Flow Computational Fluid Research Articles

Simulation of the effect of partial and total blockage of a sub-channel on two-phase flow through a 5 x 5 square rod bundle

The study of the effect of blockage on two-phase flow parameters in rod bundles is of significance to the prediction of the thermal-hydraulic behavior of fuel assembly under hypothetical accident conditions. The two-phase flow computational fluid dynamics method was used to simulate the fluid flow in 5 × 5 under partial (75%) and total blockage of a sub-channel. A scenario with no blockage was considered a baseline for further analysis into subsequent cases with blockages. The effect of the blockage on the pressure drop, void fraction, subchannel average velocities, and the turbulence intensity between the sub-channels were analyzed at different hydraulic diameters before and after the blockage, and compared. The 75% partial blockage of a single sub-channel increased the average pressure drop between 1Dh and 5Dh downstream by 0.47 kPa compared to the baseline scenario. Bubbles coalesce at the vicinity of the blockage. The effect of the blockage on other flow parameters extends to 46 Dh downstream of the blockages. The effect of these different blockage conditions can provide a reference for the parameter setting of the sub-channel analysis codes.

Statistical Descriptions of Inhomogeneous Anisotropic Turbulence

Descriptions are given of the Langevin and diffusion equation of passively marked fluid particles in turbulent flow with spatially varying and anisotropic statistical properties. The descriptions consist of the first two terms of an expansion in powers of C0−1, where C0 is an autonomous Lagrangian-based Kolmogorov constant: C0≈7. Solutions involve the application of methods of stochastic analysis while complying with the basic laws of physics. The Lagrangian-based descriptions are converted into Eulerian-based fixed-point expressions through asymptotic matching. This leads to novel descriptions for the mean values of the fluctuating convective terms of the conservation laws of continua. They can be directly implemented in CFD codes for calculating fluid flows in engineering and environmental analysis. The solutions are verified in detail through comparison with direct numerical simulations of turbulent channel flows at large Reynolds numbers.

On the onset of nonlinear fluid flow transition in rock fracture network: Theoretical and computational fluid dynamic investigation

Fluid flow regimes affect the determination of hydraulic conductivity of fractured rocks, and the critical criteria for the onset of nonlinear fluid flow transitions in discrete fracture networks (DFNs) of rocks have yet to be established. First, the factors causing the fluid flow transition regime of fracture intersections and rough surface fractures are theoretically and numerically analyzed. This reveals that the fluid flow regime is governed by the fracture aperture, density of fracture intersections, surface roughness, and Reynolds number (Re). Then, these identified parameters are redefined in DFN models, and their influence on the onset of nonlinear fluid flow is further investigated by performing computational fluid dynamic analysis. The results show that the fracture intersection and aperture play a more significant role in the linear-to-nonlinear fluid flow transition than the fracture aperture heterogeneity. With the increase in the fracture aperture, unevenness of fracture surfaces, and connectivity of DFNs, the onset of the nonlinear fluid flow appeared at the lower flow velocity. With the Forchheimer equation, it is found that the critical hydraulic gradient Jc, defined as the hydraulic gradient at which inertial effects assume 10% of the total pressure loss, is highly correlated with the fracture aperture, fracture intersection, and roughness of the surface. Finally, the mathematical expression of Jc and the Forchheimer coefficients are formulated based on the regression analysis of fluid dynamic computation results, which provides an approach to determine whether the cubic law should be applied as governing equations for the computation of fluid flow in DFNs.

Recent Developments in Design and Operations of Solar dryer.

Solar drying is useful in several residential, commercial and industrial applications. Proper dryer design, operation and maintenance are essential for enhancing the dryer performance and better quality of the dried product. Solar dryer is beneficial as it protects the product from adverse climatic conditions and quickly economically dry the product. The dryer design has undergone significant shapes, dimensions, and storage features leading to better reliability. Researchers worldwide have reported higher energetic and exergetic performance through advanced design, measurement, and control features. Conventional dryer’s efficiency deterioration is attributed to the lack of proper material selection and computational fluid flow techniques. Traditional dryer designs have focused on product drying rate while advanced sustainable solar dryer designs increased solar thermal utilization in a limited area with locally available materials based on fluid flow conditions suitable to the microclimatic conditions. Analysis using computational fluid dynamics (CFD) software has assisted in refining dryer designs to be compact with the complex flow and quick product drying features. The present paper tracks developments in solar dryer designs for different dryer classifications. Recommendations for energy-efficient design and measures for improvement in existing dryers are also discussed.

CAGE22-3 Cruise Report: EMAN7 cruise

The cruise mainly focused on two areas in the Hola Trough: at node 7 where methane seeps have been mapped, and further north (node 1) where are located cold coral reefs. The aim of the cruise was to characterize the seafloor and local biology, bubbles characteristics, the water column chemistry and methane dynamic. We also performed corals transplant to study adaptation and installed Piezometers and temperature probes for fluid flow calculation.
 The cruise may be known as: CAGE22_3

Computational Fluid Flow Model for the Development of an Arterial Bypass Graft

Intimal hyperplasia, an aberrant proliferation of smooth muscle cells that causes stenosis and graft occlusion, is the primary cause of the failure of grafts after a few years. In this way, and using the constructal design method, this study looks at how the stenosis degree, junction angle, and diameter ratio affect the flow through a bypass graft that goes around an idealised coronary artery partially blocked. The results show that the flow effect by several factors: Stenosis ratio (S), Bypass Attachment Point (L2), Bypass Angle (α), and Bypass Diameter (D1). The results indicate that the pressure drop is reduced when S is low, the optimal attachment point L2 = 4 Artery diameter (D), the optimal angle α = 30°, and the optimal D1 = 1.25D.

Methane flow in nanopores: Analytical approximation based on MD simulations

In the current study by means of molecular dynamics (MD) computer simulation, we have investigated pressure-driven flow of methane at high temperature and pressures (398.15 K and 400–500 bar) in circular, square, and triangular nanopores of different size and different fluid–wall interaction strength. To generalize the results obtained in MD simulation, we have proposed an analytical approximation (ansatz) of fluid flow dependence on pore size, while the numerical coefficients have been derived from the fitting of computer simulation data. The formulae obtained take into consideration such nanoflow features as adsorption of fluid on the pore walls and high slip velocity, which are usually neglected in petrophysical simulators operating with continuum fluid approach. These analytical formulae can be then incorporated into various larger scale models used for calculation of fluid flow in nanoporous media. This study is aiming the investigation of general phenomena occurring near the fluid–wall interfaces at nanoscale.

Velocity vector comparison between vector flow imaging and computational fluid dynamics in the carotid bifurcation

It has been largely documented that local hemodynamic conditions, characterized by low and oscillating wall shear stresses, play a key role in the initiation and progression of vascular atherosclerotic lesions. Thus, investigation of the flow field in the carotid bifurcation can lead to early identification of vulnerable plaques. In this scenario, the development of novel non-invasive imaging tools that can be used in routine clinical practice to identify disturbed and recirculating blood flow becomes crucial. In this context, Vector Flow Imaging is becoming a relevant tool as it provides an angle independent assessment of blood flow velocity and multidimensional flow vector visualization. The purpose of the present study was to validate, in several locations of the carotid bifurcation, the high-frame rate vector flow imaging (HiFR-VFI) technique by comparing with computational fluid dynamic simulations (CFD).In all eight carotid bifurcations, HiFR-VFI accurately detected regions of laminar flow as well as recirculation and unsteady flow areas. An accurate and statistically significant agreement was observed between velocity vectors obtained by HiFR-VFI and those computed by CFD, both for vector magnitude (R = 0.85) and direction (R = 0.74).Our study demonstrated that HiFR-VFI is a valid technique for rapid and advanced visual representation of velocity field in large arteries. Thus, it has a great potential in research-based clinical practice for the identification of flow recirculation, low and oscillating velocity gradients near vessel wall. The use of HiFR-VFI may provide a great improvement in the investigation of the role of local hemodynamics in vascular pathologies, as well in the assessment of the effect of pharmacological treatments.

Spectral relaxation computation of Maxwell fluid flow from a stretching surface with quadratic convection and non-Fourier heat flux using Lie symmetry transformations

A mathematical model for nonlinear quadratic convection with non-Fourier heat flux in coating boundary layer flow of a Maxwell viscoelastic fluid is presented. Nonlinear quadratic thermal radiation and heat source/ sink effects are also considered. The transformations of Lie symmetry are employed. The resultant nonlinear differential equations with defined boundary conditions are numerically solved using the spectral relaxation technique (SRM), a robust computational methodology. Graphical visualization of the velocity and temperature profiles is included for a range of different emerging parameters. For skin friction and the Nusselt number, numerical data are also provided. There is a very strong correlation between the outcomes of this study and those published in the literature. Higher values of the nonlinear thermal radiation, mixed convection, thermal conductivity, nonlinear convection and heat source/ generation parameters increase temperature as well as the thickness of the thermal boundary layer. However, a higher Prandtl number, thermal relaxation parameter and heat sink/ absorption parameter all reduce temperature. Deborah number causes velocity to be raised (and momentum boundary layer thickness to be lowered), whereas raising nonlinear mixed convection parameter causes velocity to be decreased (and momentum boundary layer thickness to be increased), and a velocity overshoot is calculated. The models are applicable to simulations of high-temperature polymeric coatings in material processing.

Poro-Mechanical Coupling for Flow Diagnostics

Accounting for poro-mechanical effects in full-field reservoir simulation studies and uncertainty quantification workflows using complex reservoir models is challenging, mainly because of the high computational cost. We hence introduce an alternative approach that couples hydrodynamics through existing flow diagnostics simulations with poro-mechanics to screen the impact of coupled poro-mechanical processes on reservoir performance without significantly increasing computational overheads. In flow diagnostics, time-of-flight distributions and influence regions can be used to characterise the flow field in the reservoir, which depends on the distribution of petrophysical properties that are altered due to production-induced changes in pore pressure and effective stress. These extended flow diagnostics calculations hence enable us to quickly screen how the dynamics in the reservoirs (e.g. reservoir connectivity, displacement efficiency, and well allocation factors) are affected by the complex interactions between poro-mechanics and hydrodynamics. Our poro-mechanically informed flow diagnostics account for steady-state and single-phase flow conditions based on the poro-elastic theory and assume that the reservoir does not contain fractures. Fluid flow and rock deformation calculations are coupled sequentially. The equations are discretised using a finite-volume method with two-point flux-approximation and the virtual element method, respectively. The solution of the coupled system considers stress-dependent permeabilities. Due to the steady-state nature of the calculations and the effective proposed coupling strategy, these calculations remain computationally efficient while providing first-order approximations of the interplay between poro-mechanics and hydrodynamics, as we demonstrate through a series of case studies. The extended flow diagnostic approach hence provides an efficient complement to traditional reservoir simulation and uncertainty quantification workflows and enable us to assess a broader range of reservoir uncertainties.

Estimation of the representative elementary volume of three-dimensional fracture networks based on permeability and trace map analysis: A case study

A three-dimensional (3D) discrete fracture network (DFN) was generated to evaluate the representative elementary volume (REV) of granitic rock masses based on the statistical geometry parameters mapped from outcrop surveys at the Xinchang site, China. Then, extensive numerical simulations were performed to investigate the effect of fracture density on the side length of models of the REV size (LREV). The relationship between LREV of 3D models and the geometric parameters of two-dimensional (2D) cutting planes parallel and perpendicular to the direction of hydraulic gradient variation were analyzed. This relationship was determined by calculating fluid flow through 90 original 3D models, 810 3D submodels, and 270 2D cutting models. Finally, two multivariable regression equations were proposed for predicting the LREV of the 3D fracture network and equivalent permeability of the REV size. The results show that the clustering results obtained from the 3D DFN model are similar to those obtained from field measurements. The values of LREV of the 3D DFN model based on statistical geometry parameters mapped from outcrops are 18.89 m, 19.56 m, and 24.86 m in three flow directions, respectively, which clearly highlights the anisotropic characteristics of fractured rock masses. The LREV decreases with increasing fracture density following an exponential relationship. The multivariable regression function estimates the evolution of the LREV of 3D DFN models with a wide range of fracture densities from 0.53 to 1.33 m/m2. The predicted results are more accurate than those predicted by the fitting function between the fracture density of 2D cutting planes perpendicular to the z-axis and LREV. The proposed model provides a method to approximate the LREV of 3D DFN models and equivalent permeability of the REV size (KREV) using geometric parameters of the 2D cutting planes obtained from trace map analysis of fractured rock masses.

Studies on computational fluid dynamics and flow characteristics of auto‐dripping bioelectrochemical reactor (AutoDriBER): A rational basis for e‐urinal design

Studies on computational fluid dynamics and flow characteristics of auto‐dripping bioelectrochemical reactor (AutoDriBER): A rational basis for e‐urinal design

Evaluation of a Turbulent Jet Flame Flashback Correlation Applied to Annular Flow Configurations With and Without Swirl

Abstract The adaptation of high hydrogen content fuels for low emissions gas turbines represents a potential opportunity to reduce the carbon footprint of these devices. The high flame speed of hydrogen air mixtures combined with the small quenching distances poses a challenge for using these fuels in situations where a significant premixing is desired. In particular, flashback in either the core flow or along the walls (i.e., boundary layer flashback) can be exacerbated with high hydrogen content fuels. In this work, the ability of a flashback correlation previously developed for round jet flames is evaluated for its ability to predict flashback in an annular flow. As a first step, an annular flow is generated with a centerbody located at the centerline of the original round jet flame. Next, various levels of axial swirl is added to that annular flow. Additional flashback data are obtained for various mixtures of hydrogen and methane and hydrogen and carbon monoxide for all the annular flow configurations. Pressures from 3 to 8 bar are tested with mixture temperatures up to 750 K. Flashback is induced by slowly increasing the equivalence ratio. The results obtained show that the same form of the correlation developed for round jet flames can be used to correlate flashback behavior for the annular flow case with and without swirl despite the presence of the centerbody. Adjustments to some of the constants in the original model were made to obtain the best fit, but in general, the correlation is quite similar to that developed for the round jet flame. A significant difference with the annular flow configurations is the determination of the appropriate gradient at the wall, which in the present case is determined using a cold flow computational fluid dynamics simulation.

CFD modelling to analyze the droplets deposition behavior on vibrating rice leaves

The external airflow and the auxiliary airflow of air-assisted application make the leaves vibrate during operation. Such vibration affects the deposition behavior of spray droplets on the leaves. In this study, A multiphase flow computational fluid dynamics (CFD) simulation model was developed and validated. Dynamic process simulations were performed using a validated CFD simulation model, which set spray droplet particle size, impact velocity, leaf tilt angle, and rotational angular velocity as variables. The simulation test results revealed that: the droplet velocity and leaf tilt angle were crucial factors affecting the deposition behavior of droplet adhesion, bouncing, and splitting; compared to static leaves, leaf vibration did not impact the deposition behavior selection of droplets under the influence of 5–10 m/s air velocity. However, the increase of leaves rotation angular velocity increased the droplet spreading to a certain degree. The CFD model validation test results conform with the simulation results. More specifically, the relative error of the longitudinal spreading factor was within 10% and the relative error of the spray droplet spreading diameter was within 20%. This study provides CFD models that can be used for the study of spray droplet deposition behavior on the surface of vibrating rice leaves, thereby providing a reference for rice plant protection and the selection of pesticide application parameters.

Accelerated sequences of 4D flow MRI using GRAPPA and compressed sensing: A comparison against conventional MRI and computational fluid dynamics.

To evaluate hemodynamic markers obtained by accelerated GRAPPA (R= 2, 3, 4) and compressed sensing (R= 7.6) 4D flow MRI sequences under complex flow conditions. The accelerated 4D flow MRI scans were performed on a pulsatile flow phantom, along with a nonaccelerated fully sampled k-space acquisition. Computational fluid dynamics simulations based on the experimentally measured flow fields were conducted for additional comparison. Voxel-wise comparisons (Bland-Altman analysis, -norm metric), as well as nonderived quantities (velocity profiles, flow rates, and peak velocities), were used to compare the velocity fields obtained from the different modalities. 4D flow acquisitions and computational fluid dynamics depicted similar hemodynamic patterns. Voxel-wise comparisons between the MRI scans highlighted larger discrepancies at the voxels located near the phantom's boundary walls. A trend for all MR scans to overestimate velocity profiles and peak velocities as compared to computational fluid dynamics was noticed in regions associated with high velocity or acceleration. However, good agreement for the flow rates was observed, and eddy-current correction appeared essential for consistency of the flow rates measurements with respect to the principle of mass conservation. GRAPPA (R= 2, 3) and highly accelerated compressed sensing showed good agreement with the fully sampled acquisition. Yet, all 4D flow MRI scans were hampered by artifacts inherent to the phase-contrast acquisition procedure. Computational fluid dynamics simulations are an interesting tool to assess these differences but are sensitive to modeling parameters.

Numerical simulations of the effect of spacer filament geometry and orientation on the performance of the reverse osmosis process

Momentum and mass transfer in reverse osmosis membrane modules must be optimized to design efficient and economical designs. The feed spacers are used to improve mixing, to minimize fouling and concentration polarization. This computational study analyses fluid flow, mass transfer, concentration polarization and pressure drop in the feed channel of a reverse osmosis membrane module with different spacer filament geometry (cylindrical, diamond, pentagonal, and triangular), orientation and Reynolds numbers. The computational model solves three-dimensional fluid flow and mass transfer while considering the complex water and solute flux boundary conditions. The cylindrical spacer filaments configurations showed the lowest water flux, followed by pentagonal, diamond, and triangular spacer filaments configurations. The concentration boundary layer thickness was high near the spacer surface due to the stagnant velocity zone. The results indicated that the concentration polarization on the membrane surface increases with a decrease in Reynolds numbers. The channel containing triangular filament spacers gave the maximum water flux and minimum concentration polarization. The spacer configurations with an attack angle of 45° and a transverse angle of 135° showed the highest water flux; however, the pressure drop was also high. The channel containing triangular spacer filament (T1A0) showed the least CP. However, the concentration boundary layer thickness was higher at the bottom side of the channel. The developed model could be useful for predicting the reverse osmosis membrane process performance.

Experimental and Numerical Study on the Dynamic and Flow Characteristics of a Reciprocating Pump Valve

The structure and dynamics of a reciprocating pump liquid end affect the volumetric efficiency and net positive suction head. To match the kinematics with theoretical parameters, reciprocating pump valve motion and flow visualization tests and computational fluid dynamics (CFD) analyses were performed on a wing-guided bevel discharge valve in a horizontal quintuple single-acting reciprocating pump. The valve motion test results showed that the maximum pump valve displacement and the pump valve opening and closing durations were approximately 8.3 mm, 29 ms, and 38 ms, respectively. The corresponding flow visualization test results were 11.4 mm, 9.5 ms, and 35.5 ms. The valve closing durations obtained from the valve motion and flow visualization tests are approximately twice as high as the U-Adolph prediction. The maximum displacement obtained from the valve motion test is consistent with the U-Adolph prediction. Three-dimensional CFD analyses were performed to investigate the flow states, pressure, and velocity characteristics of the discharge valve opening. Finally, the proposed method was applied to develop a new horizontal quintuple single-acting reciprocating pump with a rated flow rate of 1250 L/min and pressure of 40 MPa. This developed pump exhibited good performance and excellent reliability.

An investigation of a gas–liquid swirling flow with shear-thinning power-law liquids

A gas–liquid swirling flow with shear-thinning liquid rheology exhibits complex behavior. In order to investigate its flow characteristics, experiments and computational fluid dynamics (CFD) simulations are conducted based on dimensional analysis. A Malvern particle size analyzer and electrical resistance tomography are applied to obtain the bubble size distribution and section void fraction. A Coriolis mass flowmeter is applied to obtain the mixture flow rate and mixture density for an entrance gas volume fraction smaller than 7%. The CFD coupled mixture multiphase model and large eddy simulation model are applied, considering the liquid shear-thinning power-law rheology. The results show that the swirling flow can be divided into developing and decaying sections according to the swirl intensity evolution in the axial direction. A gas–liquid swirl flow with shear-thinning liquid prohibits a core-annulus flow structure. A smaller index n contributes to maintaining the development of the swirl flow field and its core-annulus flow structure so that the swirl flow can form over a shorter distance with a stronger intensity. For a more uniform distribution of the apparent viscosity, the gas column in the pipe center is thinner. On the other hand, a larger consistency k enlarges the stress tensor. The amplitude of the velocity and the pressure of the core-annulus flow structure are reduced. A weaker swirl intensity appears with a wider gas column appearing as a consequence. Furthermore, the swirl number decays with an exponential behavior with parameters sensitive to the consistency k and index n of the decaying section of the swirling flow field. These are beneficial to gas–liquid separator design and optimization when encountering the shear-thinning power-law liquid phase in the petroleum industry.

Dynamic Characteristic Analysis of Centrifugal Pump Impeller Based on Fluid-Solid Coupling

Purpose: Centrifugal pumps are prone to vibration problems during operation due to poor dynamic characteristics of its impellers that serve as the only running parts of such devices; the dynamic characteristics of the impeller during operation are the main reasons for the vibration of the centrifugal pump. Therefore, it is important to study the internal fluid flow and its influence on the dynamic characteristics of the pump impeller and to explore the causes of vibration during the transient start-up process. The understanding of such phenomena may lead to better design of such impellers. Methods: The geometry of the flow channel inside the centrifugal pump is established using Creo 4.0 software (American PTC company). The internal fluid flow computer simulation is carried out using Flomaster V9 software (UK Flowmaster company) to obtain the variation law of speed and flow during the start-up of the centrifugal pump, which is loaded into the simulation calculation of the centrifugal pump. The variation of speed and flow during the start-up process was further processed using the fluid-structure coupling method, and the structural vibration characteristics of the impeller under transient radial force are obtained by harmonic response analysis. Results: During the starting process of the centrifugal pump, the speed and flow first increased sharply and then decreased until reaching a stable process. During this period, the impeller vibration changed sharply; the overall vibration amplitude increased and fluctuated stably at the amplitude of 0.01 mm. In the unsteady numerical simulation of the centrifugal pump, the radial force on the impeller changes periodically. The time domain signal is transformed into a frequency domain signal, and the fundamental frequency of the impeller and the passing frequency of the blade are 101.67 Hz and 610 Hz, respectively. Conclusions: The radial force is the main cause of impeller vibration, and the transient radial force has the least dynamic impact on the impeller structure under the design condition and has a relatively large impact under the off-design condition. In order to ensure the stable operation of the centrifugal pump, it is necessary to avoid the centrifugal pump working under the non-standard flow condition, especially the small flow condition.

Observing capture with a colloidal model membrane channel

We use video microscopy to study the full capture process for colloidal particles transported through microfluidic channels by a pressure-driven flow. In particular, we obtain trajectories for particles as they move from the bulk into confinement, using these to map in detail the spatial velocity and concentration fields for a range of different flow velocities. Importantly, by changing the height profiles of our microfluidic devices, we consider systems for which flow profiles in the channel are the same, but flow fields in the reservoir differ with respect to the quasi-2D monolayer of particles. We find that velocity fields and profiles show qualitative agreement with numerical computations of pressure-driven fluid flow through the systems in the absence of particles, implying that in the regimes studied here particle-particle interactions do not strongly perturb the flow. Analysis of the particle flux through the channel indicates that changing the reservoir geometry leads to a change between long-range attraction of the particles to the pore and diffusion-to-capture-like behaviour, with concentration fields that show qualitative changes based on device geometry. Our results not only provide insight into design considerations for microfluidic devices, but also a foundation for experimental elucidation of the concept of a capture radius. This long standing problem plays a key role in transport models for biological channels and nanopore sensors.