in pressure is proportional to the burnup time of a grain of oxidizer. The authors considered the dependence of the duration of the transition regime of combustion on the size of PCA crystals during a pressure drop. The results indicate that, both for a pressure drop and for a rise in pressure, the time required to arrive at a steady-state combustion regime is proportional to the burnup time of a grain of oxidizer, rather than to the warm up time of the condensed phase as is the case for homogeneous fuels.

Principles and mechanism of combustion in the zirconium—Carbon—Hydrogen system

The authors examine the principles and mechanism of combustion in the Zr-C-H/sub 2/ system in order to clarify the generality of the effects observed previously. The experiments were performed in a constant-pressure vessel with type M-41 powdered zirconium, carbon black, and gaseous hydrogen. Chemical analysis was performed for the total and free contents of carbon, zirconium and hydrogen, and x-ray studies were performed using a DRON-0.5 analyzer. In the Zr-C-H/sub 2/ system the general tendency inherent in Me-C-H/sub 2/ systems toward completion of the hydrogenation and carbonization reactions is maintained. In this mixture, as in Ti-C-H/sub 2/, the steady-state combustion regimes are not the same when ignition sources of different temperatures are used.

Combustion of aluminized condensed systems under acceleration loading

1. As the acceleration increases, so do the burning rate and the mass of the slaggy residue, the more rapidly burning compositions being less sensitive to acceleration loading. 2. As the length of the specimen increases, so does the mass of the slag, whereas the mean burning rate decreases. 3. The proposed theoretical model of the combustion of aluminized systems in the presence of acceleration is in good agreement with the data of our experiments and those of other authors. 4. The maximum and minimum increases in the burning rate and the duration of the transition regime can be theoretically predicted from the acceleration and the characteristics of the system together with the size of the entrained particles and the mass of the slag in the steady-state combustion regime.