3 research outputs found
EXPERIMENTAL CHARACTERIZATION AND MODELING OF FLAME HEAT FEEDBACK AND OXIDATIVE PYROLYSIS FOR SIMULATION OF BENCH SCALE FIRE TESTS
Two important bench scale fire tests, the cone calorimeter test and UL-94V, werecharacterized experimentally to allow for predictions using a numerical pyrolysis
solver, ThermaKin2Ds with pyrolysis parameter sets. Flame heat feedback was
measured in cone calorimeter tests for several polymers to develop a generalized flame
model. Flame heat flux was measured in the center and near one side and was found to
be 11–23 kW m-2 and 32–49 kW m-2, respectively. Based on the difference in measured
heat flux, a center zone and a side zone were defined and separate models developed.
The final model was an area-weighted combination of the center and side zone
simulations. Heat release rate data were predicted well by the final model. Ignition
times for low irradiation were not predicted well initially but a correction was made to
account for the effect of oxygen. The UL-94V test required characterization of the
flame heat feedback but also of the burner flame (temperature, heat flux, and oxygen
content). UL-94V tests were performed using polymers of different flammability ratings to evaluate the model; some samples had insulated sides to investigate edge
effects. Additional UL-94V tests performed with an embedded heat flux gauge served
to measure polymer flame heat feedback. All UL-94V tests were recorded on video
using a 900-nm narrow-band filter to focus on emissions from soot for tracking flame
length over time. Flame heat fluxes of insulated PMMA samples confirmed a
previously developed wall flame submodel, while non-insulated PMMA samples had
significantly greater heat fluxes; the wall flame submodel was scaled accordingly.
Burner flame oxygen content was measured to be about 5 vol% and was found to
enhance decomposition of two materials; oxidation submodels were then developed
accordingly. Overall, the model predicted flame spread on insulated UL-94V samples
reasonably well but significantly underpredicted the results on non-insulated samples.
Discrepancies were attributed to burning and spread on the edges which were not
modeled explicitly. Finally, given the importance of oxidation on predictions of
ignition time, oxidative pyrolysis was studied both in mg-scale and gram-scale
pyrolysis experiments. Kinetic parameters were first developed based on inverse
analysis of TGA tests in atmospheres of varied oxygen content. Two models were
developed: a surface reaction model and a volumetric model. Mass flux data from
gram-scale gasification tests were used to evaluate the models. The anaerobic model
gave the best predictions of mass flux for 15 kW m-2 gasification tests, but the oxidative
models gave better predictions for the 25 kW m-2 gasification tests. The volumetric
model gives better predictions unless mass transport of oxygen is considered in which
case, the surface model gives better predictions
Improved Venting for Flammability Limit Testing Using ASTM E681 Apparatus
The literature on the determination of flammability limits was reviewed and experts on the ASTM E681 standard were interviewed to identify new means of improving the reproducibility of the ASTM E681 test. Venting was identified as a variable of flammability limits not yet addressed. Limitations of the current system for sealing and venting (a rubber stopper) were identified and addressed by the development of a custom burst disc. The burst disc was evaluated for its ability to hold and maintain a vacuum, its ability to vent at pressures of interest, and for its venting phenomena. The burst disc was deemed to be a satisfactory alternative to the rubber stopper and is recommended to be included in the ASTM E681 standard
Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies
Flare frequency distributions represent a key approach to addressing one of
the largest problems in solar and stellar physics: determining the mechanism
that counter-intuitively heats coronae to temperatures that are orders of
magnitude hotter than the corresponding photospheres. It is widely accepted
that the magnetic field is responsible for the heating, but there are two
competing mechanisms that could explain it: nanoflares or Alfv\'en waves. To
date, neither can be directly observed. Nanoflares are, by definition,
extremely small, but their aggregate energy release could represent a
substantial heating mechanism, presuming they are sufficiently abundant. One
way to test this presumption is via the flare frequency distribution, which
describes how often flares of various energies occur. If the slope of the power
law fitting the flare frequency distribution is above a critical threshold,
as established in prior literature, then there should be a
sufficient abundance of nanoflares to explain coronal heating. We performed
600 case studies of solar flares, made possible by an unprecedented number
of data analysts via three semesters of an undergraduate physics laboratory
course. This allowed us to include two crucial, but nontrivial, analysis
methods: pre-flare baseline subtraction and computation of the flare energy,
which requires determining flare start and stop times. We aggregated the
results of these analyses into a statistical study to determine that . This is below the critical threshold, suggesting that Alfv\'en
waves are an important driver of coronal heating.Comment: 1,002 authors, 14 pages, 4 figures, 3 tables, published by The
Astrophysical Journal on 2023-05-09, volume 948, page 7