Investigation of carbon dioxide transport in life support helmets

This research explores carbon dioxide transport in life support helmet annular space using new theoretical and experimental techniques. Increased transport from next generation helmets is necessary to allow reduction of fresh gas flow and associated noise. Conventional helmet noise interferes with communications and some underwater helmets even approach hearing threshold shift levels. Helmet flow is three dimensional, unsteady, and turbulent; this research is the first known effort to identify the fundamental mechanisms of CO2 transport. An analytical model is developed which predicts average inhaled CO2 concentration for generic helmet geometry using a mixing volume approach. The model includes sensitivity to supply flow, breath rate, metabolic CO2 production, inhalation and exhalation mixing volumes, and breathing symmetry. Numerical sensitivity analysis using the model indicates optimum design paths. Nominal head-helmet-lung geometry is identified. An experimental nominal model was developed which supports inhaled concentration measurements with air-CO2 or water-dye as working fluids. Water modeling provides flow visualization which is used to identify complex convective and turbulent CO2 transport mechanisms. Correlation of water-dye and air-CO2 results indicates conditions when molecular diffusion of CO2 is significant. The research was directed primarily toward diving helmets but is applicable to spacesuit and firefighter helmets, as well as any situation involving mass transport in a periodic mixing chamber. New analytical and experimental models are substantially more accurate than the conventional steady state helmet mixing model, and provide direction for improved helmet design.