Impingement Mode Research Articles

Investigation of droplet splashing behavior during oblique jet impact onto a wall

For the issue of jet impingement on the wall in industrial cooling processes, an experimental setup based on high-speed photography for oblique jet impingement onto the wall was constructed. The experimental focus was on the study of liquid droplet splashing behavior after oblique jet impingement on the wall, discussing the liquid droplet splashing behavior under three jet impingement modes: Rayleigh regime, first wind-induced regime, and secondary wind-induced regime. By employing methods such as trajectory imaging and particle image velocity for liquid droplet parameter image measurement, obtaining the particle size, velocity, and distribution of splashed droplets after oblique jet impact on walls under different working conditions. The impact of jet impingement velocity and angle on droplet splashing parameters was analyzed. The results showed that when the impingement point is before the breakup length, with increasing flow velocity, the surface wave of the liquid column, and the spreading liquid film became more pronounced, but the loss of liquid-phase components due to splashing was relatively small. When the impingement point is after the breakup length, the secondary breakup resulting in a “crown”-shaped liquid film after droplet impingement leads to a significant loss of liquid-phase components through splashing. As the inlet velocity of the jet increases, there is a decreasing trend in droplet size and an increasing trend in droplet velocity. With an increase in jet angle, there is a decreasing trend in droplet size and velocity. Based on the concentration, size, and velocity distribution characteristics of splashing droplets, the area after oblique jet impingement on the wall can be divided into the impingement zone, low-concentration low-velocity zone, high-concentration high-velocity zone, and lateral splashing zone. This has significant implications for understanding the splashing mechanism after oblique jet impingement on the wall and optimizing operating conditions.

Optimising the hydraulic performance of a jet impingement sprinkler by varying elevation angle: A Comparative study with a non-impingement sprinkler

A novel jet impingement sprinkler was designed to replace the additional water-dispersing devices usually used in rotating sprinklers to adapt performance for low-pressure conditions. Utilising the asymmetric impingement mode of primary and secondary jets, the design prioritises the secondary nozzle elevation angle. Theoretical expressions were establish between the primary and secondary jet momentum rates. Hydraulic performance experiments, were conducted under intermediate and low-pressure conditions, examining water application rate, wetted radius, and Christiansen's uniformity coefficient at various elevation angles. High-speed photography (HSP) experiments elucidate differences in hydraulic performance, revealing that the impingement jet generally featured a lower maximum water application rate but a smoother distribution compared to its non-impingement counterpart. Using the CRiteria Importance Through Intercriteria Correlation (CRITIC) method, comprehensive scores highlight an optimal arrangement with a 34.5° secondary nozzle elevation angle, illustrating superior performance under low pressure. Comparative analyses between the wetted radius and Christiansen's uniformity coefficient uncover a trade-off, with jet impingement sacrificing wetted radius for increased uniformity. However, this efficiency diminished as jet impingement effects intensify. HSP experiments validate theoretical calculations, with a relative error of less than 4% for jet deflection angle and jet breakup length. Non-linear curve fitting establishes relationships between jet breakup length, mean observed jet deflection angle, operating pressure, and wetted radius. Calculated and measured values exhibited a relative error within 5%, affirming the applicability of the developed equation for predicting wetted radius in jet impingement sprinklers.

Numerical study on jet and stretch behaviors of an impingement leaky-dielectric droplet under electric field

The droplet breakup and ejection after impact under electric fields has attracted increasing attention in industrial applications, such as spray cooling, 3D printing, anti-icing of powerlines, etc. We numerically investigated the leaky-dielectric droplet impact on the hydrophilic surface in perfect dielectric surrounding under vertical electric field with OpenFOAM. The coupled volume of fluid (VOF) and leaky dielectric model have been used to solve two-phase electrohydrodynamic (EHD) problems. Impact behaviors are most strongly influenced by the electric capillary number, dielectric permittivity and electrical conductivity. Four impact modes were registered: no-stretch-no-jet, single-droplet split, stretch-no-jet and stretch-jet. The mechanism of the four impingement modes is explained through the relationship between normal electric stress, tangential electric stress and charge relaxation. The stronger distribution of normal electric stress near the cylindrical tip of the droplet leads to a stretching behavior. Then, the stretched filament must be sustained by tangential electric stresses. The surface charge distribution influences the electric stresses, which depend on charge relaxation, charge convection and tip curvature. Furthermore, the relationship between the electric capillary number, dielectric permittivity, electrical conductivity and the breakup modes was obtained. These findings could facilitate the design of electrospray cooling, deicing for power transmission lines, and 3D printing.

Study on flow and heat transfer characteristics of 3D molten aluminum droplet printing process

The molten metal droplet printing technology can effectively realize a precise preparation of complex electronic devices at the micro scale by relying on the metallurgical bonding among the tiny molten metal droplets, which can improve the metallographic structure and mechanical properties of the fabricated parts. In this paper, the interaction process between metal droplets and movable substrates during the preparation of electronic devices is studied based on the coupled level set volume of fluid method (CLSVOF) and equivalent heat capacity method. The influence of the micro-bubbles and wall infiltration characteristics on the impingement spreading flow of metal droplets and their heat transfer cooling process is also investigated. The evolution mechanisms of droplet geometry and the distribution characteristics of droplet internal temperature and heat flow density during the impingement spreading process are discussed. The mechanism of high-temperature region formation under superhydrophobic wall conditions is analyzed. Results show that the microbubbles inhibit the spread of molten metal droplet. Under the same conditions, the spreading radius of hollow metal droplet is smaller than that of solid droplet, and their spreading height is higher than that of solid droplet. Different wall infiltration characteristics have similar degrees of influence on the spread of solid and hollow droplets. A larger wall contact angle for the same impingement mode at the same moment corresponds to a smaller droplet spreading radius and the higher the spreading height. With the participation of microbubbles, the heat transfer cooling process inside metal droplet becomes slow and demonstrates a less uniform temperature and heat flow density distribution. A high-temperature region is easily formed inside droplet under superlyophobic wall conditions, which triggers an uneven heat transfer cooling process at the bottom of the droplet. The results benefit to reveal the law of metal microdroplet impingement molding on different working surfaces and provide a theoretical reference for the preparation of electronics-related devices.

Effect of material removal characteristics with varying incident angle in fluid jet polishing process

Fluid jet polishing process is a non-contact type polishing process which is used to manufacture different kind of precise components in various engineering sectors. Such processes require more deterministic material removal distribution to better understand the position of the workpiece material and impingement angle. In this paper attempt is made to simulate the three dimensional Fluid Jet Polishing (FJP) model at vertical and oblique impingement mode. It was observed from the simulation that the shape of Tool Influence Function (TIF) is unsymmetrical and achieve more material removal when jet strikes at oblique impingement mode. Also, concluded that material removal rate becomes more when impingement angle increases from normal axis of rotation.

Modification of subcritical cylinder flow with an upstream rod

In the current research, a small circular rod is placed upstream of the main cylinder. This is used to change the incoming flow and, thus, impact the vortex shedding process of the wake as well as the resultant aerodynamic forces. The experiments are carried out in a wind tunnel with a subcritical Reynolds number of Re = 32 000. The sectional pressure distribution around the main cylindrical model is obtained to calculate unsteady aerodynamic forces. The experimental results show that the gap ratio markedly affects the aerodynamic forces exerting on the main cylinder. The optimal gap ratios for drag reduction are found to be G/D = 1.5 and 2.0, where the average drag and unsteady lift forces exerting on the main cylinder are suppressed. In addition to pressure measurements, we also apply the high-speed particle image velocimetry system to obtain flow characteristics of the cylinder wake as well as the gap flow. Two different modes of gap flow are found, i.e., the cavity mode and the wake impingement mode, and the transition value between them is around G/D = 1.5. Moreover, at the appropriate gap ratio, the wake vortex shedding pattern of the main cylindrical model changes from antisymmetric to symmetric.

INVESTIGATE THE APPROPRIATE MODEL FOR DESCRIBING THE CRYSTALLIZATION PROCESS OF Te65Sb25Ga10 THIN FILM

The sheet resistance, Rs , measurements versus temperature, T, for 1000 nm Te - Sb -Ga thin films were performed at different heating rates (   5, 10, 20, 30 and 40 K/min) in the temperature range from 300 to 673 K. The kinetic transformation parameters of Te65Sb25Ga10 thin film were examined via the theoretical method developed (TMD) and the Johnson–Mehl–Avrami model (JMA). The crystallization activation energy Ec with the transformed fraction is not constant but change with the conversion fraction. These results refer to the rate constant of the transformation is in fact determined through the nucleation and diffusion processes. The results of the impingement exponent γi dependence of the conversion fraction (χ) revealed that the mode of impingement is due to “anisotropic growth”. In addition to that the average values of kinetic exponent parameters, n and m obtained by (TMD) and JMA models. are accounted for two and three-dimensional crystal growth with heterogeneous nucleation. Moreover, comparing the experimental curve of the transformed fraction indicated that the crystallization process of Te65Sb25Ga10 thin film cannot be satisfactorily described by Johnson-Mehl-Avrami (JMA) model. On contrary, the theoretical curves of the developed model are more realistic for describing the crystallization process of the investigated chalcogenide bulk glass.

Investigating the Kinetic Parameters of SiO2-Al2O3-CaO-CaF2-K2O Oxyfluoride Glass

Oxyfluoride glass-ceramics containing CaF2 nanocrystals are kind of attractive materials for the optical applications due to their low phonon energy and high transparency. Moreover, the crystallization control and consequently, the kinetic properties are important for oxyfluoride glasses. Therefore, in the present research, the crystallization kinetics of isochronal transformation of the 37.26SiO2-28.11Al2O3-7.73CaO-26.89CaF2-4.5 K2O (wt%) glass have been determined upon the basis of maximum transformation rate using Differential Thermal Analysis (DTA) technique. Hence, it is concluded that the crystallization of the mentioned glass is a process controlled by Avrami nucleation, three-dimensional diffusion-controlled growth, and anisotropic growth impingement mode. The effective activation energy Qp =181 kJ.mol-1, growth exponent n=2.272, nucleation activation energy QN= 123, and growth activation energy QG=211 have been determined.

Numerical Simulation of Flow Control around a Circular Cylinder by Installing a Wedge-Shaped Device Upstream

The effect of a triangular wedge upstream of a circular cylinder has been investigated, and the findings are presented herein. The triangular wedge is equilateral in plan form, and the Reynolds number based on the diameter of the main cylinder is approximately 200. Contours of vorticity clearly show that two entirely different wake patterns exist between the wedge and the main cylinder. There also exists a critical spacing ratio and side length ratio at which the wake flow pattern shifts from one within the cavity mode to one within the wake impingement mode. For a relatively small side length ratio of l w / D = 0.20 and 0.27, where the side length refers to the length of one side of the triangular wedge, the drag and lift coefficients decrease monotonically with the spacing ratio. There is a sudden jump of the drag and lift coefficients at larger side length ratios of l w / D = 0.33 and 0.40. This study shows that at a spacing ratio of L/D = 2.8 (where L is the distance between the vertex of the wedge and the center of the cylinder) and a wedge side length of l w / D = 0.40, the reduction of the amplitude of lift and mean drag coefficient on the main cylinder are 71.9% and 60.1%, respectively.

Kinetics of the fcc\xa0\u2192\xa0hcp Phase Transformation in Cu-Ge Solid Solutions Upon Isothermal Aging

The kinetics of the ζ-phase formation from a supersaturated α-Cu(Ge) solid solution (i.e., transformation from the fcc crystal structure to the hcp crystal structure) containing 10.8 at. pct Ge [at isothermal temperatures of 573 K, 613 K, and 653 K (300 °C, 340 °C, and 380 °C)] were studied by X-ray diffraction (XRD) for phase fraction determination. Both in situ and ex situ annealing experiments were performed. The transformation kinetics were modeled on the basis of a versatile modular model. The transformation kinetics complied with a site-saturation nucleation mode and strongly anisotropic interface-controlled growth mode in association with a corresponding impingement mode: diffusion of Ge (towards the stacking faults, SFs) does not control the transformation rate. Transmission electron microscopy (TEM) investigations showed that segregation of Ge at the stacking faults (SFs) takes place (relatively fast) prior to the structural transformation (fcc → hcp).

Calorimetric studies on Ge23Se67Sb10-0.5%RbI glass

Understanding the crystallization mechanism of chalcogenide glass can help in directing the subsequent annealing processing, thus tuning the microstructures and physical properties. In this work, the transformation kinetics of Ge23Se67Sb10-0.5%RbI was investigated under non-isothermal conditions. From VFT equation, the ideal glass transition temperature was determined as T0g=427.74K. The fragility index m was estimated to be 64.74, indicating a relatively fragile liquid. From the classical JMA theory, the activation energy was determined as 222.9kJ/mol, growth index was 1.26, while from the model considering the impingement modes(MCI) activation energy was determined as 90.99kJ/mol and growth index was 1.38, which indicated that the growth mode of crystallization of the present glass was between one dimension and two dimension. The result shows that addition of RbI can reduce the growth dimension and activation energy.

Effects of Y2O3 on crystallization kinetics of SiO2-Al2O3-CaO-CaF2 oxy-fluoride glass-ceramic system

Differential thermal analysis (DTA) patterns of SiO2-Al2O3-CaO-CaF2 oxy-fluoride glass system with adding different amounts of Y2O3 from 0.5(wt%) to 1.5(wt%) have been studied under isochronal circumstances. It is concluded that, the crystallization of the glasses is a process controlled by Avrami nucleation, three dimensional diffusion controlled growth and anisotropic growth impingement mode. Afterwards, the effective activation energy Qp, growth exponent, nucleation activation energy QN, and growth activation energy QG, have been determined and were in the range of 283–321kJmol−1, 2.05–2.25, 190–249kJmol−1 and 325–349kJmol−1, respectively. Qp, QN, QG and np are increased with increase of Y2O3 amount, which is related to the network forming role of Y2O3 in these systems.

Scaling laws for drop impingement on porous films and papers

This study investigates drop impingement on highly wetting porous films and papers. Experiments reveal previously unexplored impingement modes on porous surfaces designated as necking, spreading, and jetting. Dimensional analysis yields a nondimensional parameter, denoted the Washburn-Reynolds number, relating droplet kinetic energy and surface energy. The impingement modes correlate with Washburn-Reynolds number variations spanning four orders of magnitude and a corresponding energy conservation analysis for droplet spreading shows good agreement with the experimental results. The simple scaling laws presented will inform the investigation of dynamic interactions between porous surfaces and liquid drops.

Evaluation of the maximum transformation rate for determination of impingement mode upon near-equilibrium solid-state phase transformation

Considering the role of hard impingement mode in analyzing phase transformation kinetics, a maximum transformation rate analysis method was developed for pre-selection of impingement mode. Based on an extended analytical model, this method proposed for transformations with very large undercooling is reevaluated for transformations in the vicinity of equilibrium transformation temperature. Accordingly, a new peak maximum analysis method for both isothermal and non-isothermal transformations conducting near or far from the equilibrium state is proposed to determine the prevailing mode of impingement. The results obtained by application of the new method to numerically calculated transformations are in good agreement with the input values. Then, the method was applied to the isothermal ferrite/austenite transformation in Fe–3.28at.%Mn alloy as measured by dilatometry and non-isothermal hcp/bcc phase transformation in pure Zr as measured by differential scanning calorimetry.

Normalized treatment for isothermally conducted phase transformation with variable kinetic parameters

On the basis of a general, flexible, analytical transformation model, normalized treatment is extended to isothermally conducted transformation with transformed fraction-dependent kinetic parameters. A characteristic time that corresponds to the extended transformed fraction reaching to unit is selected for the treatment to give a simple expression for the transformed fraction. It is found that the transformed fraction at the characteristic time is correlated with impingement mode. Accordingly, a straightforward method is proposed to determine the prevailing impingement mode from the data at half maximum of the transformation rate. Applying the newly proposed method to numerically calculated transformation, the result is in good agreement with the value for the assigned model parameter. Then, the method is applied to the isothermal crystallization of Cu-Zr-Al-Y bulk metallic glass at different annealing temperatures.

Effect of reheat temperature on continuous cooling bainite transformation behavior in low carbon microalloyed steel

The effect of reheat temperature on continuous cooling bainite transformation in a low carbon microalloyed steel was investigated using a dilatometer based on welding thermal simulation process. The variation of microstructure was analyzed in detail by means of optical microscope and transmission electron microscope (TEM). The results showed that the morphology of the main microstructure changes from polygonal ferrite to granular bainite with increasing reheat temperature at a given lower cooling rate. For the higher cooling rate, the microstructure is predominantly lath bainite irrespective of the reheat temperature. The specimens with the relatively fine austenite grain size have the lowest bainite start and finish temperatures among the simulated sub-zones of heat affected zone, which is consistent with the result of the bainite lath width size observed using the TEM. Meanwhile, although the prevailing type of impingement mode of transformation is anisotropic growth impingement for all heat treatment processes, the reheat temperature has some influence on the maximum transformation rate and effective activation energy of bainite transformation.

Application of improved analytical model for non-isothermal phase transformation

For the transformations with different overall effective activation energies occurring at different transformation temperatures, the effect of initial transformation temperature T0 has been quantitatively studied. Consequently, a guideline for selecting a proper model (analytical model or improved analytical model) to describe the transformation kinetics was given. For transformations with low overall effective activation energy occurring at high temperature, the terms involving T0 cannot be neglected, and the application of the improved analytical model is indispensable. The second crystallisation stage of amorphous Ti50Cu42Ni8 alloy as measured by differential scanning calorimetry was described by the improved analytical model. On this basis, the prevailing mechanisms of the transformation can be described as follows: mixed nucleation (combination of site saturation and continuous nucleation), interface controlled growth and impingement mode due to random nuclei dispersion. Reasonable kinetic parameters of the transformation were determined, especially the nucleation and growth activation energies (i.e. QN = 233 kJ mol−1 and QG = 288 kJ mol−1).

Analytical Approach for Describing Solid-State Phase Transformation

A general analytical phase transformation model has been proposed and successfully applied to describe the crystallization of amorphous alloys. The “additivity rule” is proved to be compatible with the analytical model; the effects of anisotropic growth based on Monte Carlo (MC) simulations is reinterpreted using the analytical approach; and an improved temperature integral is also proved to be compatible with the analytical model. Kinetic analysis basing on the analytical model declares the transformation mechanism, e.g. nucleation, growth and impingement mode. On this basis, the kinetic behaviors of isothermal and non-isothermal crystallization of amorphous Zr50Al10Ni40are analyzed.

DSC studies on the transformation kinetics of two separated crystallization peaks of Si 12.5Te 87.5 chalcogenide glass: An application of the theoretical method developed and isoconversional method

The transformation kinetics from glass to crystalline of two clearly separated crystallization peaks of Si 12.5Te 87.5 chalcogenide glass were studied using DSC technique. The kinetic parameters of these two separated crystallization peaks of the present chalcogenide glass under non-isothermal conditions are analyzed by the theoretical method developed (TMD) and the isoconversional (model-free) with the help of model-fitting method for the heating rates range of 5–90 K min −1. The average values of the effective activation energies, E eff. , are 133.27 ± 5.29 kJ mol −1 for the first crystallization peak (FCP) and 198.99 ± 4.81 kJ mol −1 for the second crystallization peak (SCP). The dependence of the transformed fraction, α P , corresponding to the maximum transformation rate, on impingement exponent, γi, indicated that the mode of impingement is due to “anisotropic growth”. The average value of kinetic exponent, n P , calculated using TMD method is 1.53 ± 0.07 for FCP, while, the average value of n P for SCP is 5.55 ± 0.52. The unexpected high value of the kinetic exponent for SCP could be explained as both nucleation frequency and crystal growth rate depend on the time as a power law. The effective activation energy of crystallization, E eff. ( α), using the isoconversional method, shows a little variation throughout the entire interval of transformations for FCP compared to that for SCP. Whereas, the dependence of E eff. ( α) for SCP shows a strong dependence on the degrees of transformation and temperature. The reaction model that may describe transformation process of Si 12.5Te 87.5 chalcogenide glass for FCP is Avrami–Erofeev model ( g( α) = [−ln(1 − α)] 1/ n ) with n equal to 1.5 for all heating rates used (5–90 K min −1). On the other hand, the reaction model that may describe transformation process of SCP is a power law model g( α) = α 1/ ξ with ξ = 0.25 for heating rate range of 5–35 K min −1 and ξ = 0.50 for the heating rate range of 40–90 K min −1. Finally, the obtained results of transformation kinetic parameters using TMD analysis are in good agreement with that obtained according to the isoconversional one under non-isothermal regime. These results indicate that each method confirms and complements the other, and using the two methods for evaluating the transformation kinetic parameters to describe the glass-crystalline transformation process of Si 12.5Te 87.5 chalcogenide glass is helpful.

Solid state phase transformation kinetics: Evaluation of the modular transformation model

Abstract An evaluation of the (most) recent developments of the modular transformation model, for describing the kinetics of solid–solid phase transformations, has been made. As a result a two-step procedure for its practical application has been proposed. On this basis the operating nucleation, growth and impingement modes can be identified and quantified. The model has been shown to be applicable to very diverse transformations, as demonstrated by examples taken up in this paper.