Unexpected trapping of particles at a T junction

Abstract

A common element in physiological flow networks, as well as most domestic and industrial piping systems, is a T junction that splits the flow into two nearly symmetric streams. It is reasonable to assume that any particles suspended in a fluid that enters the bifurcation will leave it with the fluid. Here we report experimental evidence and a theoretical description of a trapping mechanism for low-density particles in steady and pulsatile flows through Tshaped junctions. This mechanism induces accumulation of particles, which can form stable chains, or give rise to significant growth of bubbles due to coalescence. In particular, low-density material dispersed in the continuous phase fluid interacts with a vortical flow that develops at the T junction. As a result suspended particles can enter the vortices and, for a wide range of common flow conditions, the particles do not leave the bifurcation. Via 3D numerical simulations and a model of the two-phase flow we predict the location of particle accumulation, which is in excellent agreement with experimental data. We identify experimentally, as well as confirm by numerical simulations and a simple force balance, that there is a wide parameter space in which this phenomenon occurs. The trapping effect is expected to be important for the design of particle separation and fractionation devices, as well as used for better understanding of system failures in piping networks relevant to industry and physiology.fluid dynamics | bubble trapping | vortex breakdown | 3D simulations

Accumulation of Colloidal Particles in Flow Junctions Induced by Fluid Flow and Diffusiophoresis

“…However, recent observations suggest that this may not always be the case; particles that are less dense than the surrounding fluid may be trapped in flow junctions via the vortex breakdown phenomenon [1][2][3]. Such an unexpected trapping of particles in simple flow geometries implies unrecognized ways that flow systems can fail or respond in unusual ways.…”

Vortex-Breakdown-Induced Particle Capture in Branching Junctions

“…For example, for flow through a T junction in which flow enters the base of the T and splits between the two symmetric outlets, it is natural to believe that suspended particles entering the system will find the junction to be a kinematically unstable stagnation region and be swept downstream through the outlets. However, it has been shown that, in fact, bubbles can be trapped in these regions within flow features that resemble vortex breakdown [1,2], which refers to a phenomenon where internal stagnation points develop, followed by regions of reversed flow with limited axial extent [3].Despite the fact that this capture mechanism depends strongly on the swirling motion of flow in the junction through the interplay of centrifugal, pressure gradient, and drag forces [1], the effect of varying the junction angle has not been explored. We introduce this geometric change, systematically varying the junction angle θ, which introduces significant changes in the secondary swirl velocities [4,5] (Fig.…”

“…For example, for flow through a T junction in which flow enters the base of the T and splits between the two symmetric outlets, it is natural to believe that suspended particles entering the system will find the junction to be a kinematically unstable stagnation region and be swept downstream through the outlets. However, it has been shown that, in fact, bubbles can be trapped in these regions within flow features that resemble vortex breakdown [1,2], which refers to a phenomenon where internal stagnation points develop, followed by regions of reversed flow with limited axial extent [3].…”

“…Despite the fact that this capture mechanism depends strongly on the swirling motion of flow in the junction through the interplay of centrifugal, pressure gradient, and drag forces [1], the effect of varying the junction angle has not been explored. We introduce this geometric change, systematically varying the junction angle θ, which introduces significant changes in the secondary swirl velocities [4,5] (Fig.…”

“…The capture limits are not seen to depend strongly on ρ p =ρ f . A maximum particle density of ρ p ≈ 0.7ρ f for capture in a T junction has been suggested [1]. Figure 5 may be used to design systems that maximize or suppress particle capture by selecting appropriate values of Re and θ. Qualitatively, the most particles appear to be trapped around Re ¼ 230 and θ ¼ 70°.…”