Istraživanje turbulentnog dvofaznog strujanja u pravim kanalima pneumatskog transporta granularnog materijala nekružnog poprečnog preseka

Abstract

The two-phase turbulent flows of gas-solid particles, in the channels of noncircular cross-section (square, rectangular, etc.), are very often encountered in engineering practice. Examples of this flows type can be found in ventilation systems and air conditioning, in systems for pneumatic transport of granular materials, in turbo machines, heat exchangers, nuclear reactors, open channels, etc. Two-phase flows of gas-solids are characterized by a variety of phenomena which are a consequence of interaction the gas and the solid phases. In these channels beside the main stream along the channel occur secondary flows in the plane of the channel cross section. The mechanisms that lead to the secondary flows are different (curved channels, different temperature load of channel walls etc.). This type of flows, known as Prandtl’s secondary flow of the first kind, was the subject of a large number of researchers. A special flow phenomenon which occurs in straight channels in the regime of fully developed turbulent flow is a secondary flow of the second kind, known in the literature as secondary flow of Prandtl's second kind. To this problem is a very little attention has been paid so far in research practice. This flow has a significant impact in the total mechanism of mass, impulse and heat transfer in the channel and in environment. Transfers of high intensity impulses in the channel have as a consequence large gradients of transversal velocity in the plane of channel cross section. Produced secondary flow increase shear stress in the corners of the channels and has a significant effect on the intensity of heat transfer from fluid to the wall of the channel, and vice versa. These are some of the reasons why special attention to the phenomenon of secondary currents of the second kind in the two-phase system gas-solid particles is given in this paper. Considered flow is turbulent, steady, incompressible and fully developed in the straight channel of square cross-section, whose walls have a constant temperature different from the environmental temperature.