The relation between the coefficients for heat exchange by convection and by evaporation in man

Abstract

The exchange of heat between an object and the surrounding air may be considered to take place across a boundary air layer whose thickness depends on the ambient air movement and on the shape, size and surface characteristics of the object. If water vapour is being exchanged between the object and the air, this diffusion must also take place through the boundary air layer, and it might be expected that the rates of exchange of heat and of water vapour should both be related in the same way to the thickness of the boundary air layer. Thus if a certain change in boundary air layer thickness doubled the heat exchange by convection for a given temperature difference, it might be supposed that the water-vapour exchange for a given vapour pressure difference would also be doubled. This proposition involves the assumption that the diffusivities of heat and of water vapour in air are the same, and it happens that they are in fact nearly equal. Discrepancies are to be expected when the temperature difference is great or when the vapour pressures considered are comparable with the atmospheric pressure, but these effects are likely to be small in most conditions encountered in 'hot climate' physiology (Jakob, 1949).The assumption of a constant ratio between the coefficients for heat exchange by evaporation and by convection is implicit in Brunt's analysis of human heat balance (Brunt, 1947), and has been stated axiomatically in that of Woodcock, Powers & Breckenridge (1956), who give the ratio as 2-0, when the units used are kcal/m2.hr.mm Hg and kcal/m2.hr.°C respectively. Observations on three inert bodies, a thermometer bulb, a sphere of 6 in. diameter and a full-sized dummy man, have confirmed this value (Kerslake & Waddell, 1957). The present paper describes the results of an investigation of the ratio in human subjects.

Heated, perspiring manikin headform for the measurement of headgear ventilation characteristics

“…One possible explanation for the difference between ALEX and clothing manikins is that perspiration is simulated in most of the modern manikins by expelling water vapour though a porous membrane; this could entail a lower pressure gradient at the water surface, and therefore a higher water vapour resistance than a more open, wet surface as employed here. For a wet surface or wet human skin, the values obtained are expected (Brebner et al 1958) to be inversely proportional to the thermal resistances given in table 1 above, with a proportionality constant of approximately two if the pressure is expressed in Torr. The results here imply instead factors of approximately 1.5 and three, which are nevertheless greater than unity, and not greatly different from two.…”

The effect of wind speed on maximum evaporative capacity in man

“…Experiments were conducted in a booth situated in a wind tunnel (Brebner et al 1958). This contained a recording balance, on the pan of which was a dish of liquid paraffin.…”

Heat and Water Rate Transfer Processes in the Human Respiratory Tract at Various Altitudes

“…The parameter h A and evaporative heat transfer coe$cient h C are connected by the Lewis relation (Brebner et al, 1958;Rapp, 1970) h…”