Two-dimensional Computational Fluid Dynamics Simulations Research Articles

CFD-aided contraction-expansion static mixer design for oil-in-water emulsification

Abstract Contraction-expansion (CE) static mixers can enable solid-liquid and liquid-liquid dispersion with low energy dissipation, low risk of obstruction, and without moving parts. In this work, the influence of CE elements of different geometries on the imposed turbulence of a flowing liquid has been assessed by a two-dimensional computational fluid dynamic (2D-CFD) simulation. The effect of CE on the dispersion of droplets of an immiscible liquid has also been analysed from simulations, using the volume of fluid (VOF) approach. Direct numerical simulation (DNS) performed by the open-source Gerris Flow Solver software was used to get the velocity fields and turbulence characteristics. Different ratios of CE diameters and lengths were analysed for liquid Reynolds numbers from 500 to 20,000. From simulations, the CE geometry that maximised the average root mean square velocity, as an indicator of turbulence, was determined for different liquid flow rates. It was found that the average RMS had a maximum for a wide range of liquid flow rates when the CE diameter ratio was between 0.55 and 0.59 and the length ratio was between 0.2 and 0.3. Then, a device with seven CE elements with geometrical features within this range was built and used for preparing an oil-in-water emulsion. The test system contained water and sunflower oil (5 % v/v) with the further addition of TritonX100 (0.5 % in volume of the solution) as surfactant. The stability of the emulsions was assessed by measuring the time evolution of turbidity (absorbance at 860 nm), to get the initial separation velocities. The emulsions prepared using the CE device showed initial phase separation rates lower than the one obtained in a stirred flask, evidencing the feasibility of using CE static mixers for preparing emulsions with relatively low energy consumption. Moreover, the emulsions obtained with the CE device, although dependent on the flow rate, showed similar features when obtained with 10, 100 and 250 passes through the CE static mixer.

Computational fluid dynamics analysis on performance assessment of Darrieus-type vertical axis wind turbine using NACA0016, NACA0019 and NACA0020 airfoil sections

The efficiency of a vertical-axis wind turbine is remarkably less than a horizontal-axis wind turbine and consequently, the most important challenge of the former one is to enhance its aerodynamic performance. An earlier study shows that the selection of an efficient airfoil shape is an important criterion for enhancing the overall performance of a wind turbine. The objective of the present work is to perform the two-dimensional Computational Fluid Dynamics simulation of Darrieus Vertical axis wind turbine using NACA0016, NACA0019 and NACA0020 airfoils which are neither too thick nor too thin. Furthermore, the performance of these three airfoils is compared with two established airfoils, namely NACA0015 and NACA0018. The result reveals that the overall power coefficient of NACA0019 airfoils resulting from the CFD simulation is significantly higher than that of NACA0015. Therefore, the NACA0019 airfoil may be considered one of the effective alternative airfoils for the Darrieus VAWT. Highlights Simulation of vertical axis wind turbine using NACA0016, NACA0019 and NACA0020 airfoils Unsteady URANS studies are carried out to investigate the performance of the airfoils Among the blade profiles studied in this work, NACA 0019 offers superior fluid mechanical performance

Simulation of Heat and Mass Transfer in a Moving Bed Part-Fluidized Boiler

Abstract Moving bed part-fluidized boiler is a new type of furnace. The new combustion method in the furnace has attracted a lot of attention and shown attractive prospects. Two-dimensional computational fluid dynamic (CFD) simulations were performed for a 116 MW moving bed part-fluidized boiler to investigate the different combustion patterns of coal particles of different particle sizes inside the furnace chamber. A low-NOX combustion method based on the combination of laminar combustion and fluidized combustion is proposed. By comparing the effects of different air distributions on the fluidization state of coal particles, the air distribution values required for optimal fluidized combustion were obtained. The temperature field and pollutant distribution in the furnace chamber for the conventional combustion method and the new combustion method were also simulated. The results show that the combustion technology combining laminar combustion and fluidization of a moving bed part-fluidized boiler can significantly improve the combustion rate and reduce the NOX concentration at the furnace exit. When the secondary air speed is up to 15 m/s, the coal particles smaller than 5 mm are fully fluidized and burned in the whole furnace chamber. The coal particles larger than 5 mm are burned on the bed. The pollutant emission of the boiler can reach the best condition. The new type of boiler can reach a super clean emission in which the NOX emission value is below 47 mg/m3, and the SO2 emission value is reduced to 0.15 mg/m3.

Quasi One-Dimensional characteristic analysis of ejector nozzle for turbofan propulsion system

To investigate the impact of the ejector characteristics of the exhaust system on the overall performance of a turbofan engine, this paper presents a study on the overall performance of the turbofan engine equipped with an ejector nozzle, focusing on its characteristics. Firstly, the design of the ejector exhaust system is presented and its ejector characteristics are analyzed through two-dimensional computational fluid dynamics simulations. Subsequently, a one-dimensional theoretical analysis of the ejector is conducted, and the pump suction and ejector nozzle’s thrust characteristics are determined based on the analysis results. Furthermore, a two-dimensional interpolation method is proposed to calculate the ejector coefficient, which utilizes the ejector's pump suction characteristics. By applying the matching principle of the ejector coefficient, a component-level model of the turbofan engine with an ejector nozzle is established. Based on this model, a method for predicting the backward infrared radiation intensity of the ejector exhaust system is proposed to investigate the influence of the ejector nozzle characteristics on the overall engine performance and infrared signature. The results demonstrate that the installation of the ejector exhaust system leads to a slight decrease in the overall engine performance, in which the average decrease of thrust is about 1.5%, and the average increase in fuel consumption is about 0.5%.but significantly reduces the infrared stealth characteristics, the average reduction is about 20%, which has good infrared stealth effect.

Non-equilibrium aerodynamics between ionization-wave and shock-wave fronts in millimetre-wave supported detonation

A shock wave driven by discharge plasma propagating in a high-power millimetre-wave beam was investigated both experimentally and numerically. A shadowgraph experiment was used to visualize the shock wave and plasma. The ionization wave front of the plasma preceded the shock wave by 2.0–4.0 mm under conditions of 170 GHz beam frequency, atmospheric pressure, and 1.1 GW m−2 peak power density (power density at the beam axis). Furthermore, a two-dimensional computational fluid dynamics (CFD) simulation considering non-equilibrium temperatures and filamentary plasma structures revealed that an ionization-wave front precedes a shock wave front because the energy relaxation requires a few micro-seconds from vibrational excitation at a plasma front to translational excitation for the shock wave formation. Consequently, the distance between an ionization wave front and shock wave front is 3.8 mm in the two-dimensional CFD simulation, which agreed with the experimental results.

Optimal configuration of oblique air curtains for heat loss suppression from vertical solar-thermal receiver surfaces

An external solar-thermal receiver is critical to the operation of a solar-thermal power plant because it converts concentrated solar radiation into useful heat. However, as these receivers are surrounded by relatively cold air, they suffer from significant convective heat loss that is detrimental to their thermal performance. Air curtains, in the form of air jets directed obliquely from the top of the receiver surface, have been shown to provide significant local reductions in receiver heat loss. This study presents, for the first time, a rigorous optimisation of air curtain operation to increase the mitigation of overall convective heat loss. Two-dimensional computational fluid dynamics simulations in OpenFOAM were used to optimise jet speed, angle, position, and thickness. Performance was measured based on global effectiveness, with a maximum of over 18% achieved for a jet with speed Uac=5.5ms−1, angle α=45°, thickness b=30mm, and gap G=30mm. Increasing jet thickness yielded the greatest gains in overall effectiveness. An equivalent air curtain effectiveness was defined, with the optimal case having an effectiveness of around 14%. The effect of jet outlet temperatures on effectiveness was also examined, with effectiveness gradually decreasing with increasing jet outlet temperature.

Investigation of the thermal response characteristics of the loop thermosyphon under convective boundary conditions

The two-phase loop thermosyphon (TPLT) is a highly efficient passive heat transfer device that is widely applied in various fields. This paper experimentally and numerically investigates the thermal response performance of a large-scale TPLT under various convective boundary conditions. Experimental studies have been carried out to explore the effects of different working fluids, filling ratios, cold and heat source temperatures, and mass flow rates on the thermosyphon’s performance. Meanwhile, a two-dimensional Computational Fluid Dynamics (CFD) simulation model of the TPLT is established and validated. The results indicate that the start-up procedure of the TPLT is highly dependent on the heat source temperature and the working fluid. Normally, the TPLT can start when the heat source reaches a certain temperature. In general, using methanol as the working fluid is more favorable to start-up compared to the water and ethanol TPLTs. The experimental results also show that the heat transfer capacity of the TPLT increases with increasing heat source temperature while increasing cooling temperature decreases heat transfer capacity. The best heat transfer performance of a TPLT charging methanol is observed at a filling ratio of 70 % with a heating mass flow rate of 1 L/min. At that point, the convective heat transfer coefficient of outside the evaporator is 1036.7 W/(m2·K). Moreover, the simulation results reveal that the average wall heat flux of the TPLT’s evaporator initially decreases and then gradually stabilizes with time. During stable operation, the wall heat flux density of the evaporator exhibits an initial increase in the pool section followed by a subsequent decrease with increasing wall height. This work provides insight into the heat and fluid flow characteristics of the TPLT under convective boundary conditions.

Numerical explorations of the vortex-induced vibration of a streamlined box girder with water-filled barriers

The present study aims to investigate the charactefristics of vortex-induced vibrations (VIVs) of a streamlined box girder equipped with water-filled barriers (WFBs) through two-dimensional computational fluid dynamics (CFD) simulations. Initially, the VIV behavior of the bridge section is examined by considering various positions of WFBs and angles of attack (α), focusing on parameters involving vibration amplitude, surface pressure distribution, and flow field. Subsequently, in order to reveal the underlying VIV mechanism, the bridge section with WFBs commonly positioned was chosen to evaluate the contribution or inhibition of wind loads on specific regions around the bridge's surface to VIV, based on the pressure distribution and energy input process characteristics. The numerical results demonstrate that if the WFBs are positioned closer to the railing on the windward side of the bridge, as α shifts from negative to positive, the separated vortex on the upper surface of the bridge section tends to be larger. Additionally, the shedding of vortices at the tail of the deck becomes more pronounced, creating a greater periodic force on the bridge section and making it more vulnerable to VIV. The mean and standard deviations of pressure coefficients, as well as the predominant frequencies of the distributed pressures, are closely correlated with various stages of the VIV development process. The occurrence of VIV is actively triggered by the wind load acting on the upper surface upstream and lower surface downstream, while conversely, the wind load acting on the downstream of the upper surface and the upstream of the lower surface tends to inhibit the VIV. Moreover, it is worth noting that the growth rate of energy consumption on the downstream of upper surface is more significant than that of energy consumption in other regions, which makes a main effect on limiting the amplitude of VIV.

Computational fluid dynamics simulations of silicon μ-structure formation during excimer laser modification of silicon-nanoparticle layers

The laser modification of silicon-nanoparticle layers with a nanosecond pulsed excimer laser leads to the self-organized formation of crystalline, μ-cone-shaped silicon structures with good electronic properties, which have allowed the demonstration of their potential for printed flexible electronics. With the current nanosecond laser process, silicon exhibits only short melting times, resulting in a method-defined substrate contact angle, instead of this parameter being defined by the substrate surface energy as expected for equilibrium conditions. This substrate material-independent non-equilibrium contact angle of the silicon melt was experimentally determined in this study to be Θ = 68 ± 10°. To gain deeper insight into the process of the sequential melting and the formation of the silicon μ-cone structures during laser modification, a two-dimensional computational fluid dynamics simulation was performed in COMSOL Multiphysics® solving the Navier–Stokes equation for incompressible fluids. The simulation uses an effective medium approach by applying the conservative level set method to describe the porous silicon-nanoparticle layer. Its sequential melting during the pulsed laser modification is modeled using a newly developed simulation methodology, which uses a time- and depth-dependent dynamic viscosity of the molten silicon. The simulation was carried out for different laser energy densities and verified using scanning electron microscopy images of corresponding laser-modified samples. The simulation results agree well with the experiment and were subsequently used to optimize the laser process.

CFD-VOF-DPM numerical simulation of enhanced boiling heat transfer characteristics of microencapsulated phase change material slurry

Two-dimensional (2D) computational fluid dynamics (CFD) simulations of water-based microencapsulated phase change material suspension (MPCMS) on the flow boiling in a gas–liquid–solid flow system was performed with volume of fluid (VOF) method and discrete particle model (DPM). The Lagrangian particles were linked to the Eulerian phases through the interchange terms such as the drag force in the respective momentum equations. Influences of particle properties including mass fraction and core phase transition temperature, fluid properties including liquid surface tension force and viscosity, detachment time and rise velocity of gas bubbles and particle entrainment in the gas–liquid–solid flow under ambient conditions were numerically investigated. The effects of particles on bubble nucleation, growth and rupture during boiling were studied by visualization. The results show that MPCM can enhance the boiling heat transfer ability of the base liquid. The maximum heat transfer enhancement rate of MPCMS (28 °C) is 4.8%, the maximum heat transfer enhancement rate of MPCMS (90 °C) is 5.1%, the maximum heat transfer enhancement rate of MPCMS (110 °C) can reach 6.7%. MPCM can promote the formation of new bubbles and the rupture of large bubbles, reduce the departure diameter of bubbles, and enhance the boiling heat transfer capacity of the base liquid. The MPCM with core phase change temperature higher than the boiling temperature of base fluid has the best enhancement effect. Through the combination of numerical simulation methods such as VOF and DPM, the complex phase transition heat transfer process of gas-liquid-solid particle coupling of latent thermal functional thermal fluid can be accurately simulated. The work lays a foundation for further explorations on the gas–liquid–solid flows and possible industry applications.

Numerical simulation of piggyback pipelaying under current loadings

For piggyback pipelaying operations, current-induced force and its effect on the piggyback pipe have not been thoroughly studied. In the present study, an improved method in hydrodynamic load calculation and structural modelling is proposed to simulate the pipelaying of a piggyback pipeline. In order to obtain the mean drag and lift force coefficients for the piggyback pipeline subjected to different inflow angles, two-dimensional Computational Fluid Dynamics (CFD) simulations are performed by modelling the piggyback pipeline as two cylinders attached to each other without gap. Then, the acquired force coefficients are used to calculate the hydrodynamic loads through a user-defined function in OrcaFlex based on a cross-flow principle approach. The interaction between the pipeline and the piggyback cable is modelled using two types contact elements which are ring penetrator and non-penetrating contact. The present proposed method is compared with other two widely used engineering methods based on (1) the equivalent diameter and (2) two separate cylinders without accounting for hydrodynamic interaction, in terms of the top tension, and the bending moments at Hang-off Clamps (HOC) and sagbend of the pipeline. The comparison shows that the two widely used engineering methods are not always conservative in force and response predictions. Hence, it is important to consider the hydrodynamic and structural interactions between the piggyback cable and the pipeline. With different current directions, the bending moments at the HOC predicted by the present method vary from 40% lower to 100% higher than those predicted by the two widely used engineering methods.

Performance characteristics of flapping foil flow energy harvester that mimics movement of swimming fish

Many fish swim using various kinematic patterns of head and tail combinations to change their body posture, which reduces drag while swimming. This study proposes a new method to improve the energy-harvesting performance of a flapping foil by actively controlling the combined movements of the leading- and trailing-edge flaps. These movements allow the foil to deform to a shape with preferable hydrodynamic performance during the flapping motion inspired by fish swimming modes. The study employs two-dimensional (2D) computational fluid dynamics (CFD) simulations to investigate the effect of the forced oscillation of a leading-edge flap on the energy-harvesting performance of a flapping foil. Consequently, the optimal combination of operating parameters of the leading-edge flap leading to greater efficiency and an apparent increase in power production from the flapping foil can be determined. Therefore, the energy-harvesting performance of a flapping foil with dynamic leading- and trailing-edge flaps was investigated numerically. The results suggest that synchronized movement between the leading- and trailing-edge flaps, which mimic the movement of live fish, can increase the camber of the foil and effectively manage the boundary layer by adjusting the frequency and amplitude of the oscillations, thereby generating elevated hydrodynamic forces acting on the foil. Therefore, the proposed flapping foil with swinging leading- and trailing-edge flaps can exhibit superior energy-harvesting performance than its conventional alternative across the full spectrum of operations.

Estimation of the wind drift factor and uncertainty analysis based on CFD computer simulations

PurposeThe purpose of this work is to present a procedure for determining the wind drift factor through two-dimensional computational fluid dynamics (CFD) simulations of the wind acting on a wavy sea surface, such that the subjectivity of its estimation is reduced.Design/methodology/approachThe wind drift factor was determined by two-dimensional CFD analyses with open-channel condition. The characteristic wave was determined by the Sverdrup–Munk–Bretschneider (SMB) method. The uncertainty analysis is based on convergence studies using a single parameter refinement (grid and time step).FindingsThis procedure allows the estimation of the wind drift factor in a fetch-limited domain. The domain's value in the analyzed region is 0.0519 ± 4.92% which is consistent with the upper values of the wind drift factors reported in the literature.Research limitations/implicationsThe use of a three-dimensional domain was impractical with the available computational resources because of the fine mesh required for wave modeling. The uncertainty analysis consisted only of a verification procedure. Validation against real data was not possible because of the lack of measured data in the analyzed region.Originality/valueThe wind drift factor is usually estimated based on either experience or random sampling. The original contribution of this work is the presentation of a CFD procedure for estimating the wind drift factor, in which the domain inlet is subjected to a wave boundary condition and to a wind velocity.

Dynamic Response of Circulation Control for Step and Sinusoidal Inputs

A numerical study of the dynamic performance of a circulation controlled (CC) airfoil is performed to explore the feasibility of using circulation control as means of gust suppression. Steady and unsteady two-dimensional computational fluid dynamics (CFD) simulations were initially validated using published data. The dynamic performance of a CC airfoil was subsequently assessed. The response of the CC airfoil to step and sinusoidal inputs were obtained from unsteady CFD simulations. It was found that the actuation speed of circulation control is significantly faster than a conventional mechanical flap at the same flight condition. Additionally, the lift response to a step input was found to be very similar to the Küssner function.

Performance enhancement of savonius wind turbine by multicurve blade shape

ABSTRACT This study proposes a novel blade configuration, made with a multicurve profile, to further maximize the advanced benefits and the performance of a Savonius turbine for energy harvesting applications. The advantages of the presently designed blade are proved by a detailed analysis of the flow mechanism and the performance through a sequence of two-dimensional (2D) unsteady computational fluid dynamics simulation. The remarkable finding states that the main blade section with a length ratio of 0.9 combined with an elliptical auxiliary section effectively enhances the positive wind flow phenomena around the blade, especially for the returning position, which either smooths the flow or reduces the pressure on the blade’s convex side. As a result, greater torque is obtained by the present configuration, responding to the improvement of the power generation to 0.24 – about 5.5% at the low TSR = 0.8 and up to 0.28 - about 185% at the high TSR = 1.5 over the conventional configurations. The effectiveness of the presently designed blade is further proved with more than 20% improvement in self-staring speed through six degrees of freedom analysis. The results above state a high potential application of the turbine with the presently designed blade for renewable energy applications and for supporting vulnerable communities without access to electricity.

Wind turbine concepts for domestic wind power generation at low wind quality sites

An overview of extant wind turbine designs is presented. Each concept is evaluated for potential for micro-wind production under poor wind quality situations, such as those seen in densely populated, suburban, or sparsely forested regions. It was concluded that the Savonius turbine and the Savonius-Darrieus Hybrid turbines were the most suited designs out of the seven that were investigated. In addition, several types of materials for micro-wind turbines were explored. Two-dimensional Computational Fluid Dynamic (CFD) simulations were run with the Savonius, and Savonius-Darrieus hybrid turbines with wind speeds 3 m/s, 5 m/s, 7 m/s and 10 m/s. Power generated by the turbines under each wind speed as well as the power coefficients and Tip Speed Ratios of the turbines were determined, and it was found that the Savonius turbine simulated had the maximum power coefficient of 0.31 at TSR = 0.97 and the Hybrid turbine had 0.32 at TSR = 2.24. It was also concluded that the Hybrid turbine generates more power than the Savonius turbine for all wind speeds except for 3 m/s. The maximum power output by any turbine simulated in this study was 157 W.

The impact of the wind attack angle on a typical bridge deck’s flutter behavior by the distributed aerodynamic characteristics method

To investigate the influence of the wind attack on a bridge deck’s flutter performance, the concept of “pressure flutter derivatives” was introduced as a part of the “distributed aerodynamic characteristics” method, which can be used to visualize the dominant fluctuations of the pressure field. The distributions of the aerodynamic damping and stiffness under different wind attack angles were visualized, instead of using an overall force perspective. The analysis was carried out through two-dimensional computational fluid dynamics simulations based on a typical box deck section. Necessary data were obtained through the forced vibrations of the deck. The dominant pressure fluctuations were demonstrated, around which the wind attack angles’ impact on the Scanlan flutter derivatives was discussed. For the deck section used in this study, the wind attack angle was found to most affect the windward parts of the deck. When wind attack angle ranged from − 1.5 ° to 3 ° , two regions contributed the most: one located behind the windward faring vertex and the other located behind the first vertex of the upper slab. When the wind attack angle decreased to − 3 ° , a new dominant region appeared behind the first vertex of the lower slab, which caused a sudden reduction of the flutter critical speed.

Effects of heave motion of an elastically supported floating oscillating water column device on wave energy harvesting efficiency

The main aim of this paper is to analyze the effects of the heave motion of an elastically supported floating oscillating water column device (OWC) on wave energy harvesting efficiency through two-dimensional computational fluid dynamics simulations. After the numerical model is validated using experimental data, it is used to analyze the effect of the natural frequency ratio on the hydrodynamic efficiency of the OWC. The natural frequency ratio is the ratio of the natural frequency to the wave frequency. The numerical results prove that the natural frequency ratio must be greater than 3 for achieving the best hydrodynamic efficiency. The best hydrodynamic efficiency decreases with the decrease in the natural frequency ratio when the latter is 3. If the natural frequency ratio is smaller than 3, both the amplitudes of wave surface elevation and the displacement of the OWC chamber increase but the harvested energy reduces because the relative motion of the wave surface and the OWC device is small. The energy dissipation due to vortices does not have a strong contribution to the reduction of energy at small natural frequencies.

Modelling of Cantilever-Based Flow Energy Harvesters Featuring C-Shaped Vibration Inducers: The Role of the Fluid/Beam Interaction

Flow Energy Harvesters (FEHs), equipped with piezoelectric active layers, are designed to extract energy from non-pulsating flows. FEHs featuring cantilevers with tip-mounted Vibration Inducers (VIs) are designed to develop a galloping motion. In this paper, we present the modelling of a recently introduced VI shape, featuring semitubular-shaped winglets, which do not produce a wake interacting with the cantilever. Such peculiarity allows (i) to exploit the contribution of the wake to the formation of the lift, therefore opening to a more compact design; (ii) its performance to be analyzed by means of simple two-dimensional Computational Fluid Dynamics (CFD) simulations. By comparison with experimental data, we show that the minimal framework for the modelling of such new class of VIs needs to account for both the direct action of the fluid onto the cantilever and the drag on the VI, which are usually negligible for other VI shapes.

Pulsatile gas-liquid flow resembling Decompression Sickness: Computational Fluid Dynamics simulation and experimental validation.

This work performs two-dimensional Computational Fluid Dynamics (CFD) simulations of pulsatile bubbly flow in a column resembling the flow inside human vena cava during Decompression Sickness (DCS), aiming to illustrate the effect of certain parameters in bubbly blood flow and so facilitate the design of the: a) corresponding in-vitro bubbly flow experiments under pulsatile flow conditions inside a flow loop and b) in-vivo trials on swines for assessing a novel electrical impedance spectroscopy technique on the detection of bubbles (as those found during DCS) in their bloodstream. The commercially available ANSYS 2019-R3 CFD code was employed to simulate the pulsatile bubbly flow that resembled DCS. Simulations were validated against experiments conducted in a vertical co-current upward pulsatile bubbly flow provided by a flow loop equipped with electrical, optical and pressure diagnostics. CFD simulations under pulsatile conditions were initially validated by oscillatory in-vitro bubbly flow experiments. Then, the influence of pulsation parameters on void fraction, α, and flow velocity, U, profiles was computationally investigated. Intense periodic fluctuations of void fraction were observed along the column and their intensity increases with pulsation amplitude. Moreover, U and α radial profiles were uniform for bubbles 30 μm but showed a core-peaking profile for bubbles 300 μm. CFD simulations of pulsatile bubbly flow resembling DCS provided unconventional information about the influence of different-sized sub-millimetre bubbles on the flow velocity and void fraction profiles, which are expected to improve the design of in-vitro and in-vivo trials for the detection of bubbles such as those found in DCS.