Pub Date : 2024-12-13DOI: 10.1016/j.ijheatfluidflow.2024.109719
M.S. Alqurashi , Hina Gul , Irshad Ahmad , Afraz Hussain Majeed , Hamiden Abd El-Wahed Khalifa
Although human tissues already have blood-based temperature regulation, nanofluids can make this process even more powerful. In this study, we aim to examine the flow of a hybrid nanoliquid, consisting of the Yamda-Ota (Y-O) and Hamilton-Crosser (H-C) models, through an inclined magnetic field, past a horizontally extending sheet, including silver and gold nanoparticle, with blood as the base liquid. Heat transport, homogeneous-heterogeneous (homo-hetero) reactions, and magneto-hydrodynamic (MHD) effects are all taken into consideration. Heat generation and absorption, temperature stratification, and linear radiation are all taken into account. The second law of thermodynamics is used to conduct an analysis of irreversibility. Also examined are the effects of stratification, as well as heat sources and sinks with varying thermal conductivity. Numerical solutions to the mathematical model are found using the bvp4c package in MATLAB. As a means of presenting and analyzing the results, tables and figures are employed. As you change the parameters, you can see the force coefficient graphically. The outcomes demonstrate that, when comparing the two models, the Y-O approach to introducing hybrid nanofluid (HNF) flow is more advantageous. The results show that an augmentation in the nanomaterial volume fraction decline the velocity profile.
{"title":"A study of thermal conductivity enhancement in magnetic blood flow: Applications of medical engineering","authors":"M.S. Alqurashi , Hina Gul , Irshad Ahmad , Afraz Hussain Majeed , Hamiden Abd El-Wahed Khalifa","doi":"10.1016/j.ijheatfluidflow.2024.109719","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109719","url":null,"abstract":"<div><div>Although human tissues already have blood-based temperature regulation, nanofluids can make this process even more powerful. In this study, we aim to examine the flow of a hybrid nanoliquid, consisting of the Yamda-Ota (Y-O) and Hamilton-Crosser (H-C) models, through an inclined magnetic field, past a horizontally extending sheet, including silver and gold nanoparticle, with blood as the base liquid. Heat transport, homogeneous-heterogeneous (homo-hetero) reactions, and magneto-hydrodynamic (MHD) effects are all taken into consideration. Heat generation and absorption, temperature stratification, and linear radiation are all taken into account. The second law of thermodynamics is used to conduct an analysis of irreversibility. Also examined are the effects of stratification, as well as heat sources and sinks with varying thermal conductivity. Numerical solutions to the mathematical model are found using the bvp4c package in MATLAB. As a means of presenting and analyzing the results, tables and figures are employed. As you change the parameters, you can see the force coefficient graphically. The outcomes demonstrate that, when comparing the two models, the Y-O approach to introducing hybrid nanofluid (HNF) flow is more advantageous. The results show that an augmentation in the nanomaterial volume fraction <span><math><mrow><mo>(</mo><msub><mi>ϕ</mi><mn>1</mn></msub><mo>,</mo><msub><mi>ϕ</mi><mn>2</mn></msub><mo>)</mo></mrow></math></span> decline the velocity profile.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109719"},"PeriodicalIF":2.6,"publicationDate":"2024-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140451","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
This study presents the application of an artificial neural network (ANN) model to predict the Nusselt number of CuO-water nanofluid in a rectangular grooved channel under the effect of a magnetic field. In the developed ANN model, while Reynolds number (250 ≤ Re ≤ 1250), volume fraction of nanofluids (0 ≤ Φ ≤ 5), and Hartmann numbers (0 ≤ Ha ≤ 28) were taken as input parameters, Nusselt number was selected as the output parameter. Data were generated from a computational fluid dynamics (CFD) code by discretizing equations using the finite difference method. Therefore, the outcomes acquired from numerical simulations using CFD code were used for training and testing the generated ANN model. According to the results the generated ANN model can accurately predict the Nusselt number with a mean absolute percentage error (MAPE) of 0.4288 %, mean absolute error (MAE) of 0.0351, and root mean square error (RMSE) of 0.0540 in testing and of 0.3177 % MAPE, 0.0249 MAE and 0.0328 RMSE in the training. Furthermore, the correlation coefficient (R) values are observed as 0.9998 and 0.9988 in training and testing phases, which demonstrate the prediction success of the generated ANN model. Notably, the ANN model reduced computational time from 8 h, using CFD methods, to just 10 min for testing cases, showcasing its efficiency in handling nonlinear flow cases where traditional CFD methods may struggle. This study represents a novel contribution to the field as one of the first to apply ANN techniques for predicting heat transfer in grooved channels under magnetic fields and nanofluid flow, offering potential applications in the design of thermal systems in industries such as electronics cooling, nuclear reactors, and metallurgy.
{"title":"Evaluation of heat transfer characteristics of a rectangular grooved heat exchanger under magnetic field using artificial neural network","authors":"Sergen Tumse , Atakan Tantekin , Mehmet Bilgili , Besir Sahin","doi":"10.1016/j.ijheatfluidflow.2024.109712","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109712","url":null,"abstract":"<div><div>This study presents the application of an artificial neural network (ANN) model to predict the Nusselt number of CuO-water nanofluid in a rectangular grooved channel under the effect of a magnetic field. In the developed ANN model, while Reynolds number (250 ≤ Re ≤ 1250), volume fraction of nanofluids (0 ≤ Φ ≤ 5), and Hartmann numbers (0 ≤ Ha ≤ 28) were taken as input parameters, Nusselt number was selected as the output parameter. Data were generated from a computational fluid dynamics (CFD) code by discretizing equations using the finite difference method. Therefore, the outcomes acquired from numerical simulations using CFD code were used for training and testing the generated ANN model. According to the results the generated ANN model can accurately predict the Nusselt number with a mean absolute percentage error (MAPE) of 0.4288 %, mean absolute error (MAE) of 0.0351, and root mean square error (RMSE) of 0.0540 in testing and of 0.3177 % MAPE, 0.0249 MAE and 0.0328 RMSE in the training. Furthermore, the correlation coefficient (R) values are observed as 0.9998 and 0.9988 in training and testing phases, which demonstrate the prediction success of the generated ANN model. Notably, the ANN model reduced computational time from 8 h, using CFD methods, to just 10 min for testing cases, showcasing its efficiency in handling nonlinear flow cases where traditional CFD methods may struggle. This study represents a novel contribution to the field as one of the first to apply ANN techniques for predicting heat transfer in grooved channels under magnetic fields and nanofluid flow, offering potential applications in the design of thermal systems in industries such as electronics cooling, nuclear reactors, and metallurgy.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109712"},"PeriodicalIF":2.6,"publicationDate":"2024-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140443","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-12DOI: 10.1016/j.ijheatfluidflow.2024.109688
Xiang Lu , Yongbin Ji , Bing Ge , Shusheng Zang
The swirl impact on the overall effusion cooling characteristics is investigated. Two typical hole configurations, cylindrical and fan-shaped holes, are studied under different blowing ratios (BR). The overall cooling effectiveness is obtained by IR camera and the flow features are analyzed by simulations. The result shows that the typical cooling characteristic is the existence of low-effectiveness regions at where the swirl impact the wall. The reason for this is that the swirl flows destroy the cooling film and suppress the cooling air, resulting in poor cooling effectiveness. At the plate beginning, the cooling effectiveness is high because the corner recirculation pushes the cooling air upstream to cover the wall. The ending of the plate has the best cooling performance due to the gradually superimposed cooling film. Increasing the BR weakens the influence of the swirl impact and improves the cooling effects, therefore the area average cooling effectiveness of the cylindrical holes increases from 0.334 to 0.688 and the cooling effectiveness unevenness decreases from 0.042 to 0.036. The fan-shaped holes have a maximum of 0.063 higher cooling effectiveness due to better film coverage. However, the fan-shaped holes have a stronger cooling effectiveness unevenness, which is 0.012 higher. And the lowest cooling effectiveness is 0.133 and 0.146 lower than the area average value for the cylindrical and fan-shaped holes, respectively. The reason for this is that the fan-shaped holes have similarly poor film cooling in the swirl impact zone but better film cooling effectiveness in the other regions.
{"title":"Swirl impact on the overall cooling characteristics of an effusion -cooled combustor liner with cylindrical and fan-shaped holes","authors":"Xiang Lu , Yongbin Ji , Bing Ge , Shusheng Zang","doi":"10.1016/j.ijheatfluidflow.2024.109688","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109688","url":null,"abstract":"<div><div>The swirl impact on the overall effusion cooling characteristics is investigated. Two typical hole configurations, cylindrical and fan-shaped holes, are studied under different blowing ratios (<em>BR</em>). The overall cooling effectiveness is obtained by IR camera and the flow features are analyzed by simulations. The result shows that the typical cooling characteristic is the existence of low-effectiveness regions at where the swirl impact the wall. The reason for this is that the swirl flows destroy the cooling film and suppress the cooling air, resulting in poor cooling effectiveness. At the plate beginning, the cooling effectiveness is high because the corner recirculation pushes the cooling air upstream to cover the wall. The ending of the plate has the best cooling performance due to the gradually superimposed cooling film. Increasing the <em>BR</em> weakens the influence of the swirl impact and improves the cooling effects, therefore the area average cooling effectiveness of the cylindrical holes increases from 0.334 to 0.688 and the cooling effectiveness unevenness decreases from 0.042 to 0.036. The fan-shaped holes have a maximum of 0.063 higher cooling effectiveness due to better film coverage. However, the fan-shaped holes have a stronger cooling effectiveness unevenness, which is 0.012 higher. And the lowest cooling effectiveness is 0.133 and 0.146 lower than the area average value for the cylindrical and fan-shaped holes, respectively. The reason for this is that the fan-shaped holes have similarly poor film cooling in the swirl impact zone but better film cooling effectiveness in the other regions.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109688"},"PeriodicalIF":2.6,"publicationDate":"2024-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140446","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-12DOI: 10.1016/j.ijheatfluidflow.2024.109679
A. Tomboulides , N. Saini , D.R. Shaver , A.V. Obabko , H. Yuan , E. Merzari , P.F. Fischer
The Reynolds Averaged Navier Stokes (RANS) model is one of the industry standard approaches for modeling of turbulent flows. It performs better than the model for low Reynolds number flows and is also more suitable for boundary layers with adverse pressure gradients. Major drawback of the model, however, is that the asymptotic value of at the walls is singular, necessitating the use of a contrived “sufficiently” large value for as the boundary condition for its transport equation. This invariably leads to the solution being sensitive to near wall grid spacing. While an acceptable solution for low order (finite volume) methods, the excessive near wall gradients lead to persistent numerical stability issues in high order codes. To alleviate the problem, specifically in the context of the high order spectral element code Nek5000, a regularized approach was formulated in our prior work (Tomboulides et al., 2018). The formulation, however, relies on the use of wall distance and its gradients for modeling the closure terms and can pose problems for simulations in complex geometries. This work presents a novel implementation of the RANS model in Nek5000, where , eliminating the need for regularization, owing to the asymptotically bounded behavior of the source terms in the transport equation, and also eliminating dependence on wall distance. Robustness and stability of the model is ensured through implicit treatment of the source terms and their careful numerical implementation and demonstrated through several cases aimed at verification and validation. Studies include both canonical and engineering relevant problems, viz., turbulent channel flow, pipe flow, backward facing step, flow over NACA 0012 airfoil and flow in a T-junction. Results from the model are shown to be consistent with regularized model and also with the SST model in OpenFOAM (for select studies). Comparison with experimental data is also shown, where available, to bolster validation efforts for the model implementation through prediction of key turbulent quantities of interest.
{"title":"A robust spectral element implementation of the k−τ RANS model in Nek5000/NekRS","authors":"A. Tomboulides , N. Saini , D.R. Shaver , A.V. Obabko , H. Yuan , E. Merzari , P.F. Fischer","doi":"10.1016/j.ijheatfluidflow.2024.109679","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109679","url":null,"abstract":"<div><div>The <span><math><mrow><mi>k</mi><mo>−</mo><mi>ω</mi></mrow></math></span> Reynolds Averaged Navier Stokes (RANS) model is one of the industry standard approaches for modeling of turbulent flows. It performs better than the <span><math><mrow><mi>k</mi><mo>−</mo><mi>ϵ</mi></mrow></math></span> model for low Reynolds number flows and is also more suitable for boundary layers with adverse pressure gradients. Major drawback of the model, however, is that the asymptotic value of <span><math><mi>ω</mi></math></span> at the walls is singular, necessitating the use of a contrived “sufficiently” large value for <span><math><mi>ω</mi></math></span> as the boundary condition for its transport equation. This invariably leads to the solution being sensitive to near wall grid spacing. While an acceptable solution for low order (finite volume) methods, the excessive near wall gradients lead to persistent numerical stability issues in high order codes. To alleviate the problem, specifically in the context of the high order spectral element code Nek5000, a regularized <span><math><mrow><mi>k</mi><mo>−</mo><mi>ω</mi></mrow></math></span> approach was formulated in our prior work (Tomboulides et al., 2018). The formulation, however, relies on the use of wall distance and its gradients for modeling the closure terms and can pose problems for simulations in complex geometries. This work presents a novel implementation of the <span><math><mrow><mi>k</mi><mo>−</mo><mi>τ</mi></mrow></math></span> RANS model in Nek5000, where <span><math><mrow><mi>τ</mi><mo>=</mo><mn>1</mn><mo>/</mo><mi>ω</mi></mrow></math></span>, eliminating the need for regularization, owing to the asymptotically bounded behavior of the source terms in the <span><math><mi>τ</mi></math></span> transport equation, and also eliminating dependence on wall distance. Robustness and stability of the <span><math><mrow><mi>k</mi><mo>−</mo><mi>τ</mi></mrow></math></span> model is ensured through implicit treatment of the source terms and their careful numerical implementation and demonstrated through several cases aimed at verification and validation. Studies include both canonical and engineering relevant problems, viz., turbulent channel flow, pipe flow, backward facing step, flow over NACA 0012 airfoil and flow in a T-junction. Results from the <span><math><mrow><mi>k</mi><mo>−</mo><mi>τ</mi></mrow></math></span> model are shown to be consistent with regularized <span><math><mrow><mi>k</mi><mo>−</mo><mi>ω</mi></mrow></math></span> model and also with the <span><math><mrow><mi>k</mi><mo>−</mo><mi>ω</mi></mrow></math></span> SST model in OpenFOAM (for select studies). Comparison with experimental data is also shown, where available, to bolster validation efforts for the <span><math><mrow><mi>k</mi><mo>−</mo><mi>τ</mi></mrow></math></span> model implementation through prediction of key turbulent quantities of interest.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109679"},"PeriodicalIF":2.6,"publicationDate":"2024-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140444","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-12DOI: 10.1016/j.ijheatfluidflow.2024.109656
Frank G. Jacobitz , Ian Sysyn , Jacob Ryan , Jack Comfort , Patrick Bonner , Dylan Poole , Jonathan Lemarechal , Marco Costantini
The aim of this study is a direct comparison of experimental and simulation results of two flows, a laminar boundary layer developing on a flat heated plate and the interaction of a laminar boundary layer with a single cylindrical roughness element of small aspect ratio with a height similar to the boundary layer thickness. The experiments were performed in a water channel using temperature-sensitive paint (TSP) on a heated flat plate on which the boundary layer develops. The numerical simulations are meant to complement the experimental data, allowing for a direct comparison with the experiment and adding additional information not easily accessible from the experiment. In the case of the laminar boundary layer developing over a flat heated surface, experimental TSP measurements and simulation results of the surface temperature show strong agreement and a correlation coefficient for the two temperature fields of 0.99 is obtained. In the case of a laminar boundary layer interacting with a low aspect ratio roughness element, the comparison between experimental and numerical data revealed the role played by buoyancy effects even at the small implemented temperature differences between surface and fluid. With consideration of buoyancy in the simulations, again good agreement between the experimental and simulation results is obtained with a correlation coefficient of 0.95 for the respective temperature fields. The complex vortical system identified in the flow field via the simulations was shown to be consistent with the thermal footprints measured on the heated wall in the experiments.
{"title":"On the interaction of a laminar heated boundary layer with a roughness element: A comparison of experiments and simulations for steady flow","authors":"Frank G. Jacobitz , Ian Sysyn , Jacob Ryan , Jack Comfort , Patrick Bonner , Dylan Poole , Jonathan Lemarechal , Marco Costantini","doi":"10.1016/j.ijheatfluidflow.2024.109656","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109656","url":null,"abstract":"<div><div>The aim of this study is a direct comparison of experimental and simulation results of two flows, a laminar boundary layer developing on a flat heated plate and the interaction of a laminar boundary layer with a single cylindrical roughness element of small aspect ratio with a height similar to the boundary layer thickness. The experiments were performed in a water channel using temperature-sensitive paint (TSP) on a heated flat plate on which the boundary layer develops. The numerical simulations are meant to complement the experimental data, allowing for a direct comparison with the experiment and adding additional information not easily accessible from the experiment. In the case of the laminar boundary layer developing over a flat heated surface, experimental TSP measurements and simulation results of the surface temperature show strong agreement and a correlation coefficient for the two temperature fields of 0.99 is obtained. In the case of a laminar boundary layer interacting with a low aspect ratio roughness element, the comparison between experimental and numerical data revealed the role played by buoyancy effects even at the small implemented temperature differences between surface and fluid. With consideration of buoyancy in the simulations, again good agreement between the experimental and simulation results is obtained with a correlation coefficient of 0.95 for the respective temperature fields. The complex vortical system identified in the flow field via the simulations was shown to be consistent with the thermal footprints measured on the heated wall in the experiments.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109656"},"PeriodicalIF":2.6,"publicationDate":"2024-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140445","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-11DOI: 10.1016/j.ijheatfluidflow.2024.109668
Taihang Zhu, Georgios Rigas, Jonathan F. Morrison
The coherent structures of a turbulent axisymmetric bluff body wake are investigated based on synchronised near-wake velocity and base pressure measurements. The proper orthogonal decomposition (POD) analysis confirms the persistence of the laminar spatio-temporal symmetry breaking instabilities at high Reynolds numbers (here ). The laminar rotational symmetry breaking mode randomly meanders in the azimuthal direction, and the unsteady laminar unstable eigenmodes manifest as asymmetric unsteady vortex shedding. Additionally, a coherent streamwise wake pulsation is identified (bubble pumping). Based on the symmetry-breaking property of the turbulent wake, the vector field is decomposed into two antisymmetric components and to perform conditional POD (CPOD) with a comparison of and , extracting antisymmetric modes in the stable asymmetric wake states. The asymmetry of the wake due to rotational symmetry break, quantified using the centre of pressure (CoP), is correlated to the base pressure using conditional averaging. The most probable symmetry-breaking wake state corresponds to a low-pressure (high drag) region, and two high-pressure (low drag) regions at the limit of axisymmetric (CoP ) and highly asymmetric (CoP ) wake states are identified. The high-pressure wake state in the highly asymmetric wake is caused by the backflow, which results in a high-pressure region near the base edge. Conditional averaging based on the base pressure shows that the transition between high- and low-pressure conditions is coupled with wake asymmetry.
{"title":"Near wake coherent structures of a turbulent axisymmetric bluff body wake","authors":"Taihang Zhu, Georgios Rigas, Jonathan F. Morrison","doi":"10.1016/j.ijheatfluidflow.2024.109668","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109668","url":null,"abstract":"<div><div>The coherent structures of a turbulent axisymmetric bluff body wake are investigated based on synchronised near-wake velocity and base pressure measurements. The proper orthogonal decomposition (POD) analysis confirms the persistence of the laminar spatio-temporal symmetry breaking instabilities at high Reynolds numbers (here <span><math><mrow><mi>R</mi><msub><mrow><mi>e</mi></mrow><mrow><mi>D</mi></mrow></msub><mo>=</mo><mn>1</mn><mo>.</mo><mn>88</mn><mo>×</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mn>5</mn></mrow></msup></mrow></math></span>). The laminar rotational symmetry breaking mode randomly meanders in the azimuthal direction, and the unsteady laminar unstable eigenmodes manifest as asymmetric unsteady vortex shedding. Additionally, a coherent streamwise wake pulsation is identified (bubble pumping). Based on the symmetry-breaking property of the turbulent wake, the vector field <span><math><mi>q</mi></math></span> is decomposed into two antisymmetric components <span><math><msup><mrow><mi>q</mi></mrow><mrow><mo>+</mo></mrow></msup></math></span> and <span><math><msup><mrow><mi>q</mi></mrow><mrow><mo>−</mo></mrow></msup></math></span> to perform conditional POD (CPOD) with a comparison of <span><math><msup><mrow><mi>q</mi><msup><mrow></mrow><mrow><mo>′</mo></mrow></msup></mrow><mrow><mo>+</mo></mrow></msup></math></span> and <span><math><msup><mrow><mi>q</mi></mrow><mrow><mo>+</mo><mo>′</mo></mrow></msup></math></span>, extracting antisymmetric modes in the stable asymmetric wake states. The asymmetry of the wake due to rotational symmetry break, quantified using the centre of pressure (CoP), is correlated to the base pressure using conditional averaging. The most probable symmetry-breaking wake state corresponds to a low-pressure (high drag) region, and two high-pressure (low drag) regions at the limit of axisymmetric (CoP <span><math><mrow><mo>→</mo><mn>0</mn></mrow></math></span>) and highly asymmetric (CoP <span><math><mrow><mo>→</mo><mi>∞</mi></mrow></math></span>) wake states are identified. The high-pressure wake state in the highly asymmetric wake is caused by the backflow, which results in a high-pressure region near the base edge. Conditional averaging based on the base pressure shows that the transition between high- and low-pressure conditions is coupled with wake asymmetry.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109668"},"PeriodicalIF":2.6,"publicationDate":"2024-12-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140140","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-10DOI: 10.1016/j.ijheatfluidflow.2024.109672
Gerardo Zampino , Marco Atzori , Elias Zea , Evelyn Otero , Ricardo Vinuesa
The wake topology behind a wall-mounted square cylinder immersed in a turbulent boundary layer is investigated using high-resolution large-eddy simulations (LES). The boundary-layer thickness at the obstacle location is fixed, with a Reynolds number based on cylinder height and free-stream velocity of 10,000 while the aspect ratio (AR), defined as obstacle height divided by its width, ranges from 1 to 4. The mesh resolution is comparable to DNS standards used for similar wall-mounted obstacles, though with relatively lower Reynolds numbers. The effects of AR on wake structures, turbulence production, and transport are analyzed via Reynolds stresses, anisotropy-invariant maps (AIM), and the turbulent kinetic energy (TKE) budget. In particular, the transition from “dipole” to a “quadrupole” wake is extensively examined as AR increases. With increasing AR, the wake shrinks in both the streamwise and spanwise directions, attributed to the occurrence of the base vortices ( and 4). This change in the flow structure also affects the size of the positive-production region that extends from the roof and the flank of the obstacle to the wake core. The AIMs confirm three-dimensional wake features, showing TKE redistribution in all directions (Simonsen and Krogstad, 2005). Stronger turbulence production in and 4 cases highlights the role of tip and base vortices behind the cylinder. The overall aim is to refine the dipole-to-quadrupole transition as a function of AR and accounting for the incoming TBL properties. The novelty relies on proposing the momentum-thickness-based Reynolds number as a discriminant for assessing TBL effects on turbulent wake structures.
{"title":"Aspect-ratio effect on the wake of a wall-mounted square cylinder immersed in a turbulent boundary layer","authors":"Gerardo Zampino , Marco Atzori , Elias Zea , Evelyn Otero , Ricardo Vinuesa","doi":"10.1016/j.ijheatfluidflow.2024.109672","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109672","url":null,"abstract":"<div><div>The wake topology behind a wall-mounted square cylinder immersed in a turbulent boundary layer is investigated using high-resolution large-eddy simulations (LES). The boundary-layer thickness at the obstacle location is fixed, with a Reynolds number based on cylinder height <span><math><mi>h</mi></math></span> and free-stream velocity <span><math><msub><mrow><mi>u</mi></mrow><mrow><mi>∞</mi></mrow></msub></math></span> of 10,000 while the aspect ratio (AR), defined as obstacle height divided by its width, ranges from 1 to 4. The mesh resolution is comparable to DNS standards used for similar wall-mounted obstacles, though with relatively lower Reynolds numbers. The effects of AR on wake structures, turbulence production, and transport are analyzed via Reynolds stresses, anisotropy-invariant maps (AIM), and the turbulent kinetic energy (TKE) budget. In particular, the transition from “dipole” to a “quadrupole” wake is extensively examined as AR increases. With increasing AR, the wake shrinks in both the streamwise and spanwise directions, attributed to the occurrence of the base vortices (<span><math><mrow><mi>AR</mi><mo>=</mo><mn>3</mn></mrow></math></span> and 4). This change in the flow structure also affects the size of the positive-production region that extends from the roof and the flank of the obstacle to the wake core. The AIMs confirm three-dimensional wake features, showing TKE redistribution in all directions (Simonsen and Krogstad, 2005). Stronger turbulence production in <span><math><mrow><mi>AR</mi><mo>=</mo><mn>3</mn></mrow></math></span> and 4 cases highlights the role of tip and base vortices behind the cylinder. The overall aim is to refine the dipole-to-quadrupole transition as a function of AR and accounting for the incoming TBL properties. The novelty relies on proposing the momentum-thickness-based Reynolds number <span><math><msub><mrow><mi>Re</mi></mrow><mrow><mi>θ</mi></mrow></msub></math></span> as a discriminant for assessing TBL effects on turbulent wake structures.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109672"},"PeriodicalIF":2.6,"publicationDate":"2024-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140142","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-10DOI: 10.1016/j.ijheatfluidflow.2024.109682
Michael Mansour , Dominique Thévenin
The performance characteristics of a centrifugal pump were investigated experimentally for different conditions of air–water two-phase flow. A 3D semi-open impeller (with twisted blades) and a geometrically similar 2D semi-open impeller (with straight elliptical blades) were employed to compare the pump behavior using each of them, revealing the influence of the blade twisting. Additionally, the effect of the tip clearance gap on the performance of each impeller was investigated by employing either a standard or an increased gap. A rotational speed of 1450 rpm was set for all experiments. The performance of the pump was reported and described for either a constant gas volume fraction or a constant air flow rate at the pump inlet. Possible hysteresis effects were studied by approaching the desired operating conditions using different procedures. The head degradation behavior, the pump surging conditions, and the flow instabilities were considered as well. The two-phase flow regimes were recorded and identified for all the considered cases using a high-speed recording system. The results show that for single-phase flow, both impellers perform similarly, with the 3D impeller showing slight advantages at part-load and the 2D impeller excelling at near-optimal and overload conditions. For two-phase flows with high gas volume fractions, the 3D impeller maintains better performance at overload, while the 2D impeller performs better at part-load conditions. Increasing the gap slightly improves performance for gas volume fractions between 5% and 7% in both impellers. Additionally, the 2D impeller demonstrates a stronger ability to suppress and delay surging and flow instabilities compared to the 3D impeller, making it preferable for applications requiring stable operation across a range of gas volume fractions and load conditions.
{"title":"Impact of blade twisting and tip clearance on centrifugal pump performance under air–water two-phase flow","authors":"Michael Mansour , Dominique Thévenin","doi":"10.1016/j.ijheatfluidflow.2024.109682","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109682","url":null,"abstract":"<div><div>The performance characteristics of a centrifugal pump were investigated experimentally for different conditions of air–water two-phase flow. A 3D semi-open impeller (with twisted blades) and a geometrically similar 2D semi-open impeller (with straight elliptical blades) were employed to compare the pump behavior using each of them, revealing the influence of the blade twisting. Additionally, the effect of the tip clearance gap on the performance of each impeller was investigated by employing either a standard or an increased gap. A rotational speed of 1450 rpm was set for all experiments. The performance of the pump was reported and described for either a constant gas volume fraction or a constant air flow rate at the pump inlet. Possible hysteresis effects were studied by approaching the desired operating conditions using different procedures. The head degradation behavior, the pump surging conditions, and the flow instabilities were considered as well. The two-phase flow regimes were recorded and identified for all the considered cases using a high-speed recording system. The results show that for single-phase flow, both impellers perform similarly, with the 3D impeller showing slight advantages at part-load and the 2D impeller excelling at near-optimal and overload conditions. For two-phase flows with high gas volume fractions, the 3D impeller maintains better performance at overload, while the 2D impeller performs better at part-load conditions. Increasing the gap slightly improves performance for gas volume fractions between 5% and 7% in both impellers. Additionally, the 2D impeller demonstrates a stronger ability to suppress and delay surging and flow instabilities compared to the 3D impeller, making it preferable for applications requiring stable operation across a range of gas volume fractions and load conditions.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109682"},"PeriodicalIF":2.6,"publicationDate":"2024-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140143","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-10DOI: 10.1016/j.ijheatfluidflow.2024.109680
M. Dellacasagrande , A. Ghidoni , G. Noventa , D. Simoni
Separation-induced transition showed to be the weakness of the transition models based on the laminar kinetic energy concept. In fact, these models contemplate only the Tolmien–Schlichting waves, for the natural mode, and the Klebanoff streaks, for the bypass mode. Literature is very poor about the use of these models to cases with the separation-induced mode and in all these works no proofs of the phenomenological agreement between the models and the physics of the flow are spotlighted. A further improvement for the Reynolds-Averaged Navier–Stokes equations in separated and transitional shear layers needs more accurate models to describe the physics behind the phenomena, e.g., the introduction of ad-hoc designed terms for the Kelvin–Helmholtz instability in the transport equations. The objective of this work is to assess a phenomenological and local transition model based on the laminar kinetic energy concept, implemented in a high-order discontinuous Galerkin solver, for the simulation of transitional flows. The prediction capabilities of the model are proved with the simulations of the flow over the ERCOFTAC and UNIGE flat plates, characterized by the bypass and separation-induced mode of transition. The education of the model is not only based on integral coefficients and first-order statistics, but also on the turbulence intensity, laminar and turbulent kinetic energy distributions extracted from finely processed experimental data.
{"title":"Transition model based on the laminar kinetic energy concept for the prediction of all transition modes","authors":"M. Dellacasagrande , A. Ghidoni , G. Noventa , D. Simoni","doi":"10.1016/j.ijheatfluidflow.2024.109680","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109680","url":null,"abstract":"<div><div>Separation-induced transition showed to be the weakness of the transition models based on the laminar kinetic energy concept. In fact, these models contemplate only the Tolmien–Schlichting waves, for the natural mode, and the Klebanoff streaks, for the bypass mode. Literature is very poor about the use of these models to cases with the separation-induced mode and in all these works no proofs of the phenomenological agreement between the models and the physics of the flow are spotlighted. A further improvement for the Reynolds-Averaged Navier–Stokes equations in separated and transitional shear layers needs more accurate models to describe the physics behind the phenomena, e.g., the introduction of ad-hoc designed terms for the Kelvin–Helmholtz instability in the transport equations. The objective of this work is to assess a phenomenological and local transition model based on the laminar kinetic energy concept, implemented in a high-order discontinuous Galerkin solver, for the simulation of transitional flows. The prediction capabilities of the model are proved with the simulations of the flow over the ERCOFTAC and UNIGE flat plates, characterized by the bypass and separation-induced mode of transition. The education of the model is not only based on integral coefficients and first-order statistics, but also on the turbulence intensity, laminar and turbulent kinetic energy distributions extracted from finely processed experimental data.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109680"},"PeriodicalIF":2.6,"publicationDate":"2024-12-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140447","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-12-09DOI: 10.1016/j.ijheatfluidflow.2024.109687
Shuang-gen Yang , Huan-ling Liu
With the miniaturization of electronic chips, and the complexity and diversity of functions, the application of multiple heat source arrays has appeared in recent years. In order to improve the heat dissipation performance of multiple high heat flux chips, this work innovatively designs a step-by-step distributed heat exchanger (SSD), which uses ethanol as coolant and is used for two-phase flow heat dissipation. Then, in order to improve the cooling ability, a crossed step-by-step distributed heat exchanger (CSSD) is proposed by arranging cross flow channels with high heat flux. The VOF two-phase flow numerical method is used to simulate the performance of these two kinds of heat exchangers. In addition, the effects of gravity and inlet height on the performance of the heat CSSD exchanger are studied. The results show that CSSD can shorten the bubble generation time, increases the gas volume fraction in the channel, increase the gas disturbance, and intensify the heat dissipation of two-phase flow compared to the SSD design. The results show that CSSD can reduce the maximum temperature of the heat exchanger and improve the temperature uniformity. Gravity has little effect on two-phase flow heat transfer. In addition, we set up a two-phase flow experimental system to verify the correctness of the numerical simulation. The results indicates that the numerical results are consistent with the experimental results.
{"title":"Investigation of the heat transfer performance of two-phase flow in a novel step-by-step distributed heat exchanger","authors":"Shuang-gen Yang , Huan-ling Liu","doi":"10.1016/j.ijheatfluidflow.2024.109687","DOIUrl":"10.1016/j.ijheatfluidflow.2024.109687","url":null,"abstract":"<div><div>With the miniaturization of electronic chips, and the complexity and diversity of functions, the application of multiple heat source arrays has appeared in recent years. In order to improve the heat dissipation performance of multiple high heat flux chips, this work innovatively designs a step-by-step distributed heat exchanger (SSD), which uses ethanol as coolant and is used for two-phase flow heat dissipation. Then, in order to improve the cooling ability, a crossed step-by-step distributed heat exchanger (CSSD) is proposed by arranging cross flow channels with high heat flux. The VOF two-phase flow numerical method is used to simulate the performance of these two kinds of heat exchangers. In addition, the effects of gravity and inlet height on the performance of the heat CSSD exchanger are studied. The results show that CSSD can shorten the bubble generation time, increases the gas volume fraction in the channel, increase the gas disturbance, and intensify the heat dissipation of two-phase flow compared to the SSD design. The results show that CSSD can reduce the maximum temperature of the heat exchanger and improve the temperature uniformity. Gravity has little effect on two-phase flow heat transfer. In addition, we set up a two-phase flow experimental system to verify the correctness of the numerical simulation. The results indicates that the numerical results are consistent with the experimental results.</div></div>","PeriodicalId":335,"journal":{"name":"International Journal of Heat and Fluid Flow","volume":"112 ","pages":"Article 109687"},"PeriodicalIF":2.6,"publicationDate":"2024-12-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"143140133","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"工程技术","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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