The structure of the velocity and passive scalar mixing in a multiple opposed jets reactor
Seminar Room 1, Newton Institute
AbstractWe document an experimental investigation of a confined chamber in which fluid is injected through two sets of 16 opposed jets that issue from top/bottom boundary porous planes. The investigated Reynolds numbers, based on injection velocity and jet diameter are up to 28,000. The high Reynolds numbers and impinging configuration of the flow produce very intense turbulence levels and a turbulence with zero mean velocity in the central region of the reactor. The analysis is done for basically two geometries: opposed jets with strong backflow, and opposed jets with very slow backflow. Particles Image Velocimetry (PIV) measurements in different planes allowed for a characterization of the mean and fluctuating velocity fields. Fluid recirculation in the reactor creates annular shear layers. The central region of the reactor includes stagnation regions, where mean vertical velocity gradients are very strong with low local mean velocity values, leading to high rms-to-mean velocity ratios. Such gradients are responsible for considerable kinetic-energy production, that sharply peaks in the central region. A particular attention is paid to the determination of the small-scales characteristics (energy dissipation rate) in different points of the flow, which is done using both PIV (using indirect methods, i.e. inertial-range information) and LDV (Laser Doppler Anemometry). Inertial-range behaviour is discussed, in terms of second and third-order structure functions, and a critical comparison with classical (forced) flows is done.
A passive scalar (Sc=1.3) is injected in the flow, in an alternate sequence (Z=0 and Z=1) among each two opposed jets and measured using Laser-Induced Fluorescence. Scalar structure functions are investigated, as well as the extent to which isotropy and homogeneity are adequate approximations for this flow. Flow visualisations exhibit very sharp scalar gradients at the frontier among two opposed jets. The dynamics of these regions closely follows that dictated by the back-and-forth motion developed in the central region, due to the opposed jets instabilities. Thus, the scalar is directly injected at the level of small scales, whereas the velocity field itself is injected over a whole range of scales. This directly leads to a very effective mixing in the stagnation region of the opposed jets.