Velocity Reynolds Number Research Articles

Effects of magnetic fields and nanoparticle concentration on hydro-thermal performance of a surface enhanced microchannel

This article numerically investigates fluid flow and heat transfer in plain wall (PWMC) and oblique fin (OFMC) microchannels under varying magnetic fields using water- Al2O3 nanofluid as a working fluid. Effects of magnetic flux (Hartmann number, Ha = 0–30), fluid velocity (Reynolds number, Re = 100–500), and nanoparticle volume fraction (ϕ = 0%–2%) on the overall thermo-fluid performance are analyzed. Magnetic fields can significantly improve the heat transfer in OFMC in a higher rate compared to the PWMC. Also, there is a less increment on the pressure drop in the OFMC in comparison to PWMC. In addition, individual, as well as combined effect of Joule and viscous heating on the thermo-fluid performance is investigated. Due to the combined effect of Joule and viscous heating, the Performance Evaluation Criterion (PEC) in OFMC can be improved by 21%. However, the PEC in OFMC can be enhanced by 55% without considering the combined effect. The detailed characteristics flow and heat transfer are also studied by analyzing the velocity contours, isotherms, streamlines, friction factors, and Nusselt numbers. The present study concludes the importance of consideration of Joule and viscous heating for an accurate modeling, where the viscous heating plays the dominant role. The present findings can be instrumental in enhancing the performance of micro heat sinks to optimize hydrothermal performance in high powered motors, nuclear fusion reactors etc., where magnetic field is present.

Research on the generalization issue of the heterogeneous QC-EMMS drag model for gas-solid fluidization

In the circulating fluidized bed (CFB), the non-uniform mesoscale structure formed by the particle clustering effect causes a substantial decrease in the gas-solid drag. Since the degree of particle clustering varies with operating conditions, an accurate drag model needs to be universal in different heterogeneous flow conditions. In this study, the relationships between the Ψ factor in clusters' solid holdup model that characterizes the flow non-uniformity and the operating parameters of CFB (including slip velocity Reynolds number, Re*, bed-averaged solid volume fraction, εs,bed, and solid mass circulation rate, Gs) are established. It improves the adaptability of the QC-EMMS drag model under different working conditions. In previous research, we established two types of models that related Ψ factor to Re* and εs,bed respectively. However, there exists a problem of high dispersion of points, indicating that the selected parameters cannot fully describe the flow non-uniformity. Therefore, Gs is reintroduced to modify the two types of models. The results show that the prediction accuracy of the modified models is improved and the relative error is <10%, indicating that the non-uniform factor Ψ has a strong correlation with Gs. In addition, the quantitative relation between Re*, εs,bed, and Gs is derived from modified models, and the trend of relation is highly consistent with the fluidization diagram proposed by Yerushalmi J., which verifies the accuracy of modified models. Finally, numerical simulation of typical CFB cases proves the adaptability of the modified models in wide operating conditions of fluidization.

Experiments on heat transfer and film cooling characteristics of double-wall cooling structure with novel slot holes and pin-fins

The influences of blowing ratio, mainstream velocity, density ratio and jet Reynolds number on the heat transfer and film cooling properties of double-wall cooling cell (DWCC) with slot holes, impingement and diamond-shaped pin-fins were studied by means of pressure sensitive paint and transient liquid crystal. The simulation obtained the flow characteristic at a jet Reynolds number of 20000. The results indicated that with the increase of blowing ratio, the film cooling performance of DWCC structure also increased, and there was no film lift-off phenomenon which occurred in the cylindrical hole. The presence of pin-fins created a spike-shaped film in the slot outlet. Therefore, when studying the DWCC structure, it was important to consider the couple effects of impingement, pin-fins, and slot hole. As the density ratio decreased and the mainstream velocity increased, the film cooling effectiveness (FCE) gradually decreased. When jet Reynolds number increased from 20,000 to 60,000 under a density ratio of 2.0, the area-averaged FCE increased from 0.308 to 0.435, with an increase of up to 41.2%. The convective heat transfer level of impingement stagnation point was much higher than that of pin-fins region. Due to the transition between the impinging jet and the wall jet, the heat transfer coefficient (HTC) had a multi-peak distribution. In addition, several low heat transfer zones similar to wing-shaped, eye-shaped, and crescent-shaped were observed on the target surface and bottom surface inside the cooling cell. The DWCC structure studied in this paper is more efficient in cooling performance compared to traditional cooling structures. This makes it suitable for use in various fields such as heat transfer of turbine blades and electrical equipment.

Large eddy simulations of turbulent pipe flows at moderate Reynolds numbers

Wall-bounded turbulence is relevant for many engineering and natural science applications, yet there are still aspects of its underlying physics that are not fully understood, particularly at high Reynolds numbers. In this study, we investigate fully developed turbulent pipe flows at moderate-to-high friction velocity Reynolds numbers (361≤Reτ≤2000), corresponding to bulk velocity-based Reynolds numbers of 11 700≤Reb≤82 500, using wall-modeled large eddy simulations (LES) in OpenFOAM. A grid convergence study is performed for Reτ=361, followed by an investigation of the accuracy of various subgrid-scale stress models for the same Reynolds number. Results show that the wall-adapting local eddy (WALE) model performs well compared to experiments and direct numerical simulations, while one-equation eddy-viscosity model and Smagorinsky are too dissipative. LES utilizing WALE is then performed for four different Reynolds numbers with gradually refined grids, revealing excellent agreement with DNS data in the outer region. However, a significant deviation from DNS data is observed in the sub-viscous layer region, indicating the need for further mesh refinement in the wall-normal direction to accurately capture the smallest-scale motions' behavior. Additional mesh sensitivity analysis uncovered that, as the Reτ value rises, it becomes crucial for a grid to adhere to the condition of Δx+≤20−25 and Δz+≤10 in order to precisely capture substantial large and small-scale fluctuations. Overall, the WALE model enables accurate numerical simulations of high-Reynolds number, wall-bounded flows at a fraction of the computational cost required for temporal and spatial resolution of the inner layer.

Modeling and Sizing of a Y-Shaped Laminar Flow Micro-Fluidic Fuel Cell

The high demand for portable and desktop electronic devices requires mobile energy sources. Electronic devices, in general, are small when they are portable and require autonomy, sustainability, efficiency, and large enough periods of operability. In that sense, microtechnology, a type of technology related to the manufacture of prototypes or three-dimensional high-precision products in various materials ranging in size from tenths of microns to a few millimeters, has reached a boom in microfabrication of devices that are not exclusively electronic. Applications in several fields show that micro-energy sources are a focal point for commercializing prototypes in the mobility sector.The energy demand to supply micro-devices has been increasing in recent years. Considering the power output of the laminar flow microfluidic fuel cell (LFFC), it is a suitable solution to provide the required electrical energy in small devices. The absence of electrolytes and no requirement of platinum as catalytic material are two of the most important advantages of this type of fuel cell. The current study provides detailed information about the design and characteristics of an LFFC. A complete analysis of the different channel shapes is considered in this study, with the Y-shaped channel being selected as the best option. The impact of the inclination degree for the inlet channels has been considered to evaluate the average velocity the flow acquires in the channel. In addition, the voltage-current behavior considering the materials, fuel/oxidant, and design characteristics has been obtained through modeling approaches. Figure 1. Velocity profile and Reynolds number in the mid-plane of the Y-channel design Figure 1

Effect of initial velocity distribution and Reynolds number on two-dimensional wall jet of transition region impinging

Effect of initial velocity distribution and Reynolds number on two-dimensional wall jet of transition region impinging

Downstream characteristics of extended flat plate trailing edge on S809 wind turbine blade under various turbulent intensities

Purpose This study aims to investigate the effect of downstream characteristics of S809 wind turbine blade with various extended flat plate (EFP) configuration. Wind farms are recently modified to improve the power production through placing number of wind turbines and locations. Design/methodology/approach A series of wind tunnel experiments were carried out to evaluate the downstream wake characteristics of the S809 airfoil attached with various EFP (EFP, A = 0.1C, 0.2C and 0.3C) at various angles of attack corresponding to free stream velocity Reynolds number (Re) = 2.11 × 105 and various turbulence intensity (TI = 5%, 7%, 10% and 12%). Findings For the S809 wind turbine blade attached with EFP, the downstream velocity ratio decreases with increasing in angle of attack and the velocity deficit decrease with increasing turbulence intensity (TI) up to TI = 10%. The wake intensity for the S809 wind turbine blade and S809 airfoil with 10% of chord EFP performs the same for each downstream location. Practical implications Placing the wind turbine in the wind park next to another wind turbine poses a potential challenge for the park power performance. This research addresses the characteristics of the downstream turbulence intensity profile modified with the EFP in the wind turbine blade which improves the downstream characteristics of the turbine in the wind park. Originality/value The downstream velocity ratio decreases with increasing angle of attack and the velocity deficit decrease with increasing turbulence intensity (TI) up to TI = 10%.

Controlling instability waves on vertical natural convection using a buoyant impinging jet

Natural convective heat transfer is typically enhanced by installing fin arrays and other devices that increase the surface area; however, large surfaces present various problems in their implementation. In contrast, the impact of a jet on a natural convection boundary layer has been reported to produce periodically alternating T- and L-flows, whose disturbance may yield flow resonance with the periodic flows present in the transition length of the natural convection boundary layer. The control of instability waves, including flow resonance, is expected to have applications in enhancing convective heat transfer over large surfaces. This study focuses on the jet temperature (between ambient temperature 296 K and wall temperature 316 K), and jet velocity (Reynolds number 95 ≤ Re ≤ 150). Based on frequency analysis, the instability waves downstream of the heated wall are classified into three categories: resonance, convective instability, and turbulence. The analysis reveals that when resonance and turbulence take place, the heat transfer is enhanced compared to natural convection, however, when convective instability takes place, heat transfer depends on the jet temperature. Furthermore, we show that an appropriate choice of jet temperature and velocity can control the instability waves over the natural convection boundary layer.

Computational analysis and biomechanical study of Oldroyd-B fluid with homogeneous and heterogeneous reactions through a vertical non-uniform channel

Abstract Homogeneous and heterogeneous reactions play a decisive role in biological procedures such as burning, polymer creation, ceramic construction, distillation, and catalysis. The magnetic properties of hemoglobin molecules are organic. Magnetic resonance imaging (MRI) and electronic components with an electromagnetic field are now readily available, allowing for the explanation of fundamental biological processes. These ideas form the foundation of an ongoing study that attempts to look into the impact of both homogeneous and heterogeneous reactivity on the peristaltic transport of magnetohydrodynamics Oldroyd-B fluid. When convective and partial sliding conditions are present, the configuration changes to a non-uniform vertical channel. The fundamental partial differential equations are resolved utilizing the Homotopy Analysis Method. Entropy optimization has been carried out. The primary limits entering the problem are investigated, and then a graph is used to show the influences of temperature, velocity, skin fraction, Nusselt number, and pressure increase against mean circulation, trapping phenomena, homogeneous reactions, and heterogeneous way to respond. When magnetic parameter rises, the velocity of Oldroyd-B fluid and Bejan number decrease, while temperature, entropy generation, and pressure gradient increase. The tables show that the skin friction coefficient rises for accumulative values of the Grashof number and magnetic parameter, while the skin friction coefficient drops for rising values of the velocity slip parameter and Reynolds number. The Nusselt number increases for large values of Eckert, Grashof numbers, and magnetic parameters.

Particle-scale modeling study of coaxial jets of gas-solid swirling flow in an industrial-scale annular pipe via CFD-DEM

In this work, the gas-solid swirling flow characteristics of coaxial jets in an industrial-scale annular pipe with a helical swirler is numerically simulated via the CFD-DEM. After validating the numerical model, the general flow pattern of coaxial jets, and gas-particle mixing behavior together with the particle-scale information are discussed. The effect of the geometry parameters of the swirler is studied and analyzed in terms of particle azimuthal velocity and particle Reynolds number. The results demonstrate that the presence of the swirler enhances both the turbulent disturbance and gas vorticity. Increasing vane angle and diameter of swirler is beneficial for the particle dispersion. The particle velocity slightly reduces with decreasing the height in the region 0.6 m < H < 0.7 m but dramatically increases with decreasing the height in the region of height H < 0.6 m. The entrainment of particles by the swirling flow increases the distribution width and azimuthal velocity of particles with decreasing the height.

Turbulence modulation by finite-size particles of different diameters and particle–fluid density ratios in homogeneous isotropic turbulence

In this paper, the influence of particle-fluid density ratio and particle diameter on the turbulence modulation by finite-size particles in forced homogeneous isotropic turbulence is investigated. Results show that the presence of finite-size particles always attenuate the turbulence, and the attenuation is larger for particles with larger density when the particle diameter is fixed. But the attenuation is smaller for particles with larger diameter if the density is fixed, and the weaker attenuation is due to the wake fluctuation when the particle Reynolds number is large enough. The turbulence kinetic energy is attenuated at the large scales and augmented at the small scales. The radial dissipation profiles show that the region affected by the particles with same diameter is identical, but the dissipation near the particle surface is larger if the density is larger due to larger slip velocity and particle Reynolds number. For particles with same density, smaller particles have smaller dissipation near the particle surface but the influence region is larger, and the combined effect leads to the result that the contribution of dissipation in the influence region of smaller particles to the total dissipation is larger. The influence region mainly depends on the particle diameter.

Experimental characterization of a double tube heat exchanger with different corrugated tubes and shells

The aim of this paper was to study the influence of a double tube heat exchanger (DTHX) on thermo-hydraulic performance, considering 21 different shell–tube configurations including smooth and corrugated inner and outer tubes. In order to emulate the behavior of high-viscosity food-industry fluids, a 60°brix solution of water and sugar was used as the Heat Transfer Fluid (HTF). A total of 504 experimental tests were carried out, varying the flow velocity (Reynolds number) and the shell corrugation parameters (helix pitch P and groove depth d). The results from three different shells, one smooth and two corrugated shells, and seven different inner tubes, one smooth and six corrugated with a groove depth between 0.5 mm and 1.2 mm are presented. The comparison of the three shells with an inner smooth tube shows significant heat transfer rate enhancements using a light groove depth (CC1) in the outer tube of the DTHX, increasing both the average efficiency and the PEC indicator (Performance evaluation criteria) by 5% compared with the smooth shell. No improvements are found when a higher groove depth (CC2) in the outer tube of the DTHX is used. Regarding the results with different inner tubes, the tubes with the highest groove depth in the tested range (between 0.5 and 1.2 mm), CO11 and CO12 (with a groove depth of 1.1 and 1.2 mm, respectively), show the best results, regardless of the type of shell. Corrugated tube CO12 shows the best results in terms of efficiency, with an average increment of 19% compared with the smooth inner tube. Regarding PEC, CO11 shows an average increment of 9% in the analyzed PEC indicator compared with the smooth inner tube.

Evaluation of maximum wall shear stress from air impingement to remove food deposits from stainless steel surfaces

The risk of cross-contamination during manufacturing of low water activity foods requires careful attention to cleaning and sanitation. Considering the challenges encountered in such manufacturing, air impingement can provide a unique approach to remove deposits from the food-contacted surface, particularly, the crevices or cracks. The objective of this study was to evaluate the shear stress-induced by air jet impingement for removal of food residues. The wall shear stress distribution on a surface below an air jet was measured using oil-film interferometry. The air impingement was operated at three level of air velocity (Reynolds number = 7756, 13,444, and 18,787) and three different nozzle height (24 (H/D = 6), 32 (H/D = 8), 40 mm (H/D = 10)). The shear stress was found to become higher along with the increase of air velocity and decrease of nozzle height. The observed distribution of wall shear stress on the surface was fitted using the Gaussian model and Polynomial model. The Gaussian model appeared to be preferable according to the statistical analysis, which aligned with previous literature. Based on the Gaussian model, the maximum wall shear stress was estimated. The maximum wall shear stress increased with Reynolds number ( Re ) and the ratio of jet height to jet nozzle diameter. The highest maximum shear stress (10.74 Pa) occurred at a Reynolds number of 18,787 and a height of 24 mm (H/D = 6). This trend was directly correlated with removal efficiency. The greatest removal of the treatment was achieved by 40% at maximum wall shear stress. A linear relationship between percent residue removed and maximum wall shear stress was obtained from the experimental results, and a threshold maximum wall shear stress of 2.73 Pa has been proposed for removal of peanut butter residue from a stainless-steel surface. The current study demonstrates the promising of the air impingement in the cleaning operation for low water activity food. • Wall shear stress increased with an increase in velocity and decrease in height. • Removal of food deposits revealed a similar trend to velocity and height. • Linear relation was found between maximum shear stress and food removal.

Hydrodynamic Studies during Gas‐Solid Conveying in a Vertically Upward Converging Riser Tube‡

AbstractExperimental and theoretical investigations on hydrodynamics for gas‐solid particles flow were carried out in a vertical upward converging pneumatic conveying system. One‐dimensional mathematical modeling and simulation were done using the FORTRAN‐95 program. The influence of convergence geometry on the hydrodynamic parameters like particle velocity, gas‐particle slip velocity, pressure drop, and particle Reynolds number (Rep) along the riser were studied. Experiments were conducted using the dilute phase mode with three types of granular cereal seeds of different sizes. Some significant differences in all hydrodynamic parameter profiles along the riser were observed between converging and uniform risers. Enhancement of Rep in the converging riser implies a higher gas‐solid transfer coefficient.

Effectiveness of induced magnetic force and non-uniform heat source/sink features for enhancing the thermal efficiency of third grade nanofluid containing microorganisms

Owing to the multidisciplinary significance and dynamic applications of nanoparticles, the research in areas of thermal and process engineering has been expanding every day. The nanomaterials with extraordinary thermal and physical properties are being studied to pose some astonishing contributions in various fields. The nanomaterials when taken with microorganisms in a solution for collective transport, referred as mix fluids, greatly improve the thermal efficiency during heat transfer phenomena. Usually, the mixed fluids particles have greater advantage of being controlled by some physical stimuli of light, gravity and density along with electrical and magnetic forces. The current study offers some innovative applications of induced magnetic field for the bio-convection pattern third grade nanofluid under the influence of extraordinary activation energy and non-uniform hear source/sink factors. The assumptions of stagnation points are considered under the influence of stretched. The nano-materials have been mixed with microorganism to prepare a mixed fluid in order to obtain more stability and directed transportation. The flow constraints for the thermal transport phenomenon have been explained by using convective boundary approach. The couple and nonlinear equations have been put forward to present the model and the solution has been derived using shooting algorithm technique. Moreover, the numerical and graphical outcomes from the study are presented using tables and figures. The improved profile of velocity is predicted against higher values of velocity ratio, Reynolds number and third grade fluid parameter. The induced magnetic field profile enhanced for reciprocal magnetic Prandtl number and magnetic parameter. The consideration of non-uniform heat source/sink is more effective to improve and control and thermal transportation process.

Numerical analysis on hydrodynamic characteristics of a deep-sea mining vehicle under three typical motions

A numerical analysis was conducted on the hydrodynamic characteristics of a deep-sea mining vehicle. The simulations employed the Reynolds Stress Model (RSM) to focus on the complicated flow field under three typical motions. During the traveling process, the fluid separated around the vehicle, and the low-pressure areas formed behind the vehicle. The turbulent kinetic energy was large near the shape edge of the vehicle and transferred downstream along with the flow. With the increase of the velocity Reynolds number, the surface friction coefficient increased and the pressure coefficient decreased, which made a large frictional resistance and differential pressure resistance. Meanwhile, although the development processes of the flow field were similar in different motions, the spatial hydrodynamic distributions and the changes of the hydrodynamic coefficients also performed differently. This study can serve as an important reference for the structure optimization and control system development of the mining vehicle.

CFD Modelling of Non-Newtonian Fluid Flow in a Pipe Including Obstacle

Pipe flow problems are important in transportation of wastewater, oil lines and supply of water. In this study, a non-Newtonian fluid model is discussed and a numerical CFD solution is presented for flow geometry. The effects on velocity, pressure, dynamic viscosity and cell Reynolds number are discussed for different parameters of flow inside the pipe. Power Law function is considered in the analyses.

Thermal analysis and parametric optimization of plate fin heat sinks under forced air convection

Heat dissipation is becoming more and more challenging with the preface of new electronic components having staggering heat generation levels. Present day solutions should have optimized outcomes with reference to the heat sink scenarios. The experimental and theoretical results for plate type heat sink based on mathematical models have been presented in the first part of the paper. Then the parametric optimization (topology optimization) of plate type heat sink using Levenberg-Marquardt technique employed in the COMSOL Multiphysics? software is discussed. Thermal resistance of heat sink is taken as objective function against the variable length in a predefined range. Single as well as multi-parametric optimization of plate type heat sink is reported in the context of pressure drop and air velocity (Reynolds number) inside the tunnel. The results reported are compared with the numerical modeled data and experimental investigation establish the conformity of results for applied usage. Mutual reimbursements of greater heat dissipation with minimum flow rates are confidently achievable through balanced, heat sink geometry as evident by the presented simulation outcome. About 12% enhancement in pressure drop and up to 51% improvement in thermal resistance is reported for the optimized plate fin heat sink as per data manifested.

Fluid-particle drag force measurements from particle-resolved CFD simulations of flow past random arrays of ellipsoidal particles

The lack of constitutive drag laws for non-spherical particles represents a current gap in simulating fluid-particle flows, even though such non-spherical shapes are encountered in numerous applications. Some previous studies have applied spherical drag models to non-spherical particles, while others have used ad-hoc methods to account for particle shapes. However, the present work demonstrates the need to develop dedicated non-spherical drag laws, given the limitations of these previous ad-hoc approaches. In the present study, single-phase computational fluid dynamics (CFD) simulations of fluid flow over random arrays of ellipsoids are used to directly calculate the fluid-particle drag forces. By varying the particle shape, orientation, solid fraction, and fluid-flow velocity (Reynolds number), this work probes the variables that would be important for future model development. A preliminary non-spherical drag model is proposed based on the CFD results, which illustrates the functional dependency of the non-spherical drag on particle aspect ratio and orientation.

Effect of Velocity Distribution and Reynolds Number on the Diffusion Inhibition of Coaxial Jets

Effect of Velocity Distribution and Reynolds Number on the Diffusion Inhibition of Coaxial Jets