Chiara Galletti

Spectral and wavelet analysis of the flow pattern transition with impeller clearance variations in a stirred vessel

Abstract The double- to single-loop pattern transition in stirred vessels stemming from a change in the off-bottom clearance of a Rushton turbine has been investigated by laser doppler anemometry. Time-resolved data showed the transition occurring within a range of clearance values and allowed the distinction of three types of flow: the double-loop regime, the single-loop regime and an unstable one termed “transitional state”. Experiments of up to 3– 4 h duration showed that both the onset and the lifetimes of these types of flow were random; however, in the transitional state, the flow varied between the two circulation patterns in a periodic manner, with a frequency linearly related to the impeller rotational speed. The results have important implications for mixing process and vessel design as well as CFD predictions of the flows which are discussed.

Mixing of Two Miscible Liquids in T-shaped Microdevices

Numerical simulations were performed to study the flow fields and mixing characteristics of liquid flows converging in a T-shaped micromixer, when the two inlet fluids are both water or water and ethanol. We showed that at smaller Reynolds number, Re < 100, mixing is controlled by transverse diffusion, and therefore by the residence times of each fluids. Accordingly, mixing ethanol and water is slightly easier than mixing water with water, due to the fact that, as ethanol is slightly more viscous than water and therefore it is slower, the residence time of water-ethanol mixtures is larger than that of the water-water case. On the other hand, at larger Reynolds number, mixing water and ethanol may take considerably longer, as the onset of engulfment is retarded and occurs at larger Reynolds number, namely increasing from Re ≅ 140 in the water-water case to Re ≅ 230 in the water-ethanol case. This is due to the fact that a water-ethanol mixture has a viscosity that is up to almost three times larger than that of water; therefore, at the confluence of the T-mixer, the water and the ethanol streams are separated by a quite viscous layer of a water-ethanol mixture, that hampers any vortex formation, thus retarding mixing.

Modelling and experimental validation of H2S emissions in geothermal power plants

A commercial computer software, used to model and simulate geothermal power plants, has been investigated by comparing simulation outputs with field data of existing plants. Although most simulation results were in good agreement with the field data, the program failed when predicting the H2S distribution between the plant streams. The computer program has, therefore, been partly modified on the basis of pilot-scale experiments directed at investigating the H2S behaviour in the two main plant sections, i.e. the direct contact condenser and the cooling tower. The modified version fits the field data well and is a useful tool for assessing methods to reduce the environmental impact of releasing H2S into the atmosphere.

Flow instabilities associated with impeller clearance changes in stirred vessels

Abstract Flow instabilities associated with changes in the clearance (C) of a Rushton impeller from the bottom of a stirred vessel of diameter T = 294 mm were studied experimentally with laser Doppler anemometry. It is shown that for C/T = 0.17–0.18 flow changes from a double- to a single-loop pattern occur randomly. The transition from one regime to the other is characterized by periodic oscillations at a frequency (f) related to the impeller speed (N) by f/N = 0.135. The lifetimes of the stable and unstable patterns were found to be affected by fluid density and viscosity as well as clearance. The implications of the results for both mixing processes in stirred vessels and related predictions of the flows are discussed.

Mixing performance of arrow-shaped micro-devices.

The process of liquid laminar mixing in arrow-shaped micro-devices is studied by direct numerical simulations. Two different CFD codes, i.e. Fluent (based on finite volume method) and Nek5000 (based on spectral element method) have been used to investigate the flow and concentration fields. Unexpectedly we observe that within the engulfment regime, the degree of mixing first increases and then diminishes as the inlet flow rate is increased. Such reduction in the degree of mixing, not observed in T-shaped mixers, can be imputed to the presence of a strong vortical structure at the center of the mixing channel. This result is important for control operations, as it shows that on one hand arrow type mixers are characterized by higher degree of mixing with respect to T-shaped mixers, but on the other hand they present a narrower range of optimal conditions.

Effect of Composition-dependent Viscosity of Liquids on the Performance of Micro-mixers

The process of laminar mixing in a T-shaped micro-device is studied by direct numerical simulation for a model binary mixture, composed of two liquids having the same density and the same viscosity, yet presenting a strong fluidity of mixing effect, i.e. the viscosity of the mixture is a function of its composition. In particular, we consider the case where the viscosity of the mixture is up to three times larger than that of the pure liquids, with the maximum viscosity corresponding to either a 50 %-50 % (type 1 mixture), or a 25 %-75 % composition (type 2 mixture), to better emulate the behavior of real mixtures. The results are compared to the case with no fluidity of mixing effects (type 0 mixture), which has been largely investigated previously. In this latter case, the inlet streams remain separated up to a critical Reynolds number, corresponding to a strong increase of the degree of mixing. This transition is also characterized by a symmetry breaking, from a vortex flow regime, with a double mirror symmetry, to an engulfment flow regime, with a point central symmetry. When the fluid mixture has a larger viscosity than that of its pure components, a viscous layer forms at the confluence of the inlet flows, which tends to keep the two streams separated. Therefore, in this case, one would expect that the onset of the engulfment regime should be shifted to larger Reynolds numbers, in comparison with type 0 mixtures, with no sudden increase of the degree of mixing. Although this is what happens for symmetric, type 1 mixtures, for type 2 mixtures we unexpectedly find that, due to the lack of symmetry of the mixture rheology, the transition from vortex to engulfment regime, although occurring at larger Re, occurs suddenly, corresponding to a sharp increase of the degree of mixing.

Macro-Instabilities in Eccentrically Agitated Vessels

Laser Doppler anemometry and flow visualisation are used to shed light into the main turbulent flow features of an unbaffled vessel stirred by an eccentrically positioned Rushton turbine. Two main vortices, one above and one below the impeller, are present and the former vortex dominates the flow field, driving a strong circumferential flow around it. The vortices are not steady but oscillate slowly and periodically inducing a kind of flow instabilities, which may have a significant impact on macro-mixing. The characteristic frequencies of such flow instabilities were found to increase with reducing the impeller blade thickness, thus it is argued that their origin is related to the interaction between the impeller discharged stream and the vessel wall/bottom.

CFD Simulation of Natural Convection Flow in Pressurized Tanks Exposed to Fire

Computational Fluid Dynamics (CFD) is a consolidated tool to support industrial projects development and was recently adopted in the framework of consequence assessment and safety analyses. In the present study, a CFD model of pressurized vessels exposed to an accidental fire was developed, with the aim to determine the transient behavior of the stored fluid during the heat-up. An integral model for consequence assessment was adopted in order to set advanced boundary conditions to the CFD model. The results of CFD simulations allowed predicting temperature and velocity of both liquid and vapor phases, as well as the complex recirculation phenomena induced by natural convection. This allowed determining the pressurization rate in the fired tank, thus obtaining key indications for the evaluation of the residual mechanical resistance.

The effect of fluid viscosity in T-shaped micromixers

Effective mixing in small volumes is a crucial step in many chemical and biochemical processes, where microreactors are to ensure a fast homogenization of the reactants. Physically, liquid flows in microfluidic channels are characterized by low values of the Reynolds number and, in general, large values of the massive Peclet number. Accordingly, since general strategies of flow control in microfluidic devices should not depend on inertial effects, reduction of the mixing length requires that there must be transverse flow components. In this paper, three-dimensional numerical simulations were performed to study the flow dynamics and mixing characteristics of liquids flows inside T-shaped micromixers, when the two inlet fluids are either both water or water and ethanol. In particular we showed that, contrary to what one could think beforehand, the mixing efficiency of water-ethanol systems is lower than the corresponding water-water case.

Flow Instabilities in Mechanically Agitated Stirred Vessels

A detailed knowledge of the hydrodynamics of stirred vessels may help improving the design of these devices, which is particularly important because stirred vessels are among the most widely used equipment in the process industry. In the last two decades there was a change of perspective concerning stirred vessels. Previous studies were focused on the derivation of correlations able to provide global performance indicators (e.g. impeller flow number, power number and mixing time) depending on geometric and operational parameters. But recently the attention has been focused on the detailed characterization of the flow field and turbulence inside stirred vessels (Galletti et al., 2004a), as only such knowledge is thought to improve strongly the optimization of stirred vessel design. The hydrodynamics of stirred vessels has resulted to be strongly three dimensional, and characterised by different temporal and spatial scales which are important for the mixing at different levels, i.e. micro-mixing and macro-mixing. According to Tatterson (1991) the hydrodynamics of a mechanically agitated vessel can be divided at least into three flow systems: • impeller flows including discharge flows, trailing vortices behind the blades, etc.; • wall flows including impinging jets generated from the impeller, boundary layers, shed vortices generated from the baffles, etc.; • bulk tank flows such as large recirculation zones. Trailing vortices originating behind the impeller blades have been extensively studied for a large variety of impellers. For instance for a Rushton turbine (RT) they appear as a pair, behind the lower and the upper sides of the impeller blade, and provide a source of turbulence that can improve mixing. Assirelli et al. (2005) have shown how micro-mixing efficiency can be enhanced when a feeding pipe stationary with the impeller is used to release the fed reactant in the region of maximum dissipation rate behind the trailing vortices. Such trailing vortices may also play a crucial role in determining gas accumulation behind impeller blades in gas-liquid applications, thus affecting pumping and power dissipation capacity of the impeller. But in the last decade lots of investigations have pointed out that there are other important vortices affecting the hydrodynamics of stirred vessels. In particular it was found that the flow inside stirred vessels is not steady but characterised by different flow instabilities,