Modulation of turbulent flow field in an oscillating grid system owing to single bubble rise

Mohammad Mainul Hoque, Subhasish Mitra, Geoffrey M Evans, Mayur J. Sathe, Jyeshtharaj B. Joshi

Index: 10.1016/j.ces.2018.03.039

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Abstract

Modulation of turbulent flow in an oscillating grid system due to single bubble rise was experimentally investigated using particle image velocimetry (PIV) technique wherein flow field modulation was reported in single bubble resolved manner. The bubble diameter varied in the range ∼ 2.70 to 3.52 mm (∼ 26 to 34 times the single-phase Kolmogorov scale, and ∼ 1.05 to 1.35 times the single-phase integral length scale). The two-dimensional (2D) instantaneous velocity fields were obtained for both single–phase and bubble flow cases at grid Reynolds numbers (Reg) ranging from 1080 to 10800. The modulation of single-phase turbulence due to bubble has been discussed based on the turbulence intensity, isotropic ratio (IR), length scales, specific energy dissipation rates and energy spectra. The single-phase turbulence fluctuating velocity increased ∼ 5–76% in the inertial subrange region due to the single bubble rise. Presence of bubble led to enhancement in the flow field isotropy ratio due to upward acting buoyancy force. It was noted that at high Reg (6480–10800), the IR value of the flow was found to be more dependent on the grid Reynolds number compared with bubble diameter. The integral length scale of the single-phase flow was decreased in the presence of bubble following a power law dependency over the Reg. It was found that the specific energy dissipation rate of single–phase flow increased by ∼ 172% with an increase in bubble diameter. The energy spectra exhibited a slope less steep than −5/3 in the presence of bubble which indicates the additional turbulence production by the bubble in the inertial subrange region. Energy augmentation from large scale to small scale due to bubble were discussed by the dissipative spectrum which showed that a reduction of the energy on the small scale and an enhancement of the energy on both large scale and inertial subrange.