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An experimentally determined and theoretically derived relationship between convective heat transfer coefficient (hc) and barometric pressure (Pb) is applied to predict the clothing insulation property at hypobaric environments. The U.S. Army Research Institute of Environmental Medicine hypobaric chamber was used to simulate five environmental conditions: at elevations of 0, 1520 m, 3050 m, 4570 m, and 6100 m. Under constant wind speed and temperature in the chamber, hc was determined using the naphthalene sublimation technique. Circular naphthalene disks were placed at six body sites on a stationary, copper manikin, which models the human body anthropometrically. Naphthalene weight loss was then translated to hc using the Chilton-Colburn heat and mass transfer analogy. We found that as Pb decreased, hc decreased accordingly, pointing to a diminished convective transfer process. However, our data also showed that hc varied linearly with Pb. Previously, hc and Pb were thought to be related through a nonlinear power function. A linear hc-Pb dependency was also theoretically derived. Consequently, in hypobaric environments, the previously employed nonlinear function would overestimate hc when compared to a linear relationship. Overestimation of hc means an overestimation of the potential convective heat transfer, and thus a probable underestimation of clothing insulation. For the four hypobaric environments examined in this study, overestimations in hc translate to potential clothing insulation underestimations of 6.72%, 11.2%, 14.1%, and 16.1%. The degree of underestimation worsens in order of decreasing barometric pressure (or ascending terrestrial elevation). Hence, an exercising person dressed according to the conventional predictive model, may well be overdressed and suffer from too much heat storage rather than from the inherent cold of high altitude.
clothing insulation prediction, convective heat transfer, hypobaric environments, heat-mass transfer analogy, Chilton-Colburn j-factors, naphthalene sublimation
Research Biomedical Engineer, U.S. Army Research Institute of Environmental Medicine, Natick, MA
Division Chief, U.S. Army Research Institute of Environmental Medicine, Natick, MA