Grunde Jomaas

Surface Morphology and Self-Acceleration of Expanding Spherical Flames

This paper reports results on the cellular and pulsating instabilities developed over the surface of expanding spherical premixed flames. Regarding the cellular instabilities, experimental data from time-resolved images of hydrogen–air flames exhibiting hydrodynamic and diffusional-thermal cells were used to determine the flame acceleration exponent α, defined through R(t)=R0+At α A and R0 are constants. Results show that α seems to be bounded by the maximum value of 1.33 ± 0.03, which is approached regardless of the nature of the cells, and therefore suggesting the existence of a self-similar mode of propagation. Regarding pulsating instability, spiral waves and target patterns were experimentally observed over the flame surface in rich hydrogen–air, rich hydrogen-oxygen, and lean butane-oxygen-helium mixtures at elevated pressures. The transition boundary to instability was found to agree well with the Sivashinsky criterion, Ze(Le-1)>4(1+3 1/2 ) ≈ 10.9, provided local values are used in the evaluation of the Zel’dovich number, Ze, and the Lewis number, Le.

Effect of Smoke Source and Horn Configuration on Enhanced Deposition, Acoustic Agglomeration, and Chladni Figures in Smoke Detectors

A series of UL/EN based test fires was conducted in a two room/corridor enclosure to investigate the viability of methods for determining whether a smoke detector sounded under a variety of smoke conditions and to see if this methodology could be applied to a detector with a different horn configuration. The presence of enhanced deposition in the form of a black or orange-brown ring and agglomerates around the central opening of a smoke detector horn was found to be a reliable indicator that the horn sounded when it was exposed to smoke from eight standardized, single-substrate fuel sources including hydrocarbon pool, flaming polyurethane foam, and smoldering polyurethane foam fires. Determinations could generally not be made for detectors exposed to white or gray smoke generated by flaming paper, smoldering paper, flaming wood, smoldering wood, and smoldering cotton wick due to a general lack of visible soot deposition within the detector. Therefore, it is not recommended to use the absence of a black or orange-brown ring of enhanced deposition, in and of itself, as an indicator that the horn did not sound. Nevertheless, this conclusion can be reached when the absence of enhanced deposition is combined with evidence supporting the presence of flaming fuels that produce black, sooty smoke. Test series were conducted using two different smoke detector brands, each having a different horn configuration. Findings suggest that the same type of methodology for determining whether the detector sounded is applicable to both models. Chladni figures were not found on any of the smoke detectors, whether they sounded or not; hence, the absence of a Chladni figure was not an indicator that the detector did not sound. A smoke flow visualization technique was used to determine the mechanism that caused the observed enhanced deposition and agglomerates on horns that sounded during a smoke exposure. Additionally, a smoke box test series showed that the extent of observed soot deposition increased with increasing smoke exposure.

Effectiveness of a chemical herder in association with in-situ burning of oil spills in ice-infested water

The average herded slick thickness, surface distribution and burning efficiency of a light crude oil were studied in ice-infested water to determine the effectiveness of a chemical herder in facilitating the in-situ burning of oil. Experiments were performed in a small scale (1.0m2) and an intermediate scale (19m2) setup with open water and 3/10, 5/10 and 7/10 brash ice coverages. The herded slick thicknesses (3-8mm) were ignitable in each experiment. The presence of ice caused fracturing of the oil during the herding process, which reduced the size of the herded slicks and, as a consequence, their ignitability, which in turn decreased the burning efficiency. Burning efficiencies relative to the ignited fraction of the oil were in the expected range (42-86%). This shows that the herder will be an effective tool for in-situ burning of oil when the ignitability issues due to fracturing of the oil are resolved.

A study on burning behavior and convective flows in Methanol pool fires bound by ice

Abstract (ID: 2017-170) An experimental study on methanol pool fires bound by ice was carried to research the burning behavior and flow field (within the liquid-phase) of methanol. The experiments ...

Effects of oil and oil burn residues on seabird feathers.

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.

Development of Large-Scale Spacecraft Fire Safety Experiments

The status is presented of a spacecraft fire safety research project that is being developed to reduce the uncertainty and risk in the design of spacecraft fire safety systems by testing at nearly full scale in low-gravity. Future crewed missions are expected to be longer in duration than previous exploration missions outside of low-earth orbit and accordingly, more complex in terms of operations, logistics, and safety. This will increase the challenge of ensuring a fire-safe environment for the crew throughout the mission. Based on our fundamental uncertainty of the behavior of fires in low-gravity, the need for realistic scale testing at reduced gravity has been demonstrated. To address this knowledge gap, the NASA Advanced Exploration Systems Program Office in the Human Exploration and Operations Mission Directorate has established a project with the goal of substantially advancing our understanding of the spacecraft fire safety risk. The activity of this project is supported by an international topical team of fire experts from other space agencies who conduct research that is integrated into the overall experiment design. The large-scale space flight experiment will be conducted in an Orbital Sciences Corporation Cygnus vehicle after it has deberthed from the ISS. Although the experiment will need to meet rigorous safety requirements to ensure the carrier vehicle does not sustain damage, the absence of a crew removes the need for strict containment of combustion products. The tests will be fully automated with the data downlinked at the conclusion of the test before the Cygnus vehicle reenters the atmosphere. Several computer modeling and ground-based experiment efforts will complement the flight experiment effort. The international topical team is collaborating with the NASA team in the definition of the experiment requirements and performing supporting analysis, experimentation and technology development. The status of the overall experiment and the associated international technology development efforts are summarized.

An Experimental Study on the Self-Acceleration of Cellular Spherical Flames

The time-resolved images and accelerating propagation speeds of spark-ignited, expanding spherical flames exhibiting both flame-front hydrodynamic and diffusionalthermal cellular instabilities were experimentally acquired in a constant and high pressure environment. From these data the acceleration exponent , defined through R(t)=C+At   where A and C are constants, was determined for the near-equidiffusive flames of ethylene and acetylene and non-equidiffusive flames of propane and hydrogen, recognizing that the former is subjected only to hydrodynamic instability while the latter to the diffusionalthermal instability as well. Results show that the acceleration exponent seems to be bounded by the value of 1.34, which is approached for fast-propagating flames of small thicknesses, regardless of the nature of the cells. The characteristic cell sizes were also measured and were found to agree well, for the hydrodynamic cells, with the linear stability theory of Bechtold and Matalon. The possible attainment of a self-similar mode of propagation is suggested. I. Introduction ROPAGATION of laminar premixed flames is subjected to the onset of flame-front hydrodynamic, diffusionalthermal, and buoyancy instabilities that are respectively caused by the density jump across the flame, the nonequidiffusive nature of the diffusive scalars of species and heat, and body force in the presence of density gradient. The presence of cells over the flame surface increases its surface area and consequently also the global propagation speed of the flame. Furthermore, since new cells continuously evolve, it is reasonable to expect that the flame speed will also continuously increase, leading to the phenomenon of self acceleration. This self acceleration can be quantified, and an assessment of the propagation mode made, by determining the history of the flame radius, R(t) ~ t  , and thereby the flame front velocity, dR(t)/dt ~ t

Can a spreading flame over electric wire insulation in concurrent flow achieve steady propagation in microgravity

Abstract Concurrent flame spread over electric wire insulation was studied experimentally in microgravity conditions during parabolic flights. Polyethylene insulated Nickel-Chrome wires and Copper wires were examined for external flow velocities ranging from 50 mm/s to 200 mm/s. The experimental results showed that steady state flame spread over wire insulation in microgravity could be achieved, even for concurrent flow. A theoretical analysis on the balance of heat supply from the flame to the unburned region, radiation heat loss from the surface to the ambient and required energy to sustain the flame propagation was carried out to explain the presence of steady spread over insulated wire under concurrent flow. Based on the theory, the change in heat input (defined by the balance between heat supply from flame and radiation heat loss) was drawn as a function of the flame spread rate. The curve intersected the linear line of the required energy to sustain the flame. This balance point evidences the existence of steady propagation in concurrent flow. Moreover, the estimated steady spread rate (1.2 mm/s) was consistent with the experimental result by considering the ratio of the actual flame length to the theoretical to be 0.5. Further experimental results showed that the concurrent flame spread rate increased with the external flow velocity. In addition, the steady spread rate was found to be faster for Copper wires than for Nickel-Chrome wires. The experimental results for upward spreading (concurrent spreading) in normal gravity were compared with the microgravity results. In normal gravity, the flame did not reach a steady state within the investigated parameter range. This is due to the fact that the fairly large flame spread rate prevented the aforementioned heat balance to be reached, which meant that such a spread rate could not be attained within the length of the tested sample.

Experimental Procedure for Laboratory Studies of In Situ Burning : Flammability and Burning Efficiency of Crude Oil

A new method for the simultaneous study of the flammability and burning efficiency of fresh and weathered crude oil through two experimental laboratory setups is presented. The experiments are easily repeatable compared to operational scale experiments (pool diameter ≥2 m), while still featuring quite realistic in situ burning conditions of crude oil on water. Experimental conditions include a flowing water sub-layer that cools the oil slick and an external heat flux (up to 50 kW/m2) that simulates the higher heat feedback to the fuel surface in operational scale crude oil pool fires. These conditions enable a controlled laboratory study of the burning efficiency of crude oil pool fires that are equivalent to operational scale experiments. The method also provides quantitative data on the requirements for igniting crude oils in terms of the critical heat flux, ignition delay time as a function of the incident heat flux, the surface temperature upon ignition, and the thermal inertia. This type of data can be used to determine the required strength and duration of an ignition source to ignite a certain type of fresh or weathered crude oil. The main limitation of the method is that the cooling effect of the flowing water sub-layer on the burning crude oil as a function of the external heat flux has not been fully quantified. Experimental results clearly showed that the flowing water sub-layer does improve how representative this setup is of in situ burning conditions, but to what extent this representation is accurate is currently uncertain. The method nevertheless features the most realistic in situ burning laboratory conditions currently available for simultaneously studying the flammability and burning efficiency of crude oil on water.

Thermal Properties and Burning Efficiencies of Crude Oils and Refined Fuel Oil

ABSTRACT The thermal properties and burning efficiencies of fresh and weathered crude oils and a refined fuel oil were studied in order to improve the available input data for field ignition system...