Boiling remains one of the most effective processes allowing heat to be removed from heat-loaded surfaces of technological equipment. Due to the development of technologies for metal surface treatment with laser radiation to form specified near-surface properties, including the type, texture configuration and wetting properties, the task of substantiating the use of heat transfer surfaces modified by laser radiation to intensify the boiling process has become relevant. The pool boiling characteristics in distilled degassed water (before the boiling crisis onset) were compared experimentally on the surfaces of samples made of aluminum alloy, copper, and WC-coated copper. Experimental samples were treated by two different methods, including widely used polishing with abrasive materials and laser radiation. Nanosecond laser treatment was used to make micro-finned and anisotropic (developed hierarchical roughness) textures. Textured samples were then hydrophobized. As part of the experiments on pool boiling, the characteristics of the forming bubbles were recorded on the prepared surfaces, heat transfer coefficients, and critical heat flux values were determined. Experimental results were compared with predicted characteristics using well-known models. The evolution of the functional properties of laser-textured metal surfaces after prolonged exposure to heat flux, which is typical of the operating modes of modern heat-loaded equipment, was assessed.
{"title":"Pool boiling on the aluminum alloy, copper and WC-coated copper with micro-finned textures and developed multimodal roughness formed by nanosecond laser radiation","authors":"E.G. Orlova , D.O. Glushkov , A.O. Pleshko , K.N. Gulkin , A.M. Abdelmagid , P.N. Maximov , M.M. Popov , D.V. Feoktistov","doi":"10.1016/j.expthermflusci.2024.111366","DOIUrl":"10.1016/j.expthermflusci.2024.111366","url":null,"abstract":"<div><div>Boiling remains one of the most effective processes allowing heat to be removed from heat-loaded surfaces of technological equipment. Due to the development of technologies for metal surface treatment with laser radiation to form specified near-surface properties, including the type, texture configuration and wetting properties, the task of substantiating the use of heat transfer surfaces modified by laser radiation to intensify the boiling process has become relevant. The pool boiling characteristics in distilled degassed water (before the boiling crisis onset) were compared experimentally on the surfaces of samples made of aluminum alloy, copper, and WC-coated copper. Experimental samples were treated by two different methods, including widely used polishing with abrasive materials and laser radiation. Nanosecond laser treatment was used to make micro-finned and anisotropic (developed hierarchical roughness) textures. Textured samples were then hydrophobized. As part of the experiments on pool boiling, the characteristics of the forming bubbles were recorded on the prepared surfaces, heat transfer coefficients, and critical heat flux values were determined. Experimental results were compared with predicted characteristics using well-known models. The evolution of the functional properties of laser-textured metal surfaces after prolonged exposure to heat flux, which is typical of the operating modes of modern heat-loaded equipment, was assessed.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111366"},"PeriodicalIF":2.8,"publicationDate":"2024-11-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142748415","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 : 2024-11-26DOI: 10.1016/j.expthermflusci.2024.111367
Mohy S. Mansour , Mohamed K. Hasanin , Mahmoud M.A. Ahmed
The combustion characteristics and stability are affected primarily by the mixing field structure. The goal of many practical systems is to achieve higher stability by creating Inhomogeneous, Partially Premixed, and Stratified (IPPS) environments. A three-axis regime diagram is proposed in this work to describe the mixing field structure of the IPPS, the non-premixed, and the premixed environments. The proposed axes of the diagram are the mean, the fluctuations, and the 2-D gradients of the conserved scalar mixture fraction.
Highly resolved two-dimensional measurements of the mixture fraction in highly stabilized burners with well-controlled mixture inhomogeneity using advanced Rayleigh scattering measurements are used in this study to investigate the abilities of the proposed diagram. The effects of the mixing level, equivalence ratio, and Reynolds number on the mixing field structure were investigated using this diagram adequately. The three-axis diagram provided quantitative detailed information on the mixing field structure at different operating conditions. In addition, the local mixing layer thickness data are extracted from the diagram based on the mixture fraction profiles and the corresponding mixture fraction gradients profiles in the mixture fraction space.
The level of mixture inhomogeneity and equivalence ratio significantly affect the mixing field structure, while the effect of the Reynolds number in turbulent conditions is weak. The correlations between the local mixing layer thickness and the main operating parameters are observed using its PDFs. Reducing the level of mixture inhomogeneity reduces the maximum mixture fraction gradients at zero fluctuations of the mean mixture fraction. The correlations are clear for further analytical investigation. This study shows that the proposed three-axis diagram is a useful tool to investigate and analyze the mixing field structure of the IPPS regimes.
{"title":"A three-axis regime diagram for quantitative analyses of the mixing field structure in laminar and turbulent combustion","authors":"Mohy S. Mansour , Mohamed K. Hasanin , Mahmoud M.A. Ahmed","doi":"10.1016/j.expthermflusci.2024.111367","DOIUrl":"10.1016/j.expthermflusci.2024.111367","url":null,"abstract":"<div><div>The combustion characteristics and stability are affected primarily by the mixing field structure. The goal of many practical systems is to achieve higher stability by creating Inhomogeneous, Partially Premixed, and Stratified (IPPS) environments. A three-axis regime diagram is proposed in this work to describe the mixing field structure of the IPPS, the non-premixed, and the premixed environments. The proposed axes of the diagram are the mean, the fluctuations, and the 2-D gradients of the conserved scalar mixture fraction.</div><div>Highly resolved two-dimensional measurements of the mixture fraction in highly stabilized burners with well-controlled mixture inhomogeneity using advanced Rayleigh scattering measurements are used in this study to investigate the abilities of the proposed diagram. The effects of the mixing level, equivalence ratio, and Reynolds number on the mixing field structure were investigated using this diagram adequately. The three-axis diagram provided quantitative detailed information on the mixing field structure at different operating conditions. In addition, the local mixing layer thickness data are extracted from the diagram based on the mixture fraction profiles and the corresponding mixture fraction gradients profiles in the mixture fraction space.</div><div>The level of mixture inhomogeneity and equivalence ratio significantly affect the mixing field structure, while the effect of the Reynolds number in turbulent conditions is weak. The correlations between the local mixing layer thickness and the main operating parameters are observed using its PDFs. Reducing the level of mixture inhomogeneity reduces the maximum mixture fraction gradients at zero fluctuations of the mean mixture fraction. The correlations are clear for further analytical investigation. This study shows that the proposed three-axis diagram is a useful tool to investigate and analyze the mixing field structure of the IPPS regimes.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111367"},"PeriodicalIF":2.8,"publicationDate":"2024-11-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142748417","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}
The present work investigates the use of Machine Learning methods for optimizing the control of the wake behind a circular cylinder with the aim of reducing the associated aerodynamic drag using a single synthetic jet located at the rear stagnation point. Initially, a parametric study on sinusoidal shapes is performed to assess the control authority of the synthetic jet and to identify suitable initial configurations for the subsequent optimization study. This optimization leverages gradient-enriched Machine Learning (gMLC), which is based on Linear Genetic Programming, to determine the optimal waveshape for the input driving signal to the synthetic jet actuator, aiming at aerodynamic drag reduction. Machine Learning is thus exploited to overcome limitations inherent to canonical waveshapes. All the experiments are performed at a Reynolds number . Four different optimization runs are conducted to study the effect of increasing the complexity of the genetic recombination process and including a power penalty in the cost function on the control effectiveness. The maximum drag reduction is achieved when no penalty for the power consumption is included in the cost function and amounts to 9.77% with respect to the baseline case. The addition of the power penalty results in control laws comparable in both waveshape and performance to the canonical sinusoidal control laws. In the second part of this work, the ML-derived control policies are investigated via hot-wire anemometry and Particle Image Velocimetry (PIV) to understand and characterize the mechanisms responsible for the drag reduction and the control effects on the wake evolution. For this purpose, a modal analysis based on Proper Orthogonal Decomposition is performed to comparatively assess the control laws and evaluate their capability of weakening and mitigating the most energetic flow structures associated with the vortex shedding phenomenon.
本研究采用机器学习方法对圆柱体后方的尾流进行优化控制，目的是利用位于后停滞点的单个合成射流减少相关的气动阻力。首先，对正弦形状进行参数研究，以评估合成射流的控制能力，并为随后的优化研究确定合适的初始配置。该优化利用基于线性遗传编程的梯度丰富机器学习（gMLC）来确定合成喷气致动器输入驱动信号的最佳波形，目的是减少气动阻力。因此，机器学习被用来克服典型波形固有的局限性。所有实验均在雷诺数 Re=1.9×104 的条件下进行。进行了四次不同的优化运行，以研究增加基因重组过程的复杂性以及在成本函数中加入功率惩罚对控制效果的影响。当成本函数中不包含功耗惩罚时，阻力降低幅度最大，与基线情况相比达到 9.77%。增加功率惩罚后，控制法则在波形和性能上都可与正弦控制法则相媲美。在这项工作的第二部分，通过热丝风速测量法和粒子图像测速仪（PIV）对 ML 衍生的控制策略进行了研究，以了解和描述造成阻力减少的机制以及对尾流演变的控制效果。为此，基于适当正交分解进行了模态分析，以比较评估控制法则，并评估其削弱和减轻与涡流脱落现象相关的高能流动结构的能力。
{"title":"Genetically-based active flow control of a circular cylinder wake via synthetic jets","authors":"Alessandro Scala, Gerardo Paolillo, Carlo Salvatore Greco, Tommaso Astarita, Gennaro Cardone","doi":"10.1016/j.expthermflusci.2024.111362","DOIUrl":"10.1016/j.expthermflusci.2024.111362","url":null,"abstract":"<div><div>The present work investigates the use of Machine Learning methods for optimizing the control of the wake behind a circular cylinder with the aim of reducing the associated aerodynamic drag using a single synthetic jet located at the rear stagnation point. Initially, a parametric study on sinusoidal shapes is performed to assess the control authority of the synthetic jet and to identify suitable initial configurations for the subsequent optimization study. This optimization leverages gradient-enriched Machine Learning (gMLC), which is based on Linear Genetic Programming, to determine the optimal waveshape for the input driving signal to the synthetic jet actuator, aiming at aerodynamic drag reduction. Machine Learning is thus exploited to overcome limitations inherent to canonical waveshapes. All the experiments are performed at a Reynolds number <span><math><mrow><mi>R</mi><mi>e</mi><mo>=</mo><mn>1</mn><mo>.</mo><mn>9</mn><mo>×</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mn>4</mn></mrow></msup></mrow></math></span>. Four different optimization runs are conducted to study the effect of increasing the complexity of the genetic recombination process and including a power penalty in the cost function on the control effectiveness. The maximum drag reduction is achieved when no penalty for the power consumption is included in the cost function and amounts to 9.77% with respect to the baseline case. The addition of the power penalty results in control laws comparable in both waveshape and performance to the canonical sinusoidal control laws. In the second part of this work, the ML-derived control policies are investigated via hot-wire anemometry and Particle Image Velocimetry (PIV) to understand and characterize the mechanisms responsible for the drag reduction and the control effects on the wake evolution. For this purpose, a modal analysis based on Proper Orthogonal Decomposition is performed to comparatively assess the control laws and evaluate their capability of weakening and mitigating the most energetic flow structures associated with the vortex shedding phenomenon.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111362"},"PeriodicalIF":2.8,"publicationDate":"2024-11-23","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142720814","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}
The uneven distribution of flow phases in evaporator channels can drop the heat exchanger efficiency up to 30%. Due to its dependence on the interaction of several coexisting variables – both geometry, operating conditions, and fluid properties – it is a complex phenomenon to analyze. Most studies focus on the effect of single parameters: this is an inefficient and expensive way of doing experiments, and the results lack in understanding how the combination of variables affects the flow distribution. This paper presents a methodology to optimally characterize and predict the distribution of flow phases in the channels of an evaporator header based on Design of Experiment (DoE) techniques. Despite the proven potential of DoE methods, they have never been applied in this field. Tests were conducted with an air–water mixture in the configuration horizontal header with vertical channels with downward flow, varying inlet pipe position, channels intrusion, presence of a splashing grid at the header inlet, and air and water flow rates. Results prove that, when working with complex processes, interaction effects between variables cannot be neglected as they significantly affect the response. The most affecting parameter was found to be the air flow rate, followed by the combination between inlet pipe position and presence of the splashing grid. With horizontal inlet, the optimal response was given by absence of intrusion, presence of the splashing grid, lowest water, and highest air flow rate. Instead, for the vertical case, the distribution was enhanced with the highest intrusion, absence of the grid, and highest water and air flow rates. Lastly a first attempt to model the process was performed. Even if a universal regression model has low accuracy (51%), restricting the area of analysis can result in valid predictive relations, with accuracies up to 91.4%.
{"title":"Experimental assessment of multi-phase flow distribution in an evaporator header through Design of Experiments techniques","authors":"Claretta Tempesti , Aude Lecardonnel , Delphine Laboureur","doi":"10.1016/j.expthermflusci.2024.111359","DOIUrl":"10.1016/j.expthermflusci.2024.111359","url":null,"abstract":"<div><div>The uneven distribution of flow phases in evaporator channels can drop the heat exchanger efficiency up to 30%. Due to its dependence on the interaction of several coexisting variables – both geometry, operating conditions, and fluid properties – it is a complex phenomenon to analyze. Most studies focus on the effect of single parameters: this is an inefficient and expensive way of doing experiments, and the results lack in understanding how the combination of variables affects the flow distribution. This paper presents a methodology to optimally characterize and predict the distribution of flow phases in the channels of an evaporator header based on Design of Experiment (DoE) techniques. Despite the proven potential of DoE methods, they have never been applied in this field. Tests were conducted with an air–water mixture in the configuration horizontal header with vertical channels with downward flow, varying inlet pipe position, channels intrusion, presence of a splashing grid at the header inlet, and air and water flow rates. Results prove that, when working with complex processes, interaction effects between variables cannot be neglected as they significantly affect the response. The most affecting parameter was found to be the air flow rate, followed by the combination between inlet pipe position and presence of the splashing grid. With horizontal inlet, the optimal response was given by absence of intrusion, presence of the splashing grid, lowest water, and highest air flow rate. Instead, for the vertical case, the distribution was enhanced with the highest intrusion, absence of the grid, and highest water and air flow rates. Lastly a first attempt to model the process was performed. Even if a universal regression model has low accuracy (51%), restricting the area of analysis can result in valid predictive relations, with accuracies up to 91.4%.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111359"},"PeriodicalIF":2.8,"publicationDate":"2024-11-22","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142748416","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 : 2024-11-21DOI: 10.1016/j.expthermflusci.2024.111363
Jaemin Sim , Hyungmin Park
While the wire-mesh sensor (WMS) has been widely used to measure the gas-phase distribution in gas–liquid two-phase flows with visually inaccessible conditions, there are still some issues to be improved such as the dependency on the bubble size and bubble-wire interaction pattern. In our previous work [Lee et al., Int. J. Multiphas. Flow, 139, 103,620 (2021)], we reported the development and validation of a set of two-layer WMS, which can simultaneously measure the velocity and size of O(10−2–100) mm bubbles. In this study, considering the above-mentioned issues, we improve the measurement accuracy of the WMS, in particular, for relatively small (less than 2 mm) bubbles that tend to have asymmetric interactions with wires. To achieve this, we devised a new parameter to address the effect of non-uniform distance (proximity) to the nodes that are affected by the bubbles to be measured. Depending on the bubble-wire interaction (i.e., bubble size), we found that the characteristic time and length scales of the bubble vary, which are correlated with the proximity of bubbles to nodes with high electrical sensitivity. By implementing this into the algorithm, compared to the previous setup, the accuracy of measuring the averaged void fraction and velocity of bubbles increased by approximately 10 % and 20 %, respectively. We believe that the present approach will be quite useful in enhancing the measurement accuracy of other types (dual-set, for example) of wire-mesh sensors.
虽然金属丝网传感器（WMS）已被广泛用于测量气液两相流中的气相分布，但仍有一些问题需要改进，如对气泡大小和气泡与金属丝网相互作用模式的依赖性。在我们之前的工作[Lee 等，Int. J. Multiphas. Flow，139，103,620 (2021)]中，我们报告了一套双层 WMS 的开发和验证，它可以同时测量 O(10-2-100) mm 气泡的速度和大小。在本研究中，考虑到上述问题，我们提高了 WMS 的测量精度，尤其是对相对较小（小于 2 毫米）的气泡的测量精度，因为这些气泡往往会与导线产生不对称的相互作用。为此，我们设计了一个新参数，以解决与待测气泡影响的节点的不均匀距离（接近度）的影响。根据气泡与导线的相互作用（即气泡大小），我们发现气泡的特征时间尺度和长度尺度各不相同，这与气泡是否靠近具有高电学灵敏度的节点相关。通过在算法中加入这一点，与之前的设置相比，测量气泡平均空隙率和速度的精度分别提高了约 10% 和 20%。我们相信，本方法对于提高其他类型（例如双组）金属丝网传感器的测量精度非常有用。
{"title":"Improvement of a wire-mesh sensor based on the bubble-wire collision kinematics","authors":"Jaemin Sim , Hyungmin Park","doi":"10.1016/j.expthermflusci.2024.111363","DOIUrl":"10.1016/j.expthermflusci.2024.111363","url":null,"abstract":"<div><div>While the wire-mesh sensor (WMS) has been widely used to measure the gas-phase distribution in gas–liquid two-phase flows with visually inaccessible conditions, there are still some issues to be improved such as the dependency on the bubble size and bubble-wire interaction pattern. In our previous work [Lee et al., Int. J. Multiphas. Flow, 139, 103,620 (2021)], we reported the development and validation of a set of two-layer WMS, which can simultaneously measure the velocity and size of <strong><em>O</em></strong>(10<sup>−2</sup>–10<sup>0</sup>) mm bubbles. In this study, considering the above-mentioned issues, we improve the measurement accuracy of the WMS, in particular, for relatively small (less than 2 mm) bubbles that tend to have asymmetric interactions with wires. To achieve this, we devised a new parameter to address the effect of non-uniform distance (proximity) to the nodes that are affected by the bubbles to be measured. Depending on the bubble-wire interaction (i.e., bubble size), we found that the characteristic time and length scales of the bubble vary, which are correlated with the proximity of bubbles to nodes with high electrical sensitivity. By implementing this into the algorithm, compared to the previous setup, the accuracy of measuring the averaged void fraction and velocity of bubbles increased by approximately 10 % and 20 %, respectively. We believe that the present approach will be quite useful in enhancing the measurement accuracy of other types (dual-set, for example) of wire-mesh sensors.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111363"},"PeriodicalIF":2.8,"publicationDate":"2024-11-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142703653","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}
Ammonia has recently gained significant attention as a zero-carbon fuel and a carrier of hydrogen. However, its flame instability and emissions remain major challenges. In this study, the effect of low swirl configuration on stability limits and emissions of premixed ammonia-methane-air swirling flames at various equivalence ratios and ammonia blending ratios were investigated. The low swirl configuration was achieved by using three center through-hole swirlers with different mass fluxes ratio (Rm) for non-swirling and swirling flows. The results showed that the flames exhibited thermoacoustic instability at Rm = 0 % and detected higher emissions for all conditions where the ammonia mixing ratios are less than 60 %. By contrast, the swirlers with Rm = 25 % and Rm = 49 % ensured flames stabilization under all operating conditions. The swirler with Rm = 25 % effectively reduced NOx emissions about 30 % while slightly narrowing the stability limits. CO and unburned NH3 emissions also decreased significantly, but N2O emissions increased. This was due to the non-swirling flow reduced the flame temperature, which inhibited the thermal cracking of N2O. As Rm = 49 %, the stability limits became very narrow, although the emissions decreased further. In conjunction, it is found that increasing Rm caused the flame structure to shift from the V-shape to the W-shape, with the flame obviously lifting and the recirculation zone in the flow field shrinking. These changes are responsible for the transformation of the stability limits and emission with different low swirl configurations, which provide a feasible idea for use of ammonia as a gas turbine fuel to improve combustion efficiency and reduce emissions.
{"title":"The effect of low swirl configurations on stability limits and emission characteristics in premixed ammonia-methane-air swirling flames","authors":"Haojie Yang, Chunyu Liu, Boshu Dong, Xuejiao Li, Liang Yu, Xingcai Lu","doi":"10.1016/j.expthermflusci.2024.111361","DOIUrl":"10.1016/j.expthermflusci.2024.111361","url":null,"abstract":"<div><div>Ammonia has recently gained significant attention as a zero-carbon fuel and a carrier of hydrogen. However, its flame instability and emissions remain major challenges. In this study, the effect of low swirl configuration on stability limits and emissions of premixed ammonia-methane-air swirling flames at various equivalence ratios and ammonia blending ratios were investigated. The low swirl configuration was achieved by using three center through-hole swirlers with different mass fluxes ratio (<em>R<sub>m</sub></em>) for non-swirling and swirling flows. The results showed that the flames exhibited thermoacoustic instability at <em>R<sub>m</sub></em> = 0 % and detected higher emissions for all conditions where the ammonia mixing ratios are less than 60 %. By contrast, the swirlers with <em>R<sub>m</sub></em> = 25 % and <em>R<sub>m</sub></em> = 49 % ensured flames stabilization under all operating conditions. The swirler with <em>R<sub>m</sub></em> = 25 % effectively reduced NO<sub>x</sub> emissions about 30 % while slightly narrowing the stability limits. CO and unburned NH<sub>3</sub> emissions also decreased significantly, but N<sub>2</sub>O emissions increased. This was due to the non-swirling flow reduced the flame temperature, which inhibited the thermal cracking of N<sub>2</sub>O. As <em>R<sub>m</sub></em> = 49 %, the stability limits became very narrow, although the emissions decreased further. In conjunction, it is found that increasing <em>R<sub>m</sub></em> caused the flame structure to shift from the V-shape to the W-shape, with the flame obviously lifting and the recirculation zone in the flow field shrinking. These changes are responsible for the transformation of the stability limits and emission with different low swirl configurations, which provide a feasible idea for use of ammonia as a gas turbine fuel to improve combustion efficiency and reduce emissions.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111361"},"PeriodicalIF":2.8,"publicationDate":"2024-11-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142703651","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 : 2024-11-17DOI: 10.1016/j.expthermflusci.2024.111360
Hui Qi , Hao Wu , Zhiyu Fan , Sining Li , Yongbin Jia , Xianglong Yang , Shiqi Zhang , Jing Guo
Our investigation of spheres entering water at high-temperature reveals that elevated temperatures modify traditional cavitation patterns and trigger novel fluid dynamic phenomena. Experimental analysis of the high-temperature sphere’s water entry process has identified four distinct cavitation morphologies: small cavities, complete cavities, dual cavities, and unstable cavities. These phenomena result from the sphere’s thermal effects altering the local flow dynamics around it, consequently impacting the hydrodynamic coefficients. Notably, thermal conditions cause the contact line from the sphere’s midpoint to transition to its tail, leading to transformations in cavity types. Furthermore, simulations employing the lattice Boltzmann method elucidate how unstable steam films formed on hot surfaces induce boundary slip, reducing pressure drag. This observation provides further insight into established mechanisms of fluid drag reduction. Our study deepens the understanding of how temperature influences water entry dynamics and offers new perspectives on reducing drag during the water entry process of objects.
{"title":"Experimental investigation on heated spheres entering water vertically at different temperatures","authors":"Hui Qi , Hao Wu , Zhiyu Fan , Sining Li , Yongbin Jia , Xianglong Yang , Shiqi Zhang , Jing Guo","doi":"10.1016/j.expthermflusci.2024.111360","DOIUrl":"10.1016/j.expthermflusci.2024.111360","url":null,"abstract":"<div><div>Our investigation of spheres entering water at high-temperature reveals that elevated temperatures modify traditional cavitation patterns and trigger novel fluid dynamic phenomena. Experimental analysis of the high-temperature sphere’s water entry process has identified four distinct cavitation morphologies: small cavities, complete cavities, dual cavities, and unstable cavities. These phenomena result from the sphere’s thermal effects altering the local flow dynamics around it, consequently impacting the hydrodynamic coefficients. Notably, thermal conditions cause the contact line from the sphere’s midpoint to transition to its tail, leading to transformations in cavity types. Furthermore, simulations employing the lattice Boltzmann method elucidate how unstable steam films formed on hot surfaces induce boundary slip, reducing pressure drag. This observation provides further insight into established mechanisms of fluid drag reduction. Our study deepens the understanding of how temperature influences water entry dynamics and offers new perspectives on reducing drag during the water entry process of objects.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111360"},"PeriodicalIF":2.8,"publicationDate":"2024-11-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142703650","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 : 2024-11-16DOI: 10.1016/j.expthermflusci.2024.111356
L.F. Olivera-Reyes, E.S. Palacios de Paz, S. Sánchez, J.F. Hernández-Sánchez
Synthetic Jet (SJ) actuators are an intrinsically complex combination of electronics, electric and mechanical systems. When studied theoretically, these elements are often simplified to coupled damped harmonic oscillators (DHO) that induce a pressure field within the cavity and drive momentum exchange. Thus, the dynamics of an SJ actuator result from coupling these DHOs, naturally leading to a few resonant modes. There is good evidence in the specialized literature of two resonant modes developing in SJ actuators: the membrane/piezoelectric mode and the Helmholtz resonance. In this work, we report on the effect of a new resonant mode that dominates the two traditional modes when it develops. We present evidence that the resonant mode develops when the cavity is much larger than the volume displaced by the actuator. The new resonant mode is biased to lower frequencies and has a flatter response along the frequency band than other resonant modes. We show that jet and vortex velocities mimic the sound pressure curve for the low-frequency range. Its effect mitigates for the higher range due to a delve through shorter stroke lengths, characterized through the well-documented formation criteria as a fixed relation between the Reynolds and the Stokes numbers. We further characterize the new resonant mode by comparing its intensity with standard room modes. We also show that the resonant mode may be dimmed and focused by adding an obstacle in different cavity positions for the lower sound intensities. We consider that the large-cavity dynamics is an additional element that, if integrated as design criteria, could extend the applicability of SJs and their optimum response.
{"title":"A novel resonant mode drives the dynamics of a large-cavity synthetic jet actuator","authors":"L.F. Olivera-Reyes, E.S. Palacios de Paz, S. Sánchez, J.F. Hernández-Sánchez","doi":"10.1016/j.expthermflusci.2024.111356","DOIUrl":"10.1016/j.expthermflusci.2024.111356","url":null,"abstract":"<div><div>Synthetic Jet (SJ) actuators are an intrinsically complex combination of electronics, electric and mechanical systems. When studied theoretically, these elements are often simplified to coupled damped harmonic oscillators (DHO) that induce a pressure field within the cavity and drive momentum exchange. Thus, the dynamics of an SJ actuator result from coupling these DHOs, naturally leading to a few resonant modes. There is good evidence in the specialized literature of two resonant modes developing in SJ actuators: the membrane/piezoelectric mode and the Helmholtz resonance. In this work, we report on the effect of a new resonant mode that dominates the two traditional modes when it develops. We present evidence that the resonant mode develops when the cavity is much larger than the volume displaced by the actuator. The new resonant mode is biased to lower frequencies and has a flatter response along the frequency band than other resonant modes. We show that jet and vortex velocities mimic the sound pressure curve for the low-frequency range. Its effect mitigates for the higher range due to a delve through shorter stroke lengths, characterized through the well-documented formation criteria as a fixed relation between the Reynolds and the Stokes numbers. We further characterize the new resonant mode by comparing its intensity with standard room modes. We also show that the resonant mode may be dimmed and focused by adding an obstacle in different cavity positions for the lower sound intensities. We consider that the large-cavity dynamics is an additional element that, if integrated as design criteria, could extend the applicability of SJs and their optimum response.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"162 ","pages":"Article 111356"},"PeriodicalIF":2.8,"publicationDate":"2024-11-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142703652","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 : 2024-11-15DOI: 10.1016/j.expthermflusci.2024.111355
Leilei Ji , Wei Pu , Wei Li , Weidong Shi , Yang Yang , Cui Xiao , Fei Tain , Jie Zhou , Ramesh Agarwal
In order to study the influence law of the impeller wake on dynamic and static interference flow field of the centrifugal pump, this paper obtains the dynamic and static interference flow field of the centrifugal pump under different flow conditions (15 m3/h, 50 m3/h, 70 m3/h) based on PIV technology, and analyzes the influence mechanism of the impeller wake change on dynamic and static interference flow field. The results show that rotating stall occurs in the centrifugal pump under low flow condition, but it has little effect on the head loss in the centrifugal pump. In the dynamic and static interference flow field, under the condition of the low flow rate, the impeller wake collides with the baffle tongue, resulting in serious velocity fluctuation, and then there will be the secondary collision with the volute wall, which will eventually cause the wake to dissipate, and the change process of the wake shows the periodic characteristic. In the design condition and the large flow condition, the spacer tongue will have the cutting effect on the wake, and the high-speed accumulation phenomenon will occur in the volute flow path near the spacer tongue. In the side flow path of the volute, under the condition of the low flow, the impeller wake is mainly located at the exit of the impeller, and the end of the impeller wake is easy to fall off and gradually break up under the impact of the main stream. Under the design condition, the flow stability is good, and the wake vortex is close to the trailing edge of the blade, and there is no obvious shedding phenomenon. Under the condition of the large flow rate, the velocity fluctuates sharply, and many large-scale vortex structures appear on the cross section of the flow channel due to the cutting of the wake near the diaphragm tongue. In the impeller passage, the movement and distribution of the wake are significantly affected by changes in flow conditions. The research results provide a basis for optimizing volute channel.
{"title":"PIV experimental study on dynamic and static interference flow field of multi-operating centrifugal pump under the influence of impeller wake","authors":"Leilei Ji , Wei Pu , Wei Li , Weidong Shi , Yang Yang , Cui Xiao , Fei Tain , Jie Zhou , Ramesh Agarwal","doi":"10.1016/j.expthermflusci.2024.111355","DOIUrl":"10.1016/j.expthermflusci.2024.111355","url":null,"abstract":"<div><div>In order to study the influence law of the impeller wake on dynamic and static interference flow field of the centrifugal pump, this paper obtains the dynamic and static interference flow field of the centrifugal pump under different flow conditions (15 m<sup>3</sup>/h, 50 m<sup>3</sup>/h, 70 m<sup>3</sup>/h) based on PIV technology, and analyzes the influence mechanism of the impeller wake change on dynamic and static interference flow field. The results show that rotating stall occurs in the centrifugal pump under low flow condition, but it has little effect on the head loss in the centrifugal pump. In the dynamic and static interference flow field, under the condition of the low flow rate, the impeller wake collides with the baffle tongue, resulting in serious velocity fluctuation, and then there will be the secondary collision with the volute wall, which will eventually cause the wake to dissipate, and the change process of the wake shows the periodic characteristic. In the design condition and the large flow condition, the spacer tongue will have the cutting effect on the wake, and the high-speed accumulation phenomenon will occur in the volute flow path near the spacer tongue. In the side flow path of the volute, under the condition of the low flow, the impeller wake is mainly located at the exit of the impeller, and the end of the impeller wake is easy to fall off and gradually break up under the impact of the main stream. Under the design condition, the flow stability is good, and the wake vortex is close to the trailing edge of the blade, and there is no obvious shedding phenomenon. Under the condition of the large flow rate, the velocity fluctuates sharply, and many large-scale vortex structures appear on the cross section of the flow channel due to the cutting of the wake near the diaphragm tongue. In the impeller passage, the movement and distribution of the wake are significantly affected by changes in flow conditions. The research results provide a basis for optimizing volute channel.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"161 ","pages":"Article 111355"},"PeriodicalIF":2.8,"publicationDate":"2024-11-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142700110","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 : 2024-11-12DOI: 10.1016/j.expthermflusci.2024.111358
Linmeng Yu , Jiao Sun , Kangfu Sun , Pengda Yuan , Wenyi Chen
The flow characteristics on a corrugated wall and the variations caused by a perforation are investigated experimentally based on two-dimensional particle image velocimetry (PIV), and the passive control mechanism of the perforation on the corrugated wall is studied. The corrugated wall has an amplitude-to-wavelength ratio 2a/λ = 0.1, with the wavelength Reynolds number Reλ = 14400 and bulk Reynolds number Reb = 17500. The perforation is located on the eleventh cycle of the corrugated wall. The results show that perforation increases the area of the recirculation zone, reduces the effect of frictional resistance, weakens the turbulence intensity and the Reynolds normal stress on the corrugated wall, but enhances the Reynolds shear stress. The POD and the Finite-Time Lyapunov Exponent(FTLE) are used to analyze the vortex structures. From the FTLE result, it can be seen that the perforation disturbs the original shear layer and redistributes the vortex structure of the flow field. The instantaneous fluctuating flow field of the first 50 % and the last 50 % of the energy content after POD mode decomposition is reconstructed to study the effect of perforation on on large and small scale structures in the flow field. It is found that the impact of perforation on small-scale structures is greater than that on large-scale structures.
基于二维粒子图像测速仪（PIV）对波纹壁上的流动特性以及穿孔引起的流动变化进行了实验研究，并对波纹壁上穿孔的被动控制机制进行了研究。波纹壁的振幅波长比为 2a/λ = 0.1，波长雷诺数 Reλ = 14400，体积雷诺数 Reb = 17500。穿孔位于波纹壁的第十一周期。结果表明，穿孔增加了再循环区的面积，降低了摩擦阻力的影响，减弱了波纹壁上的湍流强度和雷诺法向应力，但增强了雷诺剪应力。采用 POD 和有限时间 Lyapunov 指数（FTLE）分析涡流结构。从 FTLE 结果可以看出，穿孔扰动了原有的剪切层，并重新分配了流场的涡旋结构。通过重建 POD 模式分解后前 50%和后 50%能量含量的瞬时波动流场，研究穿孔对流场大小尺度结构的影响。研究发现，穿孔对小尺度结构的影响大于对大尺度结构的影响。
{"title":"Effect of a perforation on the flow characteristics of corrugated wall","authors":"Linmeng Yu , Jiao Sun , Kangfu Sun , Pengda Yuan , Wenyi Chen","doi":"10.1016/j.expthermflusci.2024.111358","DOIUrl":"10.1016/j.expthermflusci.2024.111358","url":null,"abstract":"<div><div>The flow characteristics on a corrugated wall and the variations caused by a perforation are investigated experimentally based on two-dimensional particle image velocimetry (PIV), and the passive control mechanism of the perforation on the corrugated wall is studied. The corrugated wall has an amplitude-to-wavelength ratio 2<em>a</em>/<em>λ</em> = 0.1, with the wavelength Reynolds number Re<em><sub>λ</sub></em> = 14400 and bulk Reynolds number Re<em><sub>b</sub></em> = 17500. The perforation is located on the eleventh cycle of the corrugated wall. The results show that perforation increases the area of the recirculation zone, reduces the effect of frictional resistance, weakens the turbulence intensity and the Reynolds normal stress on the corrugated wall, but enhances the Reynolds shear stress. The POD and the Finite-Time Lyapunov Exponent(FTLE) are used to analyze the vortex structures. From the FTLE result, it can be seen that the perforation disturbs the original shear layer and redistributes the vortex structure of the flow field. The instantaneous fluctuating flow field of the first 50 % and the last 50 % of the energy content after POD mode decomposition is reconstructed to study the effect of perforation on on large and small scale structures in the flow field. It is found that the impact of perforation on small-scale structures is greater than that on large-scale structures.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"161 ","pages":"Article 111358"},"PeriodicalIF":2.8,"publicationDate":"2024-11-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142699783","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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