D. N. Schiller

Computational Analysis of Flame Spread Across Alcohol Pools

Pulsating and uniform spread across n-propanot and ethanol liquid fuel pools is simulated via a two-dimensional, transient, numerical model which incorporates finite-rate chemical kinetics, variable properties, and a partially adaptive finite-difference gridding scheme. The model is compared to detailed, independent experimental data. The explanation and characterization of the pulsating and uniform flame spread phenomena are developed Pulsating flame spread requires a gas-phase recirculation cell just forward of the flame. δ flow This cell entrains evaporating fuel vapor. The size and existence of the recirculation cell is determined by the extent of liquid motion ahead of the flame (δ flow) and by opposed flow in the gas phase, naturally induced by buoyancy. The amplitude and period of the pulsations each increase with δ flow Over the range of pool depths that were investigated (2 to 10 mm) the liquid-phase flow is primarily affected by large surface-tension variations along the liquid surface and not b...

Axisymmetric flame spread across propanol pools in normal and zero gravities

Axisymmetric ignition and flame propagation across propanol pools is investigated numerically with finite-rate one-step chemical kinetics, variable properties, and an adaptive finite-difference gridding scheme without forced gas-phase flow. Propanol fuel is allowed to evaporate into the air after filling the fuel tray and before the igniter is activated. The propanol vapor profile in the gas phase is examined numerically as a function of time. At 1 -g0, a steady fuel vapor-concentration boundary layer is established in the gas phase 5 s after filling a fuel tray. Conversely, at 0-g0, the profile of fuel vapor concentration is always unsteady and keeps growing in the gas phase as time elapses. Axisymmetric flame spread over a shallow circular pool is examined as a function of oxygen concentration. The flame spread rate at \-g0 is comparable to but lower than that at 0-g 0 for χO2,∞ ≥ 21% and T0 = 22.1°C with ΔtPI = 5s, where ΔtPI denotes the time lapse between filling the fuel tray and activating the ignit...

Opposed-flow flame spread across n-propanol pools

A computational study is made of the effects of forced, opposed air flow on pulsating and uniform flame spread across n-propanol pools at either normal or zero gravity conditions. The numerical model incorporates finite-rate chemical kinetics, variable properties, and an adaptive gridding scheme in the direction of flame spread. In zero gravity, the combination of forced, opposed flow and thermocapillary-driven concurrent flow can cause a gas-phase recirculation cell to form ahead of the flame and thus lead to flame pulsations. The pulsation mechanism, in this case, is essentially the same as that previously detailed for pulsating flame spread in normal gravity without forced flow [1]. In either normal gravity or zero gravity, increasing the opposed air speed (Uopp) causes a transition from uniform flame spread to pulsating flame spread. The value of Uopp that corresponds to this transition increases with initial pool temperature (Tc). Unlike with normal gravity flame spread, the mean flame spread rate (Ūfl) in zero gravity decreases significantly because of this transition for Tc

Buoyant thermocapillary flow with nonuniform supra-heating. I - Liquid-phase behavior. II - Two-phase behavior

The present computational study of transient heat transfer and fluid flow in a circular pool of n-decane which is undergoing central radiative heating from above gives attention to the volumetric absorption of the radiation incident on the pool surface. The first part of this study notes that buoyancy influences the number and recirculation rates of the subsurface vortices by stabilizing hot subsurface fluid above the colder core fluid; this affects the liquid surface temperature profile and in turn governs the velocity profile that is due to thermocapillarity. In the second part, the effects of gas-liquid phase coupling, variable density and thermophysical properties, and vaporization are considered. 49 refs.

Buoyant-Thermocapillary Flow with Nonuniform Supra-Heating: II. Two-Phase Behavior

A computational study has been made of transient heat transfer and fluid flow in a cylindrical enclosure containing a two-layer gas-liquid system heated nonuniformly from above. The effects of gas-liquid phase coupling, variable density and thermophysical properties, and vaporization have been considered. At all gravity levels investigated (0-1 gn), the presence of the gas has little effect on the liquid phase. Fuel vapor is transported more quickly to the heat source at higher gravity levels, which indicates that ignition delay should decrease with increasing gravity level. At reduced gravity, diffusion is dominant and surface tension significantly affects the flow pattern in the gas phase. The variation of density and thermal conductivity are important in the gas phase, whereas in the liquid phase, variable viscosity is most important.

Computational predictions of flame spread over alcohol pools

The effects of buoyancy and thermocapillarity on pulsating and uniform flame spread above n-propanol fuel pools have been studied using a numerical model. Data obtained indicate that the existence of pulsating flame spread is dependent upon the formation of a gas-phase recirculation cell which entrains evaporating fuel vapor in front of the leading edge of the flame. The size of the recirculation cell which is affected by the extent of liquid motion ahead of the flame, is shown to dictate whether flame spread is uniform or pulsating. The amplitude and period of the flame pulsations are found to be proportional to the maximum extent of the flow head. Under conditions considered, liquid motion was not affected appreciably by buoyancy. Horizontal convection in the liquid is the dominant mechanism for transporting heat ahead of the flame for both the pulsating and uniform regimes.

Energetic Fuel Droplet Gasification with Liquid-Phase Reaction

Abstract An exploratory analytical and computational study of gasifying energetic liquid fuel droplets is presented. The liquid-fuel reaction, heating, and breakup are modelled and characterized. A spherically-symmetric geometry is considered, and the problem is described by considering a general formulation capable of resolving the solution domain consisting of a liquid-phase region, a bubbly two-phase region, and a gas-phase region. The transient, two-phase, governing equations are solved numerically for various values of the nondimensional rate constant, heat of decomposition, activation energy, number of bubbles per unit mass, and ratio of gas-phase to liquid-phase thermal conductivities.

Transient Heating, Gasification, and Oxidation of an Energetic Liquid Fuel

Abstract An analytical and computational study of the gasification and oxidation of an energetic liquid fuel droplet is presented. Single-step, finite-rate, Arrhenius reaction rate expressions are used for exothermic liquid-phase decomposition and gas-phase oxidation. The liquid fuel is assumed to decompose to a gaseous product at a fixed number of bubble sites per unit mass (specified a priori ) within the droplet. Decomposed gas escapes the droplet surface by: (1) decomposition (gasification) of the droplet surface, (2) decomposition at the surface of bubbles that connect with the droplet surface, and (3) escape of gas inside bubbles due to droplet surface regression. The transient, two-phase, governing equations are solved numerically for various values of the nondimensional reaction rate coefficients (for both decomposition and oxidation), heats of decomposition and oxidation, number of bubbles per unit mass ( N / m ), and ambient temperature and pressure. After an ignition delay period, the flame radius is predicted to increase nearly linearly with time until the droplet is gasified. After this time (η d ,∗ ), the flame radius decreases with time. The variation of flame radius with time differs from classical droplet burning due to the exothermic decomposition process that determines the gasification rate and influences the heat flux at the droplet surface. Simplified scaling previously derived for the droplet lifetime also correlates the effect of decomposition parameters on the flame behavior. Gas-phase oxidation does not appreciably affect the droplet lifetime, for the selected base case values of the above parameters, because droplet heating is controlled primarily by the liquid-phase decomposition. As the decomposition rate is reduced (e.g., by reducing N / m ), the time scale for heat conduction from the flame to the droplet becomes comparable to that of liquid decomposition, and hence gas-phase oxidation significantly reduces η d . For the base case, liquid-phase decomposition increases the flame temperature by approximately 6%.