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Though bubble column reactors are widely used in industry, the present design practice is still closer to an art than a desired state of science because of the complexity of its fluid mechanics. The empiricism can be reduced by understanding detailed flow pattern, turbulence characteristics and turbulent structures and their effects on the performance such as mixing and axial mixing in both the phases and rate of heat and mass transfer. For this purpose, in the present work, CFD simulations have been undertaken by using standard k-e, RSM and LES turbulence models. The cylindrical column having a computational height of HD = 900 mm with inside diameter of D = 150 mm was employed as a bubble column operated at three superficial gas velocities (20, 40 and 100 mm/s). The instantaneous three dimensional velocity field is obtained by means of two phase Eulerian-Eulerian Large Eddy Simulations (LES). The conservation equations for turbulent kinetic energy (k) and the turbulent energy dissipation rate (e) have been derived from the two fluid governing equations using the Reynolds averaging procedure. This enabled accurate estimation of convective transport, diffusive transport, turbulent transport, production and dissipation of k and e. These estimations have been compared with the modelled terms of the standard k-e and Reynolds stress models. The difference in values gives an idea about the severity of assumptions made in these models. An attempt has been made to bring out the implications of simplifying assumptions.

Gas–liquid two phase flows are widely encountered in industry. The design parameters include two phase pressure drop, mixing and axial mixing in both the phases, effective interfacial area, heat and mass transfer coefficients. Currently, there is a high degree of empiricism in the design process of such reactors owing to the complexity of coupled flow and reaction mechanism. Hence, we focus on synthesizing recent advances in computational and experimental techniques that will enable future designs of such reactors in a more rational manner by exploring a large design space with high-fidelity models (computational fluid dynamics) that are validated with high-fidelity measurements (hot film anemometry (HFA), Laser Doppler anemometry (LDA), particle image velocimetry (PIV), etc.) to provide a high degree of rigor. Understanding the spatial distributions of dispersed phases and their interaction during scale up are key challenges that were traditionally addressed through pilot scale experiments, but now can be addressed through advanced modelling. For practically complete knowledge of the fluid mechanical parameters, it is desirable to implement direct numerical simulations (DNS). However, the current computational power does not permit full DNS for real bubble columns. Therefore, we have been using simplified turbulence models (such as large eddy simulation, Reynolds stress, k–e, etc.) which need the knowledge of turbulence parameters. For the estimation of these parameters, currently semi-empirical procedures are being used pending the knowledge of turbulence. Further, the formulation of governing equations in all the CFD models (except DNS), the knowledge of interface forces (drag, lift, virtual mass, Basset, etc.) is needed and for their estimations empirical correlations are being employed, again pending the knowledge of fluid mechanics under turbulent conditions in bubble columns. In gas–liquid dispersions, the gas is sparged in the form of bubbles. During the bubble rise, the mechanism of wake detachment creates turbulence which can be called as wake generated turbulence. In addition, energy gets transferred from the gas phase to liquid phase. The quantitative amounts are negligible when bubble motion is not hindered and the gas–liquid dispersion is homogenous. The amounts increase with an increase in the extent of hindrance. However, in the homogenous regime, even under extreme conditions, the extent of energy transfer in the bulk gas–liquid dispersions (volume other than wake volume) is fairly limited. On contrast, in the heterogeneous regime, the rates of energy transfer become sizeable. The energy received by the liquid (in both the regimes) also creates turbulent motion and termed as bulk generated turbulence. In turbulent flows a compendium of eddies (flow structures) of different length and time scales contribute towards improved/enhanced mixing, momentum transfer, heat transfer, and mass transfer (transport phenomena). Hence, a proper understanding of the dynamics of these turbulent flow structures, and their role in the transport phenomena, can bring substantial improvement in the scale-up and design procedures. The present paper brings out the current status of knowledge on bubble generated turbulence. All the published literature in experimental measurements and DNS simulations has been critically analysed and coherently presented.

Spatio-temporal instability of miscible two-fluid symmetric flow in a horizontal slippery channel is considered. Both fluids have the same density but different viscosity. A smooth viscosity stratification is created by a thin mixed layer between the fluids due to the presence of two species/scalars, which are diffusing at different rates. Our study suggests the existence of a rapidly growing absolute unstable mode for higher viscosity ratio with a highly viscous fluid close to the slippery channel wall. This instability is less stronger in the case of the equivalent single component two-fluid flow. The viscosity stratified single component (SC) and double-diffusive (DD) slippery flows are absolutely unstable for a wide range of parameter values, when a highly viscous fluid is adjacent to the slippery wall and the mixed layer is close to the channel wall with slip. The instability can be either enhanced or suppressed by wall slip and this is dependent on the location of mixed layer, inertial effects, diffusivity and the log-mobility ratios of the faster and slower diffusing species. This suggests that one can achieve early transition to turbulence due to the absolute instability in a viscosity stratified channel flow by making the channel walls hydrophobic/rough/porous with small permeability, which can be modelled by the Navier-slip condition. � 2015 Elsevier Ltd.

Confined impinging jet reactors (CIJMs) are very efficient mixing devices. Due to the generation of high turbulent dissipation and, thus, intensive mixing of two colliding liquid flows CIJMs are widely used in laboratory applications. Besides low construction effort, these mixers enable a sensitive control of fast reactions, an efficient initial homogenization of educts and consequently a rapid build-up of supersaturation as the driving force for precipitation and crystallization. The understanding of fluid mechanics and mixing in those CIJMs is of fundamental interest for a proper description of parallel or consecutive processes. Therefore, this paper gives an insight into characteristic procedures, with the main focus on the turbulent flow regime in a T- and Y-CIJM. Experimental investigation on the local mixing behavior of liquid-phase impinging jets in static mixers is carried out by using an optical laser induced fluorescence (LIF) and particle image velocimetry (PIV) setup. In order to avoid optical measurement errors due to the cylindrical geometry, a refractive index correction of fluids with the ionic additive NH 4 SCN has been carried out. Due to poor information in literature, important properties of the corrected aqueous solution are listed. Another main focus of this work lies in the mathematical modeling of the flow field based on computational fluid dynamics (CFD) methods. The transient hybrid Detached Eddy-SST- k – ω turbulence approach is introduced as a computational cost-efficient highly accurate method to calculate flow conditions in turbulent CIJMs. LIF and PIV as well as pressure drop measurements provide a wide set of data that serve to validate CFD simulations.