Physical Investigations on the Exhaust Vapour issuing from the Pistons of Pile Driving Machine

[摘要] Part I. Thermodynamical and Colloid-Meteorological Problems.Fig. 2 in Pl. III shows cumulus-like clouds produced by the exhaust vapour issuing from the piston “A” of the pile driving hammer shown if fig. 1. Its formation is completed in the following three processes:-In the first of these processes a mass of the vapour which is pushed out into the air at a pressure of 6.9 atmospheres, makes adiabatic expansion, till the pressure reduces to one atmospheric pressure. By cooling the vapour condenses to water-droplets which are large enough to form drizzle.In the second process the cooling and expansion of the vapour is continued by mixing with the surrounding air, until it attains a certain temperature somewhat higher than that of air. By the condensation in this process minute cloud particles are produced.In the second process the mixing of air into vapour is based upon thermodynamical unstability, but in the third process it is forced by mechanical or hydrodynamical action and gives rise to the evaporation of the vater particles in the cloud.Part II. On the “Surface Evaporation” of clouds.Cloud particles evaporate in de_??_cending air. We call the phenomena the “volume evaporation”; on the boundary of cloud with unsatuiated air there takes place also the evaporation of the cloud We call the phenomena “surface evaporation”.Surface evaporation of clouds might be considered analogous to that of liquids from their surface, when the diffusion of cloud particles caused by turbulence of the air is very slow compared with the rate of the evaporation. As an example of it we calculated as for the exhaust clouds the time of their disappearance under the assumptionwhere V is the volume of the cloud, S its surface area, p the saturation difference of the surrounding air, and k a constant. The stronger the wind, the more the cloud is prolonged, and if its velocity be taken into consideration, a solution of the above equation is obtained in the formwhere r is the time of disappearance of the cloud, v wind velocity, A and B are constants.Our observations agree well with the calculations.Part III. On the Size Distribution of the Water-drops.In absorption method we made photographic magnification (×5) of the spots of water-droplets on filter paper to read accurately their diameters up to decimal fractions of a millimeter, and at the same time to measure such a small spot as 0.2mm in diameter.In the size distribution of the drizzle from “A” cloud, we found a group of drops which lacks the second term, that is, the water-droplets which have the ratio of 1:4:8:16:32 in weight predominate.This might be explained from the view-point of coagulation by the consideration of the effect of the wind upon the falling paths of waterdroplets. They are driven leeward by the wind, and the droplets of different sizes go down in different paths. Let it be assumed, that a group of uniform water-droplets starts together to fall down to the ground, and that on the way they coagulate once, twice, and more. And if the exposure of the receiving filter paper is short, the droplets which have coagulated twice (correspond to the second term in the group distribution) are observed not at the same time with the others.

[学科分类] 大气科学