1,720,960 research outputs found
Reacting Flows and the Interaction between Turbulence and Chemistry
No full textThis contribution summarizes the state of the art concerning the simulation with computational fluid dynamics of reacting flows with a particular focus on the interaction between turbulent mixing and chemistry. When dealing with nonisothermal gas-phase reactions (e.g., combustion) and rapid reactions (e.g., acid-base) in the liquid phase, the calculation of the chemical source term becomes challenging. The different computational methods proposed in the last 3 decades to deal with this problem are discussed and their application in the context of direct numerical simulation, Reynolds-averaged Navier-Stokes equation, and large-eddy simulation approaches are illustrated
Multivariate Quadrature-Based Moments Methods for turbulent polydisperse gas-liquid systems
No full textThe Conditional Quadrature Method of Moments (CQMOM) and the Direct Quadrature Method of Moments (DQMOM) are compared with Direct Simulation Monte Carlo (DSMC) for the description of gas bubble coalescence, breakage and mass transfer with the surrounding continuous liquid phase. CQMOM and DQMOM are both moment methods based on the idea of overcoming the closure problem by using a quadrature approximation. The methods are compared and performances evaluated for spatially homogeneous and inhomogeneous systems. Eventually CQMOM and DQMOM are implemented in a commercial CFD code to simulate a realistic two-dimensional bubble column. Particular attention is paid to the impossibility of conserving moments with DQMOM in the presence of numerical diffusion. To cure this problem a fully-conservative DQMOM formulation is presented and tested. The relationship
between the two methods is investigated, showing that under particular conditions CQMOM is identical to DQMOM. The different methods are employed under a number of different conditions including very fast chemical reactions, in order to highlight if the problem of bubble coalescence, breakage and mass transfer really needs a bivariate population balance to be tackled and what is the optimal number of nodes for the quadrature approximation
Modeling of nanoparticles precipitation in a Confined Impinging Jets Reactor by means of Computational Fluid Dynamics
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Investigation of the flow field in a three-dimensional Confined Impinging Jets Reactor by means of microPIV and DNS
No full textMicro-mixer devices, such as the Confined Impinging Jets Reactor (CIJR) are currently under study, in particular for precipitation processes of micro- and nano-particles, employed in a variety of applications that include pharmaceuticals, cosmetics, dyes and pesticides. In this work, with the purpose of gaining a better understanding of the main mixing mechanisms occurring in a CIJR, the flow field was studied at four inlet flow rates ranging from Re = 62 to Re = 600. These conditions correspond to regimes with incipient turbulence in the chamber. Micro-Particle Image Velocimetry (microPIV) experiments and Direct Numerical Simulations (DNS) were performed and compared. MicroPIV is an innovative experimental technique that allows measurement of the instantaneous velocity fields in microfluidic devices. The coupled numerical-experimental approach was found to be essential in understanding and explaining the flow behaviour and the development of turbulence, in particular with respect to the important effects of the inlet boundary conditions. Oscillations present in the inlet flow of the device are in fact primarily responsible for the chaotic and turbulent effects in the reactor. These results provide insights that are important in the development of appropriate computational models for this type of micro-reactor or mixer
Validation of LES predictions for turbulent flow in a Confined Impinging Jets Reactor
No full textThis work focuses on the prediction of the turbulent flow in a three-dimensionial Confined Impinging Jets Reactor with a cylindrical reaction chamber by using Large Eddy Simulation. Three-dimensional unsteady simulations with different sub-grid scale models, numerical schemes and boundary conditions were performed for various flow rates, covering different flow regimes. First, a qualitative analysis of the flow field was carried out and then predictions of the mean and fluctuating velocities were compared with micro Particle Image Velocimetry data. Good agreement was found both for the mean velocity components and the fluctuations. For low to moderate Reynolds numbers the sub-grid scale model was found not to be very relevant, since small scales are of less importance, as long as scalar transport and chemical reaction are not in play. An important finding is the good prediction of the high velocity fluctuations detected in particular at higher Reynolds number due to the natural instability of the system, strongly enforced by the jets unsteadines
Derivation and applications of the generalized master equation
Full text linkCall number: LD2668 .T4 1985 F69Master of Scienc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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