Fluids

This paper deals with computational fluid dynamics (CFD) to improve the design of a new scalable photochemical reactor which uses the Taylor–Couette flow principle. This study aims to investigate the ways to improve the mixing efficiency (M_eff) within the reactor, as it is a key parameter to increase the productivity and inform the future scale-up of the novel reactor. The investigated design parameters are the gap size (d) between the reactor cylinders, the rotational speed (Ω) of the inner cylinder, the flow rate of the reagent ($\dot{V}$), and the dynamic viscosity of the mixture (μ). For all the investigated cases, the results show that the temporal evolution of the M_eff increases and then becomes steady after a maximum level is reached. The point of the maximum M_eff is called the equilibration time. It is revealed that the M_eff is mainly affected by the flow rate increase as it contracts the Taylor vortices and consequently the mixing deteriorates.

In this work, for the first time, the dependence of the mass transfer (MT) ratio (k_La coefficient to overall gas holdup) as a function of the superficial gas velocity U_G in seven organic liquids was studied. The volumetric liquid-phase MT coefficients k_La were recorded (by means of a polarographic oxygen electrode) in a bubble column (0.095 m in ID) equipped with a single tube (∅3.0 mm in ID) as a gas sparger. It was found that the MT ratio decreases monotonically through all main flow regimes. Both the constant and the exponent of the empirical correlation between the MT ratio and U_G were analyzed, and it was found that they depended in a complicated fashion on the Schmidt number, Sc. In three different regions of the Sc number, potential new correlations were discussed. The main conclusion from this work is that the MT ratio is not constant in the heterogeneous regime as reported previously by other researchers. In the case of four binary mixtures between benzene and cyclohexane, it was also found that the MT ratio decreased monotonically as a function of the superficial gas velocity, U_G. The effects of both liquid viscosity and surface tension on the MT ratio were also investigated.

The scour process of sand particles and multi-grain size and density particles were studied to investigate the segregation process of different particles in a confined channel. The effects of jet intensity and submergence as two controlling parameters were studied, and scour characteristics and profiles were measured. The time history of the scouring process was measured and the results were compared with the scour process in a uniform sand bed as benchmark tests. Experimental data revealed that the eroded area of different particle types increased with the jet intensity, but the erosion of relatively heavier particles was limited due to jet diffusion. The local erosion was affected by the level of submergence and more erosion occurred near the nozzle at low submergence. Increasing the jet Froude number increased the area of deposition, while submergence reduced the overall area of deposition. As submergence increased from 4 to 12, the area of sand particles reduced by more than 50% while the jet intensity was constant. In shallow submergence, increasing jet intensity from 1.46 to 2.11 increased the area of lead balls by 120%, whereas in relatively deep submergence, incrementing jet intensity increased the area of lead balls by more than five times. The effect of flow intensity on variations of scour dimensions was quantified by the densimetric Froude number. While a densimetric Froude number based on mean particle size, D₅₀, was found to be suitable to estimate maximum scour bed in uniform sand beds, experimental data indicated that the best fit is achievable to predict maximum scour depth in multi-grain size and density once D₉₅ is used. Semi-empirical models were proposed to predict scour dimensions as a function of the densimetric Froude number.