An experimental study was performed to determine the requirements for sustained propagation of spark–ignited hydrogen–air and butane–air flames at atmospheric and elevated pressures. Results show ...that sustained propagation is always possible for mixtures whose Lewis number is less than unity, as long as a flame can be initially established. However, for mixtures whose Lewis number is greater than unity, sustained propagation depends on whether the initially ignited flame can attain a minimum radius. This minimum radius was determined for mixtures of different equivalence ratios and pressures, and was found to agree moderately well with the theoretically predicted critical radius beyond which there is no solution for the adiabatic, quasi-steady propagation of the spherical flame. The essential roles of pressure, detailed chemistry, and the need to use local values in the quantitative evaluation of the flame response parameters are emphasized.
Flame spread: Effects of microgravity and scale Urban, David L.; Ferkul, Paul; Olson, Sandra ...
Combustion and flame,
January 2019, 2019-01-00, 20190101, 2019-01, Volume:
199
Journal Article
Peer reviewed
For the first time, a large-scale flame spread experiment was conducted inside an orbiting spacecraft to study the effects of microgravity and scale and to address the uncertainty regarding how ...flames spread when there is no gravity and if the sample size and the experimental duration are, respectively, large enough and long enough to allow for unrestricted growth. Differences between flame spread in purely buoyant and purely forced flows are presented. Prior to these experiments, only samples of small size in small confined volumes had been tested in space. Therefore the first and third flights in the experimental series, called “Saffire,” studied large-scale flame spread over a 94 cm long by 40.6 cm wide cotton-fiberglass fabric. The second flight examined an array of nine smaller samples of various materials each measuring 29 cm long by 5 cm wide. Among them were two of the same cotton-fiberglass fabric used in the large-scale tests and a thick, flat PMMA sample (1-cm thick). The forced airflow was 20–25 cm/s, which is typical of air circulation speeds in a spacecraft. The experiments took place aboard the Cygnus vehicle, a large unmanned resupply spacecraft to the International Space Station (ISS). The experiments were carried out in orbit before the Cygnus vehicle, reloaded with ISS trash, re-entered the Earth's atmosphere and perished. The downloaded test data show that a concurrent (downstream) spreading flame over thin fabrics in microgravity reaches a steady spread rate and a limiting length. The flame over the thick PMMA sample approaches a non-growing, steady state in the 15 min burning duration and has a limiting pyrolysis length. In contrast, upward (concurrent) flame spread at normal gravity on Earth is usually found to be accelerating so that the flame size grows with time. The existence of a flame size limit has important considerations for spacecraft fire safety as it can be used to establish the heat release rate in the vehicle. The findings and the scientific explanations of this series of innovative, novel and unique experiments are presented, analyzed and discussed.
Parabolic flight experiments were carried out to investigate the effect of ambient pressure on the extinction limit for opposed flame spread over an electric wire insulation in microgravity. ...Low-density polyethylene insulated Nickel-Chrome wires with inner core diameter of 0.50 mm and insulation thickness of 0.30 mm were examined for ambient pressures ranging from 50 kPa to 140 kPa for an external opposed flow of 10 cm/s. The experiments showed that the limiting volumetric oxygen concentration (LOC) increased as the total ambient pressure decreased. This LOC trend can be explained by radiation loss from wire surface. The radiation loss increased as the ambient pressure decreased – a result that can be explained by the increase in preheat length with decreasing ambient pressure. Moreover, when the data was plotted in a partial pressure vs. total pressure space, it became evident that the limiting oxygen partial pressure (LOPP) decreased with decreasing total ambient pressure. This LOPP trend can be explained by the fact that the flame temperature increased as the ambient pressure decreased under constant oxygen partial pressure. In current fire safety design for spacecraft, tentative oxygen concentration criteria in spacecraft suggested by NASA is assumed as 30% of oxygen concentration, and this value is assumed independent of ambient pressure. However, the present result implies that consideration of the effect of ambient pressure on the flammability limit is necessary, especially with respect to the possibility of an extension of the allowable atmosphere condition for spacecraft cabin in the low pressure region.
A cross-region transfer learning method is proposed to identify community (e.g. car parks, public spaces and shopping centers) fire hazards based on text input provided by community members. The key ...component of the method, which also accounts for data imbalance, is an improved transfer component analysis that is embedded with a local discriminant analysis to transfer non-local rich knowledge to the fire hazard identification of local communities with an insufficient number of samples. In addition, a fire hazard knowledge map is established and applied to supplement the missing key features for fire hazard identification, and ontology modeling is applied to standardize the text features and reduce the effect of semantic ambiguity brought by cross-region knowledge transfer. The proposed method is verified based on the text data of nine fire hazard classes from Lanzhou and Beidaihe in China. Machine learning experiments show that fire hazard identification performance of all nine classes were improved with the overall accuracy, precision, recall, F1 score and AUC increased by 12%, 15%, 16%, 15% and 15%, respectively. Under data imbalance scenarios, the proposed method outperforms the state of the art methods, such as sampling-based methods, FastText and ULMFiT. The results also show that the proposed method can achieve desired performance with only half of the training samples. These findings illustrate that the proposed method can assist regions by improving fire identification results significantly through knowledge transfer. The proposed approach can be followed to build smart systems for community fire risk management with reasonable performance and high efficiency.
Flame spread over wire insulation plays a crucial role in spacecraft fire safety. To quantify the effect of the initial ignition condition on the limiting oxygen concentration (LOC) of spreading ...flame over wire insulation, opposed flow flame spread experiments with wire insulation were conducted in microgravity (parabolic flights). Both ignition power (32.7 W to 71.8 W) and heating time (5 s to 15 s) were varied for an external flow of 100 mm/s. The sample wires were made of polyethylene-coated nickel-chrome (NiCr) and Copper (Cu), respectively, both with inner core diameter of 0.50 mm and insulation thickness of 0.30 mm. A 0.50 mm diameter coiled Kanthal wire wrapped around the sample wire 6 times with 8 mm length was used as the igniter. The experimental results show that the LOC gradually decreases as the ignition power or heating time increases and eventually it reaches a constant value. Also, the effect of ignition condition on LOC was more pronounced for Cu wires than for NiCr wires. The variation range of LOC in the tested ignition condition in microgravity was larger than that of horizontal flame spread in normal gravity. This conclusion can have implication for future experiment in the International Space Station to avoid the wrong LOC value because of the insufficient initial ignition energy and will eventually lead to an improved fire safety in spacecrafts.
A simple framework for setting risk tolerability limits is proposed following a literature review of risk acceptance in fire safety engineering and a feedback round with international fire safety ...professionals. The framework provides practical guidance for the application of international fire safety guidance, such as the recently published UK guidance document on probabilistic risk assessment for the fire design of buildings, PD 7974-7:2019, which provides a state-of-the-art framework to assess adequate safety for complex fire engineering designs. However, the application of this guidance document is hindered by two main constraints: (1) PD7974-7:2019 requires risk tolerability limits to be set, but lacks guidance on defining them for a specific building project, and (2) no reference case studies are given that demonstrate the application of PRA methods to fire engineering design. In order to help fill this gap, a risk tolerability framework is developed and proposed herein. Finally, it is applied to a reference case study of a UK office building. Considering the developed framework, further research needs are identified. It is expected that the proposed risk tolerability framework and the presented reference case study can support the application of PRA to fire engineering design and ease stakeholder communication on setting risk tolerability limits.
It is well known, that in case of oil spill, seabirds are among the groups of animals most vulnerable. Even small amounts of oil can have lethal effects by destroying the waterproofing of their ...plumage, leading to loss of insulation and buoyancy. In the Arctic these impacts are intensified. To protect seabirds, a rapid removal of oil is crucial and in situ burning could be an efficient method. In the present work exposure effects of oil and burn residue in different doses was studied on seabird feathers from legally hunted Common eider (Somateria mollissima) by examining changes in total weight of the feather and damages on the microstructure (Amalgamation Index) of the feathers before and after exposure. The results of the experiments indicate that burn residues from in situ burning of an oil spill have similar or larger fouling and damaging effects on seabird feathers, as compared to fresh oil.
•Exposure effects of fresh oil and burn residue was studied on seabird feathers.•Burn residues had similar or more damaging effects on feathers compared to the oil.•Feather microstructure damages was measured even for thin, not visible sheens.•Grane crude was slightly more damaging than IFO30 for both fresh and burned samples.
Spacecraft fire safety is an important consideration when designing future exploration missions. Concurrent flow flame spread experiments were carried out aboard the Cygnus spacecraft while in orbit ...to address current knowledge gaps related to solid fuel combustion in microgravity. The experiments used PMMA (polymethylmethacrylate) samples that were 50 mm wide and 290 mm long with two variations—one sample was a 10 mm thick flat slab, while the other was a 10 mm thick flat sample that had a 4 mm thick grooved center section. The thickness variation had a major impact on the flame shape, and the grooved sample developed a deep inverted-V shaped notch as the thin center section burned through. This notch enhanced heat transfer from the flame to the solid through an effectively wider flame base, which is the part of the flame with the highest temperature. Unlike in normal gravity (and also likely in partial gravity), where buoyant flow promotes acceleratory upward flame growth, the microgravity flames reached a steady size (limiting length) for a fixed forced convective flow in agreement with theory (i.e. there is a zero net heat flux at the flame tip). The limiting length implies that the spread rate of the flame will be controlled by the regression rate (burnout rate) of the material because the flames remains anchored to the upstream end of the fuel samples. This is a significant finding for spacecraft fire safety, and makes the probability of flashover in a spacecraft unlikely as the flame size will be small for low convective ventilation flow environments typical in spacecraft. On the other hand, long-burning flames in small vehicles will generate significant quantities of fuel vapor that may reach the lean flammability limit and cause a backdraft. The rapid extinction of the flame when the flow was turned off also supports the existing fire mitigation strategy on the ISS to deactivate the ventilation system in the event of fire alarm. These findings can be applied to improve the safety of future space exploration missions.
Studying solid combustion phenomena in microgravity environments can be complex, and this is furthered by many limitations and constraints in the available microgravity research platforms. ...Consequently, fire safety in spacecraft is also a complex subject. The main limitations found in the field are related to the microgravity quality, the duration of microgravity conditions, the rig capabilities in volume and size, time scales, length scales and the diagnostic systems, and these are therefore the focus in the current investigation. The laboratory capacity of ground-based platforms has remained somewhat stalled since 1990s, some drop towers have recently been upgraded to extend their performance. New space-based platforms have been or are being established and could extend the windows-of-opportunity to perform research. In addition, a discussion is provided on the implications of the fact that the phenomena studied in the experimental investigations and the type of material employed covers both programmatic and scientific needs. It is found that a handful of materials are most widely studied to quantify and characterise some of the phenomena, while some materials have been employed even in single experimental efforts. The current literature review provides a very comprehensive overview of previous experimental studies and the experimental methodologies utilised. Thus, this study can become an aid to planning for future studies.
•The ground-based laboratory capacity for microgravity research has remained somewhat stalled since the 1990s.•New space-based laboratory capacity is extending the windows-of-opportunity for research.•The major constraints behind every research platform are experimental duration, sample scale and microgravity quality and levels.•Previous studies have investigated a range of diverse materials to comply with programmatic and scientific needs.•Diagnostic systems have improved the quantification of the phenomena behind solid combustion in microgravity.
The thin-layer burning behaviors of gasoline, including the heat flux feedback to the burning surface, the penetrating thermal radiation, the temperature profile of liquid layer, and the burning rate ...were studied in experiments of thin-layer pool fires in square, fireproof glass trays. Experiments with four different tray sizes (side lengths of 30 cm, 40 cm, 50 cm and 60 cm) and four different initial liquid thicknesses of 6 mm, 9 mm, 12 mm and 15 mm were conducted. The results indicate that the heat flux feedback from the flame remained approximately constant, except during the ignition and extinguishment periods, and was also independent of the initial fuel thickness. The penetrating thermal radiation, on the other hand, increased with decreasing liquid layer thickness, gradually assuming rapid exponential growth. Furthermore, a boiling layer was formed during the initial burning period and its maximum depth was close to 3.0 mm. Four typical burning phases including pre-heating burning, steady burning, thin-layer burning and extinguishment were identified. The penetrating thermal radiation was the main cause of the burning rate decrease for thin-layer burning. These findings can provide a basis on which to build a real-time burning rate model for thin-layer burning.
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