Bruna Comas

Bio: Bruna Comas is an academic researcher. The author has contributed to research in topics: Combustion & Premixed flame. The author has an hindex of 6, co-authored 6 publications receiving 62 citations.

Experimental Study of the Effects of Side-Edge Burning in the Downward Flame Spread of Thin Solid Fuels

Abstract: A comparison between the downward flame spread rate for thermally thin samples with one or two inhibited edges is done in multiple situations. The effects of atmospheric composition as well as the width and thickness of a cellulosic-type fuel are tested experimentally. We have found that the normal velocity to the inclined flame front in a side-edge burning is very similar to the downward flame front speed when the sample is inhibited by both edges. Also, the effect of locating a sidewall close to the free edge of the sample is investigated. All these results may be important in order to validate or refute possible models of downward flame spread that take into account side effects.

Experimental study of the channel effect on the flame spread over thin solid fuels

Abstract: We experimentally burn thin solid fuels and obtain the speed of the flame front when it propagates (1) within a narrow channel (closed cross section), (2) within a channel with lateral walls only and (3) through a free cross section (plain case). The latter configuration is the classical one and it has been extensively studied with analytical, numerical and experimental methods by other authors. Our experiments have been carried out at different geometrical configurations and angles of inclination of the sample and also at several values of oxygen molar fraction. All experiments are restricted to purely buoyant flow. Our main results are as follows: (1) sidewalls reduce the flame spread rate in a non-monotonous trend when varying its height; (2) in horizontal flame spread, two simultaneous flame fronts that propagate at different velocities may arise in the channel case at high oxygen levels. The fastest flame front speed may be higher than that obtained in the plain case; (3) in upward flame spread, the channel effect configuration produces the highest flame front speed. We finally analyze the correlation of the downward flame front speed data in terms of the Damkohler number.

Flame front speed and onset of instability in the burning of inclined thin solid fuel samples.

Abstract: We focus on the front propagation of diffusive flames obtained from the downward burning of inclined thermally thin solid fuels. This process consists of a pyrolysis reaction in the solid-phase and a combustion reaction in the gas phase. The solid-phase model is based on two coupled one-dimensional equations of temperature and solid density. We reduce the system into a single one-dimensional equation from which we obtain an analytical expression for the flame front speed. This expression may be understood as an upper bound of the burning spread rate in inclined samples. The gas-phase model is based on four coupled two-dimensional equations. These are employed to derive a criterion for determining the critical inclination angle beyond which the flame behavior becomes unstable. The comparison with the experiments confirms the validity of our predictions.

Energy Balance Models of Downward Combustion of Parallel Thin Solid Fuels and Comparison to Experiments

Abstract: We analyze the flame front speed in the downward combustion of multiple parallel samples of thermally thin fuels at normal gravity and far from extinction conditions. In contrast with the single sample case, where conduction through the gas-phase is the dominant heat transfer mechanism, in the multiple parallel samples case, radiative heat fluxes may become very relevant, which compromises the application of the well-known formula of de Ris for determining the burning rate. Here we study the downward combustion of multiple parallel sheets by (1) obtaining new experimental data at different oxygen atmospheric levels; (2) generalizing a previous comprehensive energy balance model now expected to be valid for a wide range of scenarios; and (3) deriving an analytical approximation for the burning rate that generalizes the classical de Ris formula for those cases where radiative effects cannot be neglected. The comparison with own as well as with external data reveals the strengths and weaknesses of these type...

Analytical expressions for the flame front speed in the downward combustion of thin solid fuels and comparison to experiments

Abstract: We derive analytical expressions for the propagation speed of downward combustion fronts of thin solid fuels with a background flow initially at rest. The classical combustion model for thin solid fuels that consists of five coupled reaction-convection-diffusion equations is here reduced into a single equation with the gas temperature as the single variable. For doing so we apply a two-zone combustion model that divides the system into a preheating region and a pyrolyzing region. The speed of the combustion front is obtained after matching the temperature and its derivative at the location that separates both regions. We also derive a simplified version of this analytical expression expected to be valid for a wide range of cases. Flame front velocities predicted by our analytical expressions agree well with experimental data found in the literature for a large variety of cases and substantially improve the results obtained from a previous well-known analytical expression.

Experimental investigation on downward flame spread over rigid polyurethane and extruded polystyrene foams

Abstract: With the development of social economy and architectural aesthetics, flame spread over exterior thermal insulation system in real high-rise building fire disasters often features inclined downward or upward propagation, however most developed models for flame behavior prediction only assume absolute vertical or horizontal direction. Some researchers have explored the behavior of inclined upward flame spread for its quick development and hazardous characteristics. However, for downward flame spread over plastic polymers, theirs flame spread characteristics can also feature a special hazardous configuration, especially for porous and thermoplastic polymers. In this study, the orientation effects during inclined downward flame spread processes were thoroughly investigated by experimental and theoretical methods. Two kinds of typical thermal insulation materials were selected, rigid polyurethane (RPU) and extruded polystyrene (XPS) foams. The mass flow rate and flame spread rate at different inclination angles were obtained and analyzed. The different flame spread behaviors of RPU and XPS foams were scrutinized. Experimental results show that a linear relationship exists between flame spread rate and cube root of sine of sample inclination angle of RPU and XPS foams when the slope is smaller than a transition angle. The mechanism of orientation effect during the flame spread process was qualitatively analyzed in detail, and simplified expressions of flame spread rate of the two insulation materials with different orientations were deduced. The results of this study have implications concerning the fire safety design of exterior thermal insulation wall.

An experimental study on combustion performance and flame spread characteristics over liquid diesel and ethanol-diesel blended fuel

Abstract: Ethanol has gradually become a commonly used additive in diesel to reduce the carbon foot print of the combustion products. In the process of production, transportation and storage, once the accidental leaked ethanol-diesel blend fuel is ignited, the flame will spread very fast and cause great threaten to people's lives and properties, the combustion properties of ethanol-diesel blends is thus a matter of great concern to us. A set of experiments was conducted to study the effects of fuel depth and ullage height on liquid flame spread of diesel and 5% ethanol-diesel. Results showed that as the fuel depth increases under ullage effect, the flame spread rate increases first and then maintain constant. For 5% ethanol-diesel, the flame spread rate decreases monotonously and then tends to be unchanged with ullage height due to the higher combustion efficiency and oxygen content of ethanol-diesel. For deep pools, the surface velocity increases with fuel depth. Moreover, the flash flame pulsation frequency is weakly affected by the fuel depth and presents a negative relationship with ullage height. The study on these issues may have the potential to benefit the current safety production, utilization and management of the energy of ethanol-diesel blends.

Twenty-Sixth Symposium (International) on Combustion, Volume 2.

Abstract: : Partial contents: Non-Premixed turbulent combustion; Premixed turbulent combustion; Elementary reactions kinetics; Kinetic mechanisms--models and experiments; Laminar premixed flames; Laminar diffusion flames; Microgravity combustion; Chemical flame inhibition; Fire safety; Spray combustion; Catalytic combustion. p1-16 pages vary. Materials synthesis; Metals combustion; Propellants; NOx formation and control; Soot formation and destruction; Incineration and fuel substitution; Diesel engines; Spark ignition engines; Gas turbines; Active combustion control; High speed combustion; Detonations; Coal and char combustion; Fluidized beds; Porous media, fixed bed combustion, and furnaces,

Experimental Study of the Effects of Side-Edge Burning in the Downward Flame Spread of Thin Solid Fuels

Abstract: A comparison between the downward flame spread rate for thermally thin samples with one or two inhibited edges is done in multiple situations. The effects of atmospheric composition as well as the width and thickness of a cellulosic-type fuel are tested experimentally. We have found that the normal velocity to the inclined flame front in a side-edge burning is very similar to the downward flame front speed when the sample is inhibited by both edges. Also, the effect of locating a sidewall close to the free edge of the sample is investigated. All these results may be important in order to validate or refute possible models of downward flame spread that take into account side effects.

Influence of vertical channel on downward flame spread over extruded polystyrene foam

Abstract: The influence of vertical channel with various structure factors (α) on downward flame spread over extruded polystyrene (XPS) foam is investigated. The flame shape, flame height (Hf), temperature in the channel (Tv) and on the curtain wall surface (Tc), flame spread rate (Vf), and induced air flow (Q) are obtained. Hf first drops and then increases as α rises, attributing to similar changing trends of induced airflow rate and Froude number. Concerning the temperature history, Tc > Tv at the initial stage while Tc T c ‾ (average value) and T v ‾ drop as α rises, and T v ‾ > T c ‾ under most conditions. A model is established to predict the total, convective and radiative heat fluxes transferred to preheating zone. The convective heat flux is more dominant than the radiative heat feedback from the curtain wall. Vf first rises and then drops as α increases. For α