An experimental study of the viscous properties of water vapour

Abstract

The study of the viscous properties of water vapour has received little attention in the past, and there is no record of any systematic attempt to obtain results over a large range of temperature. This is remarkable in view of the fact that water is such a common substance, and that a knowledge of the viscosity of steam is presumably important in engineering practice. The difficulties are admittedly considerable, particularly if the range of temperature is to be such that the vapour will behave approximately as a perfect gas. In the work to be described presently, this principal difficulty has been reduced by using the vapour at comparatively low pressures, so that considerable super ­heating may be obtained without the attainment of very high temperatures. The range of temperature actually used has been from 100°C. to 260°C., and the work produces reliable evidence that under the experimental conditions the viscosity is independent of pressure—a result only to be expected according to the Kinetic Theory, in so far as the super-heated vapour obeys the laws of a perfect gas. Thus the values of the viscosity actually obtained are presumably the same as those operative under conditions more closely corresponding to actual practice. The method used involves the transpiration of the vapour through a capillary tube. There is, of course, an objection to this method, on account of its dependence upon the assumption that Boyle’s Law is applicable to the gas or vapour under investigation. This objection, however, is met by the argument given above, and it has thus been possible to adopt this method, in preference to those depending on damping of oscillations, on account of the greater facility with which the variation of viscosity with temperature can be observed.