Search

The present study reports the thermal behavior of multiple jets on a hot concave surface with different nozzle geometries such as circular and elliptical and different orientation. Tests are performed with five different nozzles at different values of nondimensional nozzle to surface distance (z/d = 2–10) and Reynolds number (5000–28,000). Here, the parameter coefficient of variation (COV) is used to estimate the non-uniformity in the local Nu variation. For N-2, the value of $CO{V_{y,non - dim}}$COVy,non−dim is found to be 50–60% higher compared to N-3 at z/d = 2; while at z/d ≥ 8, for N-3, the value of $CO{V_{y,non - dim}}$COVy,non−dim is found to be 10–15% higher compared to N-2. The higher temperature uniformity ($CO{V_{x,non - dim}}$COVx,non−dim) is noted in the fountain region compared to impingement region for the circular and elliptical nozzles. The thermal performance of elliptical jets is found to be sensitive toward the jet orientation at lower z/d values. The magnitude of temperature non-uniformity decreases with the increase in the z/d values. Among the tested nozzles, Nozzle N-2 consistently demonstrates superior performance across all nozzle-to-plate distances and with an increase in the z/d value improvement with nozzle N-2 reduces. [ABSTRACT FROM AUTHOR]

Buoyancy-driven flow and heat transfer characteristics of the solid/hollow cylinders with concave shapes were numerically analyzed in a turbulent regime. The effects of Ra , L / D , and D th / D on heat dissipation have been studied in the range of 1010 to 1012, 2 to 10, and 0.2 to 0.9, respectively. It has been found that the Nu ¯ increases with Ra for solid/hollow cylinders. For a hollow cylinder, Nu ¯ in is always higher than Nu ¯ out except at D th / D = 0.2. The Nu ¯ in of a hollow cylinder increases up to D th / D = 0.6, irrespective of the Ra , and L / D ratios thereafter reduced drastically has been observed. Whereas, at a low D th / D , and high Ra (i.e., Ra ≥ 1011), Nu ¯ out of a hollow cylinder significantly decreases with D th / D . However, it shows a marginal decrement at a high length-to-diameter ratio. For solid and hollow cylinders, the thermo-fluid dynamics have been delineated through the temperature contours, streamline patterns, and vortex generation at the inner as well as outer wall of the hollow cylinder. This numerical study also proposes a correlation for the Nu ¯ of the solid/hollow cylinders to have an accuracy ± 6 % of relevant parameters. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Agricultural Science & Technology (1008-0864) is the property of Journal of Agricultural Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

AbstractNumerical simulations were performed to investigate the thermal and aerodynamic characteristics of an annular jet impinging on a concave surface using the Realizable k-ε model. The study examined the significance of the blockage ratio (BR) from 0.3 to 0.7 and the jet exit Reynolds number (Re) from 5000 to 35000. The investigation uses an annular jet with a 10 mm outer diameter impinging on a concave surface with 150 mm radii. Three specific dimensionless distances from the nozzle exit to the target surface (z/do) were considered: z/do = 0.5 (initial merging zone), z/do = 3 (intermediate merging zone), and z/do = 6 (fully merged zone). Furthermore, the annular jet’s performance was compared to that of a conventional circular jet under analogous conditions. At small separation distances, a pair of counter-rotating vortices formed downstream of the jet outlet, inducing a reverse flow that extended to the heated surface. Vortices and flow reversals were also identified at intermediate separation distances. At large separation distances, the flow reattached to the domain axis, resembling a circular jet. The local Nusselt number (Nu) increases as the Reynolds number increases and the blockage ratio decreases. With increasing BR, the difference between the primary and secondary Nu peaks reduces. Additionally, the average Nu and turbulent kinetic energy increase with an increase in Re. [ABSTRACT FROM AUTHOR]

The current study uses numerical methods to evaluate the thermal and flow characteristics of air jet impingement on concave multi-protruded surfaces. A two-dimensional numerical analysis is carried out with the help of commercially available FLUENT package. The impact of different geometric parameters is studied, including protrusion shape (convex and triangle), protrusion height and central angle by protrusions (α). Multiple protrusions on a surface lead to a series of peaks and valleys in the local Nu variation, which governs the overall thermal performance of jet impingement. It is found that protrusions with lower central angle by protrusions improve the heat transfer attributes compared to smooth surface, irrespective of protrusion size and shape. A drastic improvement in thermal performance is noted by replacing convex protrusion with triangle protrusion, particularly at the larger central angle by protrusions (α). [ABSTRACT FROM AUTHOR]

While the impinging sweeping jet generated by a conventional fluidic oscillator has been extensively investigated for its remarkable convective heat transfer performance, the cooling performance of the recently designed vortex-based fluidic oscillator on impinged hot plates remains unexplored. This study numerically investigates the cooling performance of oscillatory jets generated by the vortex-based fluidic oscillator compared to the conventional fluidic oscillator and a steady non-oscillatory jet impinging on a hot concave surface. The Fluent 2023R2 software was employed for solving the unsteady Reynolds-averaged Navier–Stokes equations with the k–ω SST model through the finite volume method. Numerical simulations were conducted at various dimensionless nozzle-to-surface distances (X/D = 2, 4, and 6) and Reynolds numbers (20,000, 30,000, and 40,000). The performances of both oscillators were assessed through the time-averaged velocity, temperature, and Nusselt number. The results demonstrated that the vortex-based fluidic oscillator, as an innovative type of fluidic oscillator, outperformed conventional fluidic oscillators by enhancing heat transfer and reducing pressure drop, attributed to its smaller size and higher operational frequency. At X/D values of 2, 4, and 6, the mean Nusselt number for the vortex-based oscillator exhibited respective increases of 19 %, 23 %, and 16 % compared to the conventional oscillator. In addition, these values were respectively 38 %, 34 %, and 24 % higher than those for the steady non-oscillatory jet. Furthermore, the thermal enhancement factor of the vortex-based oscillator demonstrated increases of 20 %, 23 %, and 21 %, respectively, in comparison with the conventional oscillator. These findings suggest that the novel vortex-based fluidic oscillator holds significant promise for replacing conventional oscillators in industrial cooling applications.

AbstractThe double-wall cooling system containing jet impingement and full-length pin-in cooling structures shows good performance in heat transfer enhancement and uniformity, which has-been used in the blade cooling design. However, the pressure loss induced by the full-length pin-fin is relatively high. It is very meaningful to get a cooling type which can balance the heat transfer performance of double-wall cooling and flow resistance, especially for the blade leading edge. In this study, numerical simulations have been carried out to study the influences of truncated pin-fins height and arrangement on heat transfer increment and flow behavior of jets impinging onto a concave double wall channel with varying truncated pin-fins arranged, considering the heat transfer enhancement potential and relatively weaken pressure loss penalty of truncated/broken pin-fin. Jets impingement on a smooth-concave surface is regarded as Baseline. Streamlines in two-dimensional sections in the channel and limiting streamlines near the target wall conjugated with flow characteristics figures are applied to better understand the flow structures in the channel. The local and averaged heat transfer performance is analyzed based on the flow discussions. The overall parameters, i.e., thermal performance, friction factor and pumping power, are also obtained. The results show that the introduction of relatively high truncated pin-fins located on the upper wall presents higher heat transfer enhancement and lower pressure loss comparing with the full-length case and truncated pin-fins located on pressure loss comparing with the full-length case and truncated pin-fins located on target wall cases. The truncated pin-fins placed on the upper surface with relative height of 0.75 reaches the highest overall thermal performance and uniformity in this work. [ABSTRACT FROM AUTHOR]

During high-pressure die casting, a significant amount of heat is dissipated via the liquid-cooled channels in the die. The jet cooler, also known as the die insert or bubbler, is one of the most commonly used cooling methods. Nowadays, foundries casting engineered products rely on numerical simulations using commercial software to determine cooling efficiency, which requires precise input data. However, the literature lacks sufficient investigations to describe the spatial distribution of the heat transfer coefficient in the jet cooler. In this study, we propose a solver using the open-source CFD package OpenFOAM and free library for nonlinear optimization NLopt for the inverse heat conduction problem that returns the desired distribution of the heat transfer coefficient. The experimental temperature measurements using multiple thermocouples are considered the input data. The robustness, efficiency, and accuracy of the model are rigorously tested and confirmed. Additionally, temperature measurements of the real jet cooler are presented. [ABSTRACT FROM AUTHOR]

Abstract The buoyancy-driven thermofluid dynamics of a solid/hollow cylinder with concave surfaces have been studied in a laminar regime ( 10 4 ≤ R a ≤ 10 8 ) using the finite-volume method to analyze the effects of L / D ( 2 ≤ L / D ≤ 10 ) , and D t h / D   ( 0.2 ≤ D t h / D ≤ 0.9 ) . Conservation equations for mass, momentum, and energy have also been solved in a 2-D axisymmetric geometry . Second-order upwind and central difference schemes have been deployed to discretize the convective and diffusive terms. Heat dissipation increases with R a irrespective of L / D for both cylinders. However, the N u ¯ total remains constant with the L / D ratio for a hollow concave cylinder N u ¯ i n increases with D t h / D ; whereas, N u ¯ out decreases. Multiple vortices appear at the outside surface at a low D t h / D (i.e. = 0.2), and these vortices almost disappear at high D t h / D (i.e. = 0.6), affecting the outside average Nusselt number from low to high. In all cases, N u ¯ out is higher than N u ¯ i n . The thermofluid dynamics of solid and hollow cylinders have been delineated through different countour and vector plots, and two correlations for Nu have been deveoped. [ABSTRACT FROM AUTHOR]

In this study, the center of a concave surface was analytically studied using the volume of fluid approach to simulate hollow and dense droplets on a variety of solid obstacles. OpenFoam software was used to carry out the numerical simulations. The hollow droplet's fluid phase, Glycerine, has an outer diameter of 5.25 mm and its gas phase, air, has a diameter of 4 mm. We looked at the laminar flow of an incompressible Newtonian fluid phase. Jet characteristics and droplet collision hydrodynamic behavior were investigated. Due to the interaction between droplets and shells on the obstacle's surface and the concave surface, which causes a pressure difference and improves fluid movement, the largest jet size is consequently produced in rectangular obstacles. The sharp obstacle, on the other hand, molds the jet's shortest length and height. [ABSTRACT FROM AUTHOR]

Impingement/effusion cooling is the most common cooling technique or the leading edge region of a gas turbine blade to increase the maximum allowable turbine inlet temperature and thermal efficiency. This study investigated the flow characteristics of jet impingement onto a concave target plate with effusion holes depending on the shape parameters using experimental and numerical analysis methods. The diameter of the injection plate and the arrangement of the effusion holes were varied at a jet Reynolds number of 5,000. The velocity fields were measured using the PIV technique in nine cases. The CFD results were validated through comparison with experimental results, and the 3D flow structures were estimated by computational fluid dynamics (CFD) simulations. The results are expected to provide knowledge for the design optimization of a cooling system to prevent the thermal load of the leading edge of gas turbine blades. [ABSTRACT FROM AUTHOR]

The high cost of current anti-seismic devices makes it practically impossible to use them in housing buildings in low-income communities, leaving them exposed to seismic events. For this reason, the objective is to develop a low-cost seismic isolation system, made with inexpensive and accessible materials. In the initial stage, a mathematical model was developed, using its stability by means of phase portraits and the minimum potential energy principle. Then, the influence of geometrical parameters on the dynamic response of the system was analyzed. With the numerical implementation of the model, the structural response of the system was analyzed when subjected to earthquakes with different frequency contents, obtaining as a result a reduction of total accelerations that vary between a range of 4.5 times. The system was validated through the acquisition of the dynamic properties with seismographs, of an experimental model exposed to microtremors; and its comparison with the results obtained from the mathematical model. The proposed isolation system not only reduces the accelerations under the proposed earthquakes but also possesses the main characteristic of a seismic isolation system: period elongation. [ABSTRACT FROM AUTHOR]

• Using concave surfaces as contact surfaces for sessile droplets. • The critical We for coalescence and rebound on concave surfaces are given. • Analyzed the variation patterns of droplet rebound height and compression value. In this study, we used pure water droplets to investigate the collision interactions of two droplets on curved surfaces. We captured the collision dynamics at different droplet impact velocities with the use of a high-speed camera. We observed three main modes during droplet collisions: deformation, coalescence, and rebound. We also plotted a collision regime map for different surfaces using the Weber number We and presented the critical We value for coalescence and rebound. With the increases of curvature radius, the critical We variates around 6.7 to 12.3 and presenting the tendency of increases first and then decreases. Furthermore, we quantitatively analyzed the transformation laws of droplet rebound height and compression value. Our main findings describe how the surface curvature radius influences droplet interactions, thus affecting the contact time, compression value and droplet deformation factor. The outcomes of droplet collisions on curved surfaces contribute to the development of technologies related to droplet manipulation and their applications in chemical and industrial processes. [ABSTRACT FROM AUTHOR]

Magnetic abrasive finishing process (MAF) is one of the latest advanced machining processes. After eight decades have passed since the registration of the magnetic abrasive polishing process, the applicability of this method has been proven in finishing all kinds of surfaces, including flat, cylindrical and free surfaces. In this research, the influence of MAF process movement parameters on the concave surface of cold-worked steel has been investigated experimentally using the response surface method. These parameters include rotational speed, linear speed, gap between abrasive brush and workpiece, magnetic flux density and curvature angle. For this purpose, a spherical head magnet is used and the powder used is prepared by mechanical alloying method. Cold-worked steel is used in the manufacture of roll forming molds, which is used in air engines to shape compressor and turbine blades, and also to investigate the feasibility of the MAF process on the workpiece surface with high hardness and yield stress, such as Cold work steel is selected. According to the results, the optimal value of the magnetic flux density is 0.55 tesla, and with the increase of the distance between the abrasive brush and the workpiece, the surface roughness changes initially increase and decrease after passing the optimal value. [ABSTRACT FROM AUTHOR]

The main purpose of the current work is the analysis of the pulsating effect on the flow and heat transfer from impinging jet on a concave surface. In this way, the heat flux of 2300 W/m2 has been applied constantly on the surface with the radius of 120mm. The intermittent jet has been created for the frequency range of 1-100Hz by the pulsed-jet generator. Nusselt number distribution and flow field have been investigated for dimensionless distance of nozzle to surface (H/d) 2 to 5 and Reynolds number from 7000 to13000. The comparison of the experimental data with Numerical simulation shows that the k-ε RNG turbulence model is appropriately capable of predicting the Nusselt number on the concave surface under the pulsed jet impinging. Results of the present research indicate that, pulsating the jet is more effective on the concave surface in comparison with a flat surface. Also as compared to steady jet, when pulsating applies to the inlet jet with low frequency, reduction in Nusselt number is acquired. Furthermore, at each Re number and H/d, a threshold Strouhal number is found above which the Nusselt number of the pulsed jet is greater than that of the steady jet. Moreover, for the low nozzle to surface distance, Nu of stagnation point at low and high frequency is varied with Sr0.05 and Sr 0.15, respectively. At Re=10000, pulsating the impinging jet with f =100Hz causes an increase in the time and area-averaged of Nusselt number by 22% and 20% in comparison to steady jet at H/d=5 and 2, respectively.

High-temperature nickel-based alloy, which is a valuable crucial material for fabricating functional components, is commonly used to fabricate turbine blades. However, the concave surface of the turbine blade is much difficult to be polished. In this work, shear-thickening polishing (STP), which employed the shear-thickening mechanism of non-Newtonian power-law fluid, was utilized to achieve high efficiency and high-quality polishing of the concave surface of the high-temperature nickel-based alloy turbine blade. The finite element simulation of the pressure and velocity distribution on the concave surface during STP process was carried out by ANSYS, and the effects of different polishing angles, polishing velocities, pH values of polishing slurry and Fenton's reagent on the surface morphology and roughness of the workpiece were investigated by experiment. The comparison between simulation and experimental results indicated that the suitable polishing angle is 45°. The best surface can be obtained with a pH value of 6.5. With the increase of the polishing speed, polishing efficiency improved gradually. The workpiece surface was improved by adding 0.5% hydrogen peroxide (H2O2) and 0.15% FeSO4 in the polishing slurry. In the final, surface roughness of the turbine blade was reduced rapidly from Ra = 72.3 nm to Ra = 4.2 nm after 9 min polishing under the appropriate conditions. The research results provide reference and basis for high-temperature nickel-based alloy turbine blade polishing.

A computational study is carried out of the 3-D flow field and heat transfer under a turbulent intermittent circular jet impingement on a concave surface. The control-volume procedure with the SIMPLE algorithm is employed to solve the unsteady RANS (use full form) equations. The RNG k-e model is implemented to simulate turbulence due to its success in predicting similar flows. The numerical results are validated by comparing them with the experimental data. The effects of jet Reynolds number and oscillation frequency on the flow and heat transfer are evaluated. The profiles of instantaneous and time-averaged Nusselt numbers exhibit different trends in axial, x-directions and circumferential, s-directions. It is found that increasing frequency from 50 to 200 Hz results in considerable time-averaged Nusselt number enhancement in both axial and curvature directions. The intermittent jet at a frequency of 200 Hz enhances the total average Nusselt number by 51.4%, 40%, and 33.7% compared to the steady jet values at jet Reynolds numbers of 10000, 23000, and 40000, respectively. In addition, a correlation for the average Nusselt number is proposed depending on the Reynolds number and the Strouhal number. [ABSTRACT FROM AUTHOR]

A highly sensitive reflective Fabry–Perot (FP) magnetic field sensor using magnetic fluid (MF) based on the Vernier effect is presented and experimentally verified. Capillary action acts as an injection of MF into an FP cavity, which is composed of a single-mode fiber (SMF), capillary glass tube, and a short section of SMF coated with gold film. Vernier effect is formed by the MF cavity and air cavity in the FP magnetic field sensor, where the surface of MF is concave. The refractive index (RI) of MF depends on the magnetic field, and therefore, the wavelength of the reflection spectrum of the fabricated sensor can be sensitive to the magnetic field. The magnetic field sensor’s sensitivity is up to 1.02602 nm/Gs from 118.768 to 166.261 Gs, and the resolution is 0.0078 Gs. The sensor shows good linearity within the range of 71.317–231.751 Gs, and the linearity is larger than 0.999. In this article, a new type FP cavity fabricated by splicing SMF inside the capillary glass tube is proposed, and MF can be injected into the FP cavity, which can also be applied to other liquids to generate Vernier effect. The concave surface of MF is creatively applied to the sensors, and the experimental verification is carried out in a capillary glass tube, which provides a basis on the capillary action of other fluids in fiber sensing. [ABSTRACT FROM AUTHOR]

Jet impingement heat transfer finds applications where a large heat flux is required between a fluid and a surface. Impinging jets can be implemented in Concentrating Solar Power (CSP) thermal receivers and bayonet tube heat exchangers. A simultaneous outlook on the heat transfer and total pressure loss (performance) characteristics of several jets impinging on a concave hemispherical surface are investigated experimentally and using an axisymmetric Reynolds averaged Navier Stokes (RANS) Computational Fluid Dynamics (CFD) model. The four equation Transition SST RANS turbulence CFD model demonstrates to be most suitable for this domain with a mean absolute deviation from the experimental results of < 7% for the heat transfer coefficient and < 8% for the total pressure loss. Empirical correlations for the Nusselt number as a function of the nozzle outlet Reynolds number and Prandtl number are fitted. Relatively good agreement is found between the Nusselt correlation and existing literature. An empirical correlation is also presented for the total pressure loss factor for the jet impingement domain in general because it is found that the dominating total pressure loss occurs because of rapid expansion, which occurs in any impinging free jet. The developed empirical correlations and CFD model can be used to estimate the heat transfer and pressure loss characteristics of a bayonet tube heat exchanger, a solar thermal receiver employing impinging jets as well as other jet impingement domains. [ABSTRACT FROM AUTHOR]

Numerical simulations using ANSYS Fluent 17.2 are explored for a detailed discussion on jet impingement over a compound dimpled and protrusioned concave surface. Previous researchers have proved that both dimples and protrusions help in heat transfer augmentation. However, the combined effect of dimple and protrusion over a concave surface is still not studied, which may show a significant improvement in heat transfer. Therefore, the present study is devoted to the alternate location of dimple and protrusion over a concave surface for enhancement in impingement heat transfer. Simulations are performed for several arrangements of dimples and protrusions over a concave surface. It is noticed that a particular arrangement of dimples and protrusions leads to an increase in heat transfer compared with a fully protruded or fully dimpled concave surface. Furthermore, it is also noticed that protrusions/dimples at the stagnation region degrade overall impingement heat transfer. The wall shear stress profile is found to be similar to its corresponding local Nusselt number profile. Lastly, the optimal location of dimples and protrusions is predicted using an artificial neural network and a specially formulated discrete version of genetic algorithm. [ABSTRACT FROM AUTHOR]

In this work, we attempt to precisely find out the equation describing the shape of a radially symmetric concave surface. An experimental interference pattern of Fizeau ring fringes in case of transmission is obtained using the semi-reflecting concave surface (under study) placed on a glass plate. Theoretically, we showed that concentric ring fringes can be obtained by a concave surface which is a part of an ellipsoid or a paraboloid in addition to a sphere by simulating the ring fringes in each case. The simulated interferograms were obtained considering the traced optical paths of the interfered rays which guarantee accurate calculations. From a comparison between the experimental and the estimated interference patterns, we knew which equation was adapted to represent the concave surface used in our experiment. The best surface’s equation is that whose simulated pattern satisfied the minimum percent error with the experimental pattern. Interestingly, the surface’s equation was retrieved from a single experimental interferogram which was produced by a simple well-known optical setup. The proposed method is fast, simple and accurate which make it suitable for quality control of concave surfaces manufacturing.

In this paper, combining the bilayer porous metal block of copper (BPMBC) on the concave heating surface and jet impingement of SiO2-H2O nanofluid is utilized in the flow channel. The k-ε turbulent model coupled with Brinkman Forchheimer extended Darcy equations are employed to analyze the effects of the SiO2-H2O nanofluid concentration, porosity in the porous monolayer on concave surface, thickness ratio between the upper and lower porous layer in the bilayer porous metal block as well as the curvature of concave surface on the heat transfer. In comparison with pure water as working fluid, the 5.85% rise of average heat transfer coefficient (HTC) can be obtained while the 3.0% SiO2-H2O nanofluid is utilized in the mode. The combining effects of the porosity in the monolayer porous layer and the curvature of concave surface on the heat transfer are related to the heating surface area and convection between the jet nanofluid and heating surface. More heat transfer occurs in the bilayer porous metal block with a larger porosity in the upper layer and a lower porosity in the bottom layer due to the dominant effects of the convection and thermal conductivity respectively in the different porous layers. The effects of the thickness ratio between the upper and lower layer in the bilayer porous metal block on the heat transfer are related to the influencing portion between the surface area and convection. With an increase in the curvature of concave surface from R/L(ratio of concave radius to chordal length) =0.5 to R/L = 1.1, the average Nusselt numbers go up, and their rise rates decrease by the Reynolds number gradually, but the surface area decreases, which causes the average temperature rise of copper block. Besides, the higher average temperature occurs in the mode with a flat plate than that with a concave surface. [ABSTRACT FROM AUTHOR]

Experimental investigations are performed on the single‑row jet impingement heat transfer onto a semi‑cylindrical concave surface. Two kinds of nozzle shapes are taken into consideration. They are circular nozzle and chevron nozzle. The semi‑cylindrical concave surface has a diameter ratio (D/d) of 10. The jet‑to‑jet pitch is 5d. The experiments are conducted under three Reynolds numbers (Re=5 000, 10 000, and 20 000) and four dimensionless nozzle‑to‑surface distances (H/d=2, 4, 6, and 8). The results show that both nozzles achieve their respective maximum stagnation Nusselt numbers at H/d=4. However, the longitudinally‑averaged Nusselt number in the vicinity of concave leading is monotonously reduced with the increase of nozzle‑to‑surface distance in general. The chevron nozzle is confirmed to produce higher convective heat transfer than that of circular nozzle. In comparison with the circular nozzle jet impingement, the maximum increase of longitudinally‑averaged Nusselt number at the concave leading is about 16% and the maximum increase of the locally area‑averaged Nusselt number in the vicinity of concave leading (within ±2d) is about 18%, produced by the chevron‑nozzle jet impingement. [ABSTRACT FROM AUTHOR]

Noble-metal nanocrystals with structural features such as high-index facets and twin defects on the surface are desirable for various catalytic applications. Here we report an aqueous route to the facile synthesis of Pd–M (M = Pd, Pt, and Au) decahedra covered with concave facets by controlling the deposition and diffusion of M atoms on Pd decahedral seeds. Our mechanistic studies indicate that the preferential deposition of M atoms on the twin boundaries of Pd decahedral seeds under limited surface diffusion is instrumental to their evolution into decahedra enclosed by concave facets. Due to the presence of high-index facets and twin boundaries on the surface, the Pd concave decahedra show enhanced catalytic activity towards formic acid oxidation. The present work not only offers a general route to the facile synthesis of decahedral nanocrystals with concave facets but also provides an additional knob for enhancing the catalytic performance of noble-metal nanocrystals. [ABSTRACT FROM AUTHOR]

Aiming at the problem that the front face of broach is concave and hard，it is difficult to realize the microstructure machining of tool tip surface，the machining principle of single point diamond tool (SPDT) vibrating press printing microstructure was studied. A tool tip microstructure SPDT vibratory imprinting device was proposed. 3 rows and 30 columns of round pit microstructure was successfully prepared on the front of the test broach by this device. Their maximum circular section diameter was 30 pm and the depth was 10 pm. And the test broach was compared with the conventional broach. The effect of tool tip microstructure on broaching load was studied. The results show that in the stable broaching stage，compared with the conventional broach，the average broaching load，maximum and minimum broaching load of the test broach respectively is achieved by 5. 20%，5.04% and 12. 33%. It shows that the broaching load can be effectively reduced after the microstructure is prepared at the front of the broach，which is helpful to improve the broaching performance and the existing machining technology. [ABSTRACT FROM AUTHOR]

A tri-dimensional lobed nozzle is concerned in the jet impingement on a flat target and a concave target in the current study. The jet impingement heat transfer experiments are conducted under two jet Reynolds numbers (Re=10000 and 20000) and four nozzle-to-surface distances (H/d=2, 4, 6 and 8). Simultaneously, to characterize the flow dynamics of lobed jet impingement onto different target surfaces, some computations are conducted under a specific jet Reynolds number. The results show that the lobed jet is capable of achieving an increase of stagnation Nusselt number about 25% in relative to the round jet at small nozzle-to-surface distances. However, at large nozzle-to-surface distances, the lobed jet otherwise weakens the convective heat transfer in the vicinity of jet stagnation, especially under high jet Reynolds number. When compared to the flat target, approximately a 20%–30% reduction of stagnation Nusselt number is produced on a concave target, which is attributed to the combined effect of destabilization and confinement due to the concave curvature. [ABSTRACT FROM AUTHOR]

Ａｉｍｉｎｇ ａｔ ｔｈｅ ｐｒｏｂｌｅｍ ｔｈａｔ ｔｈｅ ｆｒｏｎｔ ｆａｃｅ ｏｆ ｂｒｏａｃｈ ｉｓ ｃｏｎｃａｖｅ ａｎｄ ｈａｒｄ, ｉｔ ｉｓ ｄｉｆｆｉｃｕｌｔ ｔｏ ｒｅａｌｉｚｅ ｔｈｅ ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ ｍａｃｈｉｎｉｎｇ ｏｆ ｔｏｏｌ ｔｉｐ ｓｕｒｆａｃｅ, ｔｈｅ ｍａｃｈｉｎｉｎｇ ｐｒｉｎｃｉｐｌｅ ｏｆ ｓｉｎｇｌｅ ｐｏｉｎｔ ｄｉａｍｏｎｄ ｔｏｏｌ (ＳＰＤＴ) ｖｉｂｒａｔｉｎｇ ｐｒｅｓｓ ｐｒｉｎｔｉｎｇ ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ ｗａｓ ｓｔｕｄｉｅｄ. Ａ ｔｏｏｌ ｔｉｐ ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ ＳＰＤＴ ｖｉｂｒａｔｏｒｙ ｉｍｐｒｉｎｔｉｎｇ ｄｅｖｉｃｅ ｗａｓ ｐｒｏｐｏｓｅｄ. ３ ｒｏｗｓ ａｎｄ ３０ ｃｏｌｕｍｎｓ ｏｆ ｒｏｕｎｄ ｐｉｔ ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ ｗａｓ ｓｕｃｃｅｓｓｆｕｌｌｙ ｐｒｅｐａｒｅｄ ｏｎ ｔｈｅ ｆｒｏｎｔ ｏｆ ｔｈｅ ｔｅｓｔ ｂｒｏａｃｈ ｂｙ ｔｈｉｓ ｄｅｖｉｃｅ. Ｔｈｅｉｒ ｍａｘｉｍｕｍ ｃｉｒｃｕｌａｒ ｓｅｃｔｉｏｎ ｄｉａｍｅｔｅｒ ｗａｓ ３０ μｍ ａｎｄ ｔｈｅ ｄｅｐｔｈ ｗａｓ １０ μｍ. Ａｎｄ ｔｈｅ ｔｅｓｔ ｂｒｏａｃｈ ｗａｓ ｃｏｍｐａｒｅｄ ｗｉｔｈ ｔｈｅ ｃｏｎｖｅｎｔｉｏｎａｌ ｂｒｏａｃｈ. Ｔｈｅ ｅｆｆｅｃｔ ｏｆ ｔｏｏｌ ｔｉｐ ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ ｏｎ ｂｒｏａｃｈｉｎｇ ｌｏａｄ ｗａｓ ｓｔｕｄｉｅｄ. Ｔｈｅ ｒｅｓｕｌｔｓ ｓｈｏｗ ｔｈａｔ ｉｎ ｔｈｅ ｓｔａｂｌｅ ｂｒｏａｃｈｉｎｇ ｓｔａｇｅ, ｃｏｍｐａｒｅｄ ｗｉｔｈ ｔｈｅ ｃｏｎｖｅｎｔｉｏｎａｌ ｂｒｏａｃｈ, ｔｈｅ ａｖｅｒａｇｅ ｂｒｏａｃｈｉｎｇ ｌｏａｄ, ｍａｘｉｍｕｍ ａｎｄ ｍｉｎｉｍｕｍ ｂｒｏａｃｈｉｎｇ ｌｏａｄ ｏｆ ｔｈｅ ｔｅｓｔ ｂｒｏａｃｈ ｒｅｓｐｅｃｔｉｖｅｌｙ ｉｓ ａｃｈｉｅｖｅｄ ｂｙ ５.２０％, ５.０４％ ａｎｄ １２.３３％. Ｉｔ ｓｈｏｗｓ ｔｈａｔ ｔｈｅ ｂｒｏａｃｈｉｎｇ ｌｏａｄ ｃａｎ ｂｅ ｅｆｆｅｃｔｉｖｅｌｙ ｒｅｄｕｃｅｄ ａｆｔｅｒ ｔｈｅ ｍｉｃｒｏｓｔｒｕｃｔｕｒｅ ｉｓ ｐｒｅｐａｒｅｄ ａｔ ｔｈｅ ｆｒｏｎｔ ｏｆ ｔｈｅ ｂｒｏａｃｈ, ｗｈｉｃｈ ｉｓ ｈｅｌｐｆｕｌ ｔｏ ｉｍｐｒｏｖｅ ｔｈｅ ｂｒｏａｃｈｉｎｇ ｐｅｒｆｏｒｍａｎｃｅ ａｎｄ ｔｈｅ ｅｘｉｓｔｉｎｇ ｍａｃｈｉｎｉｎｇ ｔｅｃｈｎｏｌｏｇｙ. [ABSTRACT FROM AUTHOR]

The focus of this study is to numerically reproduce the asynchronous oscillations of an impinging slot jet on a concave surface experimental configuration. A Detached Eddy Simulation (DES) numerical simulation was realised at a Reynolds number of R e b = 6667 with respect to the slot jet width. The topological characteristics of the flow were also analyzed using the Proper Orthogonal Decomposition (POD) method. It was therefore possible to propose a sequence of flow topologies (four distinct flow states) during the changeover phenomenon of the impinging slot jet. The results show that the asynchronous phenomenon of the impinging slot jet seems to be related to the transitional characteristics of the jet flow. The flow topology of the most stable and energetic states (top and bottom jet flow states) creates a pressure imbalance inside the area of interest, resulting in the appearance of two transitional flow states. For the top or bottom impinging jet flow states, the jet remains in a relatively stationary position and the flow in the area of interest is quasi-two-dimensional. However, during the rapid changeover of the jet between these two states, small vortex structures prevail inside the area of interest for the centre jet states. [ABSTRACT FROM AUTHOR]

In the present study, flow and heat transfer characteristics of an impinging jet issuing from a circular cross-sectional pipe on a rectangular plate, convex and concave hemispheres were investigated numerically for various Reynolds numbers and jet-to-plate distances. Steady and three-dimensional Reynolds-Averaged Navier-Stokes equations were solved iteratively utilizing finite volume method. Turbulence model assessment study reveals that the transitional Shear Stress Transport k-ω turbulence model can be used for such a problem. Regardless of the geometry of the impinging surfaces, two different flow regions were detected around y/D = 3. The first region starts from the stagnation point (y/D = 0) to the edge of the rectangular plate and hemispheres (y/D = 3). In this region, the maximum heat transfer is obtained for the jet impinging on the rectangular plate. However, in the second region that covers the wall jet zone the impinging jet on the convex hemisphere provides the maximum while the concave causes the minimum heat transfer. The most efficient jet-to-plate distance is found as H/D = 2 in the first region while heat transfer does not change with distance in the wall jet zone. It was shown that Nusselt number increases with Reynolds number when the jet impinges on the concave and convex hemispheres as observed for the impinging jet on a flat plate. [ABSTRACT FROM AUTHOR]

Abstract We examined the flow-field characteristics of free jets issued from three types of jet exits opened on a convex curved surface for aspect ratios of 16, 12, and 9.6 and height h of 9, 12, and 15 mm, respectively. In addition, we examined the heat transfer characteristics of the impinging jet, the shear stress inside a half-cylinder, and the flow-field near the impinging jet region through flow visualization in cases where each jet impinges vertically onto the inner part of the half-cylinder. The impinging distance ratios H/h between the jet exit and its inside for height h were 4, 3, and 2.4; and two semicircular plates spaced at a value of W were installed parallel to both long sides of the exit. The Reynolds number of the jet based on the mean bulk velocity at the jet exit and height h was approximately 11,700, except in cases where the flow visualization was made using smoke, at which the Reynolds number was approximately 5800. The flow-field characteristics of the free jets were similar to those of a two-dimensional jet. The flow-field in the region surrounded by the inside of a half-cylinder and two semicircular plates can be classified into three flow patterns by changing the space, namely W/H. This revealed that the rate of increase in heat transfer coefficient became 20% larger than that of the jet operating power by controlling the flow-field using two semicircular plates; in this case, the impinging jet flow vibrated the surface at the center of the jet. It was also clarified that even when the local heat transfer coefficient changed when mounting semicircular plates, the time-averaged shear stress on the inside of the half-cylinder did not change within the impinging jet region. Graphical abstract Image 1 Highlights • The characteristics of free jet issued from a convex curved exit are investigated. • The impinging jet heat transfer and shear stress on a half-cylinder inside are investigated. • The flow-field characteristics of the free jets were similar to those of a two-dimensional jet. • The flow-fields can be classified into three flow patterns by changing the space between semicircular plates. • The rate of increase in the heat transfer coefficient became 20% larger than that of the jet operating power. [ABSTRACT FROM AUTHOR]

In this work, time-resolved particle image velocimetry (TR-PIV) measurements have been carried out to investigate the structural features of a submerged transverse round liquid turbulent jet discharged into a confined laminar crossflow. The duct Reynolds number based on crossflow velocity is fixed at R e = 3000 , and the impinging jet is directed to a semicylindrical concave surface located in one of the walls of a vertical square duct. The spatial development and flow dynamics are analyzed along several orthogonal planes parallel/perpendicular to the jet axis for jet Reynolds numbers based on the jet exit diameter of R e j = 3000 , 6000 and 9000 (jet-to-crossflow velocity ratios of r = 3. 27 , 6.53 and 9.80), respectively. For this purpose, mean and ensemble-averaged velocity and vorticity distributions, turbulent statistics, and Reynolds shear stresses are obtained for various injection rates. Spectral analysis reveals the shedding frequencies of the wake vortices, and the pairing and breakup processes of the evolving large-scale coherent structures have been related to the measured wake Strouhal numbers. Our results indicate that due to the curvature of the target surface and the upwash of the jet impact, the size of coherent structures decreases with increase in the value of R e j . Proper orthogonal decomposition (POD) of the flow using the snapshot method is applied to extract the dominant features of the turbulent flow. It is observed that at the symmetry plane and for R e j = 3000 , 6000 and 9000, the energy collected by the first two dominant eddies contribute to 14.98%, 18.68%, and 17.89% of the turbulence kinetic energy, respectively. Our results show that the evolution of the time-resolved temporal coefficients of degenerate pairs of modes (1, 2) represent the flapping motion of the jet. Moreover, the amplitude of the oscillations of the flapping jet and the complexity of the flow structure increase for increasing values of R e j . • Flow structural features are assessed for Re j = 3000, 6000 and 9000. • Shedding frequencies of horseshoe vortices, wake vortices, and wall jets are obtained. • POD analysis reveals the dominant flow features and flapping motion of the jet. [Display omitted] [ABSTRACT FROM AUTHOR]

In the present research, free convection heat transfer from isothermal concave and convex body shapes is studied numerically. The body shapes investigated here, are bi-sphere, cylinder, prolate and cylinder with hemispherical ends; besides, they have the same height over width (H/D = 2). A Numerical simulation is implemented to obtain heat transfer and fluid flow from all of the geometries in a wide range of Rayleigh numbers. The results show that flatness, concavity and smoothness have major effects on estimation of free convection heat transfer. As the total surface heat transfer area changed by altering the geometry, the local Nusselt number are compared for these body shapes; as well; it shows that concave surfaces has adverse influence on transferring heat. In addition, the current results reveal the average Nusselt numbers based on square surface area are not affected by the geometries for the laminar range of Rayleigh numbers. Besides, “incompressible ideal gas model” is used for the variation of density in free convection heat transfer. This model has the capability to be utilized in the cases with high temperature differences between the fluid and the bodies’ surfaces.

In the present study, we report the observations of the impact and freezing processes of a water droplet on different cold concave surfaces. During the experiment, the concave surface was put into a cover in which pressurized Argon gas was injected at the same time with a purpose to minimize the desublimation of the vapor. Then, the surface temperature was cooled down to a desirable value by a constant temperature bath circulator. After that, a deionized water droplet was released to impinge the concave surface and its impact and freezing processes were recorded by two cameras. The results showed that, during the impact process of the water droplet, the maximum contact diameter along azimuthal direction was generally smaller than that along axial direction. In addition, during the water droplet recoiling process, the residue of a ring appeared on the cold concave surface and the radius of the concave surface was found to have a minor effect on the spreading factor along axial direction. During the freezing process of the water droplet, the change of the temperature as well as the radius of the concave surface did not lead to an apparent variation of the ice bead shape. [ABSTRACT FROM AUTHOR]

In this study, the numerical analysis of turbulent flow and heat transfer of oscillating impinging circular jet on concave cylindrical surface has been investigated. In this way, the averaged Navier-Stocks equations for turbulent incompressible flow in an unsteady state with RNG k-ε turbulent model and in 3D computational space were solved. The effects of oscillation frequency, amplitude of oscillation and Reynolds number on Time-averaged Nusselt number Distribution in concave surface were studied. The obtained results show that applying the pulsating jet in the range of 50 to 200 Hz can increasing heat transfer from concave surface in comparison with the steady jet. Furthermore, increasing Re number from 10000 to 40000 and amplitude of oscillation from 0.4 to 1, leads to the increase of timeaveraged Nusselt number. [ABSTRACT FROM AUTHOR]