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

Buoyancy driven mixing of fluids of different densities ( ρ 1 and ρ 2 ) 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 = ( ρ 2 − ρ 1 ) / ( ρ 2 + ρ 1 ) ≤ 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.