Energies

Research

23 pages, 10520 KiB

Open AccessArticle

CFD and PIV Investigation of a Liquid Flow Maldistribution across a Tube Bundle in the Shell-and-Tube Heat Exchanger with Segmental Baffles

by Grzegorz Ligus, Marek Wasilewski, Szymon Kołodziej and Daniel Zając

Energies 2020, 13(19), 5150; https://doi.org/10.3390/en13195150 - 2 Oct 2020

Cited by 10 | Viewed by 3384

Abstract

The paper presents the results of research on liquid flow maldistribution in the shell side of a shell-and-tube heat exchanger (STHE). This phenomenon constitutes the reason for the formation of the velocity reduction area and adversely affects heat transfer and pressure drop. In [...] Read more.

The paper presents the results of research on liquid flow maldistribution in the shell side of a shell-and-tube heat exchanger (STHE). This phenomenon constitutes the reason for the formation of the velocity reduction area and adversely affects heat transfer and pressure drop. In order to provide details of the liquid distribution in STHE, two visualization methods were utilized. First, computational fluid dynamics (CFD) code coupled with the k-ε model and the laser-based particle image velocimetry (PIV) technique was applied. The tests were carried out for a bundle comprising 37 tubes in an in-line layout with a pitch d_z/t = 1.5, placed in a shell with D_in = 0.1 m. The STHE liquid feed rates corresponded to Reynolds numbers Re_in equal to 16,662, 24,993, and 33,324. The analysis demonstrated that the flow maldistribution in the investigated geometry originates the result of three main streams in the cross-section of the shell side: central stream, oblique stream, and bypass stream. For central and oblique streams, the largest velocity reduction areas were formed in the wake of the tubes. On the basis of the flow visualization, it was also shown that the in-line layout of the tube bundle helps to boost the wake region between successive tubes in a row. Additionally, unfavorable vortex phenomena between the last row of tubes and the lower part of the exchanger shell were identified in the investigations. The conducted studies confirmed the feasibility of both methods in the identification and assessment of fluid flow irregularities in STHE. The maximum error of the CFD method in comparison to the experimental methods did not exceed 7% in terms of the pressure drops and 11% in the range of the maximum velocities. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 6607 KiB

Open AccessArticle

Numerical Simulation of the Solid Particle Sedimentation and Bed Formation Behaviors Using a Hybrid Method

by Md Abdur Rob Sheikh, Xiaoxing Liu, Tatsuya Matsumoto, Koji Morita, Liancheng Guo, Tohru Suzuki and Kenji Kamiyama

Energies 2020, 13(19), 5018; https://doi.org/10.3390/en13195018 - 24 Sep 2020

Cited by 11 | Viewed by 2839

Abstract

In the safety analysis of sodium-cooled fast reactors, numerical simulations of various thermal-hydraulic phenomena with multicomponent and multiphase flows in core disruptive accidents (CDAs) are regarded as particularly difficult. In the material relocation phase of CDAs, core debris settle down on a core [...] Read more.

In the safety analysis of sodium-cooled fast reactors, numerical simulations of various thermal-hydraulic phenomena with multicomponent and multiphase flows in core disruptive accidents (CDAs) are regarded as particularly difficult. In the material relocation phase of CDAs, core debris settle down on a core support structure and/or an in-vessel retention device and form a debris bed. The bed’s shape is crucial for the subsequent relocation of the molten core and heat removal capability as well as re-criticality. In this study, a hybrid numerical simulation method, coupling the multi-fluid model of the three-dimensional fast reactor safety analysis code SIMMER-IV with the discrete element method (DEM), was applied to analyze the sedimentation and bed formation behaviors of core debris. Three-dimensional simulations were performed and compared with results obtained in a series of particle sedimentation experiments. The present simulation predicts the sedimentation behavior of mixed particles with different properties as well as homogeneous particles. The simulation results on bed shapes and particle distribution in the bed agree well with experimental measurements. They demonstrate the practicality of the present hybrid method to solid particle sedimentation and bed formation behaviors of mixed as well as homogeneous particles. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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32 pages, 11188 KiB

Open AccessArticle

A Theory for Power Extraction from Passive Accelerators and Confined Flows

by Robert Freda, Bradford Knight and Siddharth Pannir

Energies 2020, 13(18), 4854; https://doi.org/10.3390/en13184854 - 16 Sep 2020

Cited by 5 | Viewed by 2851

Abstract

No accepted fluid theory exists for power extraction from unpressurized confined flow. The absence of a valid model to determine baseline uniform power extraction in confined flows creates difficulties in characterizing the coefficient of power. Currently, the primary body of research has been [...] Read more.

No accepted fluid theory exists for power extraction from unpressurized confined flow. The absence of a valid model to determine baseline uniform power extraction in confined flows creates difficulties in characterizing the coefficient of power. Currently, the primary body of research has been limited to Diffuser Augmented Wind Turbines (DAWTs) and passive fluid accelerators. Fluid power is proportional to the cube of velocity; therefore, passive acceleration is a promising path to effective renewable energy. Hypothetical models and experiments for passive accelerators yield low ideal power limits and poor performance, respectively. We show that these results derive from the misapplication of Betz’s Law and lack of a general theory for confined flow extraction. Experimental performance is due to the low efficiency of DAWTs and prior hypotheses exhibit high predictive error and continuity violations. A fluid model that accurately predicts available data and new experimental data, showing disk specific maximum C_P for the confined channel at 38% of power available to disk, is presented. This is significantly lower than the 59% Betz freestream limit yielded by hypothetical models when the area ratio equals one. Experiments and their results are presented with non-DAWT accelerators, where new experimental results exceed C_P limits predicted previously and correlate with the proposed predictive model. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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13 pages, 3461 KiB

Open AccessArticle

Unsteady Numerical Calculation of Oblique Submerged Jet

by Weixuan Jiao, Di Zhang, Chuan Wang, Li Cheng and Tao Wang

Energies 2020, 13(18), 4728; https://doi.org/10.3390/en13184728 - 11 Sep 2020

Cited by 4 | Viewed by 2284

Abstract

A water jet is a kind of high-speed dynamic fluid with high energy, which is widely used in the engineering field. In order to analyze the characteristics of the flow field and the change of law of the bottom impact pressure of the [...] Read more.

A water jet is a kind of high-speed dynamic fluid with high energy, which is widely used in the engineering field. In order to analyze the characteristics of the flow field and the change of law of the bottom impact pressure of the oblique submerged impinging jet at different times, its unsteady characteristics at different Reynolds numbers were studied by using the Wray–Agarwal (W-A) turbulence model. It can be seen from the results that in the process of jet movement, the pressure at the peak of velocity on the axis was the smallest, and the velocity, flow angle, and pressure distribution remain unchanged after a certain time. In the free jet region, the velocity, flow angle, and pressure remained unchanged. In the impingement region, the velocity and flow angle decreased rapidly, while the pressure increased rapidly. The maximum pressure coefficient of the impingement plate changed with time and was affected by the Reynolds number, but the distribution trend remained the same. In this paper, the characteristics of the flow field and the law of the impact pressure changing with time are described. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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20 pages, 9972 KiB

Open AccessArticle

Numerical Investigation of the Turbulent Wake-Boundary Interaction in a Translational Cascade of Airfoils and Flat Plate

by Xiaodong Ruan, Xu Zhang, Pengfei Wang, Jiaming Wang and Zhongbin Xu

Energies 2020, 13(17), 4478; https://doi.org/10.3390/en13174478 - 31 Aug 2020

Viewed by 2307

Abstract

Rotor stator interaction (RSI) is an important phenomenon influencing performances in the pump, turbine, and compressor. In this paper, the correlation-based transition model is used to study the RSI phenomenon between a translational cascade of airfoils and a flat plat. A comparison was [...] Read more.

Rotor stator interaction (RSI) is an important phenomenon influencing performances in the pump, turbine, and compressor. In this paper, the correlation-based transition model is used to study the RSI phenomenon between a translational cascade of airfoils and a flat plat. A comparison was made between computational results and experimental results. The computational boundary layer velocity is in reasonable agreement with the experimental velocity. The thickness of boundary layer decreases as the RSI frequency increases and it increases as the fluid flows downstream. The spectral plots of velocity fluctuations at leading edge x/c = 2 under RSI partial flow condition f = 20 Hz and f = 30 Hz are dominated by a narrowband component. RSI frequency mainly affects the turbulence intensity in the freestream region. However, it has little influence on the turbulence intensity of boundary layer near the wall. A secondary vortex is induced by the wake–boundary layer interaction and it leads to the formation of a thickened laminar boundary layer. The negative-vorticity wake also facilitates the formation of a thickened boundary layer while the positive-vorticity wake has a similar effect, like a calmed region which makes the boundary layer thinner. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 5453 KiB

Open AccessArticle

An Experimental Study on the Fire Spread Rate and Separation Distance between Facing Stores in Passage-Type Traditional Markets

by Hong-Seok Yun, Dong-Gun Nam and Cheol-Hong Hwang

Energies 2020, 13(17), 4458; https://doi.org/10.3390/en13174458 - 28 Aug 2020

Cited by 2 | Viewed by 3631

Abstract

Real-scale fire experiments were conducted to understand the fire spread characteristics of the major combustibles handled in traditional markets, a space with high fire risk. The major combustibles were selected through field surveys administered at a number of traditional markets. Through real-scale fire [...] Read more.

Real-scale fire experiments were conducted to understand the fire spread characteristics of the major combustibles handled in traditional markets, a space with high fire risk. The major combustibles were selected through field surveys administered at a number of traditional markets. Through real-scale fire experiments, the horizontal fire spread rate according to the maximum heat release rate of major combustibles was examined. In addition, the separation distance to prevent fire spread to the facing store by radiant heat transfer was examined. As a result of the experiments, it was confirmed that the arrangement method of the combustibles causes a large change in the maximum heat release rate, fire growth rate, and fire spread rate. The horizontal fire spread rate showed a linear proportional relationship with respect to the maximum heat release rate regardless of the type of combustibles, and a correlation to define the relationship was proposed. A correlation equation for predicting the separation distance that can prevent fire spread by radiant heat transfer was proposed, and the curve by the correlation equation was in good agreement with the experimental results. Through this study, it is expected that the correlation proposed to examine the horizontal fire spread rate and the separation distance of major combustibles in a traditional market can be usefully used in the design of fire protection systems to reduce fire damage in the traditional market. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 3342 KiB

Open AccessArticle

Multiphysics CFD Simulation for Design and Analysis of Thermoelectric Power Generation

by Olle Högblom and Ronnie Andersson

Energies 2020, 13(17), 4344; https://doi.org/10.3390/en13174344 - 22 Aug 2020

Cited by 8 | Viewed by 4030

Abstract

The multiphysics simulation methodology presented in this paper permits extension of computational fluid dynamics (CFD) simulations to account for electric power generation and its effect on the energy transport, the Seebeck voltage, the electrical currents in thermoelectric systems. The energy transport through Fourier, [...] Read more.

The multiphysics simulation methodology presented in this paper permits extension of computational fluid dynamics (CFD) simulations to account for electric power generation and its effect on the energy transport, the Seebeck voltage, the electrical currents in thermoelectric systems. The energy transport through Fourier, Peltier, Thomson and Joule mechanisms as a function of temperature and electrical current, and the electrical connection between thermoelectric modules, is modeled using subgrid CFD models which make the approach computational efficient and generic. This also provides a solution to the scale separation problem that arise in CFD analysis of thermoelectric heat exchangers and allows the thermoelectric models to be fully coupled with the energy transport in the CFD analysis. Model validation includes measurement of the relevant fluid dynamic properties (pressure and temperature distribution) and electric properties (current and voltage) for a turbulent flow inside a thermoelectric heat exchanger designed for automotive applications. Predictions of pressure and temperature drop in the system are accurate and the error in predicted current and voltage is less than 1.5% at all exhaust gas flow rates and temperatures studied which is considered very good. Simulation results confirm high computational efficiency and stable simulations with low increase in computational time compared to standard CFD heat-transfer simulations. Analysis of the results also reveals that even at the lowest heat transfer rate studied it is required to use a full two way coupling in the energy transport to accurately predict the electric power generation. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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18 pages, 5291 KiB

Open AccessArticle

Wind Tunnel Study on Wake Instability of Twin H-Rotor Vertical-Axis Turbines

by Kun Wang, Li Zou, Aimin Wang, Peidong Zhao and Yichen Jiang

Energies 2020, 13(17), 4310; https://doi.org/10.3390/en13174310 - 20 Aug 2020

Cited by 3 | Viewed by 2213

Abstract

In recent years, the H-rotor vertical-axis turbine has attracted considerable attention in the field of wind and tidal power generation. After a series of complex spatiotemporal evolutions, the vortex shed from turbine blades forms a turbulent wake with a multi-scale coherent structure. An [...] Read more.

In recent years, the H-rotor vertical-axis turbine has attracted considerable attention in the field of wind and tidal power generation. After a series of complex spatiotemporal evolutions, the vortex shed from turbine blades forms a turbulent wake with a multi-scale coherent structure. An analysis of the wake characteristics of twin turbines forms the basis of array optimisation. This study aimed to examine the instability characteristics of a twin-turbine wake with two rotational configurations. The dynamic evolution characteristics of coherent structures with different scales in the wake were analysed via wavelet analysis. The results show that an inverse energy cascade process occurs after the high-frequency small-scale coherent structures induced by rotation lose their coherence. This self-organising characteristic is more apparent in the quasi two-dimensional wake of a forward-moving counter-rotating turbine (Array 1) than in that of a backward-moving counter-rotating turbine (Array 2). With greater organisation and coherence, the wake of Array 1 exhibits low-frequency instability characteristics dominated by a large-scale coherent structure. In addition, the signals reconstructed using wavelet transform show that asymmetric modes exist between low-frequency large-scale coherent structures. The experimental results provide a new perspective on the instability mechanism of twin-turbine wakes, as well as important data for numerical modelling. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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10 pages, 3062 KiB

Open AccessArticle

Modification of Interaction Forces between Smoke and Evacuees

by Sungryong Bae, Jun-Ho Choi and Hong Sun Ryou

Energies 2020, 13(16), 4177; https://doi.org/10.3390/en13164177 - 13 Aug 2020

Cited by 5 | Viewed by 2076

Abstract

The most used fire effect models on evacuees are only focused on the physical capacity of the evacuees. However, some of the evacuees in a fire situation continuously move through the familiar route, although the familiar route is smoke-filled and they know that [...] Read more.

The most used fire effect models on evacuees are only focused on the physical capacity of the evacuees. However, some of the evacuees in a fire situation continuously move through the familiar route, although the familiar route is smoke-filled and they know that they are moving towards the fire source. Thus, the additional evacuation models are required for considering the behavioral changes due to the psychological pressure when the evacuees are moving through the smoke or towards the fire source. In this study, the inner smoke region force is modified to improve the accuracy and practicality of the BR-smoke model by varying the walking speed according to the smoke density. Additionally, the BR-smoke model is applied to FDS+Evac to compare the simulation results of the modified BR-smoke model with those of existing models. Based on the results, the evacuation characteristics inside the smoke region can be improved by using the modified BR-smoke model because the evacuees are continuously influenced by the modified inner smoke force inside the smoke region. However, additional studies for determining more reliable evacuee psychological factors are required to improve the reality of the modified BR-smoke model. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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11 pages, 1692 KiB

Open AccessArticle

Comparative Study of CFD and LedaFlow Models for Riser-Induced Slug Flow

by Rasmus Thy Jørgensen, Gunvor Rossen Tonnesen, Matthias Mandø and Simon Pedersen

Energies 2020, 13(14), 3733; https://doi.org/10.3390/en13143733 - 20 Jul 2020

Cited by 2 | Viewed by 2907

Abstract

The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas [...] Read more.

The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas industry to consider making the switch from LedaFlow to CFD. Presently, the industry use relatively simple 1D-models, such as LedaFlow, to predict flow patterns in pipelines. The reduction in cost of computational power in recent years have made it relevant to compare the performance of these codes with high fidelity CFD simulations. A laboratory test facility was used to obtain data for pressure and mass flow rates for the two-phase flow of air and water. A benchmark case of slug flow served for evaluation of the numerical models. A 3D unsteady CFD simulation was performed based on Reynolds-Averaged Navier-Stokes (RANS) formulation and the Volume of Fluid (VOF) model using the open-source CFD code OpenFOAM. Unsteady simulations using the commercial 1D LedaFlow solver were performed using the same boundary conditions and fluid properties as the CFD simulation. Both the CFD and LedaFlow model underpredicted the experimentally determined slug frequency by 22% and 16% respectively. Both models predicted a classical blowout, in which the riser is completely evacuated of water, while only a partial evacuation of the riser was observed experimentally. The CFD model had a runtime of 57 h while the LedaFlow model had a runtime of 13 min. It can be concluded that the prediction capabilities of the CFD and LedaFlow models are similar for riser-induced slug flow while the CFD model is much more computational intensive. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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11 pages, 3074 KiB

Open AccessArticle

Energy Efficiency of Pneumatic Cylinder Control with Different Levels of Compressed Air Pressure and Clamping Cartridge

by Vladislav Blagojevic, Dragan Seslija, Slobodan Dudic and Sasa Randjelovic

Energies 2020, 13(14), 3711; https://doi.org/10.3390/en13143711 - 19 Jul 2020

Cited by 13 | Viewed by 3182

Abstract

Since pneumatic systems are widely used in various branches of industry, the need to find ways to reduce energy consumption in these systems has become very pressing. The reduction in energy consumption in these systems is reflected in the reduction of compressed air [...] Read more.

Since pneumatic systems are widely used in various branches of industry, the need to find ways to reduce energy consumption in these systems has become very pressing. The reduction in energy consumption in these systems is reflected in the reduction of compressed air consumption. The paper presents a cylinder control system with a piston rod on one side, in which the reduction in energy consumption is ensured by using different levels of supply pressure in the working and the return stroke, and by holding the cylinder piston rod in its final positions with a clamping cartridge. Clamping and holding the piston rod in its final position further affects the reduction in energy consumption. Experimental data show that the application of the proposed control leads to a decrease in compressed air consumption of 25.54% to 32.97%, depending on the compressed air pressure used in the return stroke. The cost-effectiveness of the proposed cylinder control with different levels of compressed air pressure and holding the final position by clamping cartridge is presented. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 7057 KiB

Open AccessArticle

Analysis on the Fire Growth Rate Index Considering of Scale Factor, Volume Fraction, and Ignition Heat Source for Polyethylene Foam Pipe Insulation

by Jung Wook Park, Ohk Kun Lim and Woo Jun You

Energies 2020, 13(14), 3644; https://doi.org/10.3390/en13143644 - 15 Jul 2020

Cited by 12 | Viewed by 44725

Abstract

The fire growth rate index (FIGRA), which is the ratio of the maximum value of the heat release rate (Q_max) and the time (t_max) to reach the maximum heat release rate, is a general method [...] Read more.

The fire growth rate index (FIGRA), which is the ratio of the maximum value of the heat release rate (Q_max) and the time (t_max) to reach the maximum heat release rate, is a general method to evaluate a material in the fire-retardant performance in fire technology. The object of this study aims to predict FIGRA of the polyethylene foam pipe insulation in accordance with the scale factor (S_f), the volume fraction of the pipe insulation (VF) and the ignition heat source (Q_ig). The compartments made of fireboard have been mock-up with 1/3, 1/4, and 1/5 reduced scales of the compartment as specified in ISO 20632. The heat release rate data of the pipe insulation with the variation of S_f, VF, and Q_ig are measured from 33 experiments to correlate with FIGRA. Based on a critical analysis of the heat transfer phenomenon from previous research literature, the predictions of Q_max and t_max are presented. It is noticeable that the fire-retardant grade of the polyethylene foam pipe insulation could have Grade B, C, and D in accordance with the test conditions within ±15% deviation of the predicted FIGRA. In case of establishing the database of various types of insulation, the prediction models could apply to evaluate the fire-retardant performance. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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13 pages, 4744 KiB

Open AccessArticle

A Numerical Analysis of the Fire Characteristics after Sprinkler Activation in the Compartment Fire

by Ho Trong Khoat, Ji Tea Kim, Tran Dang Quoc, Ji Hyun Kwark and Hong Sun Ryou

Energies 2020, 13(12), 3099; https://doi.org/10.3390/en13123099 - 15 Jun 2020

Cited by 7 | Viewed by 5549

Abstract

Understanding fire characteristics under sprinkler spray is valuable for performance-based safety design. However, fire characteristics during fire suppression by sprinkler spray has seldom been studied in detail. In order to present a fire suppression model by sprinkler spray and determine the fire characteristics [...] Read more.

Understanding fire characteristics under sprinkler spray is valuable for performance-based safety design. However, fire characteristics during fire suppression by sprinkler spray has seldom been studied in detail. In order to present a fire suppression model by sprinkler spray and determine the fire characteristics after sprinkler activation in a compartment, a numerical analysis was conducted using a fire dynamics simulator (FDS). A simple fire suppression model by sprinkler spray was calibrated by comparing ceiling temperatures from experimental data. An extinguishing coefficient of 3.0 was shown to be suitable for the fire suppression model. The effect of sprinkler spray on the smoke layer during fire suppression was explained, revealing a smoke logging phenomenon. In addition, the smoke, which spread under the influence of the sprinkler spray, was also investigated. The temperature, velocity, and mass flow rate of the smoke layer through the doorway was significantly reduced during fire suppression compared to a free burn case. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 4593 KiB

Open AccessArticle

Analysis of the Thermal Characteristics of Welding Spatters in SMAW Using Simplified Model in Fire Technology

by Yeon Je Shin and Woo Jun You

Energies 2020, 13(9), 2266; https://doi.org/10.3390/en13092266 - 4 May 2020

Cited by 5 | Viewed by 3904

Abstract

In this study, the thermal characteristics of welding spatters were analyzed to predict the risk of fire spread in the shield metal arc welding. The mean diameters and the distribution shapes of the particles were investigated with the variation of the distribution coefficients [...] Read more.

In this study, the thermal characteristics of welding spatters were analyzed to predict the risk of fire spread in the shield metal arc welding. The mean diameters and the distribution shapes of the particles were investigated with the variation of the distribution coefficients using the modified gamma distribution function. To quantify the heat source of the welding spatters, the previous empirical equation for the heat transfer coefficient of Ranz and Marshall was analyzed regarding the particle velocity and surface temperature. The order of magnitude for the convection and the radiation were as performed to the particles and the base metal, respectively. The results, which are only valid for T_p,m = 750 K and T_b,s = 2300 K, show that the radiation term is only 10% for a particle but the convective term is only 6% for the base metal. Finally, the simplified model for the temperature of the welding spatters was obtained and the validation results were within ±13%. The variations of electrical power, droplet size, number of particles, and surface temperature were systematically analyzed with the prediction model. The importance of safety conditions to prevent the fires spread by welding spatter was suggested with electrical power, particle velocity, and numerical density of the particles. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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25 pages, 18583 KiB

Open AccessArticle

Optimized Design of a Swirler for a Combustion Chamber of Non-Premixed Flame Using Genetic Algorithms

by Daniel Alejandro Zavaleta-Luna, Marco Osvaldo Vigueras-Zúñiga, Agustín L. Herrera-May, Sergio Aurelio Zamora-Castro and María Elena Tejeda-del-Cueto

Energies 2020, 13(9), 2240; https://doi.org/10.3390/en13092240 - 3 May 2020

Cited by 11 | Viewed by 9852

Abstract

Recirculation in a combustion chamber is required for stabilizing the flame and reducing pollutants. The swirlers can generate recirculation in a combustion chamber, inducing a swirling flow that breaks vorticity and improves the mixing of air and fuel. The swirl number (S [...] Read more.

Recirculation in a combustion chamber is required for stabilizing the flame and reducing pollutants. The swirlers can generate recirculation in a combustion chamber, inducing a swirling flow that breaks vorticity and improves the mixing of air and fuel. The swirl number (S_n) is related to the formation of recirculation in conditions of high-intensity flows with S_n > 0.6. Thus, the optimized design of a swirler is necessary to generate enough turbulence that keeps the flame stable. We present the optimized design of a swirler considering the main parameters for a non-premixed combustion chamber. This optimization is made with genetic algorithms to ensure the generation of a recirculation zone in the combustion chamber. This recirculation phenomenon is simulated using computational fluid dynamics (CFD) models and applying the renormalization group (RNG) k-ε turbulence method. The chemistry is parameterized as a function of the mixture fraction and dissipation rate. A CFD comparison of a baseline swirler model and the proposed optimized swirler model shows that a recirculation zone with high intensity and longer length is generated in the primary zone of the combustion chamber when the optimized model is used. Furthermore, the CFD models depict swirling effects in the turbulent non-premixed flame, in which the stabilization is sensitive to the recirculation zone. The temperature results obtained with the CFD models agree well with the experimental results. The proposed design can help designers enhance the performance of combustion chambers and decrease the generation of CO and NO_x. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 5785 KiB

Open AccessArticle

Numerical Study of the Normal Impinging Water Jet at Different Impinging Height, Based on Wray–Agarwal Turbulence Model

by Hongliang Wang, Zhongdong Qian, Di Zhang, Tao Wang and Chuan Wang

Energies 2020, 13(7), 1744; https://doi.org/10.3390/en13071744 - 5 Apr 2020

Cited by 34 | Viewed by 3223

Abstract

As a kind of water jet technology with strong impinging force and simple structure, the submerged impinging water jet can produce strong scouring action on subaqueous sediments. In order to investigate the flow field characteristics and impinging pressure of submerged impinging water jets [...] Read more.

As a kind of water jet technology with strong impinging force and simple structure, the submerged impinging water jet can produce strong scouring action on subaqueous sediments. In order to investigate the flow field characteristics and impinging pressure of submerged impinging water jets at different impinging heights, the Wray-Agarwal (W-A) turbulence model is used for calculation. The velocity distribution and flow field structure at different impinging heights (1 ≤ H/D ≤ 8), and the impinging pressure distribution at the impingement plate under different Reynolds numbers (11, 700 ≤ Re ≤ 35100) are studied. The results show that with the increase of the impinging height, the diffusion degree increases and the velocity decreases gradually when the jet reaches the impingement region. The fluid accelerates first and then decelerates near the stagnation point. The maximum impinging pressure and the impinging pressure coefficient decrease with the increase of the impinging height, but the effective impinging pressure range remains unchanged. In this paper, the distribution characteristics of the impinging pressure in the region of the impingement plate at different heights are clarified, which provides theoretical support for the prediction method of the impinging pressure. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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15 pages, 6407 KiB

Open AccessArticle

Numerical Investigation of Vertical Crossflow Jets with Various Orifice Shapes Discharged in Rectangular Open Channel

by Hao Yuan, Ruichang Hu, Xiaoming Xu, Liang Chen, Yongqin Peng and Jiawan Tan

Energies 2020, 13(6), 1505; https://doi.org/10.3390/en13061505 - 22 Mar 2020

Cited by 1 | Viewed by 2663

Abstract

Vertical jet in flowing water is a common phenomenon in daily life. To study the flow and turbulent characteristics of different jet orifice shapes and under different velocity ratios, the realizable k-ε turbulent model was adopted to analyze the three-dimensional (3D) [...] Read more.

Vertical jet in flowing water is a common phenomenon in daily life. To study the flow and turbulent characteristics of different jet orifice shapes and under different velocity ratios, the realizable k-ε turbulent model was adopted to analyze the three-dimensional (3D) flow, turbulence, and vortex characteristics using circular, square, and rectangular jet orifices and velocity ratios of 2, 5, 10, and 15. The following conclusions were drawn: The flow trajectory of the vertical jet in the channel exhibits remarkable 3D characteristics, and the jet orifice and velocity ratio have a significant influence on the flow characteristics of the channel. The heights at which the spiral deflection and maximum turbulent kinetic energy (TKE) occur for the circular jet are the smallest, while those for square jets are the largest. As the shape of the jet orifice changes from a circle to a square and then to a rectangle, the shape formed by the plane of the kidney vortices and the region above it gradually changes from a circle to a pentagon. With the increase in the velocity ratio, the 3D characteristics, maximum TKE, and kidney vortex coverage of the flow all gradually increase. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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16 pages, 6353 KiB

Open AccessArticle

Numerical Study of the Effects of the Jet Fan Speed, Heat Release Rate and Aspect Ratio on Smoke Movement in Tunnel Fires

by Ha Thien Khieu, Young Man Lee, Ji Tae Kim and Hong Sun Ryou

Energies 2020, 13(5), 1206; https://doi.org/10.3390/en13051206 - 5 Mar 2020

Cited by 10 | Viewed by 3433

Abstract

In this study, the effects of the jet fan speed, heat release rate and aspect ratio on smoke movement in tunnel fires have been investigated. The jet fan speed was changed from 6.25 (25%) to 12.5 m/s (50%), 18.75 m/s (75%), and 25 [...] Read more.

In this study, the effects of the jet fan speed, heat release rate and aspect ratio on smoke movement in tunnel fires have been investigated. The jet fan speed was changed from 6.25 (25%) to 12.5 m/s (50%), 18.75 m/s (75%), and 25 m/s (100%). The heat release rate was set up from 3.9 to 6 MW and 16 MW, the aspect ratio was changed from 0.6 to 1 and 1.5, respectively. The lower the jet fan speed is, the longer the smoke back-layering length is. With a higher velocity, the smoke tends to move out of the tunnel quickly; however, smoke stratification also occurs, and this reduces visibility. This could make it difficult for people to evacuate. With a higher heat release rate, the smoke tends to move far away from the fires quickly when compared with other cases. Additionally, the higher the heat release is, the longer the smoke back-layering is. Finally, with a higher aspect ratio, the smoke back layering length in the tunnel is also longer. The smoke layer thickness is also larger than in other cases. The correlation of velocity, heat release rate and aspect ratio has been investigated to avoid the smoke back layer length in tunnel fires. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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19 pages, 3401 KiB

Open AccessArticle

Numerical Simulation of Coherent Structures in the Turbulent Boundary Layer under Different Stability Conditions

by Shujin Laima, Hehe Ren, Hui Li and Jinping Ou

Energies 2020, 13(5), 1068; https://doi.org/10.3390/en13051068 - 1 Mar 2020

Cited by 2 | Viewed by 2379

Abstract

Coherent structures in the turbulent boundary layer were investigated under different stability conditions. Qualitative analyses of the flow field, spatial correlation coefficient field and pre-multiplied wind velocity spectrum showed that the dominant turbulent eddy structure changed from small-scale motions to large- and very-large-scale [...] Read more.

Coherent structures in the turbulent boundary layer were investigated under different stability conditions. Qualitative analyses of the flow field, spatial correlation coefficient field and pre-multiplied wind velocity spectrum showed that the dominant turbulent eddy structure changed from small-scale motions to large- and very-large-scale motions and then to thermal plumes as the stability changed from strong stable to neutral and then to strong unstable. A quantitative analysis of the size characteristics of the three-dimensional turbulent eddy structure based on the spatial correlation coefficient field showed that under near-neutral stability, the streamwise, wall-normal and spanwise extents remained constant at approximately 0.3

δ

, 0.1

δ

and 0.2

δ

(

δ

, boundary layer height), respectively, while for other conditions, the extent in each direction varied in a log-linear manner with stability; only the spanwise extent under stable conditions was also independent of stability. The peak wavenumber of the pre-multiplied wind velocity spectrum moves towards small values from stable conditions to neutral condition and then to unstable conditions; thus, for the wind velocity spectrum, another form is needed that takes account the effects of the stability condition. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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12 pages, 3095 KiB

Open AccessArticle

Surrogate Model with a Deep Neural Network to Evaluate Gas–Liquid Flow in a Horizontal Pipe

by Yongho Seong, Changhyup Park, Jinho Choi and Ilsik Jang

Energies 2020, 13(4), 968; https://doi.org/10.3390/en13040968 - 21 Feb 2020

Cited by 18 | Viewed by 2755

Abstract

This study developed a data-driven surrogate model based on a deep neural network (DNN) to evaluate gas–liquid multiphase flow occurring in horizontal pipes. It estimated the liquid holdup and pressure gradient under a slip condition and different flow patterns, i.e., slug, annular, stratified [...] Read more.

This study developed a data-driven surrogate model based on a deep neural network (DNN) to evaluate gas–liquid multiphase flow occurring in horizontal pipes. It estimated the liquid holdup and pressure gradient under a slip condition and different flow patterns, i.e., slug, annular, stratified flow, etc. The inputs of the surrogate modelling were related to the fluid properties and the dynamic data, e.g., superficial velocities at the inlet, while the outputs were the liquid holdup and pressure gradient observed at the outlet. The case study determined the optimal number of hidden neurons by considering the processing time and the validation error. A total of 350 experimental data were used: 279 for supervised training, 31 for validating the training performance, and 40 unknown data, not used in training and validation, were examined to forecast the liquid holdup and pressure gradient. The liquid holdups were estimated within less than 8.08% of the mean absolute percentage error, while the error of the pressure gradient was 23.76%. The R² values confirmed the reliability of the developed model, showing 0.89 for liquid holdups and 0.98 for pressure gradients. The DNN-based surrogate model can be applicable to estimate liquid holdup and pressure gradients in a more realistic manner with a small amount of computating resources. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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26 pages, 7558 KiB

Open AccessArticle

Numerical Investigation of Very-Large-Scale Motions in a Turbulent Boundary Layer for Different Roughness

by Hehe Ren, Shujin Laima and Hui Li

Energies 2020, 13(3), 659; https://doi.org/10.3390/en13030659 - 4 Feb 2020

Cited by 4 | Viewed by 2411

Abstract

Wall-model large eddy simulations (WMLES) are conducted to investigate the spatial features of large-scale and very-large-scale motions (LSMs and VLSMs) in turbulent boundary flow in different surface roughnesses at a very high Reynolds number, O (10⁶–10⁷). The results of [...] Read more.

Wall-model large eddy simulations (WMLES) are conducted to investigate the spatial features of large-scale and very-large-scale motions (LSMs and VLSMs) in turbulent boundary flow in different surface roughnesses at a very high Reynolds number, O (10⁶–10⁷). The results of the simulation of nearly smooth cases display good agreement with field observations and experimental data, both dimensioned using inner and outer variables. Using pre-multiplied spectral analysis, the size of VLSMs can be reduced or even disappear with increasing roughness, which indirectly supports the concept that the bottom-up mechanism is one of the origins of VLSMs. With increases in height, the power of pre-multiplied spectra at both high and low wavenumber regions decreases, which is consistent with most observational and experimental results. Furthermore, we find that the change in the spectrum scaling law from −1 to −5/3 is a gradual process. Due to the limitations of the computational domain and coarse grid that were adopted, some VLSMs and small-scale turbulence are truncated. However, the size of LSMs is fully accounted for. From the perspective of the spatial correlation of the flow field, the structural characteristics of VLSMs under various surface roughnesses, including three-dimensional length scales and inclination angles, are obtained intuitively, and the conclusions are found to be in good agreement with the velocity spectra. Finally, the generation, development and extinction of three-dimensional VLSMs are analyzed by instantaneous flow and vorticity field, and it shows that the instantaneous flow field gives evidence of low-speed streamwise-elongated flow structures with negative streamwise velocity fluctuation component, and which are flanked on each side by similarly high-speed streamwise-elongated flow structures. Moreover, each of the low-speed streamwise-elongated flow structure lies beneath many vortices. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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18 pages, 11119 KiB

Open AccessArticle

An Experimental Study on the Jet Pressure Performance of Organ–Helmholtz (O-H), Self-Excited Oscillating Nozzles

by Xiaochuan Wang, Yueqin Li, Yi Hu, Xiaolong Ding, Meijing Xiang and Deng Li

Energies 2020, 13(2), 367; https://doi.org/10.3390/en13020367 - 12 Jan 2020

Cited by 12 | Viewed by 3182

Abstract

To make a better application of the self-excited oscillation jet, a compound organ–Helmholtz oscillation nozzle (OH nozzle) was designed to generate better pulse effects in the present study. The effects of geometric parameters on pressure characteristics of self-excited oscillation jets were investigated experimentally. [...] Read more.

To make a better application of the self-excited oscillation jet, a compound organ–Helmholtz oscillation nozzle (OH nozzle) was designed to generate better pulse effects in the present study. The effects of geometric parameters on pressure characteristics of self-excited oscillation jets were investigated experimentally. The geometric parameters of OH nozzles were determined based on the design principle of the organ-pipe and Helmholtz nozzles. Various types of OH nozzles were tested to obtain the relationship between oscillation pressure and geometric parameters. Experimental results showed that some structural parameters are sensitive to the pump pressure, while others are not. The optimum geometric optimum parameters were obtained based on experimental results. The peak pressure and pressure pulsation amplitude are closely related to the target distance. The peak pressure decreases sharply with target distance at first and decreases with a further increase of the target distance. The peak pressure becomes stable beyond a certain range of target distance. With the increase of the target distance, the pressure pulsation amplitude increases at first and then decreases. The optimum target distance was determined based on experimental results to achieve better pulsation performance and has a value of 20 mm in the present study. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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11 pages, 4849 KiB

Open AccessArticle

Investigation of Dilution Effect on CH₄/Air Premixed Turbulent Flame Using OH and CH₂O Planar Laser-Induced Fluorescence

by Li Yang, Wubin Weng, Yanqun Zhu, Yong He, Zhihua Wang and Zhongshan Li

Energies 2020, 13(2), 325; https://doi.org/10.3390/en13020325 - 9 Jan 2020

Cited by 2 | Viewed by 3097

Abstract

Diluting the combustion mixtures is one of the advanced approaches to reduce the NO_x emission of methane/air premixed turbulent flame, especially with high diluents to create a distributed reaction zone and mild combustion, which can lower the temperature of reaction zone and [...] Read more.

Diluting the combustion mixtures is one of the advanced approaches to reduce the NO_x emission of methane/air premixed turbulent flame, especially with high diluents to create a distributed reaction zone and mild combustion, which can lower the temperature of reaction zone and reduce the formation of NO_x. The effect of N₂/CO₂ dilution on the combustion characteristics of methane/air premixed turbulent flame with different dilution ratio and different exit Reynolds number was conducted by OH-PLIF and CH₂O-PLIF. Results show that the increase of dilution ratio can sharply reduce the concentration of OH and CH₂O, and postpone the burning of fuel. Compared with the ultra-lean combustion, the dilution weakens the combustion more obviously. For different dilution gases, the concentration of OH in the combustion zone varies greatly, while the concentration of CH₂O in the unburned zone is less affected by different dilution gas. The CO₂ dilution has a more significant effect on OH concentration than N₂ with the given dilution ratio, but a similar effect on the concentration of CH₂O in the preheat zone of flame. However, dilution does not have much influence on the flame structure with the given turbulent intensity. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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21 pages, 8416 KiB

Open AccessArticle

Optimization Design of the Impeller Based on Orthogonal Test in an Ultra-Low Specific Speed Magnetic Drive Pump

by Fei Zhao, Fanyu Kong, Yisong Zhou, Bin Xia and Yuxing Bai

Energies 2019, 12(24), 4767; https://doi.org/10.3390/en12244767 - 13 Dec 2019

Cited by 10 | Viewed by 2917

Abstract

To improve the hydraulic performance in an ultra-low specific speed magnetic drive pump, optimized design of impeller based on orthogonal test was carried out. Blades number Z, bias angle in peripheral direction of splitter blades θ_s, inlet diameter of splitter [...] Read more.

To improve the hydraulic performance in an ultra-low specific speed magnetic drive pump, optimized design of impeller based on orthogonal test was carried out. Blades number Z, bias angle in peripheral direction of splitter blades θ_s, inlet diameter of splitter blades D_si, and deflection angle of splitter blades α were selected as the main factors in orthogonal test. The credibility of the numerical simulation was verified by prototype experiments. Two optimized impellers were designed through the analysis of orthogonal test data. The internal flow field, pressure fluctuation, and radial force were analyzed and compared between optimized impellers and original impeller. The results reveal that impeller 7 (Z = 5, θ_s = 0.4θ, D_si = 0.75D₂, α = 0°) could increase the head and efficiency, compared to the original impeller, by 2.68% and 4.82%, respectively. Impeller 10 (Z = 5, θ_s = 0.4θ, D_si = 0.55D₂, α = 0°) reduced the head by 0.33% and increased the efficiency by 8.24%. At design flow rate condition, the internal flow of impeller 10 was the most stable. Peak-to-peak values of pressure fluctuation at the volute tongues of impeller 7 and impeller 10 were smaller than those of the original impeller at different flow rate conditions (0.6 Q_d, 1.0 Q_d and 1.5 Q_d). Radial force distribution of impeller 10 was the most uniform, and the radial force variance of impeller 10 was the smallest. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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11 pages, 3673 KiB

Open AccessArticle

Energy Dissipation in Stepped Spillways with Different Horizontal Face Angles

by Yongqin Peng, Xujin Zhang, Hao Yuan, Xia Li, Chunhang Xie, Shuqing Yang and Zhaoliang Bai

Energies 2019, 12(23), 4469; https://doi.org/10.3390/en12234469 - 23 Nov 2019

Cited by 7 | Viewed by 2593

Abstract

Energy dissipation is one of the most important factors in choosing stepped spillways. However, very few studies have investigated energy dissipation with different horizontal face angles. In this paper, the realizable k-ε turbulent model was used to study the flow field, [...] Read more.

Energy dissipation is one of the most important factors in choosing stepped spillways. However, very few studies have investigated energy dissipation with different horizontal face angles. In this paper, the realizable k-ε turbulent model was used to study the flow field, energy dissipation rates and turbulent kinetic energy and its dissipation rate for different stepped spillways with five horizontal face angles in the skimming flow regions. Results showed that the field and direction of the flow were changed by the horizontal face angles of the stepped spillway, which produced some unique characteristics and thus caused better energy dissipation. The fluctuation of free water surface will be larger with increasing horizontal face angles and the energy dissipation rate decreases with an increasing unit discharge and increases for the enlargement of the horizontal face angles. This conclusion could provide a reference for the relevant research of V shaped stepped spillways. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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22 pages, 7211 KiB

Open AccessArticle

Radiation Heat Transfer in a Complex Geometry Containing Anisotropically-Scattering Mie Particles

by Ali Ettaleb, Mohamed Ammar Abbassi, Habib Farhat, Kamel Guedri, Ahmed Omri, Mohamed Naceur Borjini, Marjan Goodarzi and M. M. Sarafraz

Energies 2019, 12(20), 3986; https://doi.org/10.3390/en12203986 - 19 Oct 2019

Cited by 5 | Viewed by 2808

Abstract

This study aims to numerically investigate the radiation heat transfer in a complex, 3-D biomass pyrolysis reactor which is consisted of two pyrolysis chambers and a heat recuperator. The medium assumes to be gray, absorbs, emits, and Mie-anisotropically scatters the radiation energy. The [...] Read more.

This study aims to numerically investigate the radiation heat transfer in a complex, 3-D biomass pyrolysis reactor which is consisted of two pyrolysis chambers and a heat recuperator. The medium assumes to be gray, absorbs, emits, and Mie-anisotropically scatters the radiation energy. The finite volume method (FVM) is applied to solve the radiation transfer equation (RTE) using the step scheme. To treat the complex geometry, the blocked-off-region procedure is employed. Mie equations (ME) are applied to evaluate the scattering phase function and analyze the angular distribution of the anisotropically scattered radiation by particles. In this study, three different states are considered to test the anisotropic scattering impacts on the temperature and radiation heat flux distribution. These states are as: (i) Isotropic scattering, (ii) forward and backward scattering and (iii) scattering with solid particles of different coals and fly ash. The outcomes demonstrate that the radiation heat flux enhances by an increment of the albedo and absorption coefficients for the coals and fly ash, unlike the isotropic case and the forward and backward scattering functions. Moreover, the particle size parameter does not have an important influence on the radiation heat flux, when the medium is thin optical. Its effect is more noticeable for higher extinction coefficients. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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18 pages, 8296 KiB

Open AccessArticle

Turbulent Flow Fields Over a 3D Hill Covered by Vegetation Canopy Through Large Eddy Simulations

by Zhenqing Liu, Yiran Hu, Yichen Fan, Wei Wang and Qingsong Zhou

Energies 2019, 12(19), 3624; https://doi.org/10.3390/en12193624 - 23 Sep 2019

Cited by 7 | Viewed by 3033

Abstract

The flow fields over a simplified 3D hill covered by vegetation have been examined by many researchers. However, there is scarce research giving the three-dimensional characteristics of the flow fields over a rough 3D hill. In this study, large eddy simulations were performed [...] Read more.

The flow fields over a simplified 3D hill covered by vegetation have been examined by many researchers. However, there is scarce research giving the three-dimensional characteristics of the flow fields over a rough 3D hill. In this study, large eddy simulations were performed to examine the coherent turbulence structures of the flow fields over a vegetation-covered 3D hill. The numerical simulations were validated by the comparison with the wind-tunnel experiments. Besides, the flow fields were systematically investigated, including the examinations of the mean velocities and root means square of the fluctuating velocities. The distributions of the parameters are shown in a three-dimensional way, i.e., plotting the parameters on a series of spanwise slices. Some noteworthy three-dimensional features were found, and the mechanisms were further revealed by assessing the turbulence kinetic energy budget and the spectrum energy. Subsequently, the instantaneous flow fields were illustrated, from which the coherent turbulence structures were clearly identified. Ejection-sweep motion was intensified just behind the hill crest, leading to a spanwise rotation. A group of vertical rotations were generated by the shedding of the vortex from the lateral sides of the hill. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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25 pages, 7273 KiB

Open AccessArticle

Large Eddy Simulations of the Flow Fields over Simplified Hills with Different Roughness Conditions, Slopes, and Hill Shapes: A Systematical Study

by Zhenqing Liu, Yiran Hu and Wei Wang

Energies 2019, 12(18), 3413; https://doi.org/10.3390/en12183413 - 4 Sep 2019

Cited by 9 | Viewed by 2606

Abstract

Turbulent flow fields over topographies are important in the area of wind energy. The roughness, slope, and shape of a hill are important parameters affecting the flow fields over topographies. However, these effects are always examined separately. The systematic investigations of these effects [...] Read more.

Turbulent flow fields over topographies are important in the area of wind energy. The roughness, slope, and shape of a hill are important parameters affecting the flow fields over topographies. However, these effects are always examined separately. The systematic investigations of these effects are limited, the coupling between these effects is still unrevealed, and the turbulence structures as a function of these effects are still unclear. Therefore, in the present study, the flow fields over twelve simplified isolated hills with different roughness conditions, slopes, and hill shapes are examined using large eddy simulations. The mean velocities, velocity fluctuations, fractional speed-up ratios, and visualizations of the turbulent flow fields are presented. It is found that as the hill slope increases, the roughness effects become weaker, and the roughness effects will further weaken as the hill changes from 3D to 2D. In addition, the fractional speed-up ratio at the summit of rough hills can even reach to three times as large as that over the corresponding smooth hills. Furthermore, the underestimation of the ratios of spanwise fluctuation to the streamwise fluctuation by International Electrotechnical Commission (IEC) 61400-1 is quite obvious when the hill shape is 3D. Finally, coherent turbulence structures can be identified for smooth hills, and as the hill slope increases, the coherent turbulence structures will experience clear evolutions. After introducing the ground roughness, the coherent turbulence structures break into small eddies. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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18 pages, 11113 KiB

Open AccessArticle

Influence of the Dynamic Motion of a Splash-Lubricated Gearbox on Churning Power Losses

by Xiaozhou Hu, Pingping Li and Minggui Wu

Energies 2019, 12(17), 3225; https://doi.org/10.3390/en12173225 - 21 Aug 2019

Cited by 19 | Viewed by 3281

Abstract

To investigate the influence of the dynamic motion of a gearbox on lubricating flow field and churning power losses under splash lubrication, a computational fluid dynamics (CFD) method based on a combination of the fluid of volume (VOF) method and turbulence model is [...] Read more.

To investigate the influence of the dynamic motion of a gearbox on lubricating flow field and churning power losses under splash lubrication, a computational fluid dynamics (CFD) method based on a combination of the fluid of volume (VOF) method and turbulence model is presented in this paper. A non-inertial coordinate system was employed to simulate the motion of the gearbox, and the feasibility and accuracy of the method was validated by the available experimental results. Numerical models of the gearbox with a spur gear pair under no load operation condition were established, and sinusoidal motions with different frequencies and amplitudes were implemented in the gearbox. The effects of the rotational speed of gears, oil immersion depth, and the frequency and amplitude of sinusoidal motions were studied. The results showed that the dynamic motion of the gearbox can exert a significant influence on churning losses and the oil supply of the gear contacting zone, and the gear pair may be in a loss-of-lubrication state. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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19 pages, 10634 KiB

Open AccessFeature PaperArticle

Unsteady RANS Simulations of Flow around a Twin-Box Bridge Girder Cross Section

by Wonmin Jeong, Shengnan Liu, Jasna Bogunovic Jakobsen and Muk Chen Ong

Energies 2019, 12(14), 2670; https://doi.org/10.3390/en12142670 - 11 Jul 2019

Cited by 12 | Viewed by 3203

Abstract

The aerodynamic performance of bridge deck girders requires a thorough assessment and optimization in the design of long-span bridges. The present paper describes a numerical investigation of the aerodynamic characteristics of a twin-box bridge girder cross section in the range of angles of [...] Read more.

The aerodynamic performance of bridge deck girders requires a thorough assessment and optimization in the design of long-span bridges. The present paper describes a numerical investigation of the aerodynamic characteristics of a twin-box bridge girder cross section in the range of angles of attack between −10.0° and +10.2°. The simulations are performed by solving 2D unsteady Reynolds-averaged Navier–Stokes (URANS) equations together with the k–ω shear stress transport (SST) turbulence model. The investigated Reynolds number (Re) based on the free stream velocity (

U_{\infty}

) and the height of the deck (D) is 31,000. The predicted aerodynamic characteristics such as the mean drag, lift and moment coefficients, are generally in good agreement with the results from the wind tunnel tests. Changes of flow patterns and aerodynamic forces with different angles of attack are investigated. Flow characteristics during one vortex shedding period are highlighted. Relative contributions of each of the two bridge decks to the overall drag and lift coefficients, with respect to the angle of attack, are also discussed. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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26 pages, 9759 KiB

Open AccessArticle

CFD Simulations of the Propagation of Free-Surface Waves Past Two Side-By-Side Fixed Squares with a Narrow Gap

by Haoyu Zhang, Shengnan Liu, Muk Chen Ong and Renqing Zhu

Energies 2019, 12(14), 2669; https://doi.org/10.3390/en12142669 - 11 Jul 2019

Cited by 6 | Viewed by 2696

Abstract

Two-dimensional computational fluid dynamics (CFD) simulations are carried out to investigate the gap resonance phenomenon that occurs when free-surface waves travel past twin squares in tandem. The volume of fluid method is used to capture the free surface. Validation studies of the present [...] Read more.

Two-dimensional computational fluid dynamics (CFD) simulations are carried out to investigate the gap resonance phenomenon that occurs when free-surface waves travel past twin squares in tandem. The volume of fluid method is used to capture the free surface. Validation studies of the present numerical model are conducted for different incident wave frequencies. The numerical results agree well with the published experimental data in terms of the free surface elevations in the gap. The hydrodynamic characteristics of the water column in the gap are investigated at different incident wave frequencies and gap widths. It is found that the free surface elevation in the gap increases and then decreases with the increasing incident wave frequency. The horizontal force on the weather side square structure (the structure in front of the gap) reaches the peak value at a larger frequency than the gap resonance frequency, whereas the variation of the horizontal force on the lee side structure (the structure behind the gap) is in-phase with the free surface elevation in the gap. Moreover, the added mass of the water column in the gap increases with the increasing gap width, which results in the decrease of resonance amplitude and frequency in the gap. However, this does not necessarily reduce the peak value of the horizontal forces on the structures. Full article

(This article belongs to the Special Issue Engineering Fluid Dynamics 2019-2020)

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