Emulation of Bubble‐Induced Turbulence Using Randomly Moving Particles in a Grid Structure

Abstract

An approach to generate turbulence with systematically controllable properties and statistically comparable to real bubble-induced turbulence is evaluated. Particles are arranged in a flexible regular grid and can move freely in the directions perpendicular to the flow direction. The flow induced by different grid configurations is characterized by particle image velocimetry and electrodiffusion measurement. With these techniques, spatially resolved velocity fields and time-resolved energy spectra are achieved. Homogeneous turbulence with the intensity comparable to regular bubble flow is obtained downstream from the particle grid. The energy spectra of these emulated turbulent flows agree with the energy spectra of real bubble-induced turbulence. The proposed concept of turbulence generation is suitable for detailed scientific investigations of bubbles in turbulent flows using sophisticated optical measurement techniques.

Experimental Assessment of an Innovative Device to Mimic Bubble Swarm Turbulence

“…The energy cascades of the higher void fraction show higher values compared to the lower void fraction. This is in agreement with the experimental results in [30].…”

“…Exemplary decay of the turbulence level in flow direction away from the grid. Different shapes and emulated gas holdups are compared to each other [30]. scale, the performance of the flexible grid matches well the bubble column behavior.…”

“…A proper positioning of the investigated bubble in the grid wake and a raise of the void fraction to increase the turbulence level beyond the natural swarm turbulence is expected to compensate for the observed differences since the induced turbulence decays from the grid downwards to the measurement position. Haase et al [30] investigated the turbulent flow fields induced by the flexible particle grid at w l,0 = 0.25 m s -1 applying planar particle image velocimetry (PIV) to cross-check the measurement methodology. The PIV was performed with an sCMOS camera from LaVision that took images of small tracer particles illuminated in a plane with a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (Innolas Spitlight 4000, l = 532 nm).…”

“…These experiments allow detecting inhomogeneities in the wake of the grid, which result in an inhomogeneous turbulence level as discussed in detail in [30]. The decay of the turbulence behind the grid and the dependence of the turbulence level on the particle size, shape, and void fraction is depicted in Fig.…”

“…Thus, the turbulence level can be increased with rising void fraction as well as particle size. The larger the particles are, the more inhomogeneous the wake becomes [30]. Evidently, elliptical particles lead to larger turbulence levels than spherical ones.…”