Observations and LDA measurements of confined turbulent vortex flow

Abstract

A series of LDA measurements and visual observations of confined turbulent vortex flow are described. The experiments were performed with water as the fluid medium in a vortex tube of length-to-diameter ratio L/D = 3.8 for a range of exit diameters De between De/D = 1 and 0.18. The experiments reveal a remarkable change in the vortex structure as De is reduced: from a thick core with an axial-velocity defect in the centre, and even reversed flow, to a thin annular jet-like core with a peak axial velocity more than an order of magnitude greater than the average value and again a central velocity deficit. The corresponding swirl profiles are not remarkable and are well-represented under all conditions by the solution of Burgers (1948), albeit with a velocity maximum which is strongly dependent upon De.

Simulations of confined turbulent vortex flow

“…In this figure, a coordinate system, and the nomenclature with respect to the geometrical dimensions (D, D e , L, L e , and t) have been defined. In [11], the definition of the Reynolds number was based on the conditions at the inlet: Re = U in t/m, with U in the (superficial) inlet velocity (U in = Q/Lt, with Q the volumetric flow rate), and m the fluidÕs kinematic viscosity. It should be noted that this Reynolds number definition is somewhat a-typical for the flow system under consideration, since the inlet conditions are not representative for the flow in the swirl tube and exit pipe.…”

“…For instance, the Reynolds number based on the exit pipe diameter, and bulk velocity in the exit pipe is 5-25 times (dependent on D e ) higher than the inlet Reynolds number. However, in order to conform with the definitions in [11], we will use the inlet Reynolds number in the remainder of this article to characterize the flow. In [11], also a geometrical swirl parameter was defined: X = pDD e /Lt.…”

“…However, in order to conform with the definitions in [11], we will use the inlet Reynolds number in the remainder of this article to characterize the flow. In [11], also a geometrical swirl parameter was defined: X = pDD e /Lt. It serves as a measure for the ratio of angular to axial momentum.…”

“…For D e /D = 0.455, and D e /D = 0.327 the LDA experiments were done at Re = 1900. In measuring the radial velocity profiles, Escudier and co-workers took care to traverse their LDA measuring volume through the vortex core center (defined as the position with zero transverse velocity), which in general does not coincide with the geometrical center of the flow facility [11]. Furthermore, in [11] (and also in [13]) flow visualization photographs were presented at various Reynolds numbers, and various D e /D ratios.…”

“…In order to study the performance of our simulation procedure applied to strongly swirling flow in more detail, and to make use of an extensive body of experimental evidence on swirling flow systems it was decided to select a geometry with great similarity to a reverse-flow cyclone: the vortex tube geometry defined in the experimental study by Escudier et al [11], see Fig. 1.…”