1,721,004 research outputs found
Hydrodynamics and Scale-up of Anaerobic Stirred Digesters
Full text linkThe investigation presented in this work is aimed at providing a detailed characterization of the hydrodynamics
in a digester of typical design, considering different scale-down criteria for the selection of the agitation
conditions, with the final purpose of suggesting a methodology for aiding in reducing the energy demand of the
digesters while optimizing the biogas production rates. A stirred tank of 40 litres having the same geometry of
an industrial digester of 1500 m3 is investigated by means of experiments and simulations. A model fluid
mimicking the rheological behaviour of the digester content stirred in a biogas production plant, which exhibits
a pseudo-plastic behaviour, is adopted. The velocity field obtained from Particle Image Velocimetry and the
results of Computational Fluid Dynamics simulations are discussed, focusing on well-known critical
hydrodynamic features for the biogas production, namely low-velocity zones, velocity gradients and shear
stresses. The detailed fluid dynamics analysis can contribute to improve the equipment design, to optimize the
energy requirement and to avoid failure of the biogas production due to poor or improper mixing of the
feedstock
Power Consumption and Fluid Mixing in a Scale-Down Geometry of a Stirred Digester for Biogas Production
Full text linkA non conventional stirred tank of geometry typically adopted for the production of biogas is experimentally investigated with pseudo-plastic model fluids. The apparent viscosities of the fluids, based on the Metzner-Otto method, is in the range 39-264 mPa·s, resulting in a range of rotational Reynolds number equal to 17-648. The power consumption of the three top-entering agitators is measured by a strain gauge technique and the power number curve is obtained in the full range of flow regimes, going from laminar to fully turbulent conditions. The flow field measured by Particle Image Velocimetry allows to observe the fluid circulation patterns and their variations in different operative conditions. The measurements reveal relatively low axial and radial velocities, especially towards the bottom of the tank, that may hinder solid feedstocks suspension and the subsequent biogas production. Significant changes in the flow patterns are observed with small variations of the impeller speed and of the mixture viscosity. The homogenization dynamics of a tracer obtained by Planar Laser Induced Fluorescence leads to estimate the dimensionless mixing time, which trend is found to be similar to that observed for conventional stirred vessel geometries. The detailed fluid dynamics information collected by a combination of different techniques can contribute to optimize the energy requirement and to avoid failure of the biogas production due to poor fluid mixing
A CFD study on the change of scale of non-Newtonian stirred digesters at low Reynolds numbers
Full text linkBiogas from anaerobic digestion of agricultural waste is proving to be a convincing way to reduce greenhouse gas emissions. To optimize the process energy efficiency, the CFD simulation of the laminar non-Newtonian fluid mixing in the digester would be an effective method, but the adoption of appropriate spatial discretization at the production scale is currently impossible. For this reason, the identification of change of scale rules for an effective design and for preliminary laboratory scale experimental investigations is still of paramount importance. This work is aimed at the identification of a methodology for the scale down of an industrial stirred anaerobic digester with a volume of 1500 m3, for which CFD simulations have an unacceptable computational cost. The investigation is based on the simulation of three different scale down geometries. The different blade rotational speeds were determined from four different change of scale approaches, which enforced constant blade tip speed, constant shear rate close to the blades, constant Reynolds number and constant power per unit volume, across the different digester sizes. The volume distributions of velocity magnitude, shear rate and shear stress can be exploited to assess the presence of dead zones or localized region where biogas production may be inhibited. The effect of the different change of scale rules on the local instantaneous fluid dynamics were quantified and discussed, finding that both the non-dimensional velocity and non-dimensional shear rate fields are constant across the different scales, when the Reynolds number, based on the Metzner and Otto concept, is constant
Advanced characterization with multimodal measurement techniques for process intensification in gas–liquid tubular reactors
Full text linkIn this work we investigate turbulent gas–liquid mixing and separation in a vertical pipeline equipped with Kenics Static Elements for process intensification applications in continuous operations. The investigation is based on Electrical Resistance Tomography, digital image analysis and pressure drop measurements to provide a comprehensive characterization of the two-phase system. The gas volume fraction distribution is calculated from the voltage difference measurement using two different reconstruction algorithms: the Sensitivity Conjugate Gradient and the Linear Back Projection. The Sensitivity Conjugate Gradient algorithm provides better resolution near the pipe wall and a clear detection of the asymmetric patterns typical of the helical elements. The bubble size distributions obtained from digital image analysis allow to assess the effectiveness of the static elements in providing mixing and separation, depending on their orientation. The spatial distributions of the gas phase measured by the optical and the tomographic techniques are adopted to obtain the average gas hold-up leading to similar results. Overall, the experimental data analysis demonstrates that optimal performance can be determined by balancing energy consumption and gas dispersion. The findings provide valuable insights for the design of in-line reactors where both efficient mixing and controlled separation are required
Ghost Particle Velocimetry implementation in millimeters devices and comparison with μPIV
Full text linkMicro/milli-fluidic devices are becoming an important reference for several disciplines and are quickly increasing their applications in scientific, as well as industrial, environment. As a consequence, the development of techniques able to analyse these kinds of systems is required to allow their progress. Here we show the implementation of the Ghost Particle Velocimetry (GPV) for the flow velocity field investigation in milli-fluidic devices. This innovative technique has been recently introduced, and has been already proven to be useful in describing rapid phenomenon at a small scale. In this work, the GPV has been used to characterize the trapping of light suspended material in a branching junction. Experiments have been performed to identify the flow velocity field close to a millimeters scale T-junction, at different Reynolds numbers. Particularly interesting are the complex structures, such as vortices and recirculation zones, induced by the vortex breakdown phenomenon. The results obtained have been deeply validated and compared with the well-established μ PIV, highlighting the differences in terms of qualitative and quantitative parameters. A performance comparison has been designed to underline the strengths and weaknesses of the two experimental techniques
Drop size distribution for the blending of immiscible fluids in static mixer using PLIF
No full textDrop size distribution in multiphase system is mostly measured using off-line measurements.
In order to understand micro and macro mechanism of drop formation, a in-situ measurement can help to identify certain phenomena which are impossible to determine using off-line measurement.
In this work, PLIF technique has been applied for the in-situ measurement of drop size distribution using static mixers in turbulent regime for the blending of oil in wate
Experimental characterization of particles trapping phenomenon at a bifurcation
No full textThe aim of the project is a deeper understanding of the par4cles trapping phenomenon. Focusing on microfluidic channels, the main interest is to characterize the flow field developed near the bifurcation, responsible of the phenomenon itself. Experiments have been performed using an innovative optical techique, Ghost Particle Velocimetry, that will be validated with the already wellknown μPIV
Mixing in Biogas Fermenters: Experimental Characterization of a Scale-down Geometry
Full text linkIn this work, the fluid dynamics features of a real industrial configuration of a biogas fermenter, which consists in a cylindrical tank stirred with three top-entering shafts with multiple impellers, are investigated. The analysis is based on the experimental characterization of a laboratory model digester of 0.49 m in tank diameter obtained from the scale-down based on the geometrical similarity criterion of a full-scale digester of diameter equal to 17 m. The aim of the work is to evaluate the appropriateness of the design for the requirements of the biogas production process and to suggest possible improvements to the overall mixing operation.
The fluid dynamics investigation is carried out using either water or an aqueous solution of xanthan gum, in order to assess the impact of the variation of the rheological properties at different impeller speeds and direction of rotation of the impellers on the mixing features. To this end, Particle Image Velocimetry is adopted to obtain the velocity fields for the different liquid phases. The data analysis allows to identify possible critical fluid dynamics characteristics that may affect the fermentation, as for example the presence of stagnant zones, where sinking layers might be expected, thus explaining the failure of the biogas production often observed in the biogas production plant
Ghost Particle Velocimetry as an alternative to μPIV for micro/milli-fluidic devices
Full text linkGhost Particle Velocimetry (GPV) has only been recently introduced and has already been proven useful in small scale phenomena investigations, such as the study of the flow field during single droplets generation in microfluidic devices. In this work, GPV was used to experimentally investigate fluid flow close to a T-shaped branched junction in a millimetre sized device. The experimental setup allowed for the first time, the study of complex fluid dynamic structures such as vortices and recirculation zones. Several experiments were performed to exploit the capability of GPV in carrying out flow field measurements, at different Reynolds numbers within the laminar flow regime and for two channel sizes. The results were validated by verifying the steady state and stability conditions and by comparing them with results obtained using the well-established micron-scale Particle Image Velocimetry (μPIV). Differences between these two velocimetry techniques were analysed in terms of qualitative and quantitative parameters, to attain a performance comparison and understand the strengths and weaknesses of each respective method
Flow visualization of the trapping induced by vortex breakdown at a junction
Full text linkHere we present experimental investigations of the vortex breakdown happening at a T-, Y- or ``arrow'' shaped junction responsible for the trapping of light material suspended in solution. Considering the ubiquitous nature of T-junctions and bifurcation in general, in industrial as well as biological environments, it is extremely interesting to better understand how this trapping phenomenon happens. In particular, we observed the flow profiles at different sections in order to perform a three-dimensional study of complex structures, such as vortices and recirculation zones, that develop at a bifurcation. We explored Reynolds number ranging from 50 to about 500 for different milli-fluidic devices. Thus we compared standard micro-PIV and a novel optical technique, the Ghost Particle Velocimetry (GPV), that was recently introduced, to investigate the onset of vortex breakdown. Moreover, the experimental results were compared with single-phase OpenFoam numerical simulations performed in the same flow conditions. Finally, we studied the mutual influence of a trapped particle on the flow field inside the recirculation zone by fully exploiting the capability of GPV to produce 3D flow field with a spatial resolution of few tens of microns
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