Pub Date : 2026-01-08DOI: 10.1016/j.expthermflusci.2026.111698
Raul Serrano-Bayona , Felipe Campuzano , Faruk Aydin , Tirthankar Mitra , Peng Liu , William L. Roberts
This study investigates the morphological and optical properties of soot aggregates in ethylene/air laminar coflow diffusion flames under partial substitution by hydrogen (H2) and ammonia (NH3), using the non-intrusive multi-angle light scattering (MALS) technique. A vertically-polarized 514.5 nm beam was focused on the flame centerline, and the scattering signal was collected between 25°-155°. A reference flame (Fuel: 60 vol% ethylene, 40 vol% nitrogen) was compared with cases where nitrogen (N2) is substituted with H2 and NH3, maintaining a constant exit flow velocity. Measurements were carried out at three heights to assess the influence of residence time. Soot properties, including scattering-to-absorption ratio (), single-scatter albedo (), mean aggregate radius of gyration (), fractal dimension (), and primary particle diameter (), were characterized using the Rayleigh-Debye-Gans theory for fractal aggregates (RDG-FA). H2 substitution increased and , and displayed a more pronounced decrease in , indicating enhanced aggregation and restructuring driven by elevated temperatures. Conversely, NH3 substitution reduced , possibly due to inhibition in soot nucleation rates and surface growth by reducing H-radical concentrations and carbon-based precursors. decreased with height, possibly due to enhanced graphitization and surface oxidation. Higher flame temperature increased and , and these ratios decreased with a reduction in carbon flux. These results offer a more integrated understanding of the links between aggregate structure, growth dynamics, and radiative behavior of soot formed under different fuel substitution conditions.
{"title":"Effects of hydrogen and ammonia substitution on morphological and optical parameters of soot aggregates in ethylene/air laminar coflow diffusion flames","authors":"Raul Serrano-Bayona , Felipe Campuzano , Faruk Aydin , Tirthankar Mitra , Peng Liu , William L. Roberts","doi":"10.1016/j.expthermflusci.2026.111698","DOIUrl":"10.1016/j.expthermflusci.2026.111698","url":null,"abstract":"<div><div>This study investigates the morphological and optical properties of soot aggregates in ethylene/air laminar coflow diffusion flames under partial substitution by hydrogen (H<sub>2</sub>) and ammonia (NH<sub>3</sub>), using the non-intrusive multi-angle light scattering (MALS) technique. A vertically-polarized 514.5 nm beam was focused on the flame centerline, and the scattering signal was collected between 25°-155°. A reference flame (Fuel: 60 vol% ethylene, 40 vol% nitrogen) was compared with cases where nitrogen (N<sub>2</sub>) is substituted with H<sub>2</sub> and NH<sub>3</sub>, maintaining a constant exit flow velocity. Measurements were carried out at three heights to assess the influence of residence time. Soot properties, including scattering-to-absorption ratio (<span><math><msub><mi>ρ</mi><mrow><mi>SA</mi></mrow></msub></math></span>), single-scatter albedo (<span><math><msub><mi>ω</mi><mi>A</mi></msub></math></span>), mean aggregate radius of gyration (<span><math><msub><mi>R</mi><mrow><mi>gm</mi></mrow></msub></math></span>), fractal dimension (<span><math><msub><mi>D</mi><mi>f</mi></msub></math></span>), and primary particle diameter (<span><math><msub><mi>d</mi><mi>p</mi></msub></math></span>), were characterized using the Rayleigh-Debye-Gans theory for fractal aggregates (RDG-FA). H<sub>2</sub> substitution increased <span><math><msub><mi>R</mi><mrow><mi>gm</mi></mrow></msub></math></span> and <span><math><msub><mi>D</mi><mi>f</mi></msub></math></span>, and displayed a more pronounced decrease in <span><math><msub><mi>d</mi><mi>p</mi></msub></math></span>, indicating enhanced aggregation and restructuring driven by elevated temperatures. Conversely, NH<sub>3</sub> substitution reduced <span><math><msub><mi>R</mi><mrow><mi>gm</mi></mrow></msub></math></span>, possibly due to inhibition in soot nucleation rates and surface growth by reducing H-radical concentrations and carbon-based precursors. <span><math><msub><mi>d</mi><mi>p</mi></msub></math></span> decreased with height, possibly due to enhanced graphitization and surface oxidation. Higher flame temperature increased <span><math><msub><mi>ρ</mi><mrow><mi>SA</mi></mrow></msub></math></span> and <span><math><msub><mi>ω</mi><mi>A</mi></msub></math></span>, and these ratios decreased with a reduction in carbon flux. These results offer a more integrated understanding of the links between aggregate structure, growth dynamics, and radiative behavior of soot formed under different fuel substitution conditions.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"174 ","pages":"Article 111698"},"PeriodicalIF":3.3,"publicationDate":"2026-01-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145941149","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}
Multiple-orifice synthetic jets have garnered significant attention in recent research due to their exceptional heat transfer performance in small installations. To achieve a larger heat dissipation area while minimizing installation height, this study employs dual synthetic jets impinging on a heated plate and investigates the influence of installation height (hi/wo) on the heat transfer characteristics. The heat transfer mechanisms associated with vortex structure evolution are analyzed using particle image velocimetry (PIV) and an infrared camera, thereby facilitating a deeper understanding of flow-thermal coupling behavior. It was found that hi/wo significantly alters the evolution of vortex structures at a nearly constant jet Reynolds number (Rej ≈ 1600). The optimal heat transfer performance (Numax ≈ 136) was achieved at hi/wo = 2–3, balancing the unsteady benefit and near-wall flow enhancement. At this installation height, the inner primary vortex (IPV) rolls up moderately and partially merges its vorticity with the wall shear layer (WSL), while secondary vortices (SVs), SV-1 and SV-2, emanate from the outer primary vortex (OPV). The appropriate vorticity distributions of SV-1 and SV-2 promote the entrainment of ambient fluid and fluctuate the WSL. The spectral analysis further substantiates the presence of unsteady benefit. Both Welch spectra and Spectral POD mode confirmed that hi/wo = 2–3 delivers the best coherence maintenance, indicating an overall optimal benefit from heat transfer. Conversely, an insufficient installation height (hi/wo = 0–1) mitigated IPV roll-up. Although higher momentum flux and periodic kinetic energy were attained near the orifice, the convective heat transfer coefficient decayed rapidly. Excessive installation height (hi/wo = 4) allowed complete IPV roll-up. IPV breaks down into secondary vortices before merging into the WSL, resulting in deteriorated heat transfer performance.
{"title":"Heat transfer mechanism of vortex structure evolution in dual synthetic jets over a heating plate","authors":"Yuanyuan Liu, Xinyu Liang, Wenqiang Peng, Zhenbing Luo, Xinyu Xie, Xiong Deng","doi":"10.1016/j.expthermflusci.2026.111689","DOIUrl":"10.1016/j.expthermflusci.2026.111689","url":null,"abstract":"<div><div>Multiple-orifice synthetic jets have garnered significant attention in recent research due to their exceptional heat transfer performance in small installations. To achieve a larger heat dissipation area while minimizing installation height, this study employs dual synthetic jets impinging on a heated plate and investigates the influence of installation height (<em>h<sub>i</sub></em>/<em>w<sub>o</sub></em>) on the heat transfer characteristics. The heat transfer mechanisms associated with vortex structure evolution are analyzed using particle image velocimetry (PIV) and an infrared camera, thereby facilitating a deeper understanding of flow-thermal coupling behavior. It was found that <em>h<sub>i</sub></em>/<em>w<sub>o</sub></em> significantly alters the evolution of vortex structures at a nearly constant jet Reynolds number (<em>Re<sub>j</sub></em> ≈ 1600). The optimal heat transfer performance (<em>Nu<sub>max</sub></em> ≈ 136) was achieved at <em>h<sub>i</sub></em>/<em>w<sub>o</sub></em> = 2–3, balancing the unsteady benefit and near-wall flow enhancement. At this installation height, the inner primary vortex (IPV) rolls up moderately and partially merges its vorticity with <del>t</del>he wall shear layer (WSL), while secondary vortices (SVs), SV-1 and SV-2, emanate from the outer primary vortex (OPV). The appropriate vorticity distributions of SV-1 and SV-2 promote the entrainment of ambient fluid and fluctuate the WSL. The spectral analysis further substantiates the presence of unsteady benefit. Both Welch spectra and Spectral POD mode confirmed that <em>h<sub>i</sub></em>/<em>w<sub>o</sub></em> = 2–3 delivers the best coherence maintenance, indicating an overall optimal benefit from heat transfer. Conversely, an insufficient installation height (<em>h<sub>i</sub></em>/<em>w<sub>o</sub></em> = 0–1) mitigated IPV roll-up. Although higher momentum flux and periodic kinetic energy were attained near the orifice, the convective heat transfer coefficient decayed rapidly. Excessive installation height (<em>h<sub>i</sub></em>/<em>w<sub>o</sub></em> = 4) allowed complete IPV roll-up. IPV breaks down into secondary vortices before merging into the WSL, resulting in deteriorated heat transfer performance.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"174 ","pages":"Article 111689"},"PeriodicalIF":3.3,"publicationDate":"2026-01-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145941150","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 : 2025-12-29DOI: 10.1016/j.expthermflusci.2025.111688
Gonçalo G. Cruz , Xavier Ottavy , Fabrizio Fontaneto
Accurately characterizing complex flow fields often requires dense measurement grids. This paper presents an active learning methodology that enables efficient and targeted flow field characterization by dynamically guiding the selection of measurement locations while sampling the flow fields. The approach leverages a Gaussian Process model to represent the flow field and its uncertainty, while the Maximize Expected Prediction Error (MEPE) acquisition function balances exploration of undersampled high uncertainty regions with exploitation in areas of potential error. The active learning methodology is validated on the engineering relevant test case of the ECL5 Ultra-High Bypass Ratio (UHBR) fan, using experimental data acquired at the outlet measurement plane. The results demonstrate that the active learning approach can accurately capture all relevant flow features, including the hub corner separation and wake structures, using only 400 measurements, which is half the number of measurements that were sampled in traditional reference tests. This reduction in measurement effort resulted in a time saving of approximately one hour compared to the three-hour reference data acquisition. Furthermore, the methodology offers researchers flexibility in customizing the data acquisition process to their specific goals through the selection of appropriate stopping criteria. By strategically combining uncertainty and error thresholds, the active learning process can be adapted to achieve a desired balance between measurement effort, accuracy, and uncertainty levels. These findings highlight the possibilities of active learning to significantly enhance the efficiency and cost-effectiveness of experimental fluid mechanics research.
{"title":"Experimental ‘In-situ’ Bayesian Active Learning: Sampling flow fields with a purpose","authors":"Gonçalo G. Cruz , Xavier Ottavy , Fabrizio Fontaneto","doi":"10.1016/j.expthermflusci.2025.111688","DOIUrl":"10.1016/j.expthermflusci.2025.111688","url":null,"abstract":"<div><div>Accurately characterizing complex flow fields often requires dense measurement grids. This paper presents an active learning methodology that enables efficient and targeted flow field characterization by dynamically guiding the selection of measurement locations while sampling the flow fields. The approach leverages a Gaussian Process model to represent the flow field and its uncertainty, while the Maximize Expected Prediction Error (MEPE) acquisition function balances exploration of undersampled high uncertainty regions with exploitation in areas of potential error. The active learning methodology is validated on the engineering relevant test case of the ECL5 Ultra-High Bypass Ratio (UHBR) fan, using experimental data acquired at the outlet measurement plane. The results demonstrate that the active learning approach can accurately capture all relevant flow features, including the hub corner separation and wake structures, using only 400 measurements, which is half the number of measurements that were sampled in traditional reference tests. This reduction in measurement effort resulted in a time saving of approximately one hour compared to the three-hour reference data acquisition. Furthermore, the methodology offers researchers flexibility in customizing the data acquisition process to their specific goals through the selection of appropriate stopping criteria. By strategically combining uncertainty and error thresholds, the active learning process can be adapted to achieve a desired balance between measurement effort, accuracy, and uncertainty levels. These findings highlight the possibilities of active learning to significantly enhance the efficiency and cost-effectiveness of experimental fluid mechanics research.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111688"},"PeriodicalIF":3.3,"publicationDate":"2025-12-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145880164","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 : 2025-12-21DOI: 10.1016/j.expthermflusci.2025.111683
Ahmed Aql , E. Al-Safran , E. Pereyra , C. Sarica
This study experimentally investigates slug flow characteristics under high-pressure conditions of 2.76 MPa (400 psi) in a 2° upward-inclined, large ID pipe with an internal diameter of 156 mm (6.1 in.), using advanced diagnostic instruments, including high-resolution wire-mesh sensors and a high-speed visualization system. The tests are performed within a broad range of superficial liquid and gas velocities of vSL = 0.01–0.35 m/s, and vSG = 1.3–2.5 m/s, allowing investigation of operational and topological features, such as film reversal and interfacial roughness, and enabling the distinction between slug and pseudo-slug flow. It is found that the strength of the hydraulic seal in the flow structure (slug and pseudo-slug) body governs the characteristics of the intermittent flow. In addition, the superficial liquid velocity vSL is found to be the major contributor to the formation and stability of the hydraulic seal, followed by the superficial gas velocity vSG. Specifically, the mean slug length, LS, and slug frequency, fS, are significantly influenced by superficial liquid velocity. For example, increasing vSL promotes longer and more frequent slugs, while the impact of superficial gas velocity on slug flow characteristics depends on the structure body liquid holdup, and hydraulic seal integrity. Overall, superficial gas velocity predominantly decreases liquid holdup in slug and film regions, due to high interfacial shear stress and film thinning.
{"title":"Intermittent two-phase flow characterization using advanced diagnostics in a high-pressure large-diameter slightly upward inclined pipe","authors":"Ahmed Aql , E. Al-Safran , E. Pereyra , C. Sarica","doi":"10.1016/j.expthermflusci.2025.111683","DOIUrl":"10.1016/j.expthermflusci.2025.111683","url":null,"abstract":"<div><div>This study experimentally investigates slug flow characteristics under high-pressure conditions of 2.76 MPa (400 psi) in a 2° upward-inclined, large ID pipe with an internal diameter of 156 mm (6.1 in.), using advanced diagnostic instruments, including high-resolution wire-mesh sensors and a high-speed visualization system. The tests are performed within a broad range of superficial liquid and gas velocities of <em>v<sub>SL</sub></em> = 0.01–0.35 m/s, and <em>v<sub>SG</sub></em> = 1.3–2.5 m/s, allowing investigation of operational and topological features, such as film reversal and interfacial roughness, and enabling the distinction between slug and pseudo-slug flow. It is found that the strength of the hydraulic seal in the flow structure (slug and pseudo-slug) body governs the characteristics of the intermittent flow. In addition, the superficial liquid velocity <em>v<sub>SL</sub></em> is found to be the major contributor to the formation and stability of the hydraulic seal, followed by the superficial gas velocity <em>v<sub>SG</sub></em>. Specifically, the mean slug length, <em>L<sub>S</sub>,</em> and slug frequency, <em>f<sub>S</sub></em>, are significantly influenced by superficial liquid velocity. For example, increasing <em>v<sub>SL</sub></em> promotes longer and more frequent slugs, while the impact of superficial gas velocity on slug flow characteristics depends on the structure body liquid holdup, and hydraulic seal integrity. Overall, superficial gas velocity predominantly decreases liquid holdup in slug and film regions, due to high interfacial shear stress and film thinning.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111683"},"PeriodicalIF":3.3,"publicationDate":"2025-12-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145836376","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 : 2025-12-18DOI: 10.1016/j.expthermflusci.2025.111687
Senthil Kumar Parimalanathan , Hatim Machrafi , Adam Chafai , Alexey Rednikov , Pierre Colinet
Evaporation of a sessile droplet is studied in a parabolic flight campaign. The setup elements are similar to some past or planned microgravity experiments in space: pure refrigerant Hydrofluoroether (HFE)-7100 for the liquid, droplet pinning at a 2 mm radius microgroove, nearly normal conditions. The droplet is injected onto a flat substrate through a small central outlet up to a volume between L and L (contact angles from to 70°) at the beginning of each ‘parabola’ (microgravity period lasting up to 20 s). Although an unfortunate ‘parasitic post-injection’ (persisting after the pump is off, a quite typical anomaly in microgravity) might have interfered with evaporation-rate measurements, such gaps are bridged using digital holographic vapor interferometry and axisymmetric simulations. Thus, we find that evaporation rates are significantly affected by residual gravity fluctuations in the plane (‘g-jitter’, g here). Quite accordingly, vapor clouds ‘dancing’ following the g-jitter are interferometrically disclosed, in good agreement with simulations. All this provides a noteworthy example of a possible difference between parabolic-flight experiments and those at other platforms (such as sounding rockets) approaching 0 g more precisely. The simulations reveal an independence with respect to high-frequency g-jitter sampling but highlight that each parabola is unique, with its own g-jitter signature. A detailed benchmark analysis is carried out motivated by the question of why the evaporation rates are appreciably higher for periods of negative (upward) g-jitter compared to positive (downward) ones. This is partly related to thermal Marangoni convection, concurrent or not to g-jitter buoyancy convection.
{"title":"Dancing vapor clouds above evaporating sessile droplets: Insights from parabolic flight microgravity experiments","authors":"Senthil Kumar Parimalanathan , Hatim Machrafi , Adam Chafai , Alexey Rednikov , Pierre Colinet","doi":"10.1016/j.expthermflusci.2025.111687","DOIUrl":"10.1016/j.expthermflusci.2025.111687","url":null,"abstract":"<div><div>Evaporation of a sessile droplet is studied in a parabolic flight campaign. The setup elements are similar to some past or planned microgravity experiments in space: pure refrigerant Hydrofluoroether (HFE)-7100 for the liquid, droplet pinning at a 2 mm radius microgroove, nearly normal conditions. The droplet is injected onto a flat substrate through a small central outlet up to a volume between <span><math><mrow><mo>∼</mo><mn>6</mn><mspace></mspace><mi>μ</mi></mrow></math></span>L and <span><math><mrow><mn>10</mn><mspace></mspace><mi>μ</mi></mrow></math></span>L (contact angles from <span><math><mrow><mo>∼</mo><mn>4</mn><msup><mrow><mn>5</mn></mrow><mrow><mo>∘</mo></mrow></msup></mrow></math></span> to 70°) at the beginning of each ‘parabola’ (microgravity period lasting up to <span><math><mo>∼</mo></math></span>20 s). Although an unfortunate ‘parasitic post-injection’ (persisting after the pump is off, a quite typical anomaly in microgravity) might have interfered with evaporation-rate measurements, such gaps are bridged using digital holographic vapor interferometry and axisymmetric simulations. Thus, we find that evaporation rates are significantly affected by residual gravity fluctuations in the plane (‘g-jitter’, <span><math><mrow><mo>∼</mo><mn>1</mn><msup><mrow><mn>0</mn></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></mrow></math></span>g here). Quite accordingly, vapor clouds ‘dancing’ following the g-jitter are interferometrically disclosed, in good agreement with simulations. All this provides a noteworthy example of a possible difference between parabolic-flight experiments and those at other platforms (such as sounding rockets) approaching 0 g more precisely. The simulations reveal an independence with respect to high-frequency g-jitter sampling but highlight that each parabola is unique, with its own g-jitter signature. A detailed benchmark analysis is carried out motivated by the question of why the evaporation rates are appreciably higher for periods of negative (upward) g-jitter compared to positive (downward) ones. This is partly related to thermal Marangoni convection, concurrent or not to g-jitter buoyancy convection.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111687"},"PeriodicalIF":3.3,"publicationDate":"2025-12-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145880163","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 : 2025-12-16DOI: 10.1016/j.expthermflusci.2025.111686
Wenjie Deng , Zhenhua Quan , Yaohua Zhao , Chunduo Song , Lincheng Wang
The flow pattern between tubes significantly influences the liquid film distribution and flow state on the lower tube bundle in the falling film evaporator, and its transition behavior is critical to the heat transfer performance of the equipment. This study develops a visualization platform to examine flow pattern transitions between horizontal flat tubes with large height-width ratio, utilizing ethylene glycol, ethanol, and water as working fluids. The findings reveal that ethylene glycol exhibits the lowest critical Reynolds number (Rec), while ethanol shows a more distinct and clearer gas–liquid interface morphology. The Rec for flow pattern transition increases with higher spray height (H), inlet liquid temperature (Tin), and circulating water temperature (Tcir), with Tin showing the most substantial impact. Flow hysteresis manifests in flow pattern transitions and strengthens with increasing H, Tin and Tcir, with water displaying the most pronounced effect. Based on experimental data, an empirical correlation and flow pattern map better suited for flow pattern transition on horizontal flat tubes is developed, with a maximum relative root mean square error of 12.76%. Research demonstrates that horizontal flat tubes show a lower Rec for transition to sheet flow compared with horizontal tubes, indicating superior liquid film flow performance. This investigation advances the understanding of flow pattern transition mechanisms on horizontal flat tubes, providing theoretical and experimental foundations for the structural optimization and efficient operation of falling film evaporators.
{"title":"Experimental study on flow pattern transition on horizontal flat tubes with large height-width ratio","authors":"Wenjie Deng , Zhenhua Quan , Yaohua Zhao , Chunduo Song , Lincheng Wang","doi":"10.1016/j.expthermflusci.2025.111686","DOIUrl":"10.1016/j.expthermflusci.2025.111686","url":null,"abstract":"<div><div>The flow pattern between tubes significantly influences the liquid film distribution and flow state on the lower tube bundle in the falling film evaporator, and its transition behavior is critical to the heat transfer performance of the equipment. This study develops a visualization platform to examine flow pattern transitions between horizontal flat tubes with large height-width ratio, utilizing ethylene glycol, ethanol, and water as working fluids. The findings reveal that ethylene glycol exhibits the lowest critical Reynolds number (<em>Re</em><sub>c</sub>), while ethanol shows a more distinct and clearer gas–liquid interface morphology. The <em>Re</em><sub>c</sub> for flow pattern transition increases with higher spray height (<em>H</em>), inlet liquid temperature (<em>T</em><sub>in</sub>), and circulating water temperature (<em>T</em><sub>cir</sub>), with <em>T</em><sub>in</sub> showing the most substantial impact. Flow hysteresis manifests in flow pattern transitions and strengthens with increasing <em>H</em>, <em>T</em><sub>in</sub> and <em>T</em><sub>cir</sub>, with water displaying the most pronounced effect. Based on experimental data, an empirical correlation and flow pattern map better suited for flow pattern transition on horizontal flat tubes is developed, with a maximum relative root mean square error of 12.76%. Research demonstrates that horizontal flat tubes show a lower <em>Re</em><sub>c</sub> for transition to sheet flow compared with horizontal tubes, indicating superior liquid film flow performance. This investigation advances the understanding of flow pattern transition mechanisms on horizontal flat tubes, providing theoretical and experimental foundations for the structural optimization and efficient operation of falling film evaporators.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111686"},"PeriodicalIF":3.3,"publicationDate":"2025-12-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145797529","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 : 2025-12-15DOI: 10.1016/j.expthermflusci.2025.111685
Gregor Bahč, Armin Hadžić, Matic Može, Matevž Zupančič, Iztok Golobič
Pool boiling is widely employed in compact thermal management systems, but its performance can deteriorate significantly under geometric confinement. This study investigates the combined effects of vertical gap height and confinement plate diameter on pool boiling of distilled water at atmospheric pressure using gold-coated copper samples. The vertical gap between the boiling surface and an overhead plate varied from 0.1λ to 20λ, where λ = 2.5 mm is the capillary length of water, and three plate diameters were examined: 14 mm, 24 mm, and 39 mm. For large gaps (kgap ≥ 2.5), the critical heat flux (CHF) remained at approximately 1100 kW m−2 and the heat transfer coefficient (HTC) was comparable to unconfined boiling. As the gap decreases below 2.5λ, both CHF and HTC decrease sharply. At the smallest gap (0.1λ), CHF and HTC were reduced by up to 78 % and 85 %, respectively, relative to the unconfined reference. In the intermediate gap range (0.5λ–2.5λ), the plate diameter had a pronounced effect, with the largest plate producing substantially lower CHF and HTC than the smallest plate for the same gap height due to more restricted radial vapor escape. High-speed visualization confirmed that strong confinement promotes bubble coalescence, vapor accumulation beneath the plate, and intermittent dryout of the boiling surface. Based on the CHF data for all gap heights and plate diameters, empirical correlations were developed using a dimensionless gap ratio and a characteristic plate-size parameter, providing a predictive framework for assessing CHF under combined vertical gap height and confinement plate diameter.
{"title":"Effect of confinement geometry on pool boiling performance of gold-coated copper surface","authors":"Gregor Bahč, Armin Hadžić, Matic Može, Matevž Zupančič, Iztok Golobič","doi":"10.1016/j.expthermflusci.2025.111685","DOIUrl":"10.1016/j.expthermflusci.2025.111685","url":null,"abstract":"<div><div>Pool boiling is widely employed in compact thermal management systems, but its performance can deteriorate significantly under geometric confinement. This study investigates the combined effects of vertical gap height and confinement plate diameter on pool boiling of distilled water at atmospheric pressure using gold-coated copper samples. The vertical gap between the boiling surface and an overhead plate varied from 0.1λ to 20λ, where λ = 2.5 mm is the capillary length of water, and three plate diameters were examined: 14 mm, 24 mm, and 39 mm. For large gaps (<em>k</em><sub>gap</sub> ≥ 2.5), the critical heat flux (CHF) remained at approximately 1100 kW<!--> <!-->m<sup>−2</sup> and the heat transfer coefficient (HTC) was comparable to unconfined boiling. As the gap decreases below 2.5λ, both CHF and HTC decrease sharply. At the smallest gap (0.1λ), CHF and HTC were reduced by up to 78 % and 85 %, respectively, relative to the unconfined reference. In the intermediate gap range (0.5λ–2.5λ), the plate diameter had a pronounced effect, with the largest plate producing substantially lower CHF and HTC than the smallest plate for the same gap height due to more restricted radial vapor escape. High-speed visualization confirmed that strong confinement promotes bubble coalescence, vapor accumulation beneath the plate, and intermittent dryout of the boiling surface. Based on the CHF data for all gap heights and plate diameters, empirical correlations were developed using a dimensionless gap ratio and a characteristic plate-size parameter, providing a predictive framework for assessing CHF under combined vertical gap height and confinement plate diameter.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111685"},"PeriodicalIF":3.3,"publicationDate":"2025-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145836375","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 study reports an experimental investigation on splashing of liquid droplets impacting on an artificial superhydrophobic (SHP) surface prepared by coating NeverWet hydrophobic solution. The prepared SHP surface exhibits micro-bumps decorated with nano-structures, as seen in Scanning Electron Microscopy (SEM) images and surface profilometry. The presence of micro-bumps causes the fragmentation of spreading lamella via hole-nucleation process, leading to a significant reduction in droplet contact time as shown in a recent study reported in the literature. The focus of the current study is on the effect of droplet viscosity on the hole-nucleation and contact time of impacting droplets. The experiments with droplet impact velocity, ranging from 0.3 m/s to 3.6 m/s are conducted using three different droplet liquids – water (W), 20% glycerine and 80% water (G20), and 50% glycerine and 50% water (G50) – varying mainly in their dynamic viscosity. From high-speed videos of droplet impact dynamics, the various impact regimes are first identified and mapped in (droplet Weber number) – (droplet Reynolds number) plane for all three droplet liquids. Quantitative measurements of the temporal variation of lamella diameter, number of holes formed, time at which the first hole nucleates, critical impact velocity at which the hole-nucleation begins, and contact time are extracted. These measurements clearly show that the contact time reduction of splashing droplets decreases with increase in droplet viscosity. Moreover, the number of holes formed in the lamella film scales with the number of micro-bumps underneath the droplet at its maximum spreading which, in turn, decreases with increase in droplet viscosity. The time instant at which the first hole nucleates on the lamella film is seen to be independent of the droplet viscosity. A modified model is proposed to describe the effects of droplet viscosity and surface micro-characteristics (height and pitch of surface micro-bumps) on the critical velocity for hole nucleation, . The predictions from this modified model seem to explain the experimental observations on in the current study as well as in the literature.
本文报道了用NeverWet疏水溶液涂覆制备的人工超疏水（SHP）表面上液滴溅射影响的实验研究。从扫描电子显微镜（SEM）图像和表面轮廓分析中可以看出，制备的SHP表面呈现出纳米结构装饰的微凸起。最近文献报道的一项研究表明，微凸起的存在导致通过孔成核过程扩展的片层破碎，导致液滴接触时间显着减少。目前研究的重点是液滴粘度对冲击液滴孔形核和接触时间的影响。液滴冲击速度Uo范围为0.3 m/s ~ 3.6 m/s，实验采用三种不同的液滴液体——水(W)、20%甘油和80%水（G20）、50%甘油和50%水（G50）——主要改变的是它们的动态粘度。从液滴撞击动力学的高速视频中，首先确定了三种液滴液体的不同撞击状态，并在We（液滴韦伯数）- Re（液滴雷诺数）平面上进行了映射。提取了片层直径的时间变化、形成的孔数、第一个孔成核的时间、孔开始成核的临界冲击速度和接触时间的定量测量结果。这些测量清楚地表明，随着液滴粘度的增加，溅射液滴的接触时间减少量减小。此外，在液滴最大扩散时，微凸点的数量随着液滴下方微凸点的数量而增加，而微凸点的数量则随着液滴粘度的增加而减少。第一个孔在片层膜上成核的时间瞬间与液滴粘度无关。提出了一个改进的模型来描述液滴粘度和表面微特征（表面微凸起的高度和节距）对孔成核临界速度Uc，h的影响。这个修正模型的预测似乎可以解释当前研究和文献中对Uc，h的实验观察结果。
{"title":"Splashing of impacting droplets on an artificial superhydrophobic surface: Effect of viscosity in hole-nucleation process","authors":"Kumar Gaurav , Visakh Vaikuntanathan , Sivakumar Deivandren","doi":"10.1016/j.expthermflusci.2025.111682","DOIUrl":"10.1016/j.expthermflusci.2025.111682","url":null,"abstract":"<div><div>The present study reports an experimental investigation on splashing of liquid droplets impacting on an artificial superhydrophobic (SHP) surface prepared by coating NeverWet hydrophobic solution. The prepared SHP surface exhibits micro-bumps decorated with nano-structures, as seen in Scanning Electron Microscopy (SEM) images and surface profilometry. The presence of micro-bumps causes the fragmentation of spreading lamella via hole-nucleation process, leading to a significant reduction in droplet contact time as shown in a recent study reported in the literature. The focus of the current study is on the effect of droplet viscosity on the hole-nucleation and contact time of impacting droplets. The experiments with droplet impact velocity, <span><math><msub><mrow><mi>U</mi></mrow><mrow><mi>o</mi></mrow></msub></math></span> ranging from 0.3 m/s to 3.6 m/s are conducted using three different droplet liquids – water (W), 20% glycerine and 80% water (G20), and 50% glycerine and 50% water (G50) – varying mainly in their dynamic viscosity. From high-speed videos of droplet impact dynamics, the various impact regimes are first identified and mapped in <span><math><mrow><mi>W</mi><mi>e</mi></mrow></math></span> (droplet Weber number) – <span><math><mrow><mi>R</mi><mi>e</mi></mrow></math></span> (droplet Reynolds number) plane for all three droplet liquids. Quantitative measurements of the temporal variation of lamella diameter, number of holes formed, time at which the first hole nucleates, critical impact velocity at which the hole-nucleation begins, and contact time are extracted. These measurements clearly show that the contact time reduction of splashing droplets decreases with increase in droplet viscosity. Moreover, the number of holes formed in the lamella film scales with the number of micro-bumps underneath the droplet at its maximum spreading which, in turn, decreases with increase in droplet viscosity. The time instant at which the first hole nucleates on the lamella film is seen to be independent of the droplet viscosity. A modified model is proposed to describe the effects of droplet viscosity and surface micro-characteristics (height and pitch of surface micro-bumps) on the critical velocity for hole nucleation, <span><math><msub><mrow><mi>U</mi></mrow><mrow><mi>c</mi><mo>,</mo><mi>h</mi></mrow></msub></math></span>. The predictions from this modified model seem to explain the experimental observations on <span><math><msub><mrow><mi>U</mi></mrow><mrow><mi>c</mi><mo>,</mo><mi>h</mi></mrow></msub></math></span> in the current study as well as in the literature.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111682"},"PeriodicalIF":3.3,"publicationDate":"2025-12-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145797531","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 : 2025-12-13DOI: 10.1016/j.expthermflusci.2025.111684
Xu Wang , Xujian Lyu , Hui Tang , Chao Qi , Ruisheng Sun
This paper presents an experimental investigation into cavity interference and surface closure characteristics of two side-by-side spheres undergoing consecutive water entries. A high-speed imaging system was employed to capture the dynamic processes of the two cavities, including the cavity elongation and their interaction with the free surface during the closure stage. Four distinct interference modes—rebound, expansion, tail, and bubble-dominated—are found primarily depending on the time interval between the two water entries, t, while showing little sensitivity to the lateral distance. Unlike the single-cavity closure, the following cavity, subject to interference, undergoes two separate detachment events from the free surface, characterized by a closure angle (defined as the acute angle between the cavity tail-end face and the free surface) whose magnitude and orientation scale with t. Statistical analysis indicates that the initial detachment time is independent of the adjacent cavity’s evolution, whereas the emergence of a closure angle significantly prolongs the complete detachment time and increases cavity length, especially at smaller lateral distances.
{"title":"Consecutive water entries of two side-by-side spheres: cavity interference and surface closure characteristics","authors":"Xu Wang , Xujian Lyu , Hui Tang , Chao Qi , Ruisheng Sun","doi":"10.1016/j.expthermflusci.2025.111684","DOIUrl":"10.1016/j.expthermflusci.2025.111684","url":null,"abstract":"<div><div>This paper presents an experimental investigation into cavity interference and surface closure characteristics of two side-by-side spheres undergoing consecutive water entries. A high-speed imaging system was employed to capture the dynamic processes of the two cavities, including the cavity elongation and their interaction with the free surface during the closure stage. Four distinct interference modes—rebound, expansion, tail, and bubble-dominated—are found primarily depending on the time interval between the two water entries, <span><math><mi>Δ</mi></math></span><em>t</em>, while showing little sensitivity to the lateral distance. Unlike the single-cavity closure, the following cavity, subject to interference, undergoes two separate detachment events from the free surface, characterized by a closure angle (defined as the acute angle between the cavity tail-end face and the free surface) whose magnitude and orientation scale with <span><math><mi>Δ</mi></math></span><em>t</em>. Statistical analysis indicates that the initial detachment time is independent of the adjacent cavity’s evolution, whereas the emergence of a closure angle significantly prolongs the complete detachment time and increases cavity length, especially at smaller lateral distances.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111684"},"PeriodicalIF":3.3,"publicationDate":"2025-12-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145797530","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 : 2025-12-12DOI: 10.1016/j.expthermflusci.2025.111680
Jitae Do , Binbin Wang
The dynamics of bursting droplets from surface bubbles is an important mechanism for water-to-air transport of small particles and contaminants. We investigated bubble-bursting droplets from bubble plumes, released from an air stone (AS) or a single nozzle (SN) at four gas flow rates ranging from 0.1 to 0.8 L min−1. Quantitative imaging was used to analyze the statistics of size and velocity distributions for rising bubbles and droplets ejected by bursting bubbles. Significantly greater droplet number, mass flow rate, and median size were observed in AS, by factors of 20, 8.6, and 1.8 compared to SN, reflecting the influence of subsurface bubble characteristics on droplet production. Log-normal distributions fit well to droplet size and velocity distribution regardless of the release mechanism. The characteristic median sizes show a bubble–droplet relationship of , which implies Bond number relationship of under constant fluid properties. Scaling analysis using visco-capillary length and capillary velocity shows , suggesting a universal scaling relationship for bursting droplets from surfacing of single bubble and bubble plumes. Involving both length and velocity scales of bubbles and droplets shows , a scaling relationship potentially used for predicting droplet dynamics from bursting bubble plumes. These findings provide the first quantitative scaling linking bubble plume and bursting droplet dynamics, with potential applications in aerosol generation, wastewater aeration, and ocean–atmosphere mass-exchange studies.
表面气泡破裂液滴的动力学是小颗粒和污染物从水到空气传输的重要机制。我们研究了气泡柱中的气泡破裂液滴，从气石（AS）或单喷嘴（SN）中以0.1至0.8 L min - 1的四种气体流速释放。利用定量成像技术对气泡上升和气泡破裂喷射出的液滴的大小和速度分布进行统计分析。与SN相比，AS的液滴数量、质量流量和中位数尺寸分别是20倍、8.6倍和1.8倍，这反映了地下气泡特性对液滴产生的影响。无论释放机制如何，对数正态分布都能很好地适应液滴大小和速度分布。特征中值尺寸表现为Rd ~ Rb1.4的气泡-液滴关系，这意味着恒定流体性质下Bod ~ Bob1.4的键数关系。用粘度-毛细长度和毛细速度进行的结垢分析显示Lad ~ Cad−1.4，表明单泡表面和泡柱表面的破裂液滴具有普遍的结垢关系。涉及气泡和液滴的长度和速度尺度显示为Frd ~ (BobWeb)−0.3，这一尺度关系可能用于预测气泡羽流破裂时的液滴动力学。这些发现首次提供了气泡羽流和破裂液滴动力学之间的定量尺度联系，在气溶胶产生、废水曝气和海洋-大气质量交换研究中具有潜在的应用前景。
{"title":"Dynamics of bursting droplets from surfacing bubble plumes","authors":"Jitae Do , Binbin Wang","doi":"10.1016/j.expthermflusci.2025.111680","DOIUrl":"10.1016/j.expthermflusci.2025.111680","url":null,"abstract":"<div><div>The dynamics of bursting droplets from surface bubbles is an important mechanism for water-to-air transport of small particles and contaminants. We investigated bubble-bursting droplets from bubble plumes, released from an air stone (AS) or a single nozzle (SN) at four gas flow rates ranging from 0.1 to 0.8 L min<sup>−1</sup>. Quantitative imaging was used to analyze the statistics of size and velocity distributions for rising bubbles and droplets ejected by bursting bubbles. Significantly greater droplet number, mass flow rate, and median size were observed in AS, by factors of 20, 8.6, and 1.8 compared to SN, reflecting the influence of subsurface bubble characteristics on droplet production. Log-normal distributions fit well to droplet size and velocity distribution regardless of the release mechanism. The characteristic median sizes show a bubble–droplet relationship of <span><math><mrow><msub><mrow><mi>R</mi></mrow><mrow><mi>d</mi></mrow></msub><mo>∼</mo><msubsup><mrow><mi>R</mi></mrow><mrow><mi>b</mi></mrow><mrow><mn>1</mn><mo>.</mo><mn>4</mn></mrow></msubsup></mrow></math></span>, which implies Bond number relationship of <span><math><mrow><mi>B</mi><msub><mrow><mi>o</mi></mrow><mrow><mi>d</mi></mrow></msub><mo>∼</mo><mi>B</mi><msubsup><mrow><mi>o</mi></mrow><mrow><mi>b</mi></mrow><mrow><mn>1</mn><mo>.</mo><mn>4</mn></mrow></msubsup></mrow></math></span> under constant fluid properties. Scaling analysis using visco-capillary length and capillary velocity shows <span><math><mrow><mi>L</mi><msub><mrow><mi>a</mi></mrow><mrow><mi>d</mi></mrow></msub><mo>∼</mo><mi>C</mi><msubsup><mrow><mi>a</mi></mrow><mrow><mi>d</mi></mrow><mrow><mo>−</mo><mn>1</mn><mo>.</mo><mn>4</mn></mrow></msubsup></mrow></math></span>, suggesting a universal scaling relationship for bursting droplets from surfacing of single bubble and bubble plumes. Involving both length and velocity scales of bubbles and droplets shows <span><math><mrow><mi>F</mi><msub><mrow><mi>r</mi></mrow><mrow><mi>d</mi></mrow></msub><mo>∼</mo><msup><mrow><mrow><mo>(</mo><mi>B</mi><msub><mrow><mi>o</mi></mrow><mrow><mi>b</mi></mrow></msub><mi>W</mi><msub><mrow><mi>e</mi></mrow><mrow><mi>b</mi></mrow></msub><mo>)</mo></mrow></mrow><mrow><mo>−</mo><mn>0</mn><mo>.</mo><mn>3</mn></mrow></msup></mrow></math></span>, a scaling relationship potentially used for predicting droplet dynamics from bursting bubble plumes. These findings provide the first quantitative scaling linking bubble plume and bursting droplet dynamics, with potential applications in aerosol generation, wastewater aeration, and ocean–atmosphere mass-exchange studies.</div></div>","PeriodicalId":12294,"journal":{"name":"Experimental Thermal and Fluid Science","volume":"173 ","pages":"Article 111680"},"PeriodicalIF":3.3,"publicationDate":"2025-12-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145797532","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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