Energies

4 pages, 170 KiB

Open AccessEditorial

Special Issue “Fluid Flow and Heat Transfer”

by Artur J. Jaworski

Energies 2019, 12(16), 3044; https://doi.org/10.3390/en12163044 - 7 Aug 2019

Cited by 1 | Viewed by 2384

Fluid flow and heat transfer processes play an important role in many areas of science and engineering from the planetary scale (e [...] Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

25 pages, 11229 KiB

Open AccessArticle

Analysis of Different POD Processing Methods for SPIV-Measurements in Compressor Cascade Tip Leakage Flow

by Lei Shi, Hongwei Ma and Lixiang Wang

Energies 2019, 12(6), 1021; https://doi.org/10.3390/en12061021 - 15 Mar 2019

Cited by 16 | Viewed by 3965

Abstract

Though the proper orthogonal decomposition (POD) method has been widely adopted in flow analysis, few publications have systematically studied the influence of different POD processing methods on the POD results. This paper investigates the effects of different decomposition regions and decomposition dimensionalities on [...] Read more.

Though the proper orthogonal decomposition (POD) method has been widely adopted in flow analysis, few publications have systematically studied the influence of different POD processing methods on the POD results. This paper investigates the effects of different decomposition regions and decomposition dimensionalities on POD decomposition and reconstruction concerning the tip flow in the compressor cascade. Stereoscopic particle image velocimetry (SPIV) measurements in the blade channel are addressed to obtain the original flow field. Through vortex core identification, development of the tip leakage vortex along the chord is described. Afterwards, each plane is energetically decomposed by POD. Using the identified vortex core center as the geometric center, the effects of different decomposition regions with respect to the vortex core are analyzed. Furthermore, the effects of different single velocity-components as well as their combination are compared. The effect of different decomposition regions on the mode 1 energy fraction mainly impacts the streamwise velocity component. Though the addition of W velocity component in the decomposition does change the spatial structures of high-order modes, it does not change the dynamic results of reconstruction using a finite number of POD modes. UV global analysis is better for capturing the kinetic physics of the tip leakage vortex (TLV) wandering. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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20 pages, 8923 KiB

Open AccessArticle

Unsteadiness of Tip Leakage Flow in the Detached-Eddy Simulation on a Transonic Rotor with Vortex Breakdown Phenomenon

by Xiangyu Su, Xiaodong Ren, Xuesong Li and Chunwei Gu

Energies 2019, 12(5), 954; https://doi.org/10.3390/en12050954 - 12 Mar 2019

Cited by 26 | Viewed by 4294

Abstract

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% [...] Read more.

Tip leakage vortex (TLV) in a transonic compressor rotor was investigated numerically using detached-eddy simulation (DES) method at different working conditions. Strong unsteadiness was found at the tip region, causing a considerable fluctuation in total pressure distribution and flow angle distribution above 80% span. The unsteadiness at near choke point and peak efficiency point is not obvious. DES method can resolve more detailed flow patterns than RANS (Reynolds-averaged Navier–Stokes) results, and detailed structures of the tip leakage flow were captured. A spiral-type breakdown structure of the TLV was successfully observed at the near stall point when the TLV passed through the bow shock. The breakdown of TLV contributed to the unsteadiness and the blockage effect at the tip region. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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15 pages, 5911 KiB

Open AccessArticle

Effect of Rotor Thrust on the Average Tower Drag of Downwind Turbines

by Shigeo Yoshida, Kazuyuki Fujii, Masahiro Hamasaki and Ao Takada

Energies 2019, 12(2), 227; https://doi.org/10.3390/en12020227 - 12 Jan 2019

Cited by 3 | Viewed by 3547

Abstract

A new analysis method to calculate the rotor-induced average tower drag of downwind turbines in the blade element momentum (BEM) method was developed in this study. The method involves two parts: calculation of the wind speed distribution using computational fluid dynamics, with the [...] Read more.

A new analysis method to calculate the rotor-induced average tower drag of downwind turbines in the blade element momentum (BEM) method was developed in this study. The method involves two parts: calculation of the wind speed distribution using computational fluid dynamics, with the rotor modeled as a uniform loaded actuator disc, and calculation of the tower drag via the strip theory. The latter calculation considers two parameters, that is, the decrease in wind speed and the pressure gradient caused by the rotor thrust. The present method was validated by a wind tunnel test. Unlike the former BEM, which assumes the tower drag to be constant, the results obtained by the proposed method demonstrate much better agreement with the results of the wind tunnel test, with an accuracy of 30%. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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18 pages, 14749 KiB

Open AccessArticle

POD Analysis of Entropy Generation in a Laminar Separation Boundary Layer

by Chao Jin and Hongwei Ma

Energies 2018, 11(11), 3003; https://doi.org/10.3390/en11113003 - 1 Nov 2018

Cited by 14 | Viewed by 3319

Abstract

Separation of laminar boundary layer is a great source of loss in energy and power machinery. This paper investigates the entropy generation of the boundary layer on the flat plate with pressure gradient. The velocity of the flow field is measured by a [...] Read more.

Separation of laminar boundary layer is a great source of loss in energy and power machinery. This paper investigates the entropy generation of the boundary layer on the flat plate with pressure gradient. The velocity of the flow field is measured by a high resolution and time related particle image velocimetry (PIV) system. A method to estimate the entropy generation of each mode extracted by proper orthogonal decomposition (POD) is introduced. The entropy generation of each POD mode caused by mean viscous, Reynolds normal stress, Reynolds sheer stress, and energy flux is analyzed. The first order mode of the mean viscous term contributes almost 100% of the total entropy generation. The first three order modes of the Reynolds sheer stress term contribute less than 10% of the total entropy generation in the fore part of the separation bubble, while it reaches to more than 95% in the rear part of the separation bubble. It indicates that the more unsteady that the flow is, the higher contribution rate of the Reynolds sheer stress term makes. The energy flux term plays an important role in the turbulent kinetic energy balance in the transition region. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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23 pages, 6584 KiB

Open AccessArticle

Experimental Investigation of Flow-Induced Motion and Energy Conversion of a T-Section Prism

by Nan Shao, Jijian Lian, Guobin Xu, Fang Liu, Heng Deng, Quanchao Ren and Xiang Yan

Energies 2018, 11(8), 2035; https://doi.org/10.3390/en11082035 - 6 Aug 2018

Cited by 19 | Viewed by 3582

Abstract

Flow-induced motion (FIM) performs well in energy conversion but has been barely investigated, particularly for prisms with sharp sections. Previous studies have proven that T-section prisms that undergo galloping branches with high amplitude are beneficial to energy conversions. The FIM experimental setup designed [...] Read more.

Flow-induced motion (FIM) performs well in energy conversion but has been barely investigated, particularly for prisms with sharp sections. Previous studies have proven that T-section prisms that undergo galloping branches with high amplitude are beneficial to energy conversions. The FIM experimental setup designed by Tianjin University (TJU) was improved to conduct a series of FIM responses and energy conversion tests on a T-section prism. Experimental results are presented and discussed, to reveal the complete FIM responses and power generation characteristics of the T-section prism under different load resistances and section aspect ratios. The main findings are summarized as follows. (1) Hard galloping (HG), soft galloping (SG), and critical galloping (CG) can be observed by varying load resistances. When the load resistances are low, HG occurs; otherwise, SG occurs. (2) In the galloping branch, the highest amplitude and the most stable oscillation cause high-quality electrical energy production by the generator. Therefore, the galloping branch is the best branch for harvesting energy. (3) In the galloping branch, as the load resistances decrease, the active power continually increases until the prism is suppressed from galloping to a vortex-induced vibration (VIV) lower branch with a maximum active power P_harn of 21.23 W and a maximum η_out of 20.2%. (4) Different section aspect ratios (α) can significantly influence the FIM responses and energy conversions of the T-section prism. For small aspect ratios, galloping is hardly observed in the complete responses, but the power generation efficiency (η_out_,0.8 = 27.44%) becomes larger in the galloping branch. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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16 pages, 4273 KiB

Open AccessArticle

Flow Structure and Heat Transfer of Jet Impingement on a Rib-Roughened Flat Plate

by Abdulrahman H. Alenezi, Abdulrahman Almutairi, Hamad M. Alhajeri, Abdulmajid Addali and Abdelaziz A. A. Gamil

Energies 2018, 11(6), 1550; https://doi.org/10.3390/en11061550 - 13 Jun 2018

Cited by 20 | Viewed by 5700

Abstract

The jet impingement technique is an effective method to achieve a high heat transfer rate and is widely used in industry. Enhancing the heat transfer rate even minimally will improve the performance of many engineering systems and applications. In this numerical study, the [...] Read more.

The jet impingement technique is an effective method to achieve a high heat transfer rate and is widely used in industry. Enhancing the heat transfer rate even minimally will improve the performance of many engineering systems and applications. In this numerical study, the convective heat transfer process between orthogonal air jet impingement on a smooth, horizontal surface and a roughened uniformly heated flat plate is studied. The roughness element takes the form of a circular rib of square cross-section positioned at different radii around the stagnation point. At each location, the effect of the roughness element on heat transfer rate was simulated for six different heights and the optimum rib location and rib dimension determined. The average Nusselt number has been evaluated within and beyond the stagnation region to better quantify the heat transfer advantages of ribbed surfaces over smooth surfaces. The results showed both flow and heat transfer features vary significantly with rib dimension and location on the heated surface. This variation in the streamwise direction included both augmentation and decrease in heat transfer rate when compared to the baseline no-rib case. The enhancement in normalized averaged Nusselt number obtained by placing the rib at the most optimum radial location R/D = 2 was 15.6% compared to the baseline case. It was also found that the maximum average Nusselt number for each location was achieved when the rib height was close to the corresponding boundary layer thickness of the smooth surface at the same rib position. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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19 pages, 7378 KiB

Open AccessArticle

Numerical Study on Thermal Hydraulic Performance of Supercritical LNG in Zigzag-Type Channel PCHEs

by Zhongchao Zhao, Yimeng Zhou, Xiaolong Ma, Xudong Chen, Shilin Li and Shan Yang

Energies 2019, 12(3), 548; https://doi.org/10.3390/en12030548 - 11 Feb 2019

Cited by 27 | Viewed by 4902

Abstract

In this paper, we study a promising plate-type heat exchanger, the printed circuit heat exchanger (PCHE), which has high compactness and is suitable for high-pressure conditions as a vaporizer during vaporization. The thermal hydraulic performance of supercritical produce liquefied natural gas (LNG) in [...] Read more.

In this paper, we study a promising plate-type heat exchanger, the printed circuit heat exchanger (PCHE), which has high compactness and is suitable for high-pressure conditions as a vaporizer during vaporization. The thermal hydraulic performance of supercritical produce liquefied natural gas (LNG) in the zigzag channel of PCHE is numerically investigated using the SST κ-ω turbulence model. The thermo-physical properties of supercritical LNG from 6.5 MPa to 10MPa were calculated using piecewise-polynomial approximations of the temperature. The effect of the channel bend angle, mass flux and inlet pressure on local convection heat transfer coefficient, and pressure drop are discussed. The heat transfer and pressure loss performance are evaluated using the Nusselt and Euler numbers. Nu/Eu is proposed to evaluate the comprehensive heat transfer performance of PCHE by considering the heat transfer and pressure drop characteristics to find better bend angle and operating conditions. The supercritical LNG has a better heat transfer performance when bend angle is less than 15° with the mass flux ranging from 207.2 kg/(m²·s) to 621.6 kg/(m²·s), which improves at bend angle of 10° and lower compared to 15° at mass flux above 414.4 kg/(m²·s). The heat transfer performance is better at larger mass flux and lower operating pressures. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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16 pages, 1223 KiB

Open AccessArticle

A Machine Learning Approach to Correlation Development Applied to Fin-Tube Bundle Heat Exchangers

by Karl Lindqvist, Zachary T. Wilson, Erling Næss and Nikolaos V. Sahinidis

Energies 2018, 11(12), 3450; https://doi.org/10.3390/en11123450 - 10 Dec 2018

Cited by 21 | Viewed by 5285

Abstract

Heat exchanger designers need reliable thermal-hydraulic correlations to optimize heat exchanger designs. This work combines an adaptive sampling method with a Computational Fluid Dynamics (CFD) simulator to obtain increased accuracy and validity range of heat transfer and pressure drop predictions using a limited [...] Read more.

Heat exchanger designers need reliable thermal-hydraulic correlations to optimize heat exchanger designs. This work combines an adaptive sampling method with a Computational Fluid Dynamics (CFD) simulator to obtain increased accuracy and validity range of heat transfer and pressure drop predictions using a limited number of data points. Correlation efficacy was evaluated based on a steam generator case study. The sensitivity to the design parameters was analyzed in detail. The RMSE (root mean square error) of the developed correlations were reduced, through CFD sampling, from 28% to 15% for pressure drop, and from 33% to 25% heat transfer, compared to regression on experimental data only. The best reference correlations have RMSE values of 43% and 33% on pressure drop and heat transfer, respectively, on an independent validation set. Indeed, a radically different fin-tube geometry was suggested for the case study, compared to results using the Escoa correlations.The developed correlations show good to excellent agreement with trends in the CFD model. The quantitative error of predicted heat transfer and pressure drop coefficients at the case study optimum, however, was as large as those of the Escoa correlations. More data are likely needed to improve accuracy for compact heat exchanger designs further. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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15 pages, 4057 KiB

Open AccessArticle

A Numerical Study on the Light-Weight Design of PTC Heater for an Electric Vehicle Heating System

by Hyun Sung Kang, Seungkyu Sim and Yoon Hyuk Shin

Energies 2018, 11(5), 1276; https://doi.org/10.3390/en11051276 - 16 May 2018

Cited by 26 | Viewed by 7569

Abstract

As the market for electric vehicles grows at a remarkable rate, various models of electric vehicles are currently in development, in parallel to the commercialization of components for diverse types of power supply. Cabin heating and heat management components are essential to electric [...] Read more.

As the market for electric vehicles grows at a remarkable rate, various models of electric vehicles are currently in development, in parallel to the commercialization of components for diverse types of power supply. Cabin heating and heat management components are essential to electric vehicles. Any design for such components must consider the requirements for heating capacity and power density, which need to reflect both the power source and weight reduction demand of any electric vehicle. In particular, design developments in electric heaters have predominantly focused on experimental values because of structural characteristics of the heater and the variability of heat sources, requiring considerable cost and duration. To meet the ever-changing demands of the market, an improved design process for more efficient models is essential. To improve the efficacy of the design process for electric heaters, this study conducted a Computational Fluid Dynamics (CFD) analysis of an electric heater with specific dimensions by changing design parameters and operating conditions of key components. The CFD analysis modeled heat characteristics through the application of user-defined functions (UDFs) to reflect temperature properties of Positive Temperature Coefficient (PTC) elements, which heat an electric heater. Three analysis models, which included fin as well as PTC elements and applied different spaces between the heat rods, were compared in terms of heating performance. In addition, the heat performance and heat output density of each analysis model was analyzed according to the variation of air flow at the inlet of the radiation section of an electric heater. Model B was selected, and a prototype was fabricated based on the model. The performance of the prototype was evaluated, and the correlation between the analysis results and the experimental ones was identified. The error rate between performance change rates was approximately 4%, which indicated that the reliability between the design model and the prototype was attained. Consequently, the design range of effective performance and the guideline for lightweight design could be presented based on the simulation of electric heaters for various electric vehicles. The fabrication of prototypes and minimum comparison demonstrated opportunities to reduce both development cost and duration. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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13 pages, 4821 KiB

Open AccessArticle

Unsteady Simulation of a Full-Scale CANDU-6 Moderator with OpenFOAM

by Hyoung Tae Kim, Se-Myong Chang and Young Woo Son

Energies 2019, 12(2), 330; https://doi.org/10.3390/en12020330 - 21 Jan 2019

Cited by 2 | Viewed by 5104

Abstract

Three-dimensional moderator flow in the calandria tank of CANDU-6 pressurized heavy water reactor (PHWR) is computed with Open Field Operation and Manipulation (OpenFOAM), an open-source computational fluid dynamics (CFD) code. In this study, numerical analysis is performed on the real geometry model including [...] Read more.

Three-dimensional moderator flow in the calandria tank of CANDU-6 pressurized heavy water reactor (PHWR) is computed with Open Field Operation and Manipulation (OpenFOAM), an open-source computational fluid dynamics (CFD) code. In this study, numerical analysis is performed on the real geometry model including 380 fuel rods in the calandria tank with the heat-source distribution to remove uncertainty of the previous analysis models simplified by the porous media approach. Realizable k-ε turbulence model is applied, and the buoyancy due to temperature variation is considered by Boussinesq approximation for the incompressible single-phase Navier-Stokes equations. The calculation results show that the flow is highly unsteady in the moderator. The computational flow visualization shows a circulation of flow driven by buoyancy and asymmetric oscillation at the pseudo-steady state. There is no region where the local temperature rises continuously due to slow circulating flow and its convection heat transfer. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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13 pages, 3230 KiB

Open AccessArticle

Visualization Study on Thermo-Hydrodynamic Behaviors of a Flat Two-Phase Thermosyphon

by Chao Wang, Feng Yao, Juan Shi, Liangyu Wu and Mengchen Zhang

Energies 2018, 11(9), 2295; https://doi.org/10.3390/en11092295 - 31 Aug 2018

Cited by 16 | Viewed by 3484

Abstract

The coupled effect of boiling and condensation inside a flat two-phase thermosyphon has a non-negligible influence on the two-phase fluid flow behavior and heat transfer process. Therefore, a flat two-phase thermosyphon with transparent wall was manufactured. Based on this device, a visualization experiment [...] Read more.

The coupled effect of boiling and condensation inside a flat two-phase thermosyphon has a non-negligible influence on the two-phase fluid flow behavior and heat transfer process. Therefore, a flat two-phase thermosyphon with transparent wall was manufactured. Based on this device, a visualization experiment system was developed to study the vapor–liquid two-phase behaviors and thermal performance of the flat two-phase thermosyphon. A cross-shaped wick using copper mesh was embedded into the cavity of two-phase thermosyphon to improve the heat transfer performance. The effects of heat flux density, working medium, and wick structure on the thermal performance are examined and analyzed. The results indicated that a strong liquid disturbance is caused by the bubble motions, leading to the enhancement of both convective boiling and condensation heat transfer. More bubbles are generated as the heat flux increases; therefore, the disturbance of bubble motion on liquid pool and condensation film becomes stronger, resulting in better thermal performance of the flat two-phase thermosyphon. The addition of the wick inside the cavity effectively reduces the temperature oscillation of the evaporator wall. In addition, the wick structure provides backflow paths for the condensate owing to the effect of capillary force and enhances the vapor–liquid phase change heat transfer, resulting in the improvement of thermal performance for the flat two-phase thermosyphon. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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23 pages, 10579 KiB

Open AccessArticle

Gas–Liquid Two-Phase Upward Flow through a Vertical Pipe: Influence of Pressure Drop on the Measurement of Fluid Flow Rate

by Tarek A. Ganat and Meftah Hrairi

Energies 2018, 11(11), 2937; https://doi.org/10.3390/en11112937 - 27 Oct 2018

Cited by 15 | Viewed by 8630

Abstract

The accurate estimation of pressure drop during multiphase fluid flow in vertical pipes has been widely recognized as a critical problem in oil wells completion design. The flow of fluids through the vertical tubing strings causes great losses of energy through friction, where [...] Read more.

The accurate estimation of pressure drop during multiphase fluid flow in vertical pipes has been widely recognized as a critical problem in oil wells completion design. The flow of fluids through the vertical tubing strings causes great losses of energy through friction, where the value of this loss depends on fluid flow viscosity and the size of the conduit. A number of friction factor correlations, which have acceptably accurate results in large diameter pipes, are significantly in error when applied to smaller diameter pipes. Normally, the pressure loss occurs due to friction between the fluid flow and the pipe walls. The estimation of the pressure gradients during the multiphase flow of fluids is very complex due to the variation of many fluid parameters along the vertical pipe. Other complications relate to the numerous flow regimes and the variabilities of the fluid interfaces involved. Accordingly, knowledge about pressure drops and friction factors is required to determine the fluid flow rate of the oil wells. This paper describes the influences of the pressure drop on the measurement of the fluid flow by estimating the friction factor using different empirical friction correlations. Field experimental work was performed at the well site to predict the fluid flow rate of 48 electrical submersible pump (ESP) oil wells, using the newly developed mathematical model. Using Darcy and Colebrook friction factor correlations, the results show high average relative errors, exceeding ±18.0%, in predicted liquid flow rate (oil and water). In gas rate, more than 77% of the data exceeded ±10.0% relative error to the predicted gas rate. For the Blasius correlation, the results showed the predicted liquid flow rate was in agreement with measured values, where the average relative error was less than ±18.0%, and for the gas rate, 68% of the data showed more than ±10% relative error. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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17 pages, 5620 KiB

Open AccessArticle

Investigation on the Handling Ability of Centrifugal Pumps under Air–Water Two-Phase Inflow: Model and Experimental Validation

by Qiaorui Si, Gérard Bois, Qifeng Jiang, Wenting He, Asad Ali and Shouqi Yuan

Energies 2018, 11(11), 3048; https://doi.org/10.3390/en11113048 - 6 Nov 2018

Cited by 25 | Viewed by 4265

Abstract

The paper presents experimental and numerical investigations performed on a single stage, single-suction, horizontal-orientated centrifugal pump in air–water two-phase non-condensable flow conditions. Experimental measurements are performed in a centrifugal pump using pressure sensor devices in order to measure the wall static pressures at [...] Read more.

The paper presents experimental and numerical investigations performed on a single stage, single-suction, horizontal-orientated centrifugal pump in air–water two-phase non-condensable flow conditions. Experimental measurements are performed in a centrifugal pump using pressure sensor devices in order to measure the wall static pressures at the inlet and outlet pump sections for different flow rates and rotational speeds combined with several air void fraction (a) values. Two different approaches are used in order to predict the pump performance degradations and perform comparisons with experiments for two-phase flow conditions: a one-dimensional two-phase bubbly flow model, and a full “Three-Dimensional Unsteady Reynolds Average Navier–Stokes” (3D-URANS) simulation using a modified k-epsilon turbulence model combined with the Euler–Euler inhomogeneous two-phase flow description. The overall and local flow features are presented and analyzed. Limitations concerning both approaches are pointed out according to some flow physical assumptions and measurement accuracies. Some additional suggestions are proposed in order to improve two-phase flow pump suction capabilities. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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20 pages, 7901 KiB

Open AccessArticle

Resonant Pulsing Frequency Effect for Much Smaller Bubble Formation with Fluidic Oscillation

by Pratik Devang Desai, Michael John Hines, Yassir Riaz and William B. Zimmerman

Energies 2018, 11(10), 2680; https://doi.org/10.3390/en11102680 - 9 Oct 2018

Cited by 23 | Viewed by 4732

Abstract

Microbubbles have several applications in gas-liquid contacting operations. Conventional production of microbubbles is energetically unfavourable since surface energy required to generate the bubbles is inversely proportional to the size of the bubble generated. Fluidic oscillators have demonstrated a size decrease for a system [...] Read more.

Microbubbles have several applications in gas-liquid contacting operations. Conventional production of microbubbles is energetically unfavourable since surface energy required to generate the bubbles is inversely proportional to the size of the bubble generated. Fluidic oscillators have demonstrated a size decrease for a system with high throughput and low energetics but the achievable bubble size is limited due to coalescence. The hypothesis of this paper is that this limitation can be overcome by modifying bubble formation dynamics mediated by oscillatory flow. Frequency and amplitude are two easily controlled factors in oscillatory flow. The bubble can be formed at the displacement phase of the frequency cycle if the amplitude is sufficient to detach the bubble. If the frequency is too low, the conventional steady flow detachment mechanism occurs instead; if the frequency is too high, the bubbles coalesce. Our hypothesis proposes the existence of a resonant mode or ‘sweet-spot’ condition, via frequency modulation and increase in amplitude, to reduce coalescence and produce smallest bubble size with no additional energy input. This condition is identified for an exemplar system showing relative size changes, and a bubble size reduction from 650 µm for steady flow, to 120 µm for oscillatory-flow, and 60 µm for resonant condition (volume average) and 250 µm for steady-flow, 15 µm for oscillatory-flow, 7 µm for the resonant condition. A 10-fold reduction in bubble size with minimal increase in associated energetics results in a substantial reduction in energy requirements for all processes involving gas-liquid operations. The reduction in the energetic footprint of this method has widespread ramifications in all gas-liquid contacting operations including but not limited to wastewater aeration, desalination, flotation separation operations, and other operations. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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13 pages, 5353 KiB

Open AccessArticle

Bubble Size and Bubble Concentration of a Microbubble Pump with Respect to Operating Conditions

by Seok-Yun Jeon, Joon-Yong Yoon and Choon-Man Jang

Energies 2018, 11(7), 1864; https://doi.org/10.3390/en11071864 - 17 Jul 2018

Cited by 27 | Viewed by 10588

Abstract

The present paper describes some aspects of the bubble size and concentration of a microbubble pump with respect to flow and pressure conditions. The microbubble pump used in the present study has an open channel impeller of a regenerative pump, which generates micro-sized [...] Read more.

The present paper describes some aspects of the bubble size and concentration of a microbubble pump with respect to flow and pressure conditions. The microbubble pump used in the present study has an open channel impeller of a regenerative pump, which generates micro-sized bubbles with the rotation of the impeller. The bubble characteristics are analyzed by measuring the bubble size and concentration using the experimental apparatus consisting of open-loop facilities; a regenerative pump, a particle counter, electronic flow meters, pressure sensors, flow control valves, a torque meter, and reservoir tanks. To control the intake, and the air flowrate upstream of the pump, a high precision flow control valve is introduced. The bubble characteristics have been analyzed by controlling the intake air flowrate and the pressure difference of the pump while the rotational frequency of the pump impeller was kept constant. All measurement data was stored on the computer through the NI (National Instrument) interface system. The bubble size and concentration are mainly affected by three operating parameters: the intake air flowrate, the pressure difference, and the water flowrate supplied to the pump. It is noted that the operating conditions that can most effectively generate microbubbles in the range of 20 to 30 micrometers are at the pressure of 5 bar and at the air flowrate ratio of 4.0 percent for the present pump. Throughout the experimental measurements, it was found that the pump efficiency changed by less than 1.2 percent, depending on the intake air supply. The performance characteristics of microbubble generation obtained by experimental measurements are analyzed and discussed in detail. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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23 pages, 20544 KiB

Open AccessArticle

Spherical Shaped ( A g − F e 3 O 4 / H 2 O ) Hybrid Nanofluid Flow Squeezed between Two Riga Plates with Nonlinear Thermal Radiation and Chemical Reaction Effects

by Naveed Ahmed, Fitnat Saba, Umar Khan, Ilyas Khan, Tawfeeq Abdullah Alkanhal, Imran Faisal and Syed Tauseef Mohyud-Din

Energies 2019, 12(1), 76; https://doi.org/10.3390/en12010076 - 27 Dec 2018

Cited by 43 | Viewed by 4189

Abstract

The main concern is to explore an electro-magneto hydrodynamic (EMHD) squeezing flow of

(A g - F e_{3} O_{4} / H_{2} O)

hybrid nanofluid between stretchable parallel Riga plates. The benefits of the use of hybrid nanofluids, and [...] Read more.

The main concern is to explore an electro-magneto hydrodynamic (EMHD) squeezing flow of

(A g - F e_{3} O_{4} / H_{2} O)

hybrid nanofluid between stretchable parallel Riga plates. The benefits of the use of hybrid nanofluids, and the parameters associated to it, have been analyzed mathematically. This particular problem has a lot of importance in several branches of engineering and industry. Heat and mass transfer along with nonlinear thermal radiation and chemical reaction effects have also been incorporated while carrying out the study. An appropriate selection of dimensionless variables have enabled us to develop a mathematical model for the present flow situation. The resulting mathematical method have been solved by a numerical scheme named as the method of moment. The accuracy of the scheme has been ensured by comparing the present result to some already existing results of the same problem, but for a limited case. To back our results further we have also obtained the solution by anther recipe known as the Runge-Kutta-Fehlberg method combined with the shooting technique. The error analysis in a tabulated form have also been presented to validate the acquired results. Furthermore, with the graphical assistance, the variation in the behavior of the velocity, temperature and concentration profile have been inspected under the action of various ingrained parameters. The expressions for skin friction coefficient, local Nusselt number and local Sherwood number, in case of

(A g - F e_{3} O_{4} / H_{2} O)

hybrid nanofluid, have been derived and the influence of various parameters have also been discussed. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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21 pages, 6163 KiB

Open AccessArticle

Numerical Study of the Magnetic Field Effect on Ferromagnetic Fluid Flow and Heat Transfer in a Square Porous Cavity

by Mohamed F. El-Amin, Usama Khaled and Abderrahmane Beroual

Energies 2018, 11(11), 3235; https://doi.org/10.3390/en11113235 - 21 Nov 2018

Cited by 9 | Viewed by 3406

Abstract

A numerical study of ferromagnetic-fluid flow and heat transfer in a square porous cavity under the effect of a magnetic field is presented. The water-magnetic particle suspension is treated as a miscible mixture and, thus, the magnetization, density and viscosity of the ferrofluid [...] Read more.

A numerical study of ferromagnetic-fluid flow and heat transfer in a square porous cavity under the effect of a magnetic field is presented. The water-magnetic particle suspension is treated as a miscible mixture and, thus, the magnetization, density and viscosity of the ferrofluid are obtained. The governing partial-differential equations were solved numerically using the cell-centered finite-difference method for the spatial discretization, while the multiscale time-splitting implicit method was developed to treat the temporal discretization. The Courant–Friedrichs–Lewy stability condition (CFL < 1) was used to make the scheme adaptive by dividing time steps as needed. Two cases corresponding to Dirichlet and Neumann boundary conditions were considered. The efficiency of the developed algorithm as well as some physical results such as temperature, concentration, and pressure; and the local Nusselt and Sherwood numbers at the cavity walls are presented and discussed. It was noticed that the particle concentration and local heat/mass transfer rate are related to the magnetic field strength, and both pressure and velocity increase as the strength of the magnetic was increased. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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18 pages, 9075 KiB

Open AccessArticle

Experimental Study of Particle Deposition on Surface at Different Mainstream Velocity and Temperature

by Fei Zhang, Zhenxia Liu, Zhengang Liu and Yanan Liu

Energies 2019, 12(4), 747; https://doi.org/10.3390/en12040747 - 24 Feb 2019

Cited by 6 | Viewed by 4126

Abstract

The effect of mainstream velocity and mainstream temperature on the behavior of deposition on a flat plate surface has been investigated experimentally. Molten wax particles were injected to generate particle deposition in a two-phase flow wind tunnel. Tests indicated that deposition occurs mainly [...] Read more.

The effect of mainstream velocity and mainstream temperature on the behavior of deposition on a flat plate surface has been investigated experimentally. Molten wax particles were injected to generate particle deposition in a two-phase flow wind tunnel. Tests indicated that deposition occurs mainly at the leading edge and the middle and backward portions of the windward side. The mass of deposition at the leading edge was far more than that on the windward and lee sides. For the windward and lee sides, deposition mass increased as the mainstream velocity was increased for a given particle concentration. Capture efficiency was found to increase initially until the mainstream velocity reaches a certain value, where it begins to drop with mainstream velocity increasing. For the leading edge, capture efficiency followed a similar trend due to deposition spallation and detachment induced by aerodynamic shear at high velocity. Deposition formation was also strongly affected by the mainstream temperature due to its control of particle phase (solid or liquid). Capture efficiency initially increased with increasing mainstream temperature until a certain threshold temperature (near the wax melting point). Subsequently, it began to decrease, for wax detaches from the model surface when subjected to the aerodynamic force at the surface temperature above the wax melting point. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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18 pages, 6385 KiB

Open AccessArticle

Development and Assessment of Two-Stage Thermoacoustic Electricity Generator

by Ahmed Hamood, Artur J. Jaworski and Xiaoan Mao

Energies 2019, 12(9), 1790; https://doi.org/10.3390/en12091790 - 11 May 2019

Cited by 12 | Viewed by 5135

Abstract

This paper presents the development and assessment of a two-stage thermoacoustic electricity generator that aims to mimic the conversion of waste heat from the internal combustion engine exhaust gases into useful electricity. The one wavelength configuration consists of two identical stages which allow [...] Read more.

This paper presents the development and assessment of a two-stage thermoacoustic electricity generator that aims to mimic the conversion of waste heat from the internal combustion engine exhaust gases into useful electricity. The one wavelength configuration consists of two identical stages which allow coupling a linear alternator in a “push-pull” mode because of the 180° out of phase acoustic excitation on two sides of the piston. This type of coupling is a possible solution for the low acoustic impedance of looped-tube traveling-wave thermoacoustic engines. The experimental set-up is 16.1 m long and runs at 54.7 Hz. The working medium is helium at maximum pressure of 28 bar. In practice, the maximum generated electric power was 73.3 W at 5.64% thermal-to-electric efficiency. The working parameters, namely load resistance, mean pressure and heating power, were investigated. System debugging illustrates the effect of local acoustic impedance of the regenerator on the start-up process of the thermoacoustic engine. The additional modelling showed that the feedback loop length can be reduced by using a combination of acoustic inertance and compliance components. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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17 pages, 3993 KiB

Open AccessArticle

Investigation of the Concepts to Increase the Dew Point Temperature for Thermal Energy Recovery from Flue Gas, Using Aspen^®

by Nataliia Fedorova, Pegah Aziziyanesfahani, Vojislav Jovicic, Ana Zbogar-Rasic, Muhammad Jehanzaib Khan and Antonio Delgado

Energies 2019, 12(9), 1585; https://doi.org/10.3390/en12091585 - 26 Apr 2019

Cited by 7 | Viewed by 5135

Abstract

Thermal energy of flue gases (FG) dissipating from industrial facilities into the environment, constitute around 20% of the total dissipated thermal energy. Being part of the FG, water vapour carries thermal energy out of the system in the form of the latent heat, [...] Read more.

Thermal energy of flue gases (FG) dissipating from industrial facilities into the environment, constitute around 20% of the total dissipated thermal energy. Being part of the FG, water vapour carries thermal energy out of the system in the form of the latent heat, which can be recovered by condensation, thus increasing the overall efficiency of an industrial process. The limiting factor in this case is the low dew point temperature (usually 40–60 °C) of the water vapour in the FG. The increase of the dew point temperature can be achieved by increasing the water content or pressure. Taking these measures as a basis, the presented work investigated the following concepts for increasing the dew point temperature: humidification of the flue gas using water, humidification using steam, compression of the FG and usage of the steam ejector. Modelling of these concepts was performed using the commercial software Aspen^®. The humidification of the FG using water resulted in the negligible increase in the dew point (3 °C). Using steam humidification the temperatures of up to 92 °C were reached, while the use of steam ejector led to few degrees higher dew point temperatures. However, both concepts proved to be energy demanding, due to the energy requirements for the steam generation. The FG compression enabled the achievement of a 97 °C dew point temperature, being both energy-efficient and exhibiting the lowest energy cost. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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16 pages, 4090 KiB

Open AccessArticle

Experimental Study on the Fire-Spreading Characteristics and Heat Release Rates of Burning Vehicles Using a Large-Scale Calorimeter

by Younggi Park, Jaiyoung Ryu and Hong Sun Ryou

Energies 2019, 12(8), 1465; https://doi.org/10.3390/en12081465 - 17 Apr 2019

Cited by 19 | Viewed by 7897

Abstract

In this article, large-scale experimental studies were conducted to figure out the fire characteristics, such as fire-spreading, toxic gases, and heat release rates, using large-scale calorimeter for one- and two-vehicle fires. The initial ignition position was the passenger seat, and thermocouples were attached [...] Read more.

In this article, large-scale experimental studies were conducted to figure out the fire characteristics, such as fire-spreading, toxic gases, and heat release rates, using large-scale calorimeter for one- and two-vehicle fires. The initial ignition position was the passenger seat, and thermocouples were attached to each compartment in the vehicles to determine the temperature distribution as a function of time. For the analysis, the time was divided into sections for the various fire-spreading periods and major changes, e.g., the fire spreading from the first vehicle to the second vehicle. The maximum temperature of 1400 °C occurred in the seats because they contained combustible materials. The maximum heat release rates were 3.5 MW and 6 MW for one and two vehicles, respectively. Since the time to reach 1 MW was about 240 s (4 min) before and after, the beginning of the car fire appears to be a medium-fast growth type. It shows the effect on the human body depending on the concentration of toxic substances such as carbon monoxide or carbon dioxide. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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15 pages, 4901 KiB

Open AccessArticle

Macro and Meso Characteristics of In-Situ Oil Shale Pyrolysis Using Superheated Steam

by Lei Wang, Dong Yang, Xiang Li, Jing Zhao, Guoying Wang and Yangsheng Zhao

Energies 2018, 11(9), 2297; https://doi.org/10.3390/en11092297 - 31 Aug 2018

Cited by 28 | Viewed by 3972

Abstract

The efficiency of oil shale pyrolysis is directly related to the feasibility of in-situ mining technology. Taiyuan University of Technology (China) proposed the technology of in-situ convective heating of oil shale, which uses superheated steam as the heat carrier to heat the oil [...] Read more.

The efficiency of oil shale pyrolysis is directly related to the feasibility of in-situ mining technology. Taiyuan University of Technology (China) proposed the technology of in-situ convective heating of oil shale, which uses superheated steam as the heat carrier to heat the oil shale’s ore-body and transport the pyrolysis products. Based on the simulated experiments of in-situ oil shale pyrolysis using superheated steam, the changes in fracture characteristics, pyrolysis characteristics and mesoscopic characteristics of the oil shale during the pyrolysis have been systematically studied in this work. The Xinjiang oil shale’s pyrolysis temperature ranged within 400–510 °C. When the temperature is 447 °C, the rate of pyrolysis of kerogen is the fastest. During the pyrolysis process, the pressure of superheated steam changes within the range of 0.1–11.1 MPa. With the continuous thermal decomposition, the horizontal stress difference shows a tendency to first increase and then, decrease. The rate of weight loss of oil shale residue at various locations after the pyrolysis is found to be within the range of 0.17–2.31%, which is much lower than the original value of 10.8%, indicating that the pyrolysis is more adequate. Finally, the number of microcracks (<50 µm) in the oil shale after pyrolysis is found to be lie within the range of 25–56 and the average length lies within the range of 53.9636–62.3816 µm. The connectivity of the internal pore groups is satisfactory, while the seepage channel is found to be smooth. These results fully reflect the high efficiency and feasibility of in-situ oil shale pyrolysis using superheated steam. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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16 pages, 5051 KiB

Open AccessArticle

A Numerical Study on Influence of Temperature on Lubricant Film Characteristics of the Piston/Cylinder Interface in Axial Piston Pumps

by Yueheng Song, Jiming Ma and Shengkui Zeng

Energies 2018, 11(7), 1842; https://doi.org/10.3390/en11071842 - 13 Jul 2018

Cited by 12 | Viewed by 4687

Abstract

The loss of kinetic energy of moving parts due to viscous friction of lubricant causes the reduction of piston pump efficiency. The viscosity of lubricant film is mainly affected by the thermal effect. In order to improve energy efficiency of piston pump, this [...] Read more.

The loss of kinetic energy of moving parts due to viscous friction of lubricant causes the reduction of piston pump efficiency. The viscosity of lubricant film is mainly affected by the thermal effect. In order to improve energy efficiency of piston pump, this research presents a numerical method to analyze the lubricant film characteristics in axial piston pumps, considering the thermal effect by the coupled multi-disciplinary model, which includes the fluid flow field expressed by Reynolds equation, temperature field expressed by energy equation and heat transfer equation, kinematics expressed by the motion equation. The velocity and temperature distributions of the gap flow of piston/cylinder interface in steady state are firstly numerically computed. Then the distributions are validated by the experiment. Finally, by changing the thermal boundary condition, the influence of thermal effect on the lubricant film, the eccentricity and the contact time between the piston and cylinder are analyzed. Results show that with the increase of temperature, the contact time increases in the form of a hyperbolic tangent function, which will reduce the efficiency of the axial piston pump. There is a critical temperature beyond which the contact time will increase rapidly, thus this temperature is the considered as a key point for the temperature design. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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20 pages, 6447 KiB

Open AccessArticle

Investigation on Water Hammer Control of Centrifugal Pumps in Water Supply Pipeline Systems

by Wuyi Wan, Boran Zhang and Xiaoyi Chen

Energies 2019, 12(1), 108; https://doi.org/10.3390/en12010108 - 29 Dec 2018

Cited by 42 | Viewed by 7874

Abstract

Water hammer control in water supply pipeline systems is significant for protecting pipelines from damage. The goal of this research is to investigate the effects of pumps moment of inertia design on pipeline water hammer control. Based on the method of characteristics (MOC), [...] Read more.

Water hammer control in water supply pipeline systems is significant for protecting pipelines from damage. The goal of this research is to investigate the effects of pumps moment of inertia design on pipeline water hammer control. Based on the method of characteristics (MOC), a numerical model is established and plenty of simulations are conducted. Through numerical analysis, it is found that increasing the pumps moment of inertia has positive effects both on water hammer control as well as preventing pumps rapid runaway speed. Considering the extra cost of space, starting energy, and materials, an evaluation methodology of efficiency on the increasing moment of inertia is proposed. It can be regarded as a reference for engineers to design the moment of inertia of pumps in water supply pipeline systems. Combined with the optimized operations of the valve behind the pumps, the pipeline systems can be better protected from accident events. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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12 pages, 1027 KiB

Open AccessArticle

One-Log Call Iterative Solution of the Colebrook Equation for Flow Friction Based on Padé Polynomials

by Pavel Praks and Dejan Brkić

Energies 2018, 11(7), 1825; https://doi.org/10.3390/en11071825 - 12 Jul 2018

Cited by 17 | Viewed by 4650

Abstract

The 80 year-old empirical Colebrook function

ξ

, widely used as an informal standard for hydraulic resistance, relates implicitly the unknown flow friction factor

λ

, with the known Reynolds number

R e

and the known relative roughness of a pipe inner surface [...] Read more.

The 80 year-old empirical Colebrook function

ξ

, widely used as an informal standard for hydraulic resistance, relates implicitly the unknown flow friction factor

λ

, with the known Reynolds number

R e

and the known relative roughness of a pipe inner surface

ε^{*}

;

λ = ξ (R e, ε^{*}, λ)

. It is based on logarithmic law in the form that captures the unknown flow friction factor

λ

in a way that it cannot be extracted analytically. As an alternative to the explicit approximations or to the iterative procedures that require at least a few evaluations of computationally expensive logarithmic function or non-integer powers, this paper offers an accurate and computationally cheap iterative algorithm based on Padé polynomials with only one

l o g

-call in total for the whole procedure (expensive

l o g

-calls are substituted with Padé polynomials in each iteration with the exception of the first). The proposed modification is computationally less demanding compared with the standard approaches of engineering practice, but does not influence the accuracy or the number of iterations required to reach the final balanced solution. Full article

(This article belongs to the Special Issue Fluid Flow and Heat Transfer)

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