Smoke and Aerosol Measurement Experiment

One of the major goals of this investigation was to quantify the sizes that the liquid smoke particulate achieves in low gravity and to relate the droplet size distribution to the smoke generation and smoke transport conditions in the experiment. A numerical code was to be developed to predict the smoke droplet growth as a function of the fuel pyrolysis rate, the thermodynamic properties of the pyrolysis vapor, and the flow environment.

The crew installed the near field hardware in the Glovebox, attached the Far Field Box to the Glovebox, and positioned the video cameras. The operator then ran the self diagnostic procedures on the two smoke detectors and activated the video cameras, turned on the igniter for a predefined period of time (15 to 60 seconds), and initiated the thermophoretic soot samplers to sample the smoke. The actual duration of each combustion event was typically 2 minutes. After flight, the particulate collected on the thermophoretic probes were analyzed using a Transmission Electron microscope (TEM) to determine primary and aggregate particle dimensions. The remains of the samples were weighed to determine their mass loss. The digital data was transcribed from the video record and analyzed to determine the response of the detectors. Once the flight mass loss rates were determined, 1-g operation conditions to produce the same loss were determined.The measurements were made using smoke generated by overheated spacecraft materials where variables such as the sample temperature, flow field, and particle aging time were more rigorously controlled.

The three hypotheses that motivated this investigation: In a μg environment, with the absence of the strong buoyancy-induced flows, smoke particle growth from a pyrolyzing/smoldering source, as a result of nucleation, condensation, and coagulation, will lead to larger particles compared to those produced in 1g. The formation of the larger particles is favored by the expected higher vapor concentration and smoke residence times near the source due to the reduced velocities and mixing at μg conditions. In particular, it is hypothesized that liquid aerosol smokes produced in microgravity will exhibit substantially larger droplet sizes than under the same conditions in normal gravity (with a corresponding decrease in the particle number density for the same mass of smoke). The change in the size distribution and number density will, in turn, affect the response of the smoke detectors and possibly result in some detectors not producing an alarm when exposed to this smoke.

The SAME experiment will provide technology for an advanced fire detector for future spacecraft that will be used for long duration missions. SAME will provide quantitative data on the sensitivity of these detectors to reduced gravity smokes that will allow evaluation of the adequacy of these existing technologies using relevant data. The current Fire Prevention, Detection, and Suppression (FPDS) program plan allows for the re-evaluation of future sensor technology, to allow new technology and capability to be utilized. The results from SAME are needed to provide the reduced gravity baseline data against which future detection technology developments can be evaluated.The smoke detectors developed from the results of SAME can also be useful in other extreme environments on Earth, such as submarines or underwater laboratories. Accurate detection of smoke in these environments can save lives.