1,721,082 research outputs found
Scale-resolving CFD modeling of a thick wind turbine airfoil with application of vortex generators: Validation and sensitivity analyses
No full textNumerical investigation of film thickness and wave statistics in gas-liquid downwards annular flows
Full text linkTwo-phase gas-liquid annular flows are observed in a broad range of industrial processes, such as production and pipeline systems for oil and gas distribution, steam generators, boiling water reactors, and emergency core cooling facilities to protect nuclear reactors.
Although the global flow characteristics of annular gas-liquid flows have been studied experimentally for more than 50 years, their numerical modelling is still immature.
We present a computational fluid dynamics model based on the volume of fluid method for simulating annular gas-liquid flows, focusing on the regular wave flow regime. We performed transient simulations on a 3-D domain using a commercial code (ANSYS Fluent 2021 R1). The mesh sensitivity analysis indicates that a very fine mesh must be used near the pipe wall to capture the liquid-gas interface correctly (Fig. 1).
The code is validated through available experimental data [1] regarding topological flow properties. In particular, we considered mean film thickness, film roughness, base film thickness, and wave film thickness. We studied two operating conditions. The first is characterized by liquid and gas Reynolds numbers of 1 250 and 25 000, respectively. The second has the same liquid Reynolds number as the first, but the gas Reynolds number is increased to 30 000.
A post-processing procedure is implemented to obtain the time traces of film thickness at 12 circumferential positions to capture the asymmetries in the flow.
The numerical values of the quantities analyzed are in good agreement with the experimental findings, with a maximum error of 21.02% concerning the wave film thickness. The errors regarding the mean film thickness and film roughness are less than 10% for both the case studies.
Considering the film thickness of time traces at different circumferential positions, we calculated the cross-correlation coefficients between them. The high values of the cross-correlation coefficients indicate that waves are coherent over the circumference of the pipe, following the experimental findings.
Finally, to better understand wave activities, we generated the power spectral density functions for the two cases studied. They are characterized by a quasi-linear power decay, similar to that of the Kolmogorov spectrum for homogeneous and isotropic turbulence, which becomes slightly steeper for the case characterized by a higher gas Reynolds number, in accordance with the experimental data
Computational fluid dynamics calculations of high efficiency heat exchangers operating in laminar motion consisting of extruded plastic profiles with insert
No full textThe use of plastic-based materials is finding new applications in the heat exchangers sector: the very high values of heat exchange surface ratio per volume unit, the lightness of the products and the lower cost achievable with plastic material are the main advantages of resorting to their use (1),(2),(3). The object of this study is to analyze and validate in terms of CFD and thermodynamic analysis a solution for the realization of high efficiency counter-flow heat exchanger (HX) consisting of extruded alveolar/multi-wall polycarbonate profiles with inserts. Bundles composed of multiple channels are commonly produced to form alveolar/multi-wall polycarbonate products/panels; several extruded bundles can be assembled to obtain HX with large sections, making it possible to require shorter paths for the fluids and lower speed of the same, and therefore less pressure losses in pumping. A further characteristic of the HX analyzed is that, in order to obtain high values of thermal power per unit of volume, instead of resorting to pumping of fluids in turbulent motion, which is an ener-getically very expensive solution, this result is achieved thanks to the introduction, inside of the little channels that make up the extruded bundle, of filling profiles; those filling profiles produce a reduction in the passage thickness of the fluids and induce some positive effects on the nature of their motion: this leads to an enormous improvement of the value of the HX power per unit of surface. Six different CFD calculations were carried out with counterflows of 25 m3/h & BULL;m2 of water at inlet temperatures of 20 & DEG;C and 100 & DEG;C with square channels of 1 meter in length, 8 mm side and three different fillings, obtaining high volumetric heat transfer coefficient (in the cases examined up to 615 kW/m3 & BULL;K) and low pressure drops
Computational Fluid Dynamics Modelling of Two-Phase Bubble Columns: A Comprehensive Review
Full text linkBubble columns are used in many different industrial applications, and their design and characterisation have always been very complex. In recent years, the use of Computational Fluid Dynamics (CFD) has become very popular in the field of multiphase flows, with the final goal of developing a predictive tool that can track the complex dynamic phenomena occurring in these types of reactors. For this reason, we present a detailed literature review on the numerical simulation of two-phase bubble columns. First, after a brief introduction to bubble column technology and flow regimes, we discuss the state-of-the-art modelling approaches, presenting the models describing the momentum exchange between the phases (i.e., drag, lift, turbulent dispersion, wall lubrication, and virtual mass forces), Bubble-Induced Turbulence (BIT), and bubble coalescence and breakup, along with an overview of the Population Balance Model (PBM). Second, we present different numerical studies from the literature highlighting different model settings, performance levels, and limitations. In addition, we provide the errors between numerical predictions and experimental results concerning global (gas holdup) and local (void fraction and liquid velocity) flow properties. Finally, we outline the major issues to be solved in future studies
Computational fluid dynamics simulation of the heterogeneous regime in a large-scale bubble column
Full text linkBubble columns are used in many industrial applications, but the complex fluid dynamics phenomena has limited their design and optimization processes. Computational Fluid Dynamics (CFD) is a promising tool to investigate the complex multi-scale flow physics characterising multiphase reactors. In this work, a CFD Eulerian-Eulerian modelling approach is developed to describe the hydrodynamics of a large-scale bubble column operated over a wide range of superficial gas velocities (0.0188 – 0.20 m/s). Available experimental results were used for the model validation. A drag law for oblate bubbles was considered and coupled with a drag modification function to include the effects of bubble–bubble interactions. The numerical approach was tested considering a mono-dispersed approximation and including coalescence and breakup by using a Population Balance Model (PBM). The role played by the lift force was investigated and, for the reactor configuration considered, it turned out to be essential in the description of the local flow properties
Bubble Column Reactors: the Variables Influencing the Mono-Dispersed Homogeneous Flow Regime
No full textThe fluid dynamics of large-diameter bubble columns explicates in six flow regimes emergin upon an increase in the gas flow rate and can be described and predicted via correct modelling of the flow regime transition coordinates. This study focuses on the transition between the mono-dispersed and poly-dispersed homogeneous flow regime and defines a statistical approach to determine the significative variables influencing the transition. The analysis is performed by coupling: (a) the Ordinary Least Squares method (OLS), to determine the relationship between the variables, (b) the Variance Inflation Factor (VIF), to check for multicollinearity issues, and (c) the Least Absolute Shrinkage and Selection Operator (LASSO), to select suitable variables. Subsequently, the Classification and Regression Tree (CART) approach has been applied to generate homogenous clusters of bubble columns in terms of flow regime transition
A Lumped Parameter Approach for Determining the Pressure Gradient in Gas-Liquid Annular Flows
Full text linkThis study defines a statistic-derived lumped parameter approach to determine the pressure gradient in two-phase annular flows. The statistical model was defined by coupling: (1) the ordinary least squares method (OLS) to determine the relationship between the variables, (2) the variance inflation factor (VIF) to check for multicollinearity issues, and (3) the least absolute shrinkage and selector operator (LASSO) to select the relevant predictors. Finally, a lumped parameter approach is derived based on the classification and regression tree (CART) approach. The model identifies the liquid and gas Reynolds numbers, the liquid phase properties, the pipe diameter, and the surface tension as significant variables influencing the two-phase pressure gradient
Integrating machine learning and image processing for void fraction estimation in two-phase flow through corrugated channels
Full text linkThere is a substantial amount of information embedded in images of two-phase flow captured through highspeed video (HSV) or high -resolution photography. However, accurate image segmentation is necessary to unlock a meaningful analysis of the data. In this study, we discuss how to estimate the flow void fraction in chevron -type corrugated channels typical of compact plate heat exchangers (CPHE) from back -lit front -view HSV images, using machine learning (ML) algorithms and data processing techniques. A U -Net neural network was employed for image segmentation, demonstrating robust performance with evaluation metrics consistently exceeding 0.9. The binary masks (indicating gas or liquid phases) derived from segmentation were processed in MATLAB (R) to estimate void fraction through a 3D reconstruction algorithm of the gas cluster's volume. In contrast to conventional void fraction estimates based on the area ratio of binary masks, this algorithm models the curvature of the liquid -vapor interface through the corrugated channel. When compared to other methods, our algorithm predicts very similar void fraction contour maps. However, the average discrepancy between our algorithm and the area -ratio approach can be as high as 80%, underscoring the importance of the processing method in analyzing the data and developing correlations. Finally, a drift flux model was introduced to predict the void fraction distribution using a two-part equation accommodating the dependency of the distribution coefficient C 0 on the liquid flow rate for a corrugation Froude number Fr similar to larger than 1. The proposed model can predict the void fraction dataset with a mean average percentage error of 8.17%. In summary, U-Net's pixel -level accuracy facilitates deep and precise post -processing of HSV images, enabling meaningful void fraction measurements. Thanks to its generality and minimal training effort requirements, the discussed methodology can be applied to estimate void fractions in various two-phase flow experiments and operating conditions
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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