水平輸送管路内の堆積層の厚みの算定式 堆積層を伴うスラリーの流動機構に関する理論的考察

Abstract

With increasing industrial applications of the method of the transporting settling slurries, a better understanding of the mechanism of the slurry flow at lower velocities has become essential. Below the critical deposition velocity Vd of flow, the deposition of solid particles occurs on the bottom of the horizontal pipe. If the deposited particles are sufficient, the plugging of the pipeline can be induced through the large surge of flow or the sudden change of flow conditions, such as a rise in concentration or a drop in velocity V. However, some papers confirm the possibility of the stable flow configuration in pipe despite of a stationary bed.One of the main problems in the hydraulic transport for V < Vd is the assessment for bed thickness h and hydraulic gradient im. The accuracy of one of the factors is interdependent upon that of the other. Since the correlations have been basically proposed by empirical methods, the bed thickness cannot be reasonably well predicted with flow conditions.The purpose of this paper is to derive the general correlation which determine the relationships between the bed thickness and the corresponding flow condition, and to give the procedure for the pipeline design with the aid of the authors' previous results. Based on the concept of the suspension behavior introduced by Bagnold, this paper estimated the normal stress on the particle at the surface of the bed. The flow condition of initial motion of the particle, then, was considered, and thus these theoretical analyses made possible to present the general correlation of h and to simulate the slurry flow for V<Vd. Experimental values obtained were fairly well coincided with the predicted values.The results of this study may be summarized as follows:(1) For a given system and particle properties the Froude number, Fr V/√4gR, is approximately as a function of concentration alone, such as Durand's results for 0.2<hr <0.6, in which hr is the ratio of the bed thickness to the pipe diameter.(2) The predictions for the critical deposition velocity Vd agree with those by Durand for particles greater than 2mm, while for ones less than 1mm both predictions are incompatible with each other, when the equivalent thickness de/2 is substituted for the bed thickness at the initial condition of Vd.(3) The relationship between hydraulic gradient and slurry velocity, which is the im-V cure predicted, shows that the critical velocities are always less than the critical deposit velocities, and considerable care, therefore, should be exercised to determine the practical velocity in the pipeline design.