Effects of Finite Sample Width on Transition and Flame Spread in Microgravity PDF Download
Are you looking for read ebook online? Search for your book and save it on your Kindle device, PC, phones or tablets. Download Effects of Finite Sample Width on Transition and Flame Spread in Microgravity PDF full book. Access full book title Effects of Finite Sample Width on Transition and Flame Spread in Microgravity by . Download full books in PDF and EPUB format.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 9
Get Book Here
Book Description
In most microgravity studies of flame spread, the flame is assumed to be two-dimensional, and two-dimensional models are used to aid data interpretation. However, since limited space is available to microgravity facilities, the flames are limited in size. It is important, therefore, to investigate the significance of three-dimensional effects. Three-dimensional and two-dimensional simulations of ignition and subsequent transition to flame spread were performed on a thermally thin cellulosic sample. Ignition occurred by applying a radiant heat flue in a strip across the center of the sample. The sample was bounded by an inert sample holder. Heat loss effects at the interface of the sample and the sample holder were tested by varying the thermal-physical properties of the sample holder. Simulations were also conducted with samples of different widths and with different ambient wind speeds (i.e., different levels of oxygen supply). The width of the sample affected both the duration of the flame transition period and the post-transition flame spread rate. Finite width effects were most significant when the ambient wind was relatively small (limited oxygen supply). In such environments, the velocity due to thermal expansion reduced the net inflow of oxygen enough to significantly affect flame behavior. for a given sample width, the influence of thermal expansion on the net incoming oxygen supply decreased as the ambient wind speed increased. Thus, both the transition and flame spread behavior of the three-dimensional flame (along the centerline) tended to that of the two-dimensional flame with increasing ambient wind speed. Heat losses to the sample holder were found to affect the flame spread rate in the case of the narrowest sample with the slowest ambient wind.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 9
Get Book Here
Book Description
In most microgravity studies of flame spread, the flame is assumed to be two-dimensional, and two-dimensional models are used to aid data interpretation. However, since limited space is available to microgravity facilities, the flames are limited in size. It is important, therefore, to investigate the significance of three-dimensional effects. Three-dimensional and two-dimensional simulations of ignition and subsequent transition to flame spread were performed on a thermally thin cellulosic sample. Ignition occurred by applying a radiant heat flue in a strip across the center of the sample. The sample was bounded by an inert sample holder. Heat loss effects at the interface of the sample and the sample holder were tested by varying the thermal-physical properties of the sample holder. Simulations were also conducted with samples of different widths and with different ambient wind speeds (i.e., different levels of oxygen supply). The width of the sample affected both the duration of the flame transition period and the post-transition flame spread rate. Finite width effects were most significant when the ambient wind was relatively small (limited oxygen supply). In such environments, the velocity due to thermal expansion reduced the net inflow of oxygen enough to significantly affect flame behavior. for a given sample width, the influence of thermal expansion on the net incoming oxygen supply decreased as the ambient wind speed increased. Thus, both the transition and flame spread behavior of the three-dimensional flame (along the centerline) tended to that of the two-dimensional flame with increasing ambient wind speed. Heat losses to the sample holder were found to affect the flame spread rate in the case of the narrowest sample with the slowest ambient wind.
Author:
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 490
Get Book Here
Book Description
Author: National Research Council
Publisher: National Academies Press
ISBN: 0309086396
Category : Science
Languages : en
Pages : 123
Get Book Here
Book Description
For thirty years the NASA microgravity program has used space as a tool to study fundamental flow phenomena that are important to fields ranging from combustion science to biotechnology. This book assesses the past impact and current status of microgravity research programs in combustion, fluid dynamics, fundamental physics, and materials science and gives recommendations for promising topics of future research in each discipline. Guidance is given for setting priorities across disciplines by assessing each recommended topic in terms of the probability of its success and the magnitude of its potential impact on scientific knowledge and understanding; terrestrial applications and industry technology needs; and NASA technology needs. At NASA's request, the book also contains an examination of emerging research fields such as nanotechnology and biophysics, and makes recommendations regarding topics that might be suitable for integration into NASA's microgravity program.
Author: Rodolfo Monti
Publisher: CRC Press
ISBN: 1482265052
Category : Science
Languages : en
Pages : 625
Get Book Here
Book Description
In a microgravity experiment, the conditions prevalent in fluid phases can be substantially different from those on the ground and can be exploited to improve different processes. Fluid physics research in microgravity is important for the advancement of all microgravity scients: life, material, and engineering. Space flight provides a uniqu
Author: W. E. Mell
Publisher:
ISBN:
Category :
Languages : en
Pages :
Get Book Here
Book Description
Author:
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages : 546
Get Book Here
Book Description
Author: Karen Woun-Tein Hung
Publisher:
ISBN:
Category :
Languages : en
Pages : 65
Get Book Here
Book Description
A Couette Flow Apparatus (CFA) was developed to study the effect of a linear velocity gradient on a flame spreading in opposed flow. This apparatus also tests the flammability of materials in a simulated microgravity flow environment. It is a channel with a rectangular cross section that produces a linear air velocity profile above a fuel sample to mimic the boundary layer velocity profile encountered by a flame in an actual microgravity fire. Similar apparatuses have been used to test materials using a parabolic velocity profile, but the purpose of this research is to determine whether or not using a linear velocity profile above the fuel surface produces different results. Simulated microgravity conditions were achieved by minimizing the space above and below the flame to reduce buoyant flow. The apparatus consists of a fixed bottom plate, two side walls, and a moving belt at the top to drive the flow. The belt is made of a Teflon-coated material to withstand the high temperature of the flame since they are in close contact. The side walls and the base plate contain sections of quartz windows so that images and videos could be recorded to show the flame behavior. First, the non-reacting flow was studied. Air velocity measurements were made using hot wire anemometers to determine the velocity profile vertically in the channel. Linear velocity profiles in the channel were not attained until after a fan was added to the outlet to help pull air through; the belt alone was not found to be sufficient. The fan speed was determined by first setting the belt speed and then adjusting the fan speed so that the velocity in the vertical center of the channel was half of the belt speed. The theoretical 2-dimensional flow field was derived analytically by solving the Navier-Stokes equation. The theory predicts that a fan is only necessary to obtain a linear air velocity profile for a smaller width-to-height ratio than what was actually found in the laboratory. The reason for this disagreement is the subject of further research being conducted by another graduate student. It was found that channel is wide enough so that the flow profile is flat over the entire width of the fuel sample. For the combustion tests, the flame speed was tracked using the videos taken through the quartz windows and Spotlight software to obtain flame spread rate. The belt velocity and channel height were varied from 8 to 65 cm/s and 4 to 11 mm, respectively, to study their effect on flame spread. Combustion results indicate that the flame spread rates in the CFA are consistently lower than the spread rates in a traditional Narrow Channel Apparatus due to the flame experiencing more heat loss to the moving boundary of the CFA. The flame spread rates were plotted against average flow velocity for both channels, with peak spread rates occurring at an average flow velocity of around 13-15 cm/s for a 5 mm gap height. Results for the CFA were also plotted against velocity gradient.
Author: Takashi Kashiwagi
Publisher:
ISBN:
Category :
Languages : en
Pages : 4
Get Book Here
Book Description
Author: Howard R. Baum
Publisher:
ISBN:
Category :
Languages : en
Pages : 16
Get Book Here
Book Description
Author: Chengyao Li
Publisher:
ISBN:
Category :
Languages : en
Pages : 189
Get Book Here
Book Description
Material flammability and burning behaviors of thin solids in concurrent flows in normal and microgravity are studied using a previously-developed transient numerical model. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase solves full Navier-Stokes equations including mass, momentum, energy and species equations, using Direct Numerical Simulation. A one-step, second-order overall Arrhenius reaction is adopted. Gas phase radiation is considered by solving the radiation transfer equation with a discrete ordinates SN approximation. In the solid phase, conservation equations of the energy and mass are solved. A cotton-fiberglass-blend fabric is considered as the solid material in this research. Test-based two-step decomposition reactions are implemented for the solid pyrolysis. In this work, the following efforts are made: (1) enhancement to the Adaptive Mesh Refinement (AMR) scheme and (2) development of a two-dimensional version of the program (based on the original three-dimensional program). The first effort allows the program to simulate and resolve multiple flames spreading along the surface of the solid combustible. The second effort dramatically reduces the computational cost when simulating flame spread over wide samples. The model is applied to simulate three scenarios: (1) upward flame spread in normal gravity, (2) purely-forced concurrent flow flame spread over a large and wide sample (41 cm wide 94 cm long), and (3) purely-forced concurrent flow flame spread over a moderate size (5 cm wide, 30 cm long) sample. In the first scenario, upward flame spread in normal gravity, the simulations follow the dimension/configuration of a standard test, NASA-STD-6001 Test #1. This test is the current ground-based screening test for materials that are intended for use in space exploration. The tested sample is 5 cm wide and 30 cm long. In the simulation, ambient pressure is the main parameter. At low pressures, the conventional upward flame spread process is observed. As the pressure increases, a special flame splitting phenomenon is observed. The splitting process is presented in details using the solid and gas profiles. It is concluded that the two-step solid pyrolysis is the cause of this special phenomenon. For the second and third scenarios, simulations are performed to support an on-going NASA project Saffire, which consists of a series of large-scale microgravity burning experiments. Concurrent flow speeds at 20 and 25 cm/s are simulated for both large and moderate sized samples. The results of both Saffire experiments and the simulations are presented and compared in detail. The numerical results are also used to interpret the phenomena observed in the experiments. For the wide sample (scenario 2), a parametric study on the sample width (5-41 cm) is conducted, and additional simulations (using the two-dimensional version of the program) at various flow conditions (different flow speeds, ambient pressures, and oxygen percentages) are performed. Based on the simulation results, analytical analysis is conducted and formulations are proposed for flame spread rate and flame length. The proposed formulation for flame spread rate is evaluated using literature data of microgravity experiments and shows seasonable performance.
