83 research outputs found
A Fundamental Study of Smoldering Combustion in Microgravity
A research program is being conducted to study smoldering combustion in microgravity. The program's final objective is to design and conduct smolder experiments in a space based laboratory, which will complement normal gravity ones, and that will help to: increase the current fundamental understanding of smoldering; predict smolder behavior in a space-based installation; and prevent and control smolder originated, ground or space based, fires
A Fundamental Study of Smoldering with Emphasis on Experimental Design for Zero-G
A research program to study smoldering combustion with emphasis on the design of an experiment to be conducted in the space shuttle was conducted at the Department of Mechanical Engineering, University of California, Berkeley. The motivation of the research is the interest in smoldering both as a fundamental combustion problem and as a serious fire risk. Research conducted included theoretical and experimental studies that have brought considerable new information about smolder combustion, the effect that buoyancy has on the process, and specific information for the design of a space experiment. Experiments were conducted at normal gravity, in opposed and forward mode of propagation and in the upward and downward direction to determine the effect and range of influence of gravity on smolder. Experiments were also conducted in microgravity, in a drop tower and in parabolic aircraft flights, where the brief microgravity periods were used to analyze transient aspects of the problem. Significant progress was made on the study of one-dimensional smolder, particularly in the opposed-flow configuration. These studies provided enough information to design a small-scale space-based experiment that was successfully conducted in the Spacelab Glovebox in the June 1992 USML-1/STS-50 mission of the Space Shuttle Columbia
Buoyancy Effects on Concurrent Flame Spread Over Thick PMMA
The flammability of combustible materials in a spacecraft is important for fire safety applications because the conditions in spacecraft environments differ from those on earth. Experimental testing in space is difficult and expensive. However, reducing buoyancy by decreasing ambient pressure is a possible approach to simulate on-earth the burning behavior inside spacecraft environments. The objective of this work is to determine that possibility by studying the effect of pressure on concurrent flame spread, and by comparison with microgravity data, observe up to what point low-pressure can be used to replicate flame spread characteristics observed in microgravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread over 10 mm thick polymethyl methacrylate (PMMA) slabs. Experiments in normal gravity were conducted over pressures ranging between 100 and 40 kPa and a forced flow velocity of 200 mm/s. Microgravity experiments were conducted during NASAs Spacecraft Fire Experiment (Saffire II), on board the Cygnus spacecraft at 100 kPa with an air flow velocity of 200 mm/s. Results show that reductions of pressure slow down the flame spread over the PMMA surface approaching that in microgravity. The data is correlated in terms of a non-dimensional mixed convection analysis that describes the convective heat transferred from the flame to the solid, and the primary mechanism controlling the spread of the flame. The extrapolation of the correlation to low pressures predicts well the flame spread rate obtained in microgravity in the Saffire II experiments. Similar results were obtained by the authors with similar experiments with a thin composite cotton/fiberglass fabric (published elsewhere). Both results suggest that reduced pressure can be used to approximately replicate flame behavior of untested gravity conditions for the burning of thick and thin solids. This work could provide guidance for potential ground-based testing for fire safety design in spacecraft and space habitats
On Simulating Concurrent Flame Spread in Reduced Gravity by Reducing Ambient Pressure
The flammability of combustible materials in spacecraft environments is of importance for fire safety applications because the environmental conditions can greatly differ from those on earth, and a fire in a spacecraft could be catastrophic. Moreover, experimental testing in spacecraft environments can be difficult and expensive, so using ground-based tests to inform microgravity tests is vital. Reducing buoyancy effects by decreasing ambient pressure is a possible approach to simulate a spacecraft environment on earth. The objective of this work is to study the effect of pressure on material flammability, and by comparison with microgravity data, determine the extent to which reducing pressure can be used to simulate reduced gravity. Specifically, this work studies the effect of pressure and microgravity on upward/concurrent flame spread rates and flame appearance of a burning thin composite fabric made of 75% cotton and 25% fiberglass (Sibal). Experiments in normal gravity were conducted using pressures ranging between 100 and 30 kPa and a forced flow velocity of 20 cm/s. Microgravity experiments were conducted during NASAs Spacecraft Fire Experiment (Saffire), on board of the Orbital Corporation Cygnus spacecraft at 100 kPa and an air flow velocity of 20 cm/s. Results show that reductions of ambient pressure slow the flame spread over the fabric. As pressure is reduced, flame intensity is also reduced. Comparison with the concurrent flame spread rates in microgravity show that similar flame spread rates are obtained at around 30 kPa. The normal gravity and microgravity data is correlated in terms of a mixed convection non-dimensional parameter that describes the heat transferred from the flame to the solid surface. The correlation provides information about the similitudes of the flame spread process in variable pressure and reduced gravity environments, providing guidance for potential on-earth testing for fire safety design in spacecraft and space habitats
Piloted Ignition Delay of PMMA in Space Exploration Atmospheres
In order to reduce the risk of decompression sickness associated with extravehicular activity (EVA), NASA is designing the next generation of exploration vehicles and habitats with a different cabin environment than used previously. The proposed environment uses a total cabin pressure of 52.7 to 58.6 kPa with an oxygen concentration of 30 to 34% by volume and was chosen with material flammability in mind. Because materials may burn differently under these conditions and there is little information on how this new environment affects the flammability of the materials onboard, it is important to conduct material flammability experiments at the intended exploration atmosphere. One method to evaluate material flammability is by its ease of ignition. To this end, piloted ignition delay tests were conducted in the Forced Ignition and Spread Test (FIST) apparatus subject to this new environment. In these tests, polymethylmethacylate (PMMA) was exposed to a range of oxidizer flow velocities and externally applied heat fluxes. Tests were conducted for a baseline case of normal pressure and oxygen concentration, low pressure (58.6 kPa) with normal oxygen (21%), and low pressure with 32% oxygen concentration conditions to determine the individual effect of pressure and the combined effect of pressure and oxygen concentration on the ignition delay. It was found that reducing the pressure while keeping the oxygen concentration at 21% reduced the ignition time by 17% on average. Increasing the oxygen concentration at low pressures reduced the ignition time by an additional 10%. It was also noted that the critical heat flux for ignition decreases at exploration atmospheres. These results show that tests conducted in standard atmospheric conditions will underpredict the ignition of materials intended for use on spacecraft and that, at these conditions, materials are more susceptible to ignition than at current spacecraft atmospheres
Applying Flammability Limit Probabilities and the Normoxic Upward Limiting Pressure Concept to NASA STD-6001 Test 1
Repeated Test 1 extinction tests near the upward flammability limit are expected to follow a Poisson process trend. This Poisson process trend suggests that rather than define a ULOI and MOC (which requires two limits to be determined), it might be better to define a single upward limit as being where 1/e (where e (approx. equal to 2.7183) is the characteristic time of the normalized Poisson process) of the materials burn, or, rounding, where approximately 1/3 of the samples fail the test (and burn). Recognizing that spacecraft atmospheres will not bound the entire oxygen-pressure parameter space, but actually lie along the normoxic atmosphere control band, we can focus the materials flammability testing along this normoxic band. A Normoxic Upward Limiting Pressure (NULP) is defined that determines the minimum safe total pressure for a material within the constant partial pressure control band. Then, increasing this pressure limit by a factor of safety, we can define the material as being safe to use at the NULP + SF (where SF is on the order of 10 kilopascal, based on existing flammability data). It is recommended that the thickest material to be tested with the current Test 1 igniter should be 3 mm thick (1/8 inches) to avoid the problem of differentiating between an ignition limit and a true flammability limit
The Application of a Genetic Algorithm to Estimate Material Properties for Fire Modeling from Bench-Scale Fire Test Data
A methodology based on an automated optimization technique that uses a genetic algorithm
(GA) is developed to estimate the material properties needed for CFD-based fire growth
modeling from bench-scale fire test data. The proposed methodology involves simulating a
bench-scale fire test with a theoretical model, and using a GA to locate a set of model parameters
(material properties) that provide optimal agreement between the model predictions and the
experimental data. Specifically, a genetic algorithm based on the processes of natural selection
and mutation is developed and integrated with the NIST FDS v4.0 pyrolysis model for thick
solid fuels. The combined genetic algorithm/pyrolysis model is used with Cone Calorimeter data
for surface temperature and mass loss rate histories to estimate the material properties of two
charring materials (redwood and red oak) and one thermoplastic material (polypropylene). This
is done by finding the parameter sets that provide near-optimal agreement between the model
predictions and experimental data given the constraints imposed by the underlying physical
model and the accuracy with which the boundary and initial conditions can be specified. The
methodology is demonstrated here with the FDS pyrolysis model and Cone Calorimeter data, but
it is general and can be used with several existing fire tests and almost any pyrolysis model.
Although the proposed methodology is intended for use in CFD-based prediction of large-scale
fire development, such calculations are not performed here and are recommended for future
work
Computational Model of Forward and Opposed Smoldering Combustion in Microgravity
A novel computational model of smoldering combustion capable of predicting both forward and
opposed propagation is developed. This is accomplished by considering the one-dimensional, transient,
governing equations for smoldering combustion in a porous fuel accounting for improved chemical
kinetics. The heterogeneous chemistry is modeled with a 5-step mechanism for polyurethane foam. The
kinetic parameters for this mechanism were obtained from thermogravimetric data in the literature and
reported by the authors elsewhere. The results from previously conducted microgravity experiments with
flexible polyurethane foam are used for calibration and testing of the numerical results. Both forward and
opposed smoldering configurations are examined. By considering the 5-step mechanism, the numerical
model is able to predict qualitatively and quantitatively the smoldering behavior, reproducing the most
important features of the process. Specifically, the model predicts the transient temperature profiles, the
overall structure of the reaction-front, the onset of smoldering ignition, and the propagation rate. The fact
that it is possible to predict the experimental observations in both opposed and forward propagation with
a single model is a significant improvement in the development of numerical models of smoldering
combustion. This is particularly relevant in multidimensional simulations where distinction between
forward and opposed modes is no longer applicable
Modelling the Propagation of Forward and Opposed Smouldering Combustion
A computational study has been carried out to investigate smouldering ignition and propagation in polyurethane foam.
The one-dimensional, transient, governing equations for smouldering combustion in a porous fuel are solved accounting for
improved solid-phase chemical kinetics. Forward and opposed smouldering modes are examine and the model describes well both
propagation modes. Specifically, the model predicts the reaction-front thermal and species structure, the onset of smouldering
ignition, and the propagation rate. This is a signifficant step forward in smouldering combustion modelling, because unification of
forward and oposed propagation modes had never been achieved before. This breakthrough is associated to the use of improved
chemical kinetics obtained with a novel metodology to establish the reaction chemistry. The corresponding kinetic parameters for a
reduced five step mechanisms of polyurethane foam smouldering kinetics are used. These kinetic mechanisms are then used to model
one-dimensional smouldering combustion, numerically solving for the solid-phase and gas-phase conservation equations. A forced
flow of oxidizer gas is considered and gravity neglected. The results from previously conducted microgravity experiments with
flexible polyurethane foam are used for calibration and testing of the model predictive capabilities
Microgravity smoldering combustion on the USML-1 Space Shuttle mission
Preliminary results from an experimental study of the smolder characteristics of a porous combustible material (flexible polyurethane foam) in normal and microgravity are presented. The experiments, limited in fuel sample size and power available for ignition, show that the smolder process was primarily controlled by heat losses from the reaction to the surrounding environment In microgravity, the reduced heat losses due to the absence of natural convection result in only slightly higher temperatures in the quiescent microgravity test than in normal gravity, but a dramatically larger production of combustion products in all microgravity tests. Particularly significant is the proportionately larger amount of carbon monoxide and light organic compounds produced in microgravity, despite comparable temperatures and similar char patterns. This excessive production of fuel-rich combustion products may be a generic characteristic of smoldering polyurethane in microgravity, with an associated increase in the toxic hazard of smolder in spacecraft
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