Carlos Fernandez-Pello

A Unified Analysis of Concurrent Modes of Flame Spread

A theoretical study is conducted of the spread of flames in gas flows moving in the direction of flame propagation. The study draws on the fact that concurrent flame spread configurations have common general characteristics, thus allowing a unified treatment of the problem. The analysis, although approximate, provides an analytic expression for the rate of flame spread over the surface of thermally thick fuels in laminar gas flow configurations that accept a similarity solution. The predicted flame spread rates for these particular flows provide limits for the rates of spread in other non-similar gas flow configurations.

An enthalpy-temperature hybrid method for solving phase change problems and its application to polymer pyrolysis and ignition

A numerical approach based on the enthalpy method is proposed for solving generalized phase-change problems. The method is applied to predict pyrolysis and ignition of polymeric combustible materials. In contrast to the traditional approach, here both enthalpy and temperature are treated as independent variables, and the conservation equations are solved simultaneously in conjunction with the constitutive equations. Also, the formulation of the constitutive equations for the phase change is not necessarily the same for all of the possible phases, but can be chosen independently according to the characteristics of the physical problem and the requirements of the numerical analysis of each respective phase. Thus with this new approach, which we refer to as the enthalpy-temperature hybrid method, the enthalpy method is applicable to the generalized phase-change problems regardless of the form of the constitutive equations. The proposed method is first applied to a one-dimensional classical freezing problem for verification. It is found that the numerical results for the temperature history and the position of the phase-change interface agree well with the analytic solution existing in the literature. The method is then applied to the numerical simulation of the pyrolysis and ignition of a composite material with a polymer as the matrix and fibreglass as the filling material. Three models of oxygen distribution in the molten layer are considered to explore the melting and oxygen effects on the polymer pyrolysis. Numerical calculation shows that high oxygen concentrations in the molten layer enhance the pyrolysis reaction, resulting in a larger amount of pyrolysate, but in lower surface temperatures of the sample. It also shows that the distribution of oxygen in the molten layer has a strong effect on the pyrolysate rate, and therefore on ignition and combustion of the polymers. Comparison with available experimental data indicates that a model of oxygen distribution in the molten layer that is limited to a thin layer near the surface best describes the ignition process for a homogeneously blended polypropylene/fibreglass composite.

Flammability limits of biogenic volatile organic compounds emitted by fire-heated vegetation (Rosmarinus officinalis) and their potential link with accelerating forest fires in canyons: a Froude-scaling approach

The accelerating forest fire phenomenon for two real accidents is studied. This phenomenon is investigated using the thermochemical hypothesis, based on the ignition of a biogenic volatile organic compounds cloud accumulated in canyons. By heating a Rosmarinus officinalis plant in a specific hermetic enclosure, a mixture of 14 biogenic volatile organic compounds is identified and their mass fractions determined as temperature functions. The theoretical flammability limits of those components are calculated by means of empirical correlations. Froude-scaling law is applied to laboratory emission results to find the concentrations of biogenic volatile organic compounds at field scale. The comparison of the flammability limits with the calculated concentrations at real scale using this changing-scale analysis shows that the emitted biogenic volatile organic compounds can lead to an accelerating forest fire.

Characterizing the Flaming Ignition of Cellulose Fuel Beds by Hot Steel Spheres

In this work, the flaming ignition of powdered cellulose fuel beds by hot steel spheres of various diameters and initial temperatures is studied. The spheres were heated in a tube furnace and then dropped onto a fuel bed, and the occurrence of flaming ignition or lack thereof was visually observed and recorded. A multivariable logistic regression method was developed and used to obtain an approximate ignition probability distribution and flaming ignition limits for the parameter space investigated. Each test was also recorded using high speed Schlieren videography, which gave qualitative insight into the ignition process. Ignition of larger spheres (>8.5 mm) appears to depend more strongly on sphere temperature than diameter. Alternatively, ignition by small spheres (<3.5 mm) appears less sensitive to temperature and more sensitive to sphere size. It was also confirmed that ignition cannot be predicted by bulk sphere energy alone. Flaming ignition occurs in the gas phase, in some cases away from the ball surface. Based on this and other evidence, it is proposed that ignition occurs when heat generation in the surrounding mixture of gasified fuel and air over-comes diffusive losses long enough for thermal runaway to occur. It is also noted that bulk energy may be important for predicting ignition by small spheres because of the increasing importance of the energy required to pyrolyze and produce an ignitable mixture.

Effect of Environmental Variables on Flame Spread Rates in Microgravity

This paper reports 2D CFD-based computer modeling of opposed flow flame spread over thick samples of polymethylmethacrylate (PMMA). Model predictions are compared with experimental data from normal-gravity experiments at multiple forced flow velocities and KC-135 parabolic flight microgravity experiments. For the normal gravity experiments, good agreement between the model predictions and experimental data is obtained at one oxygen level, but flame spread rates at other oxygen levels are not well predicted. Of the four microgravity data points, the model underpredicts the spread rate of two of the data points by 35% or less. However, the model overpredicts the other two data points by almost a factor of two. Potential reasons for the discrepancies between the model predictions and the experimental data are discussed.

Micro, Internal-Combustion Engine Fabrication With 900 µm Deep Features Via DRIE

New results are presented for the development of a micro, internal-combustion engine fabricated in a process that achieves 900 μm deep features via deep reactive ion etching (DRIE). A single-sided 900 μm deep etch process with high mask selectivity is used to generate straight sidewall structures with low sidewall roughness. This research is part of an effort to create a portable, MEMS-based Rotary Engine Power System (MEMS REPS) capable of producing power on the order of milliwatts with an energy density better than that of a conventional battery. The MEMS REPS is based on the planar geometry and self-valving operation of a Wankel engine with an integrated electrical generator. A generator and stator colocated within the engine rotor and housing eliminates the need for any external shafts, couplings, or seals. The rotary internal combustion engine is composed of 5 major comonents: a 900 μm deep rotor with soft magnetic poles and 25 μm wide in-plane cantilever beams which act as apex seals, a 900 μm deep epitrochoid housing with intake and exhaust ports, rear plate with spur gear, a top plate, and a shaft. This configuration was chosen in order to eliminate the effect of beaching during timed DRIE etches and to minimize engine leakage while maximizing spur gear teeth resolution, and simplifying engine fabrication. However, this configuration requires some assembly and optimization of DRIE parameters for each component. The rotor and epitrochoid housing are co-fabricated on the same wafer to minimize deviation in thickness and match etch behavior between mating components. This approach forces the generation of a mask with narrow, deep trenches (to define the cantilever apex seals on the rotors) in proximity to large “tub” etches (to define the engine housing). High etch cycle pressures improved etch selectivity to over 350:1 with respect to oxide and 150:1 to photoresist which is necessary for 900 μm deep features. High pressure also improved sidewall profile of the etched structures. Engine cross-sections show an 8 μm wall deviation on either side of a 250 μm trench through an etch depth of 867 μm. In addition to good sidewall straightness, these etch parameters give a low sidewall roughness through the generation of small size scallops on the sidewalls. However, the side effects of these etch parameters include silicon “grass” at the bottom of the trench, poor etch uniformity across the wafer, and increased effect of aspect ratio dependent etching. Some strategies to overcome these effects are discussed.Copyright

An experimental and numerical study of flames in narrow channels with electric fields

The advancement of microscale combustion has been limited by quenching effects as flames cease to be much smaller than combustors. The long studied sensitivity of flames to electrical effects may provide means to overcome this issue. Here we experimentally and numerically investigate the potential of electric field effects to enhance combustion. The results demonstrate that, under specific conditions, externally electric fields will sustain combustion in structures smaller than the quenching distance. The analysis proposes a reduced mechanism to model this result and provides a study of the governing parameters. We find good qualitative agreement between the model and experiments. Specifically, the model is found to successfully capture the capacity to increase and decrease flame speed according to electric field magnitude and direction. Further, in both experiments and computations the sensitivity to electrical enhancement increases for more energetic mixtures. We do find that the model underpredicts the maximum achievable speed enhancement observed, suggesting that additional phenomena should be included to expand the range of conditions that can be studied.

1 – Understanding materials flammability

Publisher Summary Flammability is the ease with which a material is ignited, the intensity with which it burns and releases heat once ignited, its propensity to spread fire, and the rate at which it generates smoke and toxic combustion products during gasification and burning. A comprehensive evaluation of a materials overall flammability may require data from several laboratory tests, perhaps combined with some form of analysis or modeling to interpret the results properly. Several of the fire properties like ignitability can be determined from bench-scale flammability tests. It is useful for establishing relative rankings or for developing input data for predictions of large-scale fire behavior. The cost of small-scale fire testing is considerably less than that of large-scale fire tests because relatively small quantities of sample material are required, and the setup and breakdown time is much shorter. These factors make bench-scale flammability testing a cost-effective screening tool and can reduce a new materials time-to-market. Due to potential time and cost savings, combined with an increased recognition of the importance of material fire properties, there is considerable interest in using data obtained from small-scale flammability tests in conjunction with correlations or models to predict large-scale fire behavior.

Biogenic volatile organic compounds emissions at high temperatures of common plants from Mediterranean regions affected by forest fires

Vegetal species emit biogenic volatile organic compounds at elevated temperatures. Because of their combustibility, biogenic volatile organic compounds can modify the wildland fires propagation dynamics, changing them from a moderate behavior to an explosive propagation. This phenomenon is known as an accelerating forest fire. The origin of such phenomena can be the accumulation of biogenic volatile organic compounds in concentrations close to their lower flammability limit in seasons where the plants are themselves very flammable. There is a lack of information on the biogenic volatile organic compounds emissions of vegetal species typically found in wildland fires at temperatures higher than ambient temperature. In this work, we used a flash pyrolysis device linked to a gas chromatograph/mass spectrometer to investigate experimentally the biogenic volatile organic compounds emissions of Thymus vulgaris, Lavandula stœchas, and Cistus albidus between 70°C and 180°C. High amounts of terpenoid compounds were found, except for C. albidus emissions, including thymol, l-fenchone, and 3-hexen-1-ol. The information provided in this work could help to improve the characterization of thermal degradation of vegetal fuels and to incorporate the biogenic volatile organic compounds combustion in physical forest fires models. They also show that under the right circumstances, biogenic volatile organic compounds from these vegetal species could contribute to the development of an accelerating forest fire.

Ignition Delay of Combustible Materials in Normoxic Equivalent Environments

Material flammability is an important factor in determining the pressure and composition (fraction of oxygen and nitrogen) of the atmosphere in the habitable volume of exploration vehicles and habitats. The method chosen in this work to quantify the flammability of a material is by its ease of ignition. The ignition delay time was defined as the time it takes a combustible material to ignite after it has been exposed to an external heat flux. Previous work in the Forced Ignition and Spread Test (FIST) apparatus has shown that the ignition delay in the currently proposed space exploration atmosphere (approximately 58.6 kPa and 32% oxygen concentration) is reduced by 27% compared to the standard atmosphere used in the Space Shuttle and Space Station. In order to determine whether there is a safer environment in terms of material flammability, a series of piloted ignition delay tests using polymethylmethacrylate (PMMA) was conducted in the FIST apparatus to extend the work over a range of possible exploration atmospheres. The exploration atmospheres considered were the normoxic equivalents, i.e. reduced pressure conditions with a constant partial pressure of oxygen. The ignition delay time was seen to decrease as the pressure was reduced along the normoxic curve. The minimum ignition delay observed in the normoxic equivalent environments was nearly 30% lower than in standard atmospheric conditions. The ignition delay in the proposed exploration atmosphere is only slightly larger than this minimum. In terms of material flammability, normoxic environments with a higher pressure relative to the proposed pressure would be desired.