Estimation of Turbulence Parameters in Pool Scrubbing Conditions

Abstract: Pool scrubbing is a widely used technique for retaining aerosol particles. It is characterized by high gas injection flow rates and substantial topological changes in different zones within the scrubber. The scrubbing process is generally divided into injection and swarm zones based on the evolution of the gas-liquid interface topology, and different mechanisms prevail in each zone. The disintegration of large globules in the injection zone forms a swarm of stable bubbles that significantly affect the retention efficiency since scrubbing largely depends on bubble size and velocity. However, the characteristics of the pool scrubbing process, such as high momentum, a wide range of bubble sizes, and complex interactions, make numerical simulations difficult. Turbulence is a key factor affecting bubble breakup, and estimating turbulence parameters properly is essential. In bubble columns, the effect of bubble-induced turbulence is dominant compared to turbulent pipe flows. However, the contribution of shear-induced turbulence is also significant due to circulation and strong oscillation formed in the pool by high-velocity gas injection. Modeling turbulence considering both shear and bubble-induced turbulence is still challenging in multiphase simulations. In this work, two turbulence models (mixtureKEpsilon and k-omega SST) that include the bubble-induced effects in OpenFOAM are evaluated for a pool scrubbing experiment from the literature. Using the two-equation models for turbulence is the widely accepted method in numerical simulations, making the accurate estimation of the turbulence boundary conditions critical. However, most of the correlations developed to estimate the turbulence boundary conditions are for fully developed pipe flow, making it harder to use them for bubble column simulations. Therefore, special attention is paid to characterizing turbulence intensity in a bubble column and estimating inlet boundary conditions for turbulence parameters such as turbulent kinetic energy and energy dissipation. The key parameter in estimating accurate turbulence parameters at the inlet is the turbulent viscosity. It is found that the classical definition of turbulence intensity, as the ratio between fluctuation and mean velocity, leads to an unreasonably large value for bubble column simulations because the mean velocity in the pool is extremely low, consequently, inaccurate estimation of inlet turbulence boundary conditions may result. Therefore, an approach is proposed to estimate the inlet turbulence boundary conditions that satisfy a condition revealed by the sensitivity tests in the current study. According to the results, the turbulent intensity can be estimated by constraining the turbulent viscosity value to be in the range of the molecular viscosity at the inlet if the inlet Reynolds number is in the range of laminar or transitional. Thus, too high or too low values for turbulent viscosity at the inlet may lead to numerical instabilities, considering that both the liquid and gas are quasi-laminar at the inlet.