Formation Of Secondary Vortices Research Articles

Innovative centrifugal airflow upgrading for coal gasification fine slag based on experiment and CFD-DEM approach

In this work, an innovative process combining centrifugal airflow upgrading and sieving was proposed to achieve enrichment of residual carbon. The separation mechanism and macroscopic flow behavior in centrifugal air classifier were explained and predicted by constructing a coupled CFD-DEM model considering particle contact parameters. Compared the grinding, dry sieving pre-treatment had a better promotion effect on the enrichment of residual carbon. The general flow pattern was composed of an upward swirling flow and a horizontal rotation flow, facilitating dispersion and secondary elution of carbon. However, the tangential inlet and horizontally mounted rotor cage were the main cause of secondary vortex formation, leading to severe particle entrainment. Increasing inlet gas velocity at appropriate rotor speed was conducive to obtaining higher carbon content and carbon yield. By regulating the inlet gas velocity and rotor speed, higher carbon content and yield could be achieved at particle size fractions of 78–160 μm.

Liquid–solid two-phase flow and separation behavior in a novel cyclone separator

In response to the prevalent issue of sand presence in liquid, particularly prominent in petroleum engineering, a novel cyclone separator has been meticulously engineered for fine-particle separation. Experiments and numerical simulation methods have provided a profound understanding of the flow-field characteristics and separation efficacy of this device. The internal architecture of the swirling flow inside the separator features a distinctive central vortex core, complemented by a turbulent secondary vortex formation in the lower section of the underflow. As the axial height increases, the secondary vortex gradually dissipates. An analysis of pressure and velocity distribution within the cyclone separator confirms the establishment of a stable cyclone field in the built-in cyclone and a tendency for the flow field within the tank to exhibit uniformity with increasing height. These flow-field characteristics show that the cyclone separator has a good separation effect on fine-rust particle impurities. Furthermore, the separation efficiency of the novel cyclone separator demonstrates a positive correlation with increasing particle size. Of the parameters studied, variation of the inlet velocity is the best method for obtaining optimum separation efficiency for a cyclone desander with a fixed particle size. Specifically, when the inlet velocity reaches 3 m/s, the desander attains an impressive separation efficiency of up to 70%.

Modeling investigation on transient behaviors of gaseous ammonia jet flow with direct injection

This study investigates the characteristics of gaseous ammonia jets under an injection pressure of 0.8 MPa and an ambient pressure of 0.1 MPa, using LES and RANS simulation approaches to inform applications of ammonia in internal combustion engines. A three-dimensional constant volume chamber model was established, and the Dynamic Structure model was applied in LES modeling with AMR techniques. The near-nozzle transient structures were successfully captured. Furthermore, a direct comparison between LES and RANS simulation was made, and main jet characteristics were investigated. LES outperforms RANS in predicting the instantaneous structures of ammonia jets and provides a more distinct visualization of oblique shock waves and diamond-like shock cells. RANS excels at displaying averaged features with a smoother distribution curve that represents self-similar characteristics. Additionally, the transient characteristics of ammonia and hydrogen jets were compared. The vortex formation and mixing process in ammonia jets occur at a much slower rate compared to hydrogen jets. Meanwhile, transient vortex ring behaviors were analyzed in detail, including their development tendencies regarding initial, secondary, and broken vortex formation. It is revealed that petal-like swirling movements are strongly correlated with the air–fuel mixing process.

MHD natural convection flow in a porous medium-filled corrugated enclosure: Effect of heat sources with different heights

A two-dimensional natural convection flow problem in a porous medium-filled enclosure with electrically conducting fluids and solid heat sources of different heights is numerically investigated in this study. The top boundary is cold, and the vertical side boundaries are insulated, while rectangular-shaped corrugations at the lower boundary are generated by placing heat sources. Three distinctive cases are considered concerning the height of the solid rectangular-shaped heat sources. The nonlinear coupled transport equations are discretized by using a higher-order compact (HOC) finite difference scheme. First, existing numerical and experimental data are used to validate the developed code. Then, the effect of key parameter values, like Hartmann number (25–150), Rayleigh number (103–106), Darcy number (10−5–10−1), and Prandtl number (0.015–10), on the convective flow in the corrugated enclosure is analysed. The influence of the permeability of the porous medium is abundantly seen for a range of significant parameter values with a fixed high Rayleigh-Darcy number. The formation of primary, secondary, tertiary, and quaternary vortices affects the thickness of the thermal boundary layer on both hot and cold surfaces in both symmetric and asymmetric patterns. In addition, possible multiple steady states are observed in the Case 2 at Pr=10.0. The local and average Nusselt numbers remains constant on the bottom cold surfaces of the corrugated enclosure, while significant variations are observed on the upper surfaces of the heat sources. These findings are analysed by conducting stream-function (ψ), isotherms (T), local (Nuh,Nuv), and average (Nū) Nusselt number plots. The intensities of heat transfer rate are also examined by analysing Nusselt numbers as a function of Prandtl, Darcy, Rayleigh and Hartmann numbers. Analysing natural convective heat transfer fluid flow in an enclosure with heat sources is prominent for ample engineering applications such as solar collectors, designing nuclear reactors, chemical energy production systems, building constructions and microelectronic equipment, to name a few.

Research on flow around two tandem rectangular cylinders

Uniform flow around two identical rectangular cylinders arranged in tandem has been investigated via the lattice Boltzmann method. With a fixed Reynolds number of 100, the effects of side ratio (γ = 1/2, 1/4, 1/8 and 1/16) and spacing ratio (S* = 0.25–12) on the fluid dynamics are investigated. Results indicate that both side ratio and spacing ratio have significant influences on the flow structure, vortex shedding frequency and hydrodynamic force. Several distinct vortex wake modes including the single bluff-body (SBB), shear-layer reattachment (SLR) and secondary vortex formation (SVF) modes are observed. The variations of Strouhal number, drag and lift coefficients versus spacing ratio are similar between all the current side ratios. In the branches of small and large spacing ratios, the fluid dynamics are obviously different and there is a critical spacing ratio, beyond which the relevant mechanical parameters jump suddenly as a result of the flow mode transition. A phenomenon of two-stage drag frequency separation between two cylinders occurs in the region of low side ratio and large spacing ratio, and its formation mechanism has also been analysed on the basis of flow characteristics and frequency spectra of force coefficients.

A comprehensive review on design and development analysis and blade material selection of helical Savonius rotor

The helical Savonius rotor (HSR) is used as green energy generation. The accurate design of HSR is essential before installing any wind farm in a region. It has been widely employed that positive static torque coefficients of helical Savonius rotor are higher compared to conventional semicircular rotor stated by several researchers. The reason is behind that the formation of secondary vortex of fluid. It is leading by vertical vortex flow, which passes over the curve surface and creates drag force. Still now, various design have been proposed to meet peak power output through optimum geometry parameters like blade profile design, aspect ratio and overlap ratio. The selection of blade material is also the influencing parameter to improve the starting torque of the rotor. In this regards present paper is aim to study the details experimental, numerical, and analytical review on HSR and selection of blade material with minimum production cost.

Hydrodynamic behavior of a novel 3D-printed nature-inspired microreactor with a high length-to-surface ratio

Microreaction technology has experienced almost exponential growth in the last decades due to significant practical benefits when compared with conventional reactors. However, the predominantly laminar flow impedes the required mixing in some processes. A novel design inspired by differential growth organic shapes in nature for improving mass transfer in microreactors is proposed. Firstly, this geometry allowed an area utilization of 16,1% and 27,2% higher than conventional serpentine and spiral channels, with the same hydraulic diameter, which can be useful for surface contact processes as in electrochemical reactions. Secondly, the performance of the microreactor was analyzed in terms of fluid flow and residence time distribution (RTD) for Reynolds in a range from 1-100. The continuous bending in the differential growth shape geometry leads to the formation of secondary vortices while approaching to the plug flow regime for the studied flow rates. These results were obtained by numerical analysis executed with the software COMSOL Multiphysics by solving three-dimensional Navier-Stokes equations with a convection-diffusion model for the species concentration. The validation of the RTD numerical model was conducted in a one-body 3D-printed microreactor by injecting a pulse of rhodamine B in water and measuring the response with colorimetric techniques. These organic shapes open a window to further research in microfluidics, as its parametric design can be adapted to a wide variety of reaction conditions.

Three-Dimensional Computational Fluid Flow Analysis of Bio-inspired Corrugated wing at Low Reynolds Number.

A computational study was performed to analyze the airflow over bio-inspired corrugated wings. The bio-inspired corrugated wing profiles were derived from the mid-span section of the forewing of Aeshna Cyanea dragonfly species. Additionally, a hybrid non-corrugated profile was also created possessing geometric similarities with the corrugated airfoils to compare and visualize the effects of corrugation on the fluid flow. The computational analysis was conducted for 4,8-, and 12-degrees angles of attack. Streamlines obtained from the computational analysis results (carried out on ANSYS CFX) showed the formation of secondary flows or vortices that are trapped in the valleys of the corrugated wings which was not observed in the hybrid airfoil. This study also compares the effects of corrugation geometry on fluid flow behavior. It was also seen that the intensity and quantity of the secondary flows increased with the increase in the angle of attack.

Investigations for efficient design of a new counter flow double-pass curved solar air heater

The aim of this paper is to numerically investigate a new curved design of a counter flow double-pass solar air heater with arched baffles placed in the second duct. Due to high inertia of the flow and curved nature of design, fluid in second channel tend to move away from the absorber plate and thus, reduces the thermal efficiency significantly. In order to overcome this deficiency, new design parameters are introduced in the second duct in terms of arched baffles and their design is optimized for various geometric parameters such as angle of attack, variable pitch ratio etc. for best thermo-hydraulic performance. It was observed that arched baffles accelerate the flow near the absorber and facilitate the formation of large secondary vortices that enables multiple attachment zones at the absorber plate. Enhancement in thermal and hydraulic performances are discussed in terms of flow visualization, local Nusselt number, thermal effectiveness and friction factor ratio. Maximum enhancement of Nu in roughened curve counter double-pass solar air heater with baffle angles α/90 = 0.5 at P/d = 6 are found in the range of 20–28%, higher than without baffles. Moreover, two new regression correlations are developed for prediction of thermo-hydraulic performance in terms of geometrical and flow parameters.

Investigation of heat transfer in wavy and dual wavy micro-channel heat sink using alumina nanoparticles

Thermal management is crucial for the proper functioning of a system whether it is in electronics, process industry, automobile, and renewable devices. Micro-channel heat exchanger has proved to be efficient in heat ejection from renewable systems due to high heat transfer surface to volume ratio. This study focuses on evaluating the cooling performance of straight, wavy, and dual wavy Micro-Channel Heat Exchanger by modelling the heat transfer model in ANSYS Fluent. The incompressible fluid is considered in the laminar regime using alumina-based nanofluids with 1%, 3%, and 6% concentration. The solution is computed by selecting SIMPLE pressure-velocity coupling scheme with second-order momentum and energy discretization. Nusselt number, pressure drop, base temperature, and Thermal Performance Factor (TPF) are used as performance parameters for comparing nanofluids performance at Reynolds number range of 100–900. For straight, wavy, and dual wavy model heat transfer, as well as pressure drop, increased with Reynolds number. It is observed that wavy and dual wavy channels compared to straight channel improved convective heat transfer due to the formation of secondary vortices at the curved section. Dual wavy with wavy base and flat base wall showed highest Nusselt number increase of more than double when compared with straight channel of equal concentration. For 6% nano particles addition in all channels, on average both dual wavy channels showed highest improvement of 8% when compared with 0% concentration channel. Dual wavy channel with a flat base and wavy base reduced the base heater temperature by 10 °C and 9 °C compared to the straight channel. A maximum Thermal Performance Factor of 2.2 is achieved for dual wavy channel with a wavy base configuration with 6% nanoparticles.

Gas flow dynamics over a plunging breaking wave prior to impact on a vertical wall

We present an experimental study on the gas flow field development over a plunging breaking wave prior to impact on a vertical wall. The variability of wave impact pressure over repeated measurements is well known (Bagnold, 1939). The formation of instabilities on the wave crest are postulated to be the main source of impact pressure variability (Dias and Ghidaglia, 2018). However, the mechanism that results in wave impact pressure variability and the influence of the gas phase in particular are relatively unknown. The velocity field of the gas phase is measured with particle image velocimetry, while simultaneously the local free surface is determined with a stereo-planar laser-induced fluorescence technique. The bulk velocity between the wave crest tip and the impact wall deviates from the mass conservation estimate based on the velocity profile between the wave crest and the impact wall. This is caused by a significant increase of the local gas velocity near the wave crest tip. The non-uniformities in the seeding concentration accumulate near the wave crest tip and reduce the accuracy of the velocity measurements. However, the bulk velocity estimate is significantly improved with a fit of the velocity profile that is based on a potential flow over a bluff body. Additionally, the development of vortex is observed and quantified for two typical measurements with either a disturbance on the wave crest or a smooth wave crest. The circulation development is comparable to the formation and separation of a vortex ring, which results in a saturated vortex that separates from the wave crest (Gharib et al., 1998). Furthermore, the impact location of the wave tip is altered by the formation of secondary vortices. The secondary vortex enhances the lift locally and alters the trajectory of the wave crest tip, which may result in additional variability of the wave impact pressure.

Effect of an upstream cylinder on the wake dynamics of two tandem cylinders with different diameters at low Reynolds numbers

This paper presents a systematic numerical study on the fluid dynamics and flow structures around a cylinder with diameter D placed in the wake of another cylinder with a smaller diameter d. Reynolds numbers of Re = 100 and 150 (based on D) are considered so the flow is physically two-dimensional. The ratios d/D and L/D vary in the ranges of 0.4–1.0 and 1.0–8.0, respectively, where L is the distance from the center of the upstream cylinder to the forward stagnation point of the downstream cylinder. The analysis focuses on how d/D and L/D influence the Strouhal number St, wake topology, and fluid forces on the downstream cylinder and links them with the flow physics. The flow is classified into the reattachment and co-shedding flow regimes, with the latter being further subdivided into the prime vortex shedding, two-layer vortex shedding, and secondary vortex shedding (SVS) modes, and the detailed aspects of the three modes are discussed based on the time-averaged flow fields. The two vortex frequencies of the downstream cylinder can be detected only in the SVS mode, and in addition to the fundamental vortex frequency f1, the shedding of the secondary vortex further results in the subharmonic frequency f2. Only when the secondary shedding length Ls* is <10 does f2 affect the downstream cylinder and lead to a pattern of alternating high- and low-amplitude peaks in the time history of the lift coefficient. A novel mechanism of secondary vortex formation is identified, and the critical spacing and the modulation of lift by f2 are also discussed.

Vortex dynamics of veno-arterial extracorporeal circulation: A computational fluid dynamics study

Veno-arterial extra corporeal membrane oxygenation (VA-ECMO) is a modified heart lung machine used for patients with both heart and lung failure. This results in retrograde supply of oxygenated blood through the femoral artery in which the unsteady pulsating antegrade flow from the aorta interacts with a steady, uniform, and retrograde flow from the femoral artery, creating a mixing zone. This work aims to provide a mechanistic interpretation of VA-ECMO by developing an in-silico framework using computational fluid dynamics. We performed several numerical simulations to investigate the effects of aortic geometry on VA-ECMO by implementing two idealized full aorta models and studied the formation of secondary flow features and vortices. We used vortex identification methods to capture the three-dimensional vortical structures formed under various ECMO support levels. Our results show that under pulsatile aortic flow and 80% of ECMO support, the streamwise vorticity and aortic arch geometry strongly influence the mixing zone. Furthermore, we found that pulsatility at the aortic inlet causes oscillation in secondary flow structures at the abdominal aorta leading to unsteadiness in ECMO flow and differential wall shear stress. We also examined the effects of VA-ECMO flow rates on secondary flow and vortical structures. We show that the location and complexity of secondary flows and vortical structures are affected by ECMO support levels and geometry of aortic segments. Together, we believe that this computational framework is a crucial step in understanding flow features and vortical structures formed during VA-ECMO administration, which can improve patient care and ECMO management.

Discrete shedding of secondary vortices along a modified Kaden spiral

Abstract

Comparative assessment of mixing characteristics and pressure drop in spiral and serpentine micromixers

In this paper, numerical simulations are carried out to analyze the mixing performance and pressure drop of water and ethyl alcohol in a spiral micromixer for Reynolds number in the range of 1 ≤ Re≤100. The characteristics are also compared with those for serpentine and straight micromixers by keeping the total flow length equal. It reveals that at lower Re ( = 1), the values of mixing index are almost same for all the micromixers, For higher Re, spiral micromixer provides better mixing than that of serpentine and straight micromixers, because of the formation of secondary Dean vortices. For straight micromixer, the mixing index decreases monotonically with Reynolds number. However, contrasting features are observed for both spiral and serpentine micromixers at higher regime of Re. The mixing index first decreases with Re and then increases although the values of critical Re beyond which the mixing index increases are different for spiral and serpentine micromixers. The pressure drops for all three micromixers are almost the same at lower values of Re, while the pressure drops for spiral and serpentine micromixers are almost equal and the values are slightly higher than those of straight micromixer at higher Re.

Thermal analysis on a criterion for interaction of secondary vortices with primary flow field under parallel mode rotation

In this paper, the effects of rotational forces such as centrifugal buoyancy and Coriolis forces caused by parallel mode channel rotation on turbulent air flow through tubes with different aspect ratios are investigated numerically. However, rotational effects does not necessarily increase heat transfer and pressure loss since a number of parameters such as main flow condition and channel aspect ratio must be regarded. This paper discusses rotational effects and establishes novel criteria that illustrate how the aspect ratio affects the formation of secondary vortices and thus convective heat transfer coefficient on different rotational conditions. The numerical modeling results are validated by the available experimental and numerical data in literature and it is shown that the normalized Nusselt number (Nu/Nu0) result points for each aspect ratio collapses on a single curve if it is plotted against GrΩα/Re which it is revealed that α depends on aspect ratio only. However, it is demonstrated that once the GrΩα/ Re < λ the effects of rotation may be neglected and thus Nusselt number is equal to stationary case counterpart while λ is also determined as a function of aspect ratio only. The variation of α and λ with respect to channel aspect ratio is discussed as well.

ОСОБЕННОСТИ ТЕРМОМЕХАНИКИ ПУЛЬСИРУЮЩИХ ПОТОКОВ ГАЗА ВО ВПУСКНЫХ СИСТЕМАХ С КАНАВКАМИ ПРИМЕНИТЕЛЬНО К АВТОМОБИЛЬНЫМ ДВИГАТЕЛЯМ

Reciprocating internal combustion engines (RICE) are widely used as heat engines for converting the chemical energy of a fuel into mechanical work on the crankshaft. Aerodynamic and thermophysical processes in gas exchange systems significantly affect the efficiency of the RICE. This paper examines the possibility of influencing the gas dynamics and heat transfer of pulsating gas flows in the inlet system by placing a channel with grooves. It is known that the presence of grooves in the channel leads to the formation of significant secondary vortices, which radically change the physical picture of the gas flow. The studies were carried out on a laboratory bench, which was a singlecylinder model of a turbocharged RICE. A system of measurements of basic physical quantities is described, taking into account their high dynamics. Techniques for processing experimental data are presented. Primary data on instantaneous values of gas-dynamic and heatexchange characteristics of pulsating flows are presented. It was found that the presence of a channel with grooves in the inlet system leads to a decrease in the degree of turbulence to 40 % and an intensification of heat transfer in the range of 5–50 % compared to the basic inlet system. A positive effect is shown in the form of an increase in engine power by 3 % when using the modernized system.

Features of thermomechanics of pulsating gas flows in intake systems with grooves in relation to turbocharged engines

Reciprocating engines (RICE) are widely used as heat engines to convert the chemical energy of fuel into mechanical work on the crankshaft. Aerodynamic and thermophysical processes in gas exchange systems significantly affect the efficiency of internal combustion RICEs. This article explores the possibility of influencing the gas dynamics and heat transfer of pulsating gas flows in the intake system by placing a channel with grooves. It is known that the presence of grooves in the channel leads to the formation of significant secondary vortices, which radically change the physical picture of the gas flow. The studies are carried out on a laboratory bench, which was a single-cylinder model of a turbocharged RICE. The system of measurements of basic physical quantities is described, taking into account their high dynamics in gas exchange systems. The experimental data processing techniques are presented. Primary data on the instantaneous values of the gas-dynamic and heat-exchange characteristics of pulsating flows are reported. It is established that the presence of a channel with grooves in the intake system leads to a decrease in the turbulence number by 40% and the intensification of heat transfer in the range of 5-50% compared with the basic intake system. A positive effect is shown in the form of an increase in engine power by 3% when using an upgraded system.

Development of a Swirl‐Induced Rotating Glow Discharge Reactor for CO2 Conversion: Fluid Dynamics and Discharge Dynamics Studies

Herein, CO2 conversion is conducted in a newly developed swirl flow–induced rotating glow discharge reactor. The swirl injector is designed to produce forward vortex by eight inclined guided vanes. A glow‐type rotating discharge is formed between two electrodes of constant height. The discharge and fluid dynamics studies reveal that the magnitude of tangential velocity and secondary vortex formation affects the rotational frequency of the discharge. It is also found that the anchoring point of the plasma column root is greatly influenced by the increased pressure gradient, especially in the turbulent regime. The conversion of CO2 as a function of flow rate is found to increase in the laminar regime and decrease in the turbulent regime. With increasing specific energy input from 0.25 to 3.4 kJ L−1, there is a rise in conversion from 1.2% to 4.6%, which indicates the high residence time and high energy deposition to lesser number of CO2 molecules. Higher energy efficiencies (54.9–63.7%) at turbulent flow rates show that this swirl design enhances the distribution of electron energy to incoming fresh molecules by formation of circulation zones and turbulence.

On the dynamics of unconfined and confined vortex rings in dense suspensions

Abstract