Heat and mass exchange processes between the surface of the human body and ambient air at various altitudes
The rates of convection and evaporation at the interface between the human body and the surrounding air are expressed by the parameters convective heat transfer coefficient hc, in W m-2°C-1 and evaporative heat transfer coefficient he, W m-2 hPa-1. These parameters are determined by heat transfer equations, which also depend on the velocity of the airstream around the body, that is still air (free convection) and moving air (forced convection). The altitude dependence of the parameters is represented as an exponential function of the atmospheric pressure p: hc pn and he p1-n, where n is the exponent in the heat transfer equation. The numerical values of n are related to airspeed: n=0.5 for free convection, n=0.618 when airspeed is below 2.0 ms-1 and n=0.805 when airspeed is above 2.0 ms-1. This study considers the coefficients hc and he with respect to the similarity of the two processes, convection and evaporation. A framework to explain the basis of established relationships is proposed. It is shown that the thickness of the boundary layer over the body surface increases with altitude. As a medium of the transfer processes, the boundary layer is assumed to be a layer of still air with fixed insulation which causes a reduction in the intensity of heat and mass flux propagating from the human body surface to its surroundings. The degree of reduction is more significant at a higher altitude because of the greater thickness of the boundary layer there. The rate of convective and evaporative heat losses from the human body surface at various altitudes in otherwise identical conditions depends on the following factors: (1) during convection - the thickness of the boundary layer, plus the decrease in air density, (2) during evaporation (mass transfer) - the thickness of the boundary layer, plus the increase with altitude in the diffusion coefficient of water vapour in the air. The warming rate of the air volume due to convection and evaporation is also considered. Expressions for the calculation of altitude dependences hc (p) and he (p) are suggested.