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•Elbows induce secondary flow and affect the flow and interfacial structure.•Elbow orientation has no effect on dissipation of elbow effect in single phase flow.•Elbow effect takes ...longer length to dissipate in two phase flow.
This study investigates elbow effects on single-phase and two-phase flows. ANSYS CFX is used to simulate the evolution of the single-phase velocity distribution and two-phase interfacial structure downstream of vertical elbows. Simulations are benchmarked with existing LDA data and detailed four-sensor conductivity probe data. It is found that elbows create secondary flow, containing two counter-rotating vortices in both single-phase and two-phase flows. The vortices shift to pipe center and develop into parabolic velocity profile downstream. The elbow effect dissipates exponentially and dissipates 90% of its initial maximum value at approximately 11D downstream, no matter the elbow direction. In two-phase flow, bubbles are entrained by the secondary flow to form double-peaked void fraction and interfacial area concentration distributions. The double-peaked distribution moves upward and becomes single-peaked distribution downstream. Due to the additional buoyant force, the elbow effect takes much longer length to dissipate in two-phase flow than in single-phase flow.
In this paper we propose some important improvements related to the single-phase Smoothed Particle Hydrodynamics (SPH) simulations and available for different SPH schemes (such as Riemann SPH or ...δ-SPH). This model is based on the Continuous Surface Force (CSF), a volumic description of the surface tension and relies on an accurate evaluation of the local normal and curvature at the interface. In particular we show that renormalized SPH operators are necessary to both improve the stability and the accuracy of the model. We also propose a solution aiming at imposing a desired equilibrium contact angle near the contact line for the Boundary Integral Method (BIM). The proposed model is called corrected CSF (C-CSF) and is validated through static and dynamic test cases, with and without solid boundaries, showing that the proposed corrections are necessary to obtained the expected solutions.
The Fracture Cut (FraC) approach to mesh three-dimensional (3D) Discrete Fracture Networks (DFN) is presented. The considered DFNs consist of a network of planar two-dimensional (2D) fractures ...sharing intersections that can in turn intersect themselves, resulting in highly complex meshing issues. The key idea of FraC is to decompose each fracture into a set of connected closed contours, with the original intersection traces located at the boundaries of the contours. Thus, intersection segments can be more easily accounted for when building a conforming mesh. Three distinct strategies for intersection points management are also proposed to enhance the quality of resulting meshes. Steady-state single-phase flow simulations are performed to validate the conform meshes obtained using FraC. The results from flow simulations as well as from a mesh quality analysis on a benchmark case show that a flexible AoM strategy (Adding or Moving intersection points) appears to be the best choice to generate ready-to-run meshes for complex DFN. This approach also allows accounting for tiny features within the fracture networks while keeping a good mesh quality and respecting DFN connectivity. Finally, a scalability of the mesh generator is conducted to assess the performance of the approach.
•An original and innovative conforming mesh approach is presented for DFN mesh generation.•Three distinct strategies for intersection points management are proposed.•This approach allows to account for tiny features within the fracture network.•High-accuracy flow simulations are obtained using Cast3M and DuMux open source code.
Single phase water flow experiments were performed on seven tightly-closed, rough model fractures. Each model has mirror-image transparent upper and opaque lower walls reproduced from the original ...single fractures created by laboratory indirect tensile tests of different rock types. The experiments were conducted under both non-loading and normal loading conditions as well as in two opposite injection directions. Surface roughness of model fractures was quantified using variogram fractal dimension.
While a short linear portion was distinguished in the beginning, pressure versus injection rate curves showed nonlinear characteristic at higher values. This nonlinearity was attributed to the predominance of fracture expansion compensating the turbulence effect. This was the case regardless of injection direction and loading. The compensation of turbulence effect developed earlier for rough fractures compared to the smooth parallel plate model and was longer for the fractures with higher fractal dimensions. Transmissivity values showed a decrease with increasing fractal dimensions and normal loading, and also exhibited directional-dependent behaviour. The percentages of water-invaded wet planar areas showed a tendency to decrease with increasing fractal dimension. This relationship was more obvious with a higher correlation when the fractal dimension was replaced by the ratio of fracture surface area to the apparent fracture area, which was used as another parameter to quantify the roughness. The percentage of the dry planar areas that do not involve any flow could be as high as 20% and is controlled by tortuous flow channeling.
•Single phase water flow experiments on seven different original replica fractures.•Zero and four different normal loading conditions are applied.•Roughness is more effective on transmissivity due to interlocking and contact between fracture walls.•The percentages of water-invaded wet planar areas is higher for lower fractal dimensions.
•Four benchmark cases for single-phase flow in fractured porous media.•Comparison of seven state-of-the-art discrete-fracture-matrix methods.•Public access to all mesh and solution data.
This paper ...presents several test cases intended to be benchmarks for numerical schemes for single-phase fluid flow in fractured porous media. A number of solution strategies are compared, including a vertex and two cell-centred finite volume methods, a non-conforming embedded discrete fracture model, a primal and a dual extended finite element formulation, and a mortar discrete fracture model. The proposed benchmarks test the schemes by increasing the difficulties in terms of network geometry, e.g. intersecting fractures, and physical parameters, e.g. low and high fracture-matrix permeability ratio as well as heterogeneous fracture permeabilities. For each problem, the results presented are the number of unknowns, the approximation errors in the porous matrix and in the fractures with respect to a reference solution, and the sparsity and condition number of the discretized linear system. All data and meshes used in this study are publicly available for further comparisons.
Flows of solid particles are known to exhibit a clustering instability-dynamic microstructures characterized by a dense region of highly concentrated particles surrounded by a dilute region with ...relatively few particles-that has no counterpart in molecular fluids. Clustering is pervasive in rapid flows. Its presence impacts momentum, heat, and mass transfer, analogous to how turbulence affects single-phase flows. Yet predicting clustering is challenging, again analogous to the prediction of turbulent flows. In this review, we focus on three key areas: (
a
) state-of-the-art mathematical tools used to study clustering, with an emphasis on kinetic theory-based continuum models, which are critical to the prediction of the larger systems found in nature and industry, (
b
) mechanisms that give rise to clustering, most of which are explained via linear stability analyses of kinetic theory-based models, and (
c
) a critical review of validation studies of kinetic theory-based models to highlight the accuracies and limitations of such theories.
•Molecular Diffusion may not be the dominant mechanism in wax deposition.•Phase transition (partial freezing), often disregarded, might be the dominant phenomena.•All evidence shows a constant ...temperature of the interface (Phase Transition!).•There are several assumptions regarding the morphology of crystals in the deposit that have never been experimentally verified.
Wax deposition is a costly problem for the O&G industry, especially for pipelines in cold environments. For at least three decades, the scientific community has overwhelmingly agreed that molecular diffusion is the main mechanism for wax deposition. There are, however, severe problems with models based on molecular diffusion. They rely on untested hypotheses and several empirical correlations; hence, they can hardly predict the experimental data from laboratory. For real fields, the prediction is no better than an educated guess – heuristic solutions. Several research areas in wax deposition need to be better understood, and these are discussed in detail here, with a highlight to the most important concern: the controlling mechanism. Is wax deposition indeed a mass transfer controlled phenomenon? What is the evidence supporting this “general knowledge”? Is it possible that, for some conditions, mass transfer is dominant, and for others, the phase transition mechanism is dominant? Apart from this, we also discuss other issues: the accuracy of empirical correlations for diffusivity, the behavior of crystals in the deposit and how that influences the general deposit behavior, non-Newtonian influence on heat transfer and mass transfer, among others. Wax deposition is a complex topic that has been reviewed over and over. In this review, however, we focus on both presenting what has been discussed in the literature and make a critical analysis. The goal is to increase the general knowledge by highlighting a number of gaps and challenges related to this complex and financially exorbitant issue.
•Numerical study of heat transfer in extended surface microchannels has been carried.•Upstream, downstream and complete finned extended microchannels have been studied.•Univariate search optimization ...of no. of fins, pitch, and fins size has been carried.•Nusselt number increases by 160% for optimized case compared to plain microchannel.
Microchannel heat sinks (MCHS’s) are currently projected as twenty first century cooling solution. In the present numerical study, heat transfer enhancement in microchannel using extended surface has been carried out. Rectangular microchannel and cylindrical microfins are used in current study. Three different configurations of extended surface microchannel; Case I (upstream finned microchannel), Case II (downstream finned microchannel) and Case III (complete finned microchannel) are compared with plain rectangular microchannel. It is found that heat transfer performance of Case I is better than Case II. Case I even performs better than Case III at low Reynolds number. Average surface temperature is also significantly reduced in case of extended surface microchannels. Optimization of extended surface microchannel has also been successively carried out following univariate search method for number of fins, pitch, diameter and height of fins. Average heat transfer enhancement in optimized case is around 160% with acceptable pressure drop penalty.
This article reviews recent studies on the hydrodynamic and thermal characteristics of micro pin fin heat sink (MPFHS). In the studies reviewed in this article, liquid coolants such as water, ...HFE-7000, HFE-7200, R-123 were tested under both single-phase and two-phase flow conditions. Analytical, computational and experimental research studies were covered with a focus on configurations with traditional arrangements of micro pin fins (MPF) as well as original designs such as oblique finned MPFs, variable density MPF, vortex generators and herringbone structures. Single-phase flow results highlighted pressure drop penalty with achieved heat transfer enhancement. Many studies revealed the inability of conventional correlations to predict the hydrodynamic and thermal characteristics and proposed new correlations for different operating conditions and geometrical specifications. Regarding the studies on two-phase flows the number of performed studies is less than the ones in single-phase flow regime although the diversity of utilized coolants is more. Under flow boiling conditions, the focus was on determining flow patterns among MPFs for different arrangements and under different operating conditions. Unlike the studies on single-phase flows, the data could be relatively well predicted using the earlier suggested model by Lockhart and Martinelli with appropriate coefficients for different arrangements of MPFs.
•Structure forms and performance comparison of different PCHE structures were summarized.•Literatures with carbon dioxide and helium as working fluids were categorized and compared.•Studies on PCHEs ...by numerical and experimental methods were summarized and analyzed.•Correlations related to flow and heat transfer in PCHEs were summarized, categorized and analyzed.•The further research directions on flow and heat transfer performances in PCHEs were prospected.
Printed circuit heat exchanger (PCHE) is a compact heat exchanger with a series of advantages, such as, high ability of heat transfer, bearing high pressure and low temperature, etc., and it presents potential application in the field of new generation of nuclear power, solar thermal power generation, and hydrogen energy in recent years. In such a development situation, this review categorizes and summarizes the recent development related to the characteristics of flow and heat transfer in PCHEs based on results from experiments and numerical simulations. It is shown that present studies are limited to single-phase flow with supercritical carbon dioxide or helium as the working fluid. So, some problems still exist to be investigated, such as, the actual evolution of flow and temperature field, multi-phase flow, high Reynolds-number flow, etc. Finally, it points out some research topics for PCHEs in the future.
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