Flow structure and momentum transport for buoyancy driven mixing flows in long tubes at different tilt angles

Abstract

Buoyancy driven mixing of fluids of different densities (ρ1 and ρ2) in a long circular tube is studied experimentally at the local scale as a function of the tilt angle from vertical (15°≤θ≤60°) and of the Atwood number [10−3≤At=(ρ2−ρ1)/(ρ2+ρ1)≤10−2]. Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) measurements in a vertical diametral plane provide the velocity and the relative concentration (and, hence, density) fields. A map of the different flow regimes observed as a function of At and θ has been determined: as At increases and θ is reduced, the regime varies from laminar to intermittent destabilizations and, finally, to developed turbulence. In the laminar regime, three parallel stable layers of different densities are observed; the velocity profile is linear and well predicted from the density profile. The thickness of the intermediate layer can be estimated from the values of At and θ. In the turbulent regime, the density varies slowly with z in the core of the flow: there, transverse turbulent momentum transfer is dominant. As At decreases and θ increases, the density gradient β in the core (and, hence, the buoyancy forces) becomes larger, resulting in higher extremal velocities and indicating a less efficient mixing. While the mean concentration varies with time in the turbulent regime, the mean velocity remains constant. In the strong turbulent regime (highest At and lowest θ values), the transverse gradient of the mean concentration and the fluctuations of concentration and velocity remain stationary, whereas they gradually decay with time when turbulence is weaker.