Heat and mass transfer mechanisms have been characterized in physical models of human cadaver airways to simulated depths of 305 m with various gas mixtures. Such characterizations offer a detailed understanding of the effects of environmental pressures, gas composition, and respiratory rates (RMV) on the body cooling capacity of the respiratory airways. Empirical heat transfer relationships in the form -Nu = AReNPr1/3 are derived for the oral and nasal passageways during inhalation and exhalation flows. -Nu, Re, and Pr are the dimensionless Nusselt, Reynolds, and Prandtl numbers, respectively. The Nusselt and Reynolds numbers are based on the diameter and gas flow rate in the trachea and are applicable to Reynolds number values up to 70,000.
The thermal insulation characteristics of two drysuit ensembles, consisting of the same tri-laminate outergarment with differing thermal undergarments, were measured on a 21-zone thermal manikin at the Navy Clothing and Textile Research Facility (NCRTF) during immersion testing when using air and argon alternatively as the suit inflation gas. Total thermal insulation values were determined for both garments utilizing ASTM test standard F 1291-standard test method for measuring the thermal insulation of clothing using a heated manikin. Improvements in localized thermal insulation values were seen throughout both drysuit ensembles when using argon as an inflation gas when compared with those while using air. Improvements with argon inflation in an experimental aerogel garment ranged from a low of 11% in the legs, 27% in the arms, and 22% in the torso. Overall, the total suit insulation increased with the aerogel garment by approximately 16%. Improvements with argon inflation in a commercial drysuit ranged from a low of 5% in the torso, 12% in the arms, to a high of 32% in the legs. Overall, the total suit insulation increased with the commercial garment by approximately 20%. This investigation demonstrated that significant improvements in drysuit thermal protection can be achieved when using argon instead of air as a drysuit inflation gas. It should be noted however that these improvements were achieved by carefully and repeatedly purging (a minimum of 6 purge cycles) with pure argon prior to water entry. It is hypothesized that reduced thermal improvements have been seen in practice due to inadequate suit purging prior to dives.
A powerful new tool for the analysis and design of underwater breathing gas systems is being developed. A versatile computer simulator is described which makes possible the modular “construction” of any conceivable breathing gas system from computer memory-resident components. The analysis of a typical breathing gas system is demonstrated using this simulation technique, and the effects of system modifications on performance of the breathing system are shown. This modeling technique will ultimately serve as the foundation for a proposed breathing system simulator under development by the Navy. The marriage of this computer modeling technique with an interactive graphics system will provide the designer with an efficient, cost-effective tool for the development of new and improved diving systems.
During January 2005, the U. S. Navy Experimental Diving Unit (NEDU) in Panama City, Florida conducted a repeated measures series of twelve test dives, each up to three hours in duration, to compare the thermal performance of a prototype diving garment using a superinsulation aerogel fabric with that of a commercially-available Thinsulate garment worn beneath a commercial dry suit. The thermal benefit of the experimental aerogel garment was determined by statistics describing psychological and physical thermal status data from the aerogel and the commercial Thinsulate garments. All tests were conducted to simulate long-duration cold water conditions in the NEDU test pool, where water temperature was maintained between 1.7 and 4.4 °C (35 and 40 °F). Divers remained immobile while either lying or sitting in chairs on the bottom of the test pool, and they subjectively reported their thermal comfort at 30-minute intervals during each dive. Mean dive durations were found to be approximately 43% longer when divers wore the prototype aerogel garment than when they wore an M400 Thinsulate liner. The prototype aerogel garment also enhanced thermal protection to the fingers and toes, although thermal stress to these body regions still remained the most frequent reason for aborting dives. Future research should include work on localized active heating of the hands and feet to augment the thermal insulation of the prototype aerogel garment.
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