Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........17f&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF VIRGINIA, 1993.Source: Dissertation Abstracts International, Volume: 54-08, Section: B, page: 427
Astronomy and Astrophysics
Astrophysics
3
Refractory
Scientific paper
The nucleation and coagulation of refractory particles from the vapor is a phenomenon which is important in many diverse fields. It is especially important in astrophysics where such information may shed insight into the origin of interstellar dust and perhaps larger bodies within the universe such as planets. Unfortunately, there are few data on refractory nucleation and little agreement between theory and the data that do exist. To provide more data on the nucleation of refractory particles from vapor and perhaps the subsequent coagulation of these particles, we have constructed a new apparatus at NASA's Goddard Space Flight Center. Previous studies of refractory nucleation using the gas evaporation method have been hampered by natural convective flows arising due to high temperatures and temperature gradients needed to produce refractory vapors. To help minimize these thermal convective currents, the new chamber has been designed to fly aboard NASA's KC-135 research aircraft which is capable of producing a series of approximately 23-second periods of microgravity. As with other gas evaporation methods, conditions such as temperature cannot be measured directly at the point of condensation since such probing may influence the nucleation process. Therefore, the conditions at the point of condensation must be estimated using a combination of mathematical models and measurements in less critical areas. This work is a summary of a numerical model developed to predict temperature and flow fields within the flight chamber in response to the changing gravitational level aboard the KC-135. Results from this model are presented and compared with experimental measurements for five cases from previous KC-135 flights. The model results indicate that, under typical operating conditions, it is possible for the natural convective flows to decay within the 23 seconds of microgravity, but the pressure must be roughly 36 torr or less for the temperature field to reach near-steady conditions within this period. In addition, both the flow and temperature fields are very sensitive to minute fluctuations in gravitational level or "g-jitter" during microgravity.
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