Jiann Yang

Detection of Smoke from Microgravity Fires

The history and current status of spacecraft smoke detection is discussed including a review of the state of understanding of the effect of gravity on the resultant smoke particle size. The results from a spacecraft experiment (Comparative Soot Diagnostics (CSD)) which measured microgravity smoke particle sizes are presented. Five different materials were tested producing smokes with different properties including solid aerosol smokes and liquid droplets aerosol smokes. The particulate size distribution for the solid particulate smokes increased substantially in microgravity and the results suggested a corresponding increase for the smokes consisting of a liquid aerosol. A planned follow on experiment that will resolve the issues raised by CSD is presented. Early results from this effort have provided the first measurements of the ambient aerosol environment on the ISS (International Space Station) and suggest that the ISS has very low ambient particle levels.

Measurement of the Optical Extinction Coefficients of Post-Flame Soot in the Infrared

Abstract The optical extinction coefficients of post-flame soot have been measured in the wavelength range 2.8 to 4.1 μm. A laminar diffusion burner was combined with an infrared spectrograph and gravimetric measurements to determine the mass specific extinction coefficient, σs, and the dimensionless extinction coefficient, Ke. Using ethene gas as the fuel, the burner was operated at four global equivalence ratios (φ = 0.8, 1.0, 2.0, and 3.0) to examine the effect of the fuel-air ratio on the extinction coefficient. The extinction coefficient was found to decrease with increasing values of the global equivalence ratio for φ = 1.0, 2.0, and 3.0. The results for φ = 0.8 and φ = 1.0 were in agreement to within the uncertainty of the measurements. Measurements were obtained using propane gas as the fuel (φ = 1.0) and resulted in extinction coefficients equivalent to those of ethene. Transmission electron microscopy (TEM) images revealed differences in the morphology of the particles, consistent with the quantitative differences observed in the extinction data. The data indicate that the equivalence ratio has a strong effect on the optical properties of post-flame soot agglomerates.

Smoke Characterization and Feasibility of the Moment Method for Spacecraft Fire Detection

The Smoke Aerosol Measurement Experiment (SAME) has been conducted twice by the National Aeronautics and Space Administration and provided real-time aerosol data in a spacecraft micro-gravity environment. Flight experiment results have been recently analyzed with respect to comparable ground-based experiments. The ground tests included an electrical mobility analyzer as a reference instrument for measuring particle size distributions of the smoke produced from overheating five common spacecraft materials. Repeatable sample surface temperatures were obtained with the SAME ground-based hardware, and measurements were taken with the aerosol instruments returned from the International Space Station comprising two commercial smoke detectors, three aerosol instruments, which measure moments of the particle size distribution, and a thermal precipitator for collecting smoke particles for transmission electron microscopy (TEM). Moment averages from the particle number concentration (zeroth moment), the diameter concentration (first moment), and the mass concentration (third moment) allowed calculation of the count mean diameter and the diameter of average mass of smoke particles. Additional size distribution information, including geometric mean diameter and geometric standard deviations, can be calculated if the particle size distribution is assumed to be lognormal. Both unaged and aged smoke particle size distributions from ground experiments were analyzed to determine the validity of the lognormal assumption. Comparisons are made between flight experiment particle size distribution statistics generated by moment calculations and microscopy particle size distributions (using projected area equivalent diameter) from TEM grids, which have been returned to the Earth. Copyright 2015 American Association for Aerosol Research

Particle Morphology and Size Results from the Smoke Aerosol Measurement Experiment-2

Results are presented from the Reflight of the Smoke Aerosol Measurement Experiment (SAME-2) which was conducted during Expedition 24 (July-September 2010). The reflight experiment built upon the results of the original flight during Expedition 15 by adding diagnostic measurements and expanding the test matrix. Five different materials representative of those found in spacecraft (Teflon, Kapton, cotton, silicone rubber and Pyrell) were heated to temperatures below the ignition point with conditions controlled to provide repeatable sample surface temperatures and air flow. The air flow past the sample during the heating period ranged from quiescent to 8 cm/s. The smoke was initially collected in an aging chamber to simulate the transport time from the smoke source to the detector. This effective transport time was varied by holding the smoke in the aging chamber for times ranging from 11 to 1800 s. Smoke particle samples were collected on Transmission Electron Microscope (TEM) grids for post-flight analysis. The TEM grids were analyzed to observe the particle morphology and size parameters. The diagnostics included a prototype two-moment smoke detector and three different measures of moments of the particle size distribution. These moment diagnostics were used to determine the particle number concentration (zeroth moment), the diameter concentration (first moment), and the mass concentration (third moment). These statistics were combined to determine the diameter of average mass and the count mean diameter and, by assuming a log-normal distribution, the geometric mean diameter and the geometric standard deviations can also be calculated. Overall the majority of the average smoke particle sizes were found to be in the 200 nm to 400 nm range with the quiescent cases producing some cases with substantially larger particles.

Spacecraft Fire Detection: Smoke Properties and Transport in Low-Gravity

Results from a recent smoke particle size measurement experiment conducted on the International Space Station (ISS) are presented along with the results from a model of the transport of smoke in the ISS. The experimental results show that, for the materials tested, a substantial portion of the smoke particles are below 500 nm in diameter. The smoke transport model demonstrated that mixing dominates the smoke transport and that consequently detection times are longer than in normal gravity.

Smoke Aerosol Measurement Experiment-2: Comparison of Flight Experiment Results with Ground Test Results

The Re-flight of the Smoke Aerosol Measurement Experiment (SAME-2) was conducted during Expedition 24 (July-September 2010) and flight experiment results have been analyzed with respect to comparable ground-based experiment results. The ground tests included a reference instrument for measuring particle size distributions of the smoke from SAME-2 materials (Teflon, Kapton ® , cotton, silicone rubber and Pyrell ® ). Repeatable sample surface temperatures were obtained with SAME engineering hardware and measurements were taken with returned flight diagnostic equipment consisting of a thermal precipitator for collecting smoke particles for Transmission Electron Microscopy (TEM), several aerosol instruments, and a commercial smoke detector. These flight units measured moments of the particle size distribution: particle number concentration (zeroth moment), the diameter concentration (first moment), and the mass concentration (third moment). The count mean diameter of the smoke particles and the diameter of average mass can be calculated from moment averages. Furthermore, the geometric mean diameter and geometric standard deviations can be calculated if the particle size distribution is assumed to be lognormal. The flight system also included International Space Station and Space Shuttle smoke detectors and the results from these devices are presented. A Scanning Mobility Particle Sizer Spectrometer (SMPS) is too large and complex to include in a flight experiment, but its use in ground-based testing determined whether the lognormal assumptions were valid, and provided high-resolution particle size distributions of the smoke from each material. SMPS particle size distributions from ground experiments were compared to particle statistics generated by moment calculations and particle size

Properties of Smoke from Overheated Materials in Low- Gravity

Smoke particle size measurements were obtained under low-gravity conditions by overheating several materials typical of those found in spacecraft. The measurements included integral measurements of the smoke particles and physical sample of the particles for Transmission Electron Microscope analysis. The integral moments were combined to obtain geometric mean particle sizes and geometric standard deviations. These results are presented with the details of the instrument calibrations. The experimental results show that, for the materials tested, a substantial portion of the smoke particles are below 500 nm in diameter.

Validation Experiments of Large Compartment Fires

In cooperation with the fire protection engineering community, a computational fire model, Fire Dynamics Simulator (FDS), is being developed at NIST to study fire behavior and to evaluate the performance of fire protection systems in buildings. The software was released into the public domain in 2000, and since then has been used for a wide variety of analyses by fire protection engineers. An on-going need is to develop and validate new sub-models. Fire experiments are conducted for a variety of reasons, and model predictions of these experiments over the past few decades have gradually improved. However, as the models become more detailed, so must the measurements. The bulk of available large scale test data consist of temperature (thermocouple) measurements made at various points above a fire or throughout an enclosure. While it is useful to compare model predictions with these measurements, one can only gauge how closely the model reproduces the given data. There is often no way to infer why the model and experiment disagree, and thus no way to improve the model. Also, it is difficult to separate various physical phenomena in a large scale fire test so that combustion, radiation and heat transfer algorithms can be evaluated independently. For example, the heat release rate of the fire governs the rate at which energy is added to the system, convective and radiative transport distribute the energy throughout, and thermal conduction drains the system of some of the energy. The measured value of a temperature, heat flux, or gas concentration at any one point depends on all the physical processes, and uncertainties in each phase of the calculation tend to combine in a non-linear way impacting the prediction.Copyright

Microgravity Superagglomerates Produced By Silane And Acetylene

The size of the agglomerates produced in the upper portion of a flame is important for a variety of applications. Soot particle size and density effect the amount of radiative heat transfer from a fire to its surroundings. Particle size determines the lifetime of smoke in a building or in the atmosphere, and exposure hazard for smoke inhaled and deposited in the lungs. The visibility through a smoke layer and dectectability of the smoke are also greatly affected by agglomerate size. Currently there is limited understanding of soot growth with an overall dimension of 10 m and larger. In the case of polystyrene, smoke agglomerates in excess of 1 mm have been observed raining out from large fires. Unlike hydrocarbon fuels, silane has the advantage that silica particles are the major combustion product resulting in a particle volume fraction a factor of ten greater than that for a carbonaceous smoke. There are two very desirable properties of silica aero-gels that are important for both space and earth based applications. The first important property is its inertness to most oxidizing and reducing atmospheres. Therefore, silica aero-gels make excellent fire ablatives and can be used in very demanding applications. The second important property is that silica aero-gels are expected to have very high porosity (greater than 0.999), making them lightweight and ideal for aerospace applications. The added benefit of the high porosity is that they can be used as extremely efficient filters for many earth based applications as well. Evidence of the formation of superagglomerates in a laminar acetylene/air diffusion flame was found by Sorensen et al. [1]. An interconnecting web of super-agglomerates was observed to span the width of the soot plume in the region just above the flame tip and described as a gel state. It was observed that this gel state immediately breaks up into agglomerates as larges as 100 m due to buoyancy induced turbulence. Large soot agglomerates were observed in microgravity butane jet diffusion flames by Ito et al.[2]. Several other works to date have studied the effect of flame structure on soot volume fraction and agglomeration size in a microgravity environment.[3-4]. In microgravity the absence of buoyant convective flows increases the residence time in the flame and causes a broadening of the high temperature region in the flame. Both of these factors play a significant role in gas phase radiation and soot formation