Pub Date : 2024-10-30DOI: 10.1016/j.expthermflusci.2024.111351
Understanding the droplet size and shape control mechanism in a magnetic field is critical for precisely manipulating ferrofluid droplets. Here, we conducted an experimental investigation on the dynamic behavior of a falling ferrofluid droplet under a nonuniform magnetic field produced by current coils. We observed an interesting phenomenon: the uneven distribution of the magnetic field and the jump in magnetic properties at fluid interfaces will cause the Laplace pressure difference on the droplet surface, stimulating the droplet’s oscillation. We also use the Laplace pressure difference equation and the interfacial tension coefficient correlation to model the deformation of ferrofluid droplets and determine the oscillation frequencies and deflection angles. The droplets’ oscillation frequency is related to the magnetic Bond number: f∼. The deflection angle of the droplet is further diminished by the superposition of a viscous shear moment and a magnetic moment (7.41°∼12.90°). Our research lays the groundwork for precise ferrofluid droplet manipulation in drug delivery and soft robots.
{"title":"The oscillation of a falling ferrofluid droplet induced by a nonuniform magnetic field","authors":"","doi":"10.1016/j.expthermflusci.2024.111351","DOIUrl":"10.1016/j.expthermflusci.2024.111351","url":null,"abstract":"<div><div>Understanding the droplet size and shape control mechanism in a magnetic field is critical for precisely manipulating ferrofluid droplets. Here, we conducted an experimental investigation on the dynamic behavior of a falling ferrofluid droplet under a nonuniform magnetic field produced by current coils. We observed an interesting phenomenon: the uneven distribution of the magnetic field and the jump in magnetic properties at fluid interfaces will cause the Laplace pressure difference on the droplet surface, stimulating the droplet’s oscillation. We also use the Laplace pressure difference equation and the interfacial tension coefficient correlation to model the deformation of ferrofluid droplets and determine the oscillation frequencies and deflection angles. The droplets’ oscillation frequency is related to the magnetic Bond number: <em>f</em>∼<span><math><mrow><msubsup><mrow><mi>Bo</mi></mrow><mrow><mi>m</mi></mrow><mrow><mo>-</mo><mn>0.523</mn><mspace></mspace><mo>∼</mo><mo>-</mo><mn>0.501</mn></mrow></msubsup></mrow></math></span>. The deflection angle of the droplet is further diminished by the superposition of a viscous shear moment and a magnetic moment (7.41<sup>°</sup>∼12.90<sup>°</sup>). Our research lays the groundwork for precise ferrofluid droplet manipulation in drug delivery and soft robots.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-30","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142579125","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-28DOI: 10.1016/j.expthermflusci.2024.111343
When a bileaflet mechanical heart valve is surgically implanted into the body, the downstream left ventricular blood flow pattern becomes complex, which is directly related to many postoperative complications. To investigate the hemodynamic properties associated with mechanical heart valve design, we built a left heart circulatory pulsatile flow generation system to simulate left ventricular flow and pressure under physiological conditions. We used time-resolved particle image velocimetry to study left ventricular blood flow downstream of two types of bileaflet mechanical heart valve: one with planar leaflets and one with cambered leaflets. Blood flow downstream of two different bileaflet mechanical valve shapes was assessed. The experimental results show that the bileaflet valve with a triple-jet pattern creates a three-dimensional vortex ring with a complex topology. In addition, the robust jet mode can introduce high shear stresses into the ventricular blood flow. Compared with the planar valve, the jet produced by the cambered valve has a more uniform velocity distribution, its vortex structure moves farther, and its shear stress distribution is more straightforward and continuous. Furthermore, the channel formed between the cambered valve vortex structure and the left ventricle wall surface is highly favorable for scouring the apical position and facilitating the transport of blood to the aortic orifice. Therefore, the shape of the leaflets of a bileaflet mechanical valve can significantly impact the left ventricular blood flow pattern and the blood transport process. Rational optimization of the design of the leaflet shape and improvement of the mechanical valve’s hemodynamic characteristics can reduce complications after valve replacement.
{"title":"Effect of leaflet shape on the left ventricular blood flow pattern in BMHVs","authors":"","doi":"10.1016/j.expthermflusci.2024.111343","DOIUrl":"10.1016/j.expthermflusci.2024.111343","url":null,"abstract":"<div><div>When a bileaflet mechanical heart valve is surgically implanted into the body, the downstream left ventricular blood flow pattern becomes complex, which is directly related to many postoperative complications. To investigate the hemodynamic properties associated with mechanical heart valve design, we built a left heart circulatory pulsatile flow generation system to simulate left ventricular flow and pressure under physiological conditions. We used time-resolved particle image velocimetry to study left ventricular blood flow downstream of two types of bileaflet mechanical heart valve: one with planar leaflets and one with cambered leaflets. Blood flow downstream of two different bileaflet mechanical valve shapes was assessed. The experimental results show that the bileaflet valve with a triple-jet pattern creates a three-dimensional vortex ring with a complex topology. In addition, the robust jet mode can introduce high shear stresses into the ventricular blood flow. Compared with the planar valve, the jet produced by the cambered valve has a more uniform velocity distribution, its vortex structure moves farther, and its shear stress distribution is more straightforward and continuous. Furthermore, the channel formed between the cambered valve vortex structure and the left ventricle wall surface is highly favorable for scouring the apical position and facilitating the transport of blood to the aortic orifice. Therefore, the shape of the leaflets of a bileaflet mechanical valve can significantly impact the left ventricular blood flow pattern and the blood transport process. Rational optimization of the design of the leaflet shape and improvement of the mechanical valve’s hemodynamic characteristics can reduce complications after valve replacement.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142572132","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-28DOI: 10.1016/j.expthermflusci.2024.111342
A comprehensive experimental study was performed to characterize the fluid transportation processes in pulsatile post-stenotic flows. This study aims to understand the effect of pulsatility on the transportation dynamics of post-stenotic flows and to establish a non-dimensional number to quantify transportation effectiveness in these flows. Two-dimensional particle tracking velocimetry measurements were conducted in a close flow loop with a symmetric stenosis model. A pathline extension algorithm is then applied to the obtained Lagrangian data, such that fluid parcels are continuously tracked as they flow through the region of interest. Pulsatile flows at Reynolds numbers , Strouhal number and amplitude ratio and 0.8 are systematically investigated to understand the influence of pulsatility on the transport and mixing dynamics. The flow structures, such as the formation and evaluation of vortex rings, Kelvin-Helmoltz instabilities, jet meandering and breakdown, are clearly revealed by the lifespan parcel trajectories and the particle residence time (PRT). These structures are closely related to the transportation behaviours of the post-stenotic flows. Using the obtained Lagrangian results, the transportation effectiveness of the post-stenotic flows is further quantified by the depletion efficiency. The results demonstrate that while post-stenotic flows transport most residual fluids under a higher amplitude ratio, the depletion efficiency itself is insensitive to the amplitude ratio. The flow system operates more efficiently with high pulsatile frequencies (). Additionally, a transportation effectiveness parameter, , is defined to evaluate the transport performance by comparing the transportation efficiency to the pressure drop. The value is optimized at a high pulsatile frequency () and a low amplitude ratio (), with being up to twice as high as its counterpart in the steady flow.
{"title":"Lagrangian analysis of fluid transport in pulsatile post-stenotic flows","authors":"","doi":"10.1016/j.expthermflusci.2024.111342","DOIUrl":"10.1016/j.expthermflusci.2024.111342","url":null,"abstract":"<div><div>A comprehensive experimental study was performed to characterize the fluid transportation processes in pulsatile post-stenotic flows. This study aims to understand the effect of pulsatility on the transportation dynamics of post-stenotic flows and to establish a non-dimensional number to quantify transportation effectiveness in these flows. Two-dimensional particle tracking velocimetry measurements were conducted in a close flow loop with a symmetric stenosis model. A pathline extension algorithm is then applied to the obtained Lagrangian data, such that fluid parcels are continuously tracked as they flow through the region of interest. Pulsatile flows at Reynolds numbers <span><math><msub><mi>Re</mi><mi>m</mi></msub><mo>=</mo><mn>1000</mn><mo>,</mo><mspace></mspace><mn>2000</mn><mo>,</mo><mspace></mspace><mn>4000</mn></math></span>, Strouhal number <span><math><mi>St</mi><mo>=</mo><mn>0.05</mn><mo>,</mo><mspace></mspace><mn>0.1</mn><mo>,</mo><mspace></mspace><mn>0.15</mn></math></span> and amplitude ratio <span><math><mrow><mi>λ</mi><mo>=</mo><mn>0.4</mn></mrow></math></span> and 0.8 are systematically investigated to understand the influence of pulsatility on the transport and mixing dynamics. The flow structures, such as the formation and evaluation of vortex rings, Kelvin-Helmoltz instabilities, jet meandering and breakdown, are clearly revealed by the lifespan parcel trajectories and the particle residence time (PRT). These structures are closely related to the transportation behaviours of the post-stenotic flows. Using the obtained Lagrangian results, the transportation effectiveness of the post-stenotic flows is further quantified by the depletion efficiency. The results demonstrate that while post-stenotic flows transport most residual fluids under a higher amplitude ratio, the depletion efficiency itself is insensitive to the amplitude ratio. The flow system operates more efficiently with high pulsatile frequencies (<span><math><mrow><mi>St</mi><mo>=</mo><mn>0.8</mn></mrow></math></span>). Additionally, a transportation effectiveness parameter, <span><math><mrow><mi>Te</mi></mrow></math></span>, is defined to evaluate the transport performance by comparing the transportation efficiency to the pressure drop. The <span><math><mrow><mi>Te</mi></mrow></math></span> value is optimized at a high pulsatile frequency (<span><math><mrow><mi>St</mi><mo>=</mo><mn>0.8</mn></mrow></math></span>) and a low amplitude ratio (<span><math><mrow><mi>λ</mi><mo>=</mo><mn>0.4</mn></mrow></math></span>), with <span><math><mrow><mi>Te</mi></mrow></math></span> being up to twice as high as its counterpart in the steady flow.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142579126","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-24DOI: 10.1016/j.expthermflusci.2024.111338
Contributing to a better understanding of spray cooling systems, the heat transfer process underlying the event of a droplet impinging onto a uniformly heated stainless steel surface (SS304) was experimentally investigated. Since the heat transfer process is linked to the droplet’s hydrodynamics, high-speed videos were recorded to measure the deformation of the droplet. A series of isothermal and non-isothermal impacts were performed for Weber numbers () within the range . A strong relationship between the maximum spreading ratio reached by the droplet and its initial kinetic energy was found. The surface temperature directly affects the droplet hydrodynamic during the impact by promoting an oscillatory behavior of the droplet after the maximum spreading is reached. Given the spatial–temporal resolution of the heat transfer process, a high-frequency phosphor thermometry technique was implemented, finding that the temperature drop upon droplet impact was independent of impact velocity. The sharp temperature drop results in an intense thermal interaction that occurred during the first 10 ms of the impact. The maximum average heat flux registered was 98.56 with a cooling effectiveness of 3.5%.
{"title":"Implementation of a high-frequency phosphor thermometry technique to study the heat transfer of a single droplet impingement","authors":"","doi":"10.1016/j.expthermflusci.2024.111338","DOIUrl":"10.1016/j.expthermflusci.2024.111338","url":null,"abstract":"<div><div>Contributing to a better understanding of spray cooling systems, the heat transfer process underlying the event of a droplet impinging onto a uniformly heated stainless steel surface (SS304) was experimentally investigated. Since the heat transfer process is linked to the droplet’s hydrodynamics, high-speed videos were recorded to measure the deformation of the droplet. A series of isothermal and non-isothermal impacts were performed for Weber numbers (<span><math><mrow><mi>W</mi><mi>e</mi></mrow></math></span>) within the range <span><math><mrow><mn>17</mn><mo>.</mo><mn>7</mn><mo>≤</mo><mi>W</mi><mi>e</mi><mo>≤</mo><mn>58</mn><mo>.</mo><mn>2</mn></mrow></math></span>. A strong relationship between the maximum spreading ratio reached by the droplet and its initial kinetic energy was found. The surface temperature directly affects the droplet hydrodynamic during the impact by promoting an oscillatory behavior of the droplet after the maximum spreading is reached. Given the spatial–temporal resolution of the heat transfer process, a high-frequency phosphor thermometry technique was implemented, finding that the temperature drop upon droplet impact was independent of impact velocity. The sharp temperature drop results in an intense thermal interaction that occurred during the first 10 ms of the impact. The maximum average heat flux registered was 98.56 <span><math><mrow><mi>W</mi><mo>/</mo><mi>c</mi><msup><mrow><mi>m</mi></mrow><mrow><mn>2</mn></mrow></msup></mrow></math></span> with a cooling effectiveness of 3.5%.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142553418","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-24DOI: 10.1016/j.expthermflusci.2024.111340
Gas-fired boilers operating at high-altitude regions often suffer from inadequate output, decreased thermal efficiency, and excessive NOx emissions. The effect of sub-atmospheric pressure on flame appearance and pollutant formation is the main reason for those problems, and thus needs to be clarified particularly under furnace combustion conditions with a fixed excess air coefficient. Inverse diffusion is a widely employed fuel–air configuration in burners of gas-fired boilers, and therefore the flame appearance, CO generation, and NO generation were experimentally investigated in this paper by adopting a low-pressure quartz tube reactor. Results show that the flame is elongated from reducing pressure under fuel-lean conditions, mainly due to the reduced oxygen mass concentration and the elevated jet velocity. Under fuel-rich combustion conditions, however, the flame is shorted at sub-atmospheric pressure from the suppressed soot formation. The reduced pressure leads to an increase in the global strain rate, making the flame more prone to uplift. With decreasing pressure, the increased air–fuel mixing and flame length facilitate the gas burnout, thus decreasing CO generation. The sub-atmospheric pressure could evidently reduce the NO generation under fuel-rich conditions, but slightly increase it under fuel-lean conditions. Under fuel-lean conditions, the NO major pathways (prompt, thermal, NNH, and N2O) are promoted which leads to an increase in NO generation with decreasing pressure. Under fuel-rich conditions, however, NO formation is suppressed from the decreased rate of reaction .
在高海拔地区运行的燃气锅炉经常会出现出力不足、热效率降低和氮氧化物排放过多的问题。亚大气压对火焰外观和污染物形成的影响是造成这些问题的主要原因，因此需要对其进行澄清，尤其是在过剩空气系数固定的炉膛燃烧条件下。反向扩散是燃气锅炉燃烧器中广泛采用的一种燃料-空气配置，因此本文采用低压石英管反应器对火焰外观、CO 生成量和 NO 生成量进行了实验研究。结果表明，在燃料贫乏的条件下，火焰因压力降低而伸长，这主要是由于氧气质量浓度降低和喷射速度提高所致。然而，在燃料丰富的燃烧条件下，由于烟尘的形成受到抑制，火焰在亚大气压下被缩短。压力降低导致整体应变率增加，使火焰更容易上浮。随着压力的降低，空气-燃料混合和火焰长度的增加会促进气体燃烧，从而减少 CO 的生成。在燃料丰富的条件下，亚大气压可以明显减少氮氧化物的生成，但在燃料贫乏的条件下，氮氧化物的生成会略有增加。在燃料贫乏的条件下，促进了 NO 的主要生成途径（瞬时、热、NNH 和 N2O），从而导致 NO 生成量随着压力的降低而增加。然而，在燃料丰富的条件下，NO2+CH═HCN+N 的反应速率降低，从而抑制了 NO 的生成。
{"title":"Effects of sub-atmospheric pressure on appearance and pollutant formation of inverse diffusion flame within a confined space","authors":"","doi":"10.1016/j.expthermflusci.2024.111340","DOIUrl":"10.1016/j.expthermflusci.2024.111340","url":null,"abstract":"<div><div>Gas-fired boilers operating at high-altitude regions often suffer from inadequate output, decreased thermal efficiency, and excessive NO<em><sub>x</sub></em> emissions. The effect of sub-atmospheric pressure on flame appearance and pollutant formation is the main reason for those problems, and thus needs to be clarified particularly under furnace combustion conditions with a fixed excess air coefficient. Inverse diffusion is a widely employed fuel–air configuration in burners of gas-fired boilers, and therefore the flame appearance, CO generation, and NO generation were experimentally investigated in this paper by adopting a low-pressure quartz tube reactor. Results show that the flame is elongated from reducing pressure under fuel-lean conditions, mainly due to the reduced oxygen mass concentration and the elevated jet velocity. Under fuel-rich combustion conditions, however, the flame is shorted at sub-atmospheric pressure from the suppressed soot formation. The reduced pressure leads to an increase in the global strain rate, making the flame more prone to uplift. With decreasing pressure, the increased air–fuel mixing and flame length facilitate the gas burnout, thus decreasing CO generation. The sub-atmospheric pressure could evidently reduce the NO generation under fuel-rich conditions, but slightly increase it under fuel-lean conditions. Under fuel-lean conditions, the NO major pathways (prompt, thermal, NNH, and N<sub>2</sub>O) are promoted which leads to an increase in NO generation with decreasing pressure. Under fuel-rich conditions, however, NO formation is suppressed from the decreased rate of reaction <span><math><mrow><mtext>N2</mtext><mo>+</mo><mtext>CH</mtext><mo>↔</mo><mtext>HCN</mtext><mo>+</mo><mtext>N</mtext></mrow></math></span>.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142572133","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-23DOI: 10.1016/j.expthermflusci.2024.111341
Superhydrophobic surfaces (SHSs) have been proven effective in reducing frictional drag force in various flow conditions. However, at high flow speeds, the air plastron on these surfaces collapses, leading to a decline in their effectiveness. In this study, we investigated the frictional drag forces of various SHSs and their combination with surface patterns across a wide range of flow conditions ( < Re < ) by using an facile coating method. Our experiments involved incorporating a superhydrophobic coating on the inner cylinder of a custom-made Taylor-Couette apparatus, integrated with a rheometer to measure torque applied on the inner rotor as a function of rotational speed. As part of our research, we calculate the effective slip length to assess the drag reduction performance of coatings, revealing an effective slip length of around 63 µm on a flat SHS. Furthermore, we explore the combined effect of superhydrophobic coatings and triangular-shaped riblets on drag reduction in Taylor-Couette flow, comparing the performance of these surfaces based on the riblet’s sharpness and the Reynolds number. Our experimental results show a reduction in measured torque of up to 24 % and 48 % on a V-grooved SHS in laminar and turbulent flow, respectively. Longevity tests confirm that the designed surfaces maintain their superhydrophobicity and drag reduction performance under turbulent flow conditions. Overall, this work introduces a passive drag reduction strategy through surface design, which substantially mitigates the frictional drag force and demonstrating considerable potential for enhanced performance and increased efficiency of Taylor-Couette systems.
{"title":"Sustainable drag reduction in Taylor-Couette flow using riblet superhydrophobic surfaces","authors":"","doi":"10.1016/j.expthermflusci.2024.111341","DOIUrl":"10.1016/j.expthermflusci.2024.111341","url":null,"abstract":"<div><div>Superhydrophobic surfaces (SHSs) have been proven effective in reducing frictional drag force in various flow conditions. However, at high flow speeds, the air plastron on these surfaces collapses, leading to a decline in their effectiveness. In this study, we investigated the frictional drag forces of various SHSs and their combination with surface patterns across a wide range of flow conditions (<span><math><mrow><mn>5.00</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mn>2</mn></msup></mrow></math></span> < Re < <span><math><mrow><mn>1.12</mn><mo>×</mo><msup><mrow><mn>10</mn></mrow><mn>5</mn></msup></mrow></math></span>) by using an facile coating method. Our experiments involved incorporating a superhydrophobic coating on the inner cylinder of a custom-made Taylor-Couette apparatus, integrated with a rheometer to measure torque applied on the inner rotor as a function of rotational speed. As part of our research, we calculate the effective slip length to assess the drag reduction performance of coatings, revealing an effective slip length of around 63 µm on a flat SHS. Furthermore, we explore the combined effect of superhydrophobic coatings and triangular-shaped riblets on drag reduction in Taylor-Couette flow, comparing the performance of these surfaces based on the riblet’s sharpness and the Reynolds number. Our experimental results show a reduction in measured torque of up to 24 % and 48 % on a V-grooved SHS in laminar and turbulent flow, respectively. Longevity tests confirm that the designed surfaces maintain their superhydrophobicity and drag reduction performance under turbulent flow conditions. Overall, this work introduces a passive drag reduction strategy through surface design, which substantially mitigates the frictional drag force and demonstrating considerable potential for enhanced performance and increased efficiency of Taylor-Couette systems.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142527907","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-21DOI: 10.1016/j.expthermflusci.2024.111337
Phase change heat transfer, crucial in thermal management systems, can be significantly enhanced through optimized surface structures. This study investigates pool boiling heat transfer enhancement using 3D printed structures with carefully designed pillar and pit geometries. We present a novel approach combining the Dual Rise model with separate liquid–vapor pathways to improve Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC). Using copper-infused Polylactic Acid (PLA) filaments, we created and sintered structured surfaces featuring pit-assisted nucleation sites, interpillar spacing for vapor escape, and pillar roughness for enhanced liquid supply. Experiments with deionized water and ethanol under atmospheric pressure demonstrated substantial improvements over plain surfaces: water showed an 87% increase in CHF and 39% in maximum HTC, while ethanol exhibited even greater enhancements of 122% in CHF and 61% in HTC. These improvements are attributed to the synergistic effects of optimized surface geometry and separated liquid–vapor pathways, reducing counterflow resistance and improving hydrodynamic stability. A theoretical framework based on the Dual Rise model explains these enhancements, providing insights into coupled capillary action and hemiwicking effects in boiling heat transfer. The study introduces predictive models for CHF and HTC enhancement, offering valuable tools for future design optimization in applications ranging from electronics cooling to power plant thermal management and advanced heat exchangers.
{"title":"Enhanced Phase Change Heat Transfer with Fused Deposition Modeling (FDM) Printed Pit and Pillar (Pi2) Arrays","authors":"","doi":"10.1016/j.expthermflusci.2024.111337","DOIUrl":"10.1016/j.expthermflusci.2024.111337","url":null,"abstract":"<div><div>Phase change heat transfer, crucial in thermal management systems, can be significantly enhanced through optimized surface structures. This study investigates pool boiling heat transfer enhancement using 3D printed structures with carefully designed pillar and pit geometries. We present a novel approach combining the Dual Rise model with separate liquid–vapor pathways to improve Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC). Using copper-infused Polylactic Acid (PLA) filaments, we created and sintered structured surfaces featuring pit-assisted nucleation sites, interpillar spacing for vapor escape, and pillar roughness for enhanced liquid supply. Experiments with deionized water and ethanol under atmospheric pressure demonstrated substantial improvements over plain surfaces: water showed an 87% increase in CHF and 39% in maximum HTC, while ethanol exhibited even greater enhancements of 122% in CHF and 61% in HTC. These improvements are attributed to the synergistic effects of optimized surface geometry and separated liquid–vapor pathways, reducing counterflow resistance and improving hydrodynamic stability. A theoretical framework based on the Dual Rise model explains these enhancements, providing insights into coupled capillary action and hemiwicking effects in boiling heat transfer. The study introduces predictive models for CHF and HTC enhancement, offering valuable tools for future design optimization in applications ranging from electronics cooling to power plant thermal management and advanced heat exchangers.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142527904","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-19DOI: 10.1016/j.expthermflusci.2024.111339
An eccentric inlet swirl, a source of circumferential non-uniformity in aero-engines, can compromise the stable operational range of the compression system and potentially jeopardize the safety of the entire flight vehicle. This study experimentally examined the additional effects of an eccentric inlet swirl on an axial fan in comparison with a concentric inlet condition. Then, the effectiveness of an impedance-boundary-controlled (IBC) casing treatment (CT) in extending the stable operating range of the fan under eccentric inlets is evaluated. Steady-state loading and prestall disturbance analyses were conducted using a five-hole probe and high-frequency response pressure transducers to elucidate the instability mechanisms of fans exposed to eccentric inlets. The findings indicate that the eccentric swirl generates localized over-loading regions around the circumference, where abnormal prestall disturbances amplify in amplitude across a frequency range of 0.3 to 0.5 times the blade passing frequency. These characteristics were mitigated when IBC CT was applied over the rotor tip, allowing the fan to operate under concentric inlet conditions. The IBC CT enhances the stall margin of the fan by 9.3–19.6% in response to a range of swirl inlet conditions, suggesting its potential to address the irregularity problems in fans/compressors. The mechanisms by which IBC CT extends the stall margin are discussed from the unique perspective of evaluating steady loading and system damping.
{"title":"Effects of impedance-boundary-controlled casing treatment on the fan performance with eccentric inlet swirl","authors":"","doi":"10.1016/j.expthermflusci.2024.111339","DOIUrl":"10.1016/j.expthermflusci.2024.111339","url":null,"abstract":"<div><div>An eccentric inlet swirl, a source of circumferential non-uniformity in aero-engines, can compromise the stable operational range of the compression system and potentially jeopardize the safety of the entire flight vehicle. This study experimentally examined the additional effects of an eccentric inlet swirl on an axial fan in comparison with a concentric inlet condition. Then, the effectiveness of an impedance-boundary-controlled (IBC) casing treatment (CT) in extending the stable operating range of the fan under eccentric inlets is evaluated. Steady-state loading and prestall disturbance analyses were conducted using a five-hole probe and high-frequency response pressure transducers to elucidate the instability mechanisms of fans exposed to eccentric inlets. The findings indicate that the eccentric swirl generates localized over-loading regions around the circumference, where abnormal prestall disturbances amplify in amplitude across a frequency range of 0.3 to 0.5 times the blade passing frequency. These characteristics were mitigated when IBC CT was applied over the rotor tip, allowing the fan to operate under concentric inlet conditions. The IBC CT enhances the stall margin of the fan by 9.3–19.6% in response to a range of swirl inlet conditions, suggesting its potential to address the irregularity problems in fans/compressors. The mechanisms by which IBC CT extends the stall margin are discussed from the unique perspective of evaluating steady loading and system damping.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142527908","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-18DOI: 10.1016/j.expthermflusci.2024.111336
Understanding co/counter-swirl or twin-swirl flames remains challenging due to the complex interaction of two swirling streams. In the present study, we investigate the heat release features of non-premixed co/counter-swirl syngas/air flames and their’ influence on combustor noise in a ∼20 kW combustor using simultaneous high-speed OH*-chemiluminescence (5 kHz) and microphone measurement (50 kHz) by varying the momentum ratio (M) from 0.4 to 0.95. Furthermore, the velocity field is examined using a low-speed two-dimensional particle image velocimetry (2D-PIV, frequency = 7 Hz). For all studied momentum ratios, the frequency spectra of noise measurements for both co/counter-swirl flames consistently exhibit a dominant frequency (∼285 Hz), close to the fundamental axial mode of the combustor. A further analysis using spectrograms, phase spaces, and recurrence plots reveals intermittent patterns in noise measurements, featuring periodic (P) and aperiodic regions (A). In periodic regions (P), noise synchronizes with the global fluctuation of the heat release rate as observed in chemiluminescence. Along with global fluctuation, the chemiluminescence also reveals a rotational component of heat release rate with distinct frequencies for co and counter-swirl configurations. This rotational motion possibly originated from a precessing vortex core (PVC) as indicated by the zig-zag arrangements of vortices in the inner shear layer. Furthermore, the impact of M on global fluctuation and rotational motion has been investigated using the frequency spectrum of OH*-intensity and the distribution of peaks in noise measurement. The global fluctuation is found to be suppressed when M increases while the rotational component becomes prominent at higher M. Therefore, the study elucidates the co-existence of global fluctuation and rotational motion and how these motions evolve with the varying momentum ratio (M), thus enhancing the understanding of combustion characteristics of the complex twin-swirl (co/counter-swirl) flames.
{"title":"Influence of momentum ratio on heat release rate and noise of co/counter-swirl non-premixed diluted CO/H2 flames","authors":"","doi":"10.1016/j.expthermflusci.2024.111336","DOIUrl":"10.1016/j.expthermflusci.2024.111336","url":null,"abstract":"<div><div>Understanding co/counter-swirl or twin-swirl flames remains challenging due to the complex interaction of two swirling streams. In the present study, we investigate the heat release features of non-premixed co/counter-swirl syngas/air flames and their’ influence on combustor noise in a ∼20 kW combustor using simultaneous high-speed OH*-chemiluminescence (5 kHz) and microphone measurement (50 kHz) by varying the momentum ratio (<em>M</em>) from 0.4 to 0.95. Furthermore, the velocity field is examined using a low-speed two-dimensional particle image velocimetry (2D-PIV, frequency = 7 Hz). For all studied momentum ratios, the frequency spectra of noise measurements for both co/counter-swirl flames consistently exhibit a dominant frequency (∼285 Hz), close to the fundamental axial mode of the combustor. A further analysis using spectrograms, phase spaces, and recurrence plots reveals intermittent patterns in noise measurements, featuring periodic (P) and aperiodic regions (A). In periodic regions (P), noise synchronizes with the global fluctuation of the heat release rate as observed in chemiluminescence. Along with global fluctuation, the chemiluminescence also reveals a rotational component of heat release rate with distinct frequencies for co and counter-swirl configurations. This rotational motion possibly originated from a precessing vortex core (PVC) as indicated by the zig-zag arrangements of vortices in the inner shear layer. Furthermore, the impact of <em>M</em> on global fluctuation and rotational motion has been investigated using the frequency spectrum of OH*-intensity and the distribution of peaks in noise measurement. The global fluctuation is found to be suppressed when <em>M</em> increases while the rotational component becomes prominent at higher <em>M</em>. Therefore, the study elucidates the co-existence of global fluctuation and rotational motion and how these motions evolve with the varying momentum ratio (<em>M</em>), thus enhancing the understanding of combustion characteristics of the complex twin-swirl (co/counter-swirl) flames.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142527906","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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 : 2024-10-16DOI: 10.1016/j.expthermflusci.2024.111332
Experiments were conducted in this study to investigate the chocked cavitation characteristics and its oscillation mechanism in jet pump cavitation reactors (JPCR) under limited operation stage (LOS) utilizing a synchronous measurement system. The pulsation characteristics of cavitation in JPCR under various inlet and outlet pressures were analyzed by the processed high-speed camera images. Furthermore, correlation between cavitation and pressure pulsation as well as the mechanism of cavitation oscillation in JPCR under LOS are elucidated based on synchronized measurements. The results reveal that the typical jet choked cavitation flow field can be divided into three characteristic regions, i.e., stability region, oscillation region and collapse region. Changes in flow parameters cause variations in the areas of these three regions and shift the initial and collapse positions of cavitation. The time-averaged length of cavitation clouds varies linearly with the absolute pressure ratio at the outlet, corresponding to both stable and unstable LOS. Notably, the results reveal a clear correlation between the grayscale of cavitation clouds and pressure fluctuations over time, identifying the inverse pressure gradient as the primary cause of cavitation oscillation in the throat tube during unstable LOS.
本研究利用同步测量系统，对喷射泵空化反应器（JPCR）在有限运行阶段（LOS）下的阻塞空化特性及其振荡机制进行了实验研究。通过处理后的高速摄像图像，分析了喷射泵空化反应器在不同入口和出口压力下的空化脉动特征。此外，基于同步测量还阐明了气蚀与压力脉动之间的相关性以及 LOS 条件下 JPCR 中气蚀振荡的机理。研究结果表明，典型的喷气窒息空化流场可分为三个特征区域，即稳定区、振荡区和崩溃区。流动参数的变化会导致这三个区域的面积发生变化，并移动空化的初始位置和塌陷位置。空化云的时间平均长度与出口处的绝对压力比呈线性变化，与稳定和不稳定 LOS 相对应。值得注意的是，结果显示空化云的灰度与压力随时间的波动之间存在明显的相关性，从而确定反向压力梯度是不稳定 LOS 期间喉管内空化振荡的主要原因。
{"title":"Experimental investigation of chocked cavitation flow and its oscillation mechanism in jet pump cavitation reactors under limited operation stage","authors":"","doi":"10.1016/j.expthermflusci.2024.111332","DOIUrl":"10.1016/j.expthermflusci.2024.111332","url":null,"abstract":"<div><div>Experiments were conducted in this study to investigate the chocked cavitation characteristics and its oscillation mechanism in jet pump cavitation reactors (JPCR) under limited operation stage (LOS) utilizing a synchronous measurement system. The pulsation characteristics of cavitation in JPCR under various inlet and outlet pressures were analyzed by the processed high-speed camera images. Furthermore, correlation between cavitation and pressure pulsation as well as the mechanism of cavitation oscillation in JPCR under LOS are elucidated based on synchronized measurements. The results reveal that the typical jet choked cavitation flow field can be divided into three characteristic regions, i.e., stability region, oscillation region and collapse region. Changes in flow parameters cause variations in the areas of these three regions and shift the initial and collapse positions of cavitation. The time-averaged length of cavitation clouds varies linearly with the absolute pressure ratio at the outlet, corresponding to both stable and unstable LOS. Notably, the results reveal a clear correlation between the grayscale of cavitation clouds and pressure fluctuations over time, identifying the inverse pressure gradient as the primary cause of cavitation oscillation in the throat tube during unstable LOS.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":null,"pages":null},"PeriodicalIF":2.8,"publicationDate":"2024-10-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142527903","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"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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