Jet Surface Research Articles

Swirling instability of viscous liquid jets with axial shear effect in gas surroundings

Linear instability analysis of a viscous swirling liquid jet surrounded by ambient gas is carried out by considering the significant influence of axial shear effect. The jet azimuthal flow is assumed as a Rankine vortex, and the non-uniform velocity distribution in the jet axial direction is approximated by parabolic and error functions. The enhancement of jet rotation is found to promote the predominant mode with larger azimuthal wavenumbers, and the mode transition is decided by the competition between centrifugal force and axial shear stress. Subsequently, the influence of the axial shear effect is examined through changing the degree of shear stress and the thickness of the gas velocity boundary layer. It is found that an increase of jet average velocity or surface velocity in the axial direction leads to the predominant mode transition to smaller azimuthal wavenumbers, due to the combined effects of shear stress and gas pressure perturbation. A larger velocity difference between ambient gas and liquid jet also promotes the predominant modes with smaller azimuthal wavenumbers, and the physical mechanism is attributed to gas pressure perturbation. Phase diagrams of different azimuthal modes are given and compared with the study of Kubitschek & Weidman (J. Fluid Mech., vol. 572, 2007, pp. 261–286), where a static swirling column without axial shear stress was considered. The strengthened axial shear effect is found to delay the transition of predominant modes with the increase of angular velocity. Experimental studies considering the swirling jets with different axial velocities are further carried out, which validate the theoretical findings. Different instability mechanisms and their transition rules are also identified through energy budget analysis. This study is expected to give scientific guidance on understanding the instability mechanisms of the swirling jets that widely exist in natural phenomena and engineering applications.

Elimination of satellite droplets in droplet streams by superposing harmonic perturbations

In tin-droplet laser-produced plasma sources, uniform droplet streams with large droplet spacing are desired to minimize the interference of explosion on neighboring droplets. Such droplet streams can be generated in low wavenumber regimes. However, satellite droplets easily appear among main droplets in those regimes, resulting in plenty of undesirable debris. Herein, a novel odd harmonic superposition perturbation method is proposed to eliminate satellite droplets and enhance droplet spacing of uniform droplet streams. The superposition number (N) and the phase difference (θ) of odd harmonic perturbations are adjusted to facilitate the coalescence of satellite droplets with main droplets. First, the superposed odd-order harmonic components could induce additional disturbance growth in jet surfaces, and finally lead to the asymmetric necking on filaments formed between two adjacent main droplets, featured as various carrot-shaped configurations. This asymmetric necking will cause unbalanced surface tension forces at the two sides of filaments, resulting in a velocity difference between satellite and main droplets. Based on this principle, by setting N and k to 3 and 0.2, respectively, satellite droplets positioned above main droplets accelerate, while those below decelerate, achieving complete coalescence between main and satellite droplets. Furthermore, the phase difference is found to determine the jet breakup location and satellite droplet merging characteristics. As θ varies from 0° to 90°, the droplet size significantly decreases while the droplet spacing remains constant since the perturbation energy increases. The merge direction of satellite droplets reverses from upward to downward due to enhanced unbalanced surface tension forces and velocity differences. As θ continuously increases to 270°, the droplet size further decreases along with a slight decrease in droplet spacing. Finally, by setting N = 3 and θ = 0°, mono-disperse tin droplet streams with a mean diameter of 31.5 μm and a maximum droplet spacing-to-diameter ratio of 17.7 are successfully formed. This work presents a novel approach for eliminating satellite droplets to achieve uniform tin droplet streams with large droplet spacing without increasing the droplet diameter.

Fluid Dynamics of Interacting Rotor Wake with a Water Surface

Rotor-type cross-media vehicles always induce considerably complex mixed air–water flows when approaching the water surface, resulting in relative thrust loss and structural damage on rotor. The interactions between a water surface and rotor wake bring potential risks to the cross-media process, which is known as the near-water effect of the rotor. In this paper, experimental investigations are used to explore the fluid dynamics of the near-water effect of the rotor. Qualitative droplet observation was carried out on the 0.25 m and 0.56 m diameter commercial rotor blades and the 0.07 m diameter ducted fan near the water surface first to gain a qualitative understanding of droplet characteristics. The results show that the rotor wake caused water surface deformation, droplet tearing off, splashing, and entrainment into the rotor disk. The depression formed by the rotor downwash flow impacting the water surface is named as three modes: dimpling, splashing, and penetrating, and the correlation between the depression modes and the aerodynamic characteristics of the rotor is primary analyzed. The flow mechanisms of dimpling mode were studied using the particle image velocimetry (PIV) technique. The results showed that the cavity and liquid crown obviously alter the flow direction of water surface jets, but not all rotors near water enter the vortex ring state. Two splashing mechanisms were revealed, including the direct ejection of droplets at the rim of depression and the tearing of liquid crown by the water surface jets. The blade tip vortex in the surface jet is a potential cause of entrainment into the rotor disk and secondary breakup of the droplet.

Numerical simulation of jet break-up in the second-wind induced regime using the local front reconstruction method

In the second wind-induced regime, the jet deformations are influenced by aerodynamic instabilities acting on the jet surface. In addition, velocity relaxation in the liquid is an important mechanism with a strong influence on the violent break-up of laminar jets (i.e., the bursting). The exact interplay between the aerodynamic forces and the velocity profile relaxation is insufficiently described in the literature. Thus, we investigate numerically cylindrical liquid jets with fully developed and uniform velocity inlet profiles for different liquid and gas properties to elucidate the physical mechanisms governing the bursting of the jet. For this study, the gas-liquid interface is tracked using a front tracking technique: the Local Front Reconstruction Method (LFRM). In contrast with traditional front tracking techniques, LFRM allows for complex topological changes (merging and break-up) while keeping a sharp representation of the interface. The comparison of the obtained results with literature suggests that LFRM is a suitable technique for capturing the complex morphological deformations of a bursting jet. Additionally, we analyzed the influence of air and liquid properties on the jet deformations, which evidenced that velocity profile relaxation plays an important role in the complex process of break-up of the jet, that in combination with the destabilizing effect of the aerodynamic forces lead to the bursting of the jet.

Coaxial air blast atomization of a particulate gel suspension jet

The objective of this study is to experimentally and theoretically explore the impact of particle volume fractions and nozzle thread structure on the coaxial air blast atomization of particulate gel suspension jets through the high-speed flow visualization technology. The four breakup modes of particulate gel suspension jets are confirmed, namely oscillation mode, membrane-type breakup, fiber-type breakup, and superpulsating submode. However, the fiber-type breakup of jets disappears as the particle concentration reaches 40%, primarily attributed to the numerous particles sharply promoting the instability on the free jet surface. The experimental statistical results demonstrate that an increase in particle concentration and the adoption of the threaded nozzle both result in an expansion of the jet spray angle. The breakup length exhibits a reduction trend with the increase of particle concentration. However, it remains nearly the same value as that in the case of volume fraction 20% when the particle concentration continues to increase, contributed by the surge of viscous dissipation counteracting the enhanced instability on the jet interface caused by a large number of particles. The breakup length of a pure gel jet is predicted through the linear instability analysis, which is further modified by the viscosity model of particle suspensions to derive the breakup lengths of particulate gel suspension jets with different concentrations. The theoretical predictions align well with the statistical results in the experiment. The research is poised to have potential implications for numerous industrial processes and engineering applications including gel propellants.

Study on the key parameters of ice particle air jet ejector structure

Existing ice particle jet surface treatment technology is prone to ice particle adhesion during application, significantly affecting surface treatment efficiency. Based on the basic structure of the jet pump, the ice particle air jet surface treatment technology is proposed for the instant preparation and utilization of ice particles, solving the problem of ice particle adhesion and clogging. To achieve efficient utilization of ice particles and high-speed jetting, an integrated jet structure for ice particle ejection and acceleration was developed. The influence of the working nozzle position (Ld), expansion ratio (n), and acceleration nozzle diameter ratio (Dn) length-to-diameter ratio (Ln) on the ice particle ejection and acceleration was systematically studied. The structural parameters of the ejector were determined using the impact kinetic energy of ice particles as the comprehensive evaluation index, and the surface treatment test was conducted to verify the results. The study shows that under 2 MPa air pressure, the ejector nozzle parameters of n = 1.5, Dn = 4.0, Ld = 4, and Ln = 0 mm can effectively eject and accelerate the ice particles. The aluminum alloy plate depainting test obtained a larger paint removal radius and resulted in a smoother aluminum alloy plate surface, reducing the surface roughness from 3.194 ± 0.489 μm to 1.156 ± 0.136 μm. The immediate preparation and utilization of ice particles solved the problems of adhesion and storage in the engineering application of ice particle air jet technology, providing a feasible technical method in the field of material surface treatment.

Dynamic Analysis of Deformation of a Droplet on a Uniform Jet Surface

Dynamic Analysis of Deformation of a Droplet on a Uniform Jet Surface

On the radial interface profile of the non-circular hydraulic jump

The radial interface profile of the non-circular hydraulic jump is analyzed based on the time-averaged film thickness profile of the liquid film formed by a round jet obliquely impinging on a horizontal plate. The influence of many factors, including the jet velocity, impingement angle, azimuthal angle, liquid viscosity, and surface tension, on the radial interface profile is considered. The interface profile is like a quasi-spherical crown when the azimuthal angle is small but is like a sewing needle when the azimuthal angle is larger than 100°. Six parameters, including the inner and outer tangential angles, width, maximum thickness, radial position of maximum thickness, and area are defined to describe the interface profile. Then, six empirical equations are developed to fit the variation of the six parameters. As the azimuthal angle increases, the inner tangential angle decreases, but the outer tangential angle increases. As the jet velocity increases to 20.3 m/s, both the maximum thickness and the area increase suddenly. All empirical equations have a prediction accuracy of about 10%, except for the empirical equation of the radial position of maximum thickness. The bubble trajectory indicates that the liquid flows radially in the thin-layer zone, deflects the flow direction within a relatively short distance in the inner half of the non-circular hydraulic jump, and then flows tangentially. The normal bulk velocity in the radial section of the non-circular hydraulic jump increases from zero at first and then decreases as the azimuthal angle increases.

Flow dynamics and mixing of jet in crossflow with cylindrical cavity

The flow dynamics and mixing characteristics of an air jet issued from a cylindrical cavity in an air crossflow are numerically studied using a large-eddy-simulation technique. The cavity, aligned concentrically with the jet, is located beneath the crossflow wall. The jet-to-crossflow velocity ratio is 4, and the Reynolds number for the jet flow is 1.39×104 based on its diameter and centerline velocity. The presence of the cavity significantly influences the early evolution of the jet and its interaction with the crossflow. Complex vortical structures are observed. Notably, windward vortices on the jet surface increase in size, accompanied by a reduction in the Strouhal number. For a deep cavity, these vortices break down and result in small vortical tubes in the jet streamwise direction due to secondary instability. Also examined are leeward shear-layer vortices, hanging vortices, wake vortices, and the recirculating flow within the cavity. Their roles in the mixing between the jet fluid and the crossflow are identified. The cavity enhances mixing. The effect is significant in the near field but diminishes in the far field. By adjusting the cavity radius and depth, it is determined that the cavity depth exercises a more profound impact on jet evolution and mixing than the cavity radius. The most substantial influence occurs when a narrow and deep cavity is implemented. These findings may serve as guidelines for optimizing cavity design for effective modulation of jet behaviors.

Breakup characteristics of a pulse jet issuing into a compressed gas environment under different injection conditions

The dynamics of jet breakup undergo significant alteration due to the influence of a compressed gas environment. In the first injection stage of an air-assisted fuel injector (AAFI), fuel is introduced into such an environment. Therefore, studying the influence of injection conditions on the jet breakup characteristics has significant importance for AAFI spray. This study utilized a high-speed camera to record the jet breakup images in a compressed gas environment. Subsequently, these images were analyzed using MATLAB to get the spray penetration distance and fuel projection area (FPA). The research findings indicate that both fuel injection pressure (FIP) and fuel–gas pressure drop (ΔP) exert influence on jet breakup characteristics, with ΔP exhibiting more significant influence. Maintaining ΔP at 1 bar, when FIP increased from 4 to 7 bar, gas Weber number (Weg) increased by 87%. While maintaining gas pressure at 5 bar, as ΔP increased from 1 to 3 bar, Weg escalated by 194%. Additionally, jet breakup length under different injection conditions followed a pattern as summarized by Bonhoeffer et al. [“Impact of formulation properties and process parameters on the dispensing and depositioning of drug nanosuspensions using micro-valve technology,” J. Pharm. Sci. 106(4), 1102–1110 (2017)]. The jet surface disturbance was enhanced by the increase in both FIP and ΔP. The detachment of the droplets from main jet stream induced by ΔP resulted in an increase in jet flow width. Furthermore, the effect of ΔP on FPA was more significant compared to FIP. As ΔP rose from 1 to 3 bar, the time-averaged FPA and area-to-mass ratio (Raq) increased 245% and 207%, respectively.

Microscopic investigation and local standard deviation analysis of liquid propane jets in supercritical environment

To improve the thermal efficiency of engines, the temperature and pressure in the combustion chamber are continually increased, potentially reaching the critical points of the fuels. Additionally, light fuels such as liquid ammonia and propane are utilized to reduce carbon emissions. The relatively low critical temperature of light fuels facilitates their transition into a supercritical state, which greatly change the jet characteristics. To gain a deeper understanding of the supercritical transition of fuels, jets of liquid propane are investigated through high-speed microscopic imaging technology, covering subcritical to supercritical environmental conditions. Liquid propane is injected through a transparent flat nozzle to observe the internal flow states within the nozzle. For image and data processing, local standard deviation (LSD) analysis and a Residual Network (ResNet-50) model are employed to identify the structures within the mixing region. Droplets counting and frequency domain analysis are also performed to reveal the features of supercritical transition. The findings highlight the importance of mask size in the jet feature extraction. Large mask highlights large scale structures including jet surface and mixing region structures, while small mask brings about full-scale feature including droplets and all other features. A criterion for mixing regime classification based on reduced temperature (Tr) and reduced pressure (Pr) is established directly from image data using the ResNet-50 model, where a condition of Tr1.6Pr0.4 ≥ 1.067 indicates a transcritical mixing regime. Both droplet counting and frequency domain analysis corroborate this mixing regime criterion. These insights enhance our understanding of supercritical transitions and are useful for simulation verifications.

Fundamental insights on turbulence characterization, vortex motion and ignition mechanism of sub/supersonic turbulent jet flames

This paper presents turbulence characterization and vortex behavior measurements of sub/supersonic jet flames subjected to extreme levels of turbulence. Specifically, sub/supersonic CH4/O2/N2 premixed jet flames were studied under calculated Reynolds (Re) number and measured jet tip velocity (Vtip) of around 20,000 and 100 m/s or greater, respectively. The vortex motion on the flame surface was measured by Schlieren image velocimetry (SIV), and ignition mechanisms of sub/supersonic jets are discussed by analyzing vortex motion and free radical reactions, respectively. Results show that the maximum vortex motion is maintained in the jet top edge region. In contrast, the velocity of vortex motion in the middle of the jet, i.e., near the side of the jet orifice, is reduced by a factor of 4–5, which visually shows a “mild” velocity gradient on the jet surface. The direction turning points of X/D = 7.25, X/D = 6.06, X/D = 6.06 and X/D = 3.40 were found in the flow direction (Y-direction) for four typical cases of supersonic jets with top primary swirling motion, supersonic jets with top secondary swirling motion, supersonic jets with weak top swirling motion and subsonic jets without top flame vortex motion. It is speculated that the ignition location is strongly correlated with the direction turning point of the vortex movement along the Y-direction. The vortex propagating along the flow direction disperses many small-scale vortices moving in the X-direction, the X-direction being perpendicular to the flow direction. Moreover, the ignition mechanisms of sub/supersonic jet were explored separately.

Flow and spray characteristics of a gas–liquid pintle injector under backpressure environment

Both the flow and spray characteristics of a gas–liquid pintle injector element under the backpressure environment were investigated experimentally and numerically. The cold atomization tests were conducted with the backpressure range from 0.50 to 1.54 MPa. Both the interaction process between gas film and liquid jets and the detail distribution of the spray were obtained by the verified volume of fluid to discrete phase model. Results showed that there is a local high-pressure zone at the root of the liquid jets resulted by the collision of gas film and liquid jet. A semi-empirical model for predicting the discharge coefficient of the orifices is proposed considering the effect of local high-pressure zones based on the experiments. It was found that the discharge coefficient is mainly affected by the local momentum ratio (LMR) and ambient pressure. The discharge coefficient increases with the increase in LMR and ambient pressure. Before the primary breakup occurs, the liquid jets deform from rectangle jets to bow-shaped liquid films under the effect of the gas film. Then, both the gas and liquid mix in the range included by the gas passed by the windward surface and side of liquid jets. The droplet size is larger at the edges of the spray and the Sauter mean diameter (SMD) is beyond 100 μm. On the contrary, it is relatively small and uniform at the spray central, and the corresponding SMD is about 50 μm.

Study on the film thickness and surface wave velocity of the thin liquid film formed by a round jet obliquely impinging on a horizontal plate

For a thin liquid film (in a supercritical flow) prior to the formation of a non-circular hydraulic jump formed by a round jet obliquely impinging on a horizontal plate, the time-averaged film thickness and the surface wave velocity are extracted based on the measured transient film thickness. On the one hand, the effect of many factors, including the jet velocity, impingement angle, azimuthal angle, liquid viscosity, and surface tension, on the time-averaged film thickness and surface wave velocity are discussed. When the jet Reynolds number increases to about 1.4×104, the film thickness profile suddenly increases, and the transition of liquid flow from laminar to turbulent occurs. Meanwhile, a rapid increase is observed downstream of the turbulent film thickness profile. The influence of surface tension on the time-averaged film thickness and surface wave velocity is negligible for thin liquid films before non-circular hydraulic jumps. Nonetheless, the surface tension has a significant influence on the interface profile of non-circular hydraulic jumps. Furthermore, a “crescent” kink region upstream of the jump can be identified when the surface tension is lower than 40.6 mN/m. On the other hand, experimental results are used to verify the prediction accuracy of existing approximate solutions. The laminar approximate solution with a quadratic boundary layer velocity profile can accurately predict the film thickness distribution of most laminar thin liquid films, except downstream of the thin liquid films with a dynamic viscosity higher than 9.71 mPa s. The surface wave velocities are found to be close to the predicted surface velocities of the approximate solutions.

Numerical analysis of jet impingement cooling with elongated nozzle holes on a curved surface roughened with V-shaped ribs

Numerical studies were performed in this study to analyze the effect of dimensionless elongated injection holes (G/d = 0.5, 2.0, 4.0, 8.0) on heat and flow characteristics on rib-roughened surfaces with an array of inclined impingement jets. Experimental and numerical data from existing literature were used to validate the numerical solution procedure’s heat transfer and flow characteristics in the semicircular test section. The turbulence equations were solved using the SST k-ω turbulence model by varying the Reynolds number from 5000 to 25,000. The curvature effect and staggered array pattern created an additional stagnation region between adjacent impinging jets on smooth surfaces, leading to a low heat transfer zone. Rectangular cross-sectional V-shaped ribs (VSR) were placed in regions where the stagnation point occurs to eliminate this disadvantage. The effect of different normalized rib heights (Hr/d = 0, 0.2, 0.3, 0.4) and rib angles (α = 30°, 45°, 60°, 90°) on the curved surface was also investigated to increase convective heat transfer performance and achieve more homogenous heat transfer distribution by relatively reducing the effect of thermal stresses. Flow properties, area-averaged and local Nusselt number variations on smooth and ribbed surfaces, and the thermal performance criterion (TPC) were investigated in detail. The results indicated an increase in overall heat transfer and a more evenly distributed measurement region compared to the conventional (unextended jet and smooth surface) jet impingement configuration. The most significant heat transfer enhancement from combining the elongated jets with VSR was 47.23% at Hr/d = 0.2 by reducing the G/d to 0.5. In addition, the highest TPC was determined as 1.07 on the proposed model with G/d = 2.0 and Hr/d = 0.2 at Re = 25,000.

Local Heat Transfer Distribution On Thin Metal Foil Impinged by Array of Free Surface Jets

Abstract The present study focuses on the measurement of the local heat transfer distribution of a smooth flat plate impinged by an array of free surface jets on a thin metal foil. Local Nusselt number distributions are measured for a fixed jet diameter (d = 3 mm). The jet arrays consist of perfectly round apertures arranged in a square pattern, with a uniform spacing of 4d between adjacent jets in both the streamwise and spanwise direction. A wide range of Reynolds numbers varying from 1000 to 12500 are covered in this study. The nozzle to plate spacing (z/d) is varied between 1 to 10. The effect of the Reynolds number and nozzle to plate spacing on the local and spanwise average Nusselt number distributions are studied. The local Nusselt number exhibits an increase with nozzle-to-plate spacing within the low Reynolds number range (Re = 1000 to 2500). However, for Reynolds numbers exceeding 2500, the influence of nozzle-to-plate spacing on Nusselt number distributions remains negligible up to a nozzle-to-plate distance (z/d) of 5. Beyond this point, there is a gradual decrease in the Nusselt number value. The Nusselt number value gradually decreases beyond z/d of 5. At a Reynolds number of 1500, the Nusselt number increases by 71% for z/d = 10 in comparison to z/d = 1. Empirical correlations for local and spanwise average Nusselt number are proposed which satisfactory predict the local as well as spanwise average Nusselt number distributions.

Fluidic Flow Control Devices for Gust Load Alleviation

A reduction in the structural weight and climate-relevant emissions of future transport aircraft can be realized through active gust load alleviation, limiting the peak aerodynamic loads experienced in flight. Two fluidic concepts, surface and Coandă jets, offer promising solutions. The surface jet induces a separated flow region on the wing’s suction side near the trailing edge, while the Coandă jet utilizes tangential blowing over a rounded trailing edge to make use of the Coandă effect for direct circulation control. This study investigates these fluidic actuation concepts using two-dimensional unsteady Reynolds-averaged Navier–Stokes simulations on a supercritical airfoil to alleviate gust-induced lift increases. Results reveal that fluidic concepts can surpass the load reduction capability of a trailing edge flap. Notably, the Coandă-type design, while more efficient, exhibits limited control authority compared to surface jet, achieving maximum gust load reductions of 40% versus 70% for surface jet. Optimizing the temporal deployment of the surface jet leads to increased peak lift reduction. Even under challenging conditions with reduced gust anticipation time and actuator placement near the wing tip with a reduced chord length, the surface jet maintains consistent performance and therefore offers a promising solution for active gust load alleviation on future aircraft.

Investigation on unstable flow characteristics and energy dissipation in Pelton turbine

The internal flow scale of a Pelton turbine is variable, and the interaction between the jet and the bucket has strong transient characteristics, resulting in an incomplete understanding of its internal vortex structure evolution and energy dissipation mechanisms. In order to reveal the influence of vortices on the flow regime and energy dissipation mechanisms in the turbine, this paper establishes the correlation between the unsteady flow characteristics and energy dissipation inside the Pelton turbine based on the energy balance equation and quantifies the energy losses inside the turbine. The results indicate that vortex structures present a non-uniform distribution inside the jet, which disrupts the uniformity of the jet velocity distribution, resulting in an uneven distribution of high-vorticity zones on the bucket surfaces and intensifying flow interference among the buckets. The strong shear flow caused by the downstream flow detachment of the needle guide, the turbulent boundary layer on the jet surface, and the wake effect downstream of the needle tip are the main reasons for the dissipation of fluid kinetic energy. The distribution range of energy loss on the rotating bucket closely corresponds to the position of the high-vorticity zone. Energy dissipation inside the turbine is primarily in the form of turbulent kinetic energy, and the injectors and runner are the primary energy dissipation components. Moreover, inside the injectors, each form of energy loss remains relatively constant, whereas inside the runner, its rotation induces fluctuating energy losses attributed to Reynolds stress work. The results contribute to an enhanced understanding of the energy dissipation characteristics and complex flow mechanisms within the turbine, providing reference for the optimisation and efficient operation of the multi-nozzle Pelton turbine.

Numerical study on laser-induced cavitation bubble dynamics inside a millimetric droplet

In this study, complicated nonlinear interactions of a single laser-induced cavitation bubble inside a millimetric water droplet were numerically investigated using a fully compressible three-phase homogeneous model. A general condensation phase-change model and high-resolution interface-capturing schemes were adopted to accurately predict the bubble collapsing and rebound stages as well as strongly deformable droplet interface evolutions. The numerical model was validated using experimental data in terms of the equivalent bubble radius until the second collapse stage, and good quantitative agreement was achieved. The variation in the droplet surface velocity was detected and could better reveal the mechanism underlying the complicated bubbles and droplet interactions, particularly in droplet surface splash dynamics. Subsequently, the complex bubble–droplet interaction phenomena were studied by investigating the ratio of the maximum bubble radius to the initial droplet radius. The numerical results show that the bubble collapsing time decreases monotonically with an increase in the bubble–droplet radius ratio. The droplet surface instabilities became more dominant as the radius ratio increased. In addition, four distinct patterns of droplet motion, namely, stable, multi-spike, ventilating jet, and splashing phenomena, were captured. Finally, the specific mechanisms leading to droplet surface jetting were identified.

Cooling enhancement for engine parts using jet impingement

A computational study has been performed to evaluate the use of jet impingement for cooling applications in the automotive industry. The current study uses an entire internal combustion engine cylinder with its components as a computational domain. An unsteady numerical solution for the Navier-Stokes equations was carried out using Improved Delayed Detached Eddy Simulation (IDDES). The volume of fluid approach is proposed to track and locate the liquid jet surface that is in contact with the air. The conjugate heat transfer approach is used to link the heat transfer solution between the fluid and the solid. The boundary conditions that are employed in the study are provided from lab experiments and one-dimensional simulations. The cooling jet in this study targets the hottest region in the piston, i.e., the region underneath the exhaust valve. Three nozzle sizes with flows at different Reynolds numbers are chosen to examine the thermal characteristics of the cooling jet. The computational study reveals that for a specific Reynolds number, the smaller diameter nozzle provides the highest heat transfer coefficient around the impingement point. The maximum relative velocity location at the impingement point slightly leads the location of the maximum Nusselt number. The maximum temperature in the piston decreases by 7% to 11% as the nozzle diameter changes from 1.0 to 3.0 mm for a jet Reynolds number of 4,500. If a correct selection is made for the nozzle size, the cooling jet can be efficiently used to reduce the temperature and alleviate the thermal stresses in the piston in the region underneath the exhaust valve where the maximum temperature occurs.