Abstract
Convective heat transfer is governed by a number of factors including various fluid properties, the presence of a thermal gradient, geometric configuration, flow condition, and gravity. Empirically-derived analytical relationships can be used to estimate convection as a function of these governing parameters. Although it is relatively straightforward to experimentally quantify the contributions of the majority of these variables, it is logistically difficult to assess the influence of reduced-gravity due to practical limitations of establishing this environment. Therefore, in order to explore this regime, a series of tests was conducted to evaluate convection under reduced-gravity conditions averaging 0.45 m/sec2 (0.05 g) achieved aboard a parabolic aircraft. The results showed a reduction in net heat transfer of approximately 61% in flight relative to a 1g terrestrial baseline using the same setup. The average experimental Nusselt Number of 19.05 ± 1.41 statistically correlated with the predicted value of 18.90 ± 0.63 (N = 13), estimated using the Churchill-Chu correlation for free convective heat transfer from a finite, flat, vertical plate. Extrapolating this to similar performance in true microgravity (10−6 g) indicates that these conditions should yield a Nusselt Number of 1.27, which is 2.6% the magnitude of free convection at 1g, or a reduction of 97.4%. With advection essentially eliminated, heat transfer becomes limited to diffusion and radiation, which are gravity-independent and nearly equivalent in magnitude in this case. These results offer a general guideline for integrating components that utilize natural (free) convective gas cooling in a spacecraft habitat and properly sizing the thermal control system.
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Acknowledgments
The authors would like to thank the NASA’s Reduced Gravity Student Flight Opportunities Program and Reduced Gravity Office for providing the parabolic aircraft platform and flight opportunity in the summer of 2012. Financial support for this project was provided by the William F. Marlar Memorial Foundation, the College of Engineering and Applied Sciences at the University of Colorado Boulder (CU), the CU Aerospace Engineering Sciences Department, the CU Engineering Council, and the CU Council of Colleges and Schools. We would also like to acknowledge Professor Jeffrey Thayer at the University of Colorado Boulder for his insight and assistance on this project, as well as Andy Broucek, Kyle Shannon and Jared Yenzer for their participation in develo** the test hardware and carrying out the flight experiments.
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Lotto, M.A., Johnson, K.M., Nie, C.W. et al. The Impact of Reduced Gravity on Free Convective Heat Transfer from a Finite, Flat, Vertical Plate. Microgravity Sci. Technol. 29, 371–379 (2017). https://doi.org/10.1007/s12217-017-9555-8
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DOI: https://doi.org/10.1007/s12217-017-9555-8