Project Overview
- Introduction
The transition from smoldering to flaming is of interest both as a
fundamental combustion problem and as a serious fire risk. It encompasses phenomena
related to the ignition of a homogeneous gas phase reaction (flaming) that is induced
by a heterogeneous surface reaction (smolder) that acts both as the source of gaseous
fuel (pyrolyzate, CO, etc.) and the source of heat to initiate the homogeneous
reaction. That it is a fire hazard is confirmed by the fact that more than
30% of US fire deaths can be attributed to smoldering. The transition from smoldering
to flaming is also of concern in the space flight program; to date there have been
a six incidents of overheated and charred cables and electrical components
reported on Space Shuttle flights and significant smolder-related incidents aboard
the Russian space station Mir. With the ongoing establishment of the International
Space Station and the planning of other long-term space missions, there is an
added need to study smoldering and its transition to flaming in microgravity
in order to prevent and minimize the effects of a smolder-initiated fire.
While considerable work has been conducted to
understand the smoldering combustion of porous fuels, there has been considerably
less research conducted on the transition from smoldering to flaming. A common observation of these works is that for the transition from smoldering to
flaming to occur the fuel samples have to be fairly large, or the process must be
assisted by increasing the oxygen concentration of the oxidizer flow and/or external
heating.
The present work is part of a
NASA-funded project. Because the microgravity experiments
are planned for the International Space Station, the polyurethane foam samples
had to be limited in size for safety and launch mass reasons. The maximum
sample size permitted for the experiments is too small for smolder to self
propagate due to heat losses to the
surroundings. Thus, the smolder propagation had to be assisted by reducing
the heat losses and by increasing the oxidizer oxygen concentration. The work has
demonstrated that both can occur with relatively small fuel samples if the
external ambient conditions are appropriate.
- Experimental Apparatus
The experiments are conducted in a small vertically oriented flow duct.
The fuel sample (flexible polyurethane foam of 50 mm x 50 mm cross section and 125 mm long) is oriented such
that its front face is flush with the wall of the flow duct. The back and side walls
of the sample holder have guard heaters.
During the tests, there is a forced flow of oxidizer through the igniter, and into
the foam. An infrared heater is mounted facing the fuel-sample free surface and
the bottom of the sample is in contact with an igniter. Smolder ignition is induced
at the bottom and smolder propagates upward in the same direction as
the oxidizer (buoyant plus forced flows), i.e., forward smolder.
Six thermocouples are located along the sample to monitor
the foam temperature, and an infrared camera monitors the free-surface
temperature of the foam. An ultrasound probing technique is used to measure
changes in permeability of the sample. In addition, a high-speed camera,
operating at 1000 frames/s, observes the free surface of the sample.
A schlieren imaging system is used to observe density gradients in the duct flow
along the free surface of the fuel sample.
Fig. 1. Schematic of the experimental apparatus
- Results
The experiments show strong evidence that a transition from smolder
to flaming occurs in the char region upstream of the smolder reaction,
agreeing with previous observations of the process. A combination of infrared
and video imaging with in-depth thermocouples adequately tracked the progress of
the smolder reaction, and captured the transition to flaming event in suficient
detail to determine the approximate location of the transition and the time delay
to the transition event.
It is shown that the transition to flaming is sensitive to the external heat losses,
and the heat generated by the secondary char reactions occurring in the char upstream
of the smolder reaction. The data show that increasing the oxygen concentration
of the oxidizer flow, reducing the velocity of the external flow, and/or increasing
the external radiant flux increase the likelihood of a transition to flaming.
These observations support the concept that the transition from smolder to flaming
is basically a spontaneous gas-phase ignition reaction that is supported by the
smolder reaction, which acts both as the source of gaseous fuel (pyrolyzate)
and of heat to support the reaction.
Fig. 2. Photographic sequences of transition from smoldering to flaming,
using (top) visible imaging, (bottom) infrared imaging.
Fig. 3. Schlieren color-map sequence of the
flow duct (profile view of the exposed sample surface) during a transition to flaming
Download Videos:
real speed transition video (.avi file).
high speed transition video, slowed down 250x (.avi file).
The transition is more likely to occur when the heat released by both the heterogeneous
smolder reaction and the homogeneous gas-phase reaction is larger than the heat losses
to the surrounding environment. These findings are supported by a simplifled energy
balance analysis that describes the transition as an autoignition process. The analysis
is able to predict the boundaries between the transition/no transition regions.
Fig. 4. Comparison of experimental results with the predictions from the
energy balance-analysis of transition/no-transition regions.
The objective of the numerical modelling part is to have a model of smolder combustion
and transition to flaming applicable to microgravity and normal gravity conditions.
The governing equations for smolder combustion
in a porous media have been derived from basic principles: solid-phase conservation of mass, species and energy;
gas-phase conservation of mass, species, energy and momentum (Darcy's Law with buoyancy included).
The boundary and initial conditions are set to mimic the actual experimental setup.
These governing equations are too complex to be fully solved analytically and therefore
a computational model is been developed.
The model requires the kinetic parameters of the
thermal-decomposition of the material. The quantification of heterogonous kinetics
requires the derivation of
rate and stoichiometric parameters by solving an inverse problem.
The efficient multidimensional optimization technique of Genetic
Algorithms has been applied to the extraction of the parameters from thermogravimetric analysis.
This procedure has allowed to propose and validate a reduced mechanism for polyurethane
foam with five reactions.
Figure 5. TGA mass-loss rate in air atmosphere as a function of temperature for three heating rates. Marks: Experiments; Lines: Reduced-mechanism (numerical).
The model is used to extend the microgravity data to be collected,
to a wider set of conditions, configurations and materials, avoiding the high
cost of extensive space-based experiments.
Scientific Publications on STAF:
- O. Putzeys, A. Bar-Ilan, G. Rein, Y. Tsuji, A.C. Fernandez-Pello and D.L. Urban, "Transition from Forward Smoldering to Flaming in Small Polyurethane Foam Samples" Western States Section Spring Meeting, The Combustion Institute, Davis (CA,USA). March 2004. Paper 04S-52.
- A. Bar-Ilan, O. Putzeys, G. Rein, A.C. Fernandez-Pello and D.L. Urban. "Transition from Forward Smoldering to Flaming in Small Polyurethane Foam Samples" Proceedings of the Combustion Institute 30 (2) pp. 2295-2302, 2005. http://repositories.cdlib.org/postprints/422
- G. Rein, A. Bar-Ilan, A.C. Fernandez-Pello, J.L. Ellzey, J.L. Torero, D.L. Urban. "Modeling of One-Dimensional Smoldering of Polyurethane in Microgravity Conditions" Proceedings of the Combustion Institute 30 (2) pp. 2327-2334, 2005.http://repositories.cdlib.org/postprints/342.
- O. Putzeys, R. Titus, A. Bar-Ilan, D.L. Urban and A.C. Fernandez-Pello, "Observations of Forward Smoldering and the Transition to Flaming in Small Polyurethane Foam Samples with Ulrasound Probing". 43rd AIAA Aerospace Sciences Meeting (2005). Paper 2005-0715.
- G. Rein, C. Lautenberger, A.C. Fernandez-Pello, J.L. Torero and D.L. Urban, "On the Derivation of Polyurethane Kinetics Parameters using Genetic Algorithms and its Application to Smoldering Combustion" 4th International Conference on Computational Heat and Mass Transfer, Paris (France), May 2005. /~reingu/papers/Rein_ICCHMT05.pdf.
More information STAF and related websites:
- NASA Glenn Research Center Experiments Branch.
- NASA FEANICS Project
- Previous results on Transition to Flaming
Other Scientific Publications on Smoldering:
- A. Bar-Ilan, G. Rein, D.C. Walther, A.C. Fernandez-Pello, J.L. Torero, D.L. Urban. “The Effect of Buoyancy on Opposed smoldering” Combustion Science and Technology, Vol. 176, 2004, pp. 2027-2055. http://repositories.cdlib.org/postprints/350
- A. Bar-Ilan, G. Rein, A.C. Fernandez-Pello, J.L. Torero and D.L. Urban “Forced Forward Smoldering Experiments in Microgravity", Experimental Thermal and Fluid Science, Vol. 28 (7), 2004, pp. 743-751. http://repositories.cdlib.org/postprints/341.
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reingu (at me.berkeley.edu)