Entropy

Research

21 pages, 12753 KiB

Open AccessArticle

A Modified Shielding and Rapid Transition DDES Model for Separated Flows

by Da Lei, Hui Yang, Yun Zheng, Qingzhe Gao and Xiubo Jin

Entropy 2023, 25(4), 613; https://doi.org/10.3390/e25040613 - 4 Apr 2023

Cited by 2 | Viewed by 1503

In this paper, the major problems associated with detached eddy simulation (DES) (namely, modeled stress depletion (MSD) and slowing of the RANS to LES transition (RLT)) are discussed and reviewed, and relevant improvements are developed. A modified version for the delayed DES (DDES) [...] Read more.

In this paper, the major problems associated with detached eddy simulation (DES) (namely, modeled stress depletion (MSD) and slowing of the RANS to LES transition (RLT)) are discussed and reviewed, and relevant improvements are developed. A modified version for the delayed DES (DDES) method with adaptive modified adequate shielding and rapid transition is proposed; this is called MSRT DDES. The modified shielding strategy can be adjusted adaptively according to the local flow conditions: keeping the RANS behavior in the whole boundary layer when there is no resolved turbulence, and weakening the shielding function when resolved turbulence exists in the mainstream over the boundary layer. This strategy can significantly ameliorate the MSD in the RANS boundary layer, regardless of the mesh refinement, and avoid excessive shielding in the fully developed resolved turbulence that may otherwise delay the development of the separated and reattached flow. Three cases are designed to test the modified DDES, namely, complete shielding in the RANS zone of a boundary layer (the zero-pressure gradient turbulent boundary layer with the refined mesh), modified adaptive improved shielding with a rapid transition (the flow over a hump), and the overall performance in a complex 3D separation (the corner separation in a compressor cascade). The results show that the modified shielding function is more physical than earlier proposals compared to shielding functions, and according to detailed comparisons of the wall skin friction coefficients, velocity profiles, total pressure-loss coefficients, entropy production analyses, and so on, the MSD and RLT problems are moderately alleviated by the MSRT DDES. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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16 pages, 10020 KiB

Open AccessArticle

Effect of the Radial Velocity Distribution on the Loss Generation of a Contra-Rotating Fan in a Ventilation System

by Xingyu Jia, Xi Zhang, Kui Guo and Xuehui Li

Entropy 2023, 25(3), 433; https://doi.org/10.3390/e25030433 - 1 Mar 2023

Cited by 1 | Viewed by 1581

Abstract

Quantification of the loss generation of ducted contra-rotating fan (CRF) blades is difficult to achieve, since there are no guide vanes between rotors. A blade design program was established to investigate the relationship between radial velocity distribution and incurred loss. Numerical and experimental [...] Read more.

Quantification of the loss generation of ducted contra-rotating fan (CRF) blades is difficult to achieve, since there are no guide vanes between rotors. A blade design program was established to investigate the relationship between radial velocity distribution and incurred loss. Numerical and experimental techniques were used to confirm the optimal configuration’s overall performance. The relationship between loss and velocity distribution under the impact of spanwise load distribution was confirmed by the entropy contour from various perspectives. The appropriate radial velocity distribution can improve the operating efficiency of a CRF by reducing the entropy around the annulus under design and near-stall conditions. This regularity could provide some strategies in the design of contra-rotating blades. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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14 pages, 5027 KiB

Open AccessArticle

Optimization of the Double-Expansion Film-Cooling Hole Using CFD

by Zhen Zhang, Tianyu Hu, Xinrong Su and Xin Yuan

Entropy 2023, 25(3), 410; https://doi.org/10.3390/e25030410 - 24 Feb 2023

Cited by 1 | Viewed by 1398

Abstract

Film cooling is a major cooling technique used in modern gas turbines and air engines. The geometry of film-cooling holes is the fundamental aspect affecting the cooling performance. In this paper, a new cooling configuration called the double-expansion film-cooling hole has been put [...] Read more.

Film cooling is a major cooling technique used in modern gas turbines and air engines. The geometry of film-cooling holes is the fundamental aspect affecting the cooling performance. In this paper, a new cooling configuration called the double-expansion film-cooling hole has been put forward, which yields better performance than the widely used shaped holes and is easy to manufacture. The double-expansion holes at inclination angles of

α = 30^{\circ}

,

45^{\circ}

, and

60^{\circ}

are optimized using the genetic algorithm and the Kriging surrogate model, which is trained by CFD data randomly sampled using the Latin hypercube method. The numerically optimized double-expansion holes at different inclination angles were experimentally evaluated and compared with the optimized single-expansion laid-back fan-shaped holes, and the optimized double-expansion hole at

α = 30^{\circ}

was manually modified based on experiment results. Compared with the optimal single-expansion holes, the area-averaged cooling effectiveness of the double-expansion holes was increased by

34.5 %

at

α = 30^{\circ}

, by

27.8 %

at

α = 45^{\circ}

, and basically the same at

α = 60^{\circ}

, showing the benefit of the double-expansion concept. The loss mechanism of film cooling was also analyzed in the perspective of the entropy generation rate, showing the optimal double-expansion holes have 21% less loss compared to a baseline narrow single-expansion hole. It was also found that CFD sometimes predicts a different trend from the experiment in optimization, and the experimental validation is necessary. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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17 pages, 6110 KiB

Open AccessArticle

Effect of Gap Length and Partition Thickness on Thermal Boundary Layer in Thermal Convection

by Zhengyu Wang, Huilin Tong, Zhengdao Wang, Hui Yang, Yikun Wei and Yuehong Qian

Entropy 2023, 25(2), 386; https://doi.org/10.3390/e25020386 - 20 Feb 2023

Cited by 1 | Viewed by 1674

Abstract

Two-dimensional direct numerical simulations of partitioned thermal convection are performed using the thermal lattice Boltzmann method for the Rayleigh number (Ra) of 10⁹ and the Prandtl number (Pr) of 7.02 (water). The influence of the partition walls on [...] Read more.

Two-dimensional direct numerical simulations of partitioned thermal convection are performed using the thermal lattice Boltzmann method for the Rayleigh number (Ra) of 10⁹ and the Prandtl number (Pr) of 7.02 (water). The influence of the partition walls on the thermal boundary layer is mainly focused on. Moreover, to better describe the spatially nonuniform thermal boundary layer, the definition of the thermal boundary layer is extended. The numerical simulation results show that the gap length significantly affects the thermal boundary layer and Nusselt number (Nu). The gap length and partition wall thickness have a coupled effect on the thermal boundary layer and the heat flux. Based on the shape of the thermal boundary layer distribution, two different heat transfer models are identified at different gap lengths. This study provides a basis for improving the understanding of the effect of partitions on the thermal boundary layer in thermal convection. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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35 pages, 12354 KiB

Open AccessArticle

Numerical Calculation of the Irreversible Entropy Production of Additively Manufacturable Off-Set Strip Fin Heat-Transferring Structures

by Marco Fuchs, Nico Lubos and Stephan Kabelac

Entropy 2023, 25(1), 162; https://doi.org/10.3390/e25010162 - 13 Jan 2023

Cited by 4 | Viewed by 1722

Abstract

In this manuscript, off-set strip fin structures are presented which are adapted to the possibilities of additive manufacturing. For this purpose, the geometric parameters, including fin height, fin spacing, fin length, and fin longitudinal displacement, are varied, and the Colburn j-factor and the [...] Read more.

In this manuscript, off-set strip fin structures are presented which are adapted to the possibilities of additive manufacturing. For this purpose, the geometric parameters, including fin height, fin spacing, fin length, and fin longitudinal displacement, are varied, and the Colburn j-factor and the Fanning friction factor are numerically calculated in the Reynolds number range of 80–920. The structures are classified with respect to their entropy production number according to Bejan. This method is compared with the results from partial differential equations for the calculation of the irreversible entropy production rate due to shear stresses and heat conduction. This study reveals that the chosen temperature difference leads to deviation in terms of entropy production due to heat conduction, whereas the dissipation by shear stresses shows only small deviations of less than 2%. It is further shown that the variation in fin height and fin spacing has only a small influence on heat transfer and pressure drop, while a variation in fin length and fin longitudinal displacement shows a larger influence. With respect to the entropy production number, short and long fins, as well as large fin spacing and fin longitudinal displacement, are shown to be beneficial. A detailed examination of a single structure shows that the entropy production rate due to heat conduction is dominated by the entropy production rate in the wall, while the fluid has only a minor influence. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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17 pages, 6417 KiB

Open AccessArticle

Optimisation and Efficiency Improvement of Electric Vehicles Using Computational Fluid Dynamics Modelling

by Darryl Afianto, Yu Han, Peiliang Yan, Yan Yang, Anas F. A. Elbarghthi and Chuang Wen

Entropy 2022, 24(11), 1584; https://doi.org/10.3390/e24111584 - 1 Nov 2022

Cited by 2 | Viewed by 3749

Abstract

Due to the rise in awareness of global warming, many attempts to increase efficiency in the automotive industry are becoming prevalent. Design optimization can be used to increase the efficiency of electric vehicles by reducing aerodynamic drag and lift. The main focus of [...] Read more.

Due to the rise in awareness of global warming, many attempts to increase efficiency in the automotive industry are becoming prevalent. Design optimization can be used to increase the efficiency of electric vehicles by reducing aerodynamic drag and lift. The main focus of this paper is to analyse and optimise the aerodynamic characteristics of an electric vehicle to improve efficiency of using computational fluid dynamics modelling. Multiple part modifications were used to improve the drag and lift of the electric hatchback, testing various designs and dimensions. The numerical model of the study was validated using previous experimental results obtained from the literature. Simulation results are analysed in detail, including velocity magnitude, drag coefficient, drag force and lift coefficient. The modifications achieved in this research succeeded in reducing drag and were validated through some appropriate sources. The final model has been assembled with all modifications and is represented in this research. The results show that the base model attained an aerodynamic drag coefficient of 0.464, while the final design achieved a reasonably better overall performance by recording a 10% reduction in the drag coefficient. Moreover, within individual comparison with the final model, the second model with front spitter had an insignificant improvement, limited to 1.17%, compared with 11.18% when the rear diffuser was involved separately. In addition, the lift coefficient was significantly reduced to 73%, providing better stabilities and accounting for the safety measurements, especially at high velocity. The prediction of the airflow improvement was visualised, including the pathline contours consistent with the solutions. These research results provide a considerable transformation in the transportation field and help reduce fuel expenses and global emissions. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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18 pages, 7119 KiB

Open AccessArticle

Entropy Production Analysis of a Vertical Mixed-Flow Pump Device with Different Guide Vane Meridians

by Yanjun Li, Yi Zhong, Fan Meng, Yunhao Zheng and Danghang Sun

Entropy 2022, 24(10), 1370; https://doi.org/10.3390/e24101370 - 27 Sep 2022

Cited by 3 | Viewed by 1549

Abstract

With the aim of investigating the influence of guide vane meridians on the external characteristics and internal flow field of the mixed-flow pump device, this research constructed seven guide vane meridians and applied computational fluid dynamic (CFD) and entropy production theory to investigate [...] Read more.

With the aim of investigating the influence of guide vane meridians on the external characteristics and internal flow field of the mixed-flow pump device, this research constructed seven guide vane meridians and applied computational fluid dynamic (CFD) and entropy production theory to investigate the spread of hydraulic loss in a mixed-flow pump. As observed, when the guide vane outlet diameter D_gvo decreased from 350 mm to 275 mm, the head and efficiency increased by 2.78% and 3.05% at 0.7 Q_des, respectively. At 1.3 Q_des, when D_gvo increased from 350 mm to 425 mm, the head and efficiency increased by 4.49% and 3.71%, respectively. At 0.7 Q_des and 1.0 Q_des, the entropy production of the guide vane increased with the increase of D_gvo due to flow separation. When D_gvo < 350 mm, at 1.0 Q_des and 1.3 Q_des, entropy production of the outlet channel increased as D_gvo decreased owing to the excessive flow rate, but at 0.7 Q_des, entropy production did not change much. When D_gvo > 350 mm, at 0.7 Q_des and 1.0 Q_des, due to the expansion of the channel section, the flow separation intensified, which resulted in an increase of the entropy production, but the entropy production decreased slightly at 1.3 Q_des. These results provide guidance for improving the efficiency of pumping stations. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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19 pages, 3698 KiB

Open AccessArticle

Development of a 3D Eulerian/Lagrangian Aircraft Icing Simulation Solver Based on OpenFOAM

by Han Han, Zifei Yin, Yijun Ning and Hong Liu

Entropy 2022, 24(10), 1365; https://doi.org/10.3390/e24101365 - 27 Sep 2022

Cited by 7 | Viewed by 2757

Abstract

A 3D icing simulation code is developed in the open-source CFD toolbox OpenFOAM. A hybrid Cartesian/body-fitted meshing method is used to generate high-quality meshes around complex ice shapes. Steady-state 3D Reynolds-averaged Navier-Stokes (RANS) equations are solved to provide the ensemble-averaged flow around the [...] Read more.

A 3D icing simulation code is developed in the open-source CFD toolbox OpenFOAM. A hybrid Cartesian/body-fitted meshing method is used to generate high-quality meshes around complex ice shapes. Steady-state 3D Reynolds-averaged Navier-Stokes (RANS) equations are solved to provide the ensemble-averaged flow around the airfoil. Considering the multi-scale nature of droplet size distribution, and more importantly, to represent the less uniform nature of the Super-cooled Large Droplets (SLD), two droplet tracking methods are realized: the Eulerian method is used to track the small-size droplets (below 50

μ

m) for the sake of efficiency; the Lagrangian method with random sampling is used to track the large droplets (above 50

μ

m); the heat transfer of the surface overflow is solved on a virtual surface mesh; the ice accumulation is estimated via the Myers model; finally, the final ice shape is predicted by time marching. Limited by the availability of experimental data, validations are performed on 3D simulations of 2D geometries using the Eulerian and Lagrangian methods, respectively. The code proves to be feasible and accurate enough in predicting ice shapes. Finally, an icing simulation result of the M6 wing is presented to illustrate the full 3D capability. Full article

(This article belongs to the Special Issue Applications of CFD in Heat and Fluid Flow Processes)

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