Abstract
This mini-review mainly focuses on the fundamental problem of jet–jet/film impingement exhibiting superior fragmentation and atomization characteristics compared to single-jet injection; this has been widely used in agricultural irrigation and combustion propulsion systems. First, it presents the main controlling parameters and spray characteristic for both jet–jet and jet–film configurations, analyzes the breakup mechanisms, and points out the coupling between jet fragmentation processes and collision-induced or externally imposed vorticity fields. Then, the atomization enhancement of jet–jet or jet–film impingement is explained from the aspects of vortex generation, evolution, identification, and the interactions between vorticity fields and spray fields. Finally, representative applications of jet–jet/film impingement in agricultural engineering and aerospace engineering are introduced so as to achieve spatially uniform spray distribution and efficient fuel/oxidizer mixing characteristics. Future advancements require breakthroughs in cross-scale vortex–ligament interaction diagnostics and intelligent control of variable-viscosity fluids to promote deep implementation of this technology in clean energy systems.
1 Introduction
Liquid jet atomization has been extensively used in various industrial applications, such as liquid fuel atomization and combustion in propulsion systems [, ], electrostatic spray coating [], high-pressure spray aspirating in mining [], pesticide spraying and agricultural irrigation [, ], fire-fighting [], spray cooling [], and respiratory disease treatment []. Liquid atomization describes the dynamic process involving the liquid jet breaking up into dispersed droplets by hydrodynamic instabilities [, ]. Examples are the primary breakup of liquid jet into filaments or large droplets by Kelvin–Helmholtz (KH) instability and then secondary breakup of filaments into small dispersed droplets by Rayleigh–Plateau (PR) instability or frequent droplet collision dynamics exerted by aerodynamic forces.
Compared to the direct injection of jet atomization [, ], jet–jet/film impingement [] can prominently increase the gas liquid surface area and enhance atomization. Jet–jet/film impingement generally involves substantial jet deformation and the unstable breakup of a liquid sheet [, ] with a broad spray distribution in which the breakup mechanism is more complicated than that of single jet fragmentation.
This mini-review introduces the jet–jet/film impingement phenomenon and the spray characteristics influenced by several main controlling parameters in Section 2. The interpretation of the atomization enhancement of jet–jet/film impingement from a vortical perspective is presented in Section 3, followed by some widely used applications in agriculture and engine systems in Section 4 and suggestions for future research in Section 5.
2 Phenomenal description of jet–jet/film impingement and atomization
A typical jet–jet impingement configuration [] is schematically shown in Figure 1a. It generally includes four main controlling parameters: jet diameter , impact velocity vector for each jet, impact angle , and liquid viscosity . The entire atomization process of jet–jet impingement can be described as the formation of a thin liquid sheet upon impingement, followed by the propagation and intensification of surface capillary waves induced by the surface KH instability along the liquid sheet as well as the liquid sheet breakup [, ] either at the center or rim to generate a large number of ligaments or dispersed droplets once the impact velocity is sufficiently large. As it increases the impact velocity at fixed impact angle, liquid sheet formation shows five distinct regimes []: liquid chain, closed edge, opening edge, unstable edges, and liquid sheet breakup.
FIGURE 1
Jet–jet impingement is symmetrically mirrored [, ] based on the symmetry plane, leading to a nonuniform distribution of the ligament and droplet formation that is merely close to the symmetry plane. Thus, Zhang and colleagues [] proposed misaligned impinging jets by defining a misalignment ratio e, where the droplet distribution becomes more uniform at a moderately misaligned impingement owing to the competition between enhanced mass stretching and reduced mass contact volume as increasing e. In addition, He and colleagues [] proposed spinning jet–jet impingement by breaking the mirror symmetry to shorten the liquid sheet breakup length and promote the ligament breakup into dispersed droplets, especially for the small flow rate conditions in practical variable-flow engines [].
Jet–film impingement [, ] is schematically shown in Figure 1b, with additional controlling parameters of liquid film thickness . Again, increasing the impact velocity shows the three distinct breakup modes [] of close arch spray, mantle sheet, and fully developed fan spray. Generally, as the jet perpendicularly impacts on the film, the spray angle [, ] can be approximately determined by the momentum ratio between the radial jet flow and axial film flow. Analogously, transverse jet injection (jet–gas crossflow) exhibits a characteristic “horseshoe vortex” [] near the jet root. When the gas velocity is sufficiently close to supersonic, a bow shock [, ] may occur around the jet that interacts with the liquid jet breakup.
3 Vortex interpretation for jet–jet/film atomization enhancement
In practical spray environments usually involving complex turbulence flow, the liquid jet atomization process is probably strongly coupled with the evolution of the vortex field. Understanding and controlling vortices can enhance jet–jet and jet–film atomization leading to finer droplets, better mixing, and higher energy efficiency; vortex field control is thus fundamental to advancing jet–jet and jet–film atomization technologies [].
3.1 Vortex formation, evolution, and identification of jet–jet/film impingement
When fluid flows through a nozzle, the presence of a boundary layer causes fluid molecules near the nozzle wall to move more slowly, while those away from the wall move faster, creating a velocity gradient [–]. As the jet exits the nozzle, shear layer instabilities cause the fluid to roll up, forming a toroidal vortex structure. There is also an obvious mushroom-shaped entrainment vortex at the jet head due to the liquid gas interaction. Vortex rings and vortex entrainment are classic coherent structures, commonly observed in free jets [–].
In jet–jet flow, two colliding jets interact, leading to complex vortex dynamics due to stagnation, shear, and flow instabilities [] (Figure 1c). First, the impingement zone forms a high-pressure stagnation region, deflecting flow radially outward. Velocity gradients between the jets generate shear layers, which roll up into vortices via KH instability. If the jets are pulsed or turbulent, coherent vortex rings may form and interact upon collision []. Wu and colleagues [] investigated a moderate Reynolds number (Re = 4,050) turbulent opposed jet flow by using direct numerical simulation. They analyzed the turbulent flow field by using the proper orthogonal decomposition (POD) technique and found that the first POD mode includes a vortex ring situated in the impinging zone which is capable of shifting the axial position of the stagnation point. In contrast, the second to fourth POD modes contain several vortices that rotate alternately in two directions along the radial axis, and these vortices primarily induce tilting and distortion effects on the stagnation plane. Vortex evolution and breakdown in opposed-jets flow are governed by shear layer instabilities, Reynolds number effects, and nozzle geometry, leading to primary vortex stretching, secondary vortex generation, and eventual turbulent dissipation [].
In jet–film impingement, the jet strikes a thin liquid film or boundary layer on a surface. The impact creates a stagnation region with high pressure, forcing the fluid to spread radially outward. The high-velocity jet interacts with the slower-moving film, creating a shear layer. KH instabilities may develop, leading to roll-up vortices at the jet–film interface [, ]. Moreover, the radial outflow from the impingement point may form a hydraulic jump (sudden increase in film thickness). Behind the jump, vortices can form due to adverse pressure gradients. The formation and evolution of the vortices are mainly influenced by jet velocity, film thickness, impingement angle, and fuel properties. Zhang and colleagues [] investigated the height and distance of the jet–film vortex core under different ambient pressures and cross-flow velocities using laser sheet technology. An indicator of the “contribution index” was proposed to evaluate the degree of influence of different influencing factors.
With advancements in vortex dynamics research, a variety of vortex identification methods [–] have been developed, including the Q, k, D, and swirling strength criteria. These techniques play a crucial role in analyzing and understanding complex vortex structures in fluid dynamics. However, the problem of finding the best vortex identification techniques is still controversial.
3.2 Vortex diagnose technics
High-speed imaging is a widely used experimental technique for diagnosing vortex dynamics in jet–jet/film impingement studies [, ]. By employing ultra-fast cameras, researchers can resolve transient flow phenomena such as vortex formation, evolution, and breakdown. The technique often incorporates tracer particles or fluorescent dyes for enhanced contrast, combined with backlight illumination or laser sheet lighting to clearly visualize thin film dynamics (radial spreading, hydraulic jumps, and vortex roll-up) and interfacial instabilities caused by jet–jet/film interactions [, ].
For the quantitative measurement of instantaneous velocity fields, particle image velocimetry (PIV) is a powerful non-intrusive flow diagnostic technique. By illuminating seeded tracer particles with a pulsed laser sheet and capturing their displacements via synchronized high-speed cameras, PIV provides two- (2D-PIV) or three-component (3D stereoscopic/volumetric PIV) velocity vector maps of the flow field [, ]. In jet–jet/film impingement research, PIV enables the precise characterization of vortex dynamics through derived quantities like vorticity, swirling strength, and Q-criteria, revealing key features such as shear layer roll-up, recirculation zones, and vortex-ring structures (Figure 1d).
Modern advances like tomographic PIV (Tomo-PIV) can reconstruct 3D vortex tubes and coherent structures [], while microscopic PIV (μPIV) resolves near-wall phenomena []. When combined with machine-learning-assisted diagnostics, PIV data can effectively identify dominant vortex modes []. This makes it an indispensable tool for validating CFD models and understanding complex vortex interactions.
3.3 Interaction between vortex field and spray characteristics
The interaction between jet–jet/film vortex fields and spray characteristics represents a complex multiphase phenomenon where coherent vortical structures fundamentally govern spray development and atomization processes [, ]. In dual-jet impingement or jet–film interaction systems, the collision and merging of vortex filaments from adjacent jets create intricate three-dimensional flow topologies that dramatically alter spray formation. Wang and colleagues [] compared the hydraulic performance between jet-impingement and non-impinging sprinklers by using highspeed photography (HSP). Their results show that the developed jet impingement sprinkler achieved a smoother water distribution trend.
The primary vortex dynamics include shear layer roll-up from individual jets, collision-induced vortex pairing, and film-driven recirculation zones which collectively control the liquid breakup mechanism through several interconnected pathways. Large-scale vortex rings generated at jet boundaries enhance primary atomization by extruding liquid sheets and promoting Rayleigh–Taylor instabilities, while small-scale turbulent vortices in the merging region drive secondary droplet fragmentation through intense velocity fluctuations []. Xia and colleagues [] found that a type of large scale instability due to a vortex ring located at the impingement zone promotes the breakup of the water sheet or ligaments, forming smaller droplets. Moreover, coherent vortices entrain droplets, altering trajectories and enhancing radial/axial spreading. These vortices preferentially transport larger droplets toward the spray periphery through centrifugal effects while simultaneously promoting more homogeneous mixing of droplet sizes [].
In propulsion systems, these vortex-mediated spray patterns directly affect combustion stability [], while in industrial coating applications and agricultural production applications they determine deposition uniformity [–]. Current research is focused on active flow control strategies to manipulate vortex interactions for optimized spray performance across different Weber and Reynolds number regimes, although the nonlinear coupling between vortex merging dynamics and droplet formation pathways remains a key challenge in predictive modeling.
4 Applications of jet–jet/film impingement
The application of sprinklers [, , 67–70] in agricultural engineering (Figure 2a) adopts the asymmetric impingement between a primary and a secondary jet to replace the traditional single water-jet-dispersing devices in rotating sprinklers [71–73] and promote atomized performance, especially for low-pressure conditions [, 67, 74]. Jiang and colleagues [] found the Christiansen’s uniformity coefficient [75] of the jet impingement sprinkler with various elevation angles of secondary nozzle greater than the non-impingement sprinkler. Many parameter optimization studies have demonstrated that the nozzle geometry [, , 76, 77], angle of dispersion [78], aspect ratio [, 79], and aperture ratio [, 80] for the primary and secondary jets can significantly influence jet instabilities and breakup characteristics in agricultural irrigation.
FIGURE 2
Apart from jet–jet impingement, some other jet-based methods can also be used in sprinklers to adapt hydraulic performance. Fan-type nozzles [, , 81] increase the total liquid surface area to facilitate atomization [82] by forming a fan-shaped liquid sheet (Figure 2b), while the penetration distance of the liquid sheet is generally smaller than the jet–jet impingement. For oil-based emulsion spray [81, 83, 84], the holes and web structures [, 85, 86] break up the inner liquid sheet differently from the water liquid sheet, generally showing the rim breakup. For larger liquid viscosity, the enhanced viscous dissipation would suppress surface capillary wave propagation and subsequent liquid sheet breakup. In addition, non-Newtonian jet–jet impingement [, 87] and two miscible liquid jets with different surface tensions [85] would further complicate the breakup mechanism [88] and enrich the breakup phenomenon.
An air-assisted nozzle [63, 89] can promote the liquid jet breakup owing to the large shearing effects and kinetic energy of gas (Figure 2c). The combination of air-assisted nozzle and electrostatic excitation [, 63, 90] can further promote atomization and reduce the need for pesticide spraying. In addition, as shown in Figure 2d, the atomization of a traditional sprinkler can be developed by using a driving arm [64] to cut the water jet periodically with appropriate driving frequency and injection pressure. Similarly, as shown in Figure 2e, a dispersion tooth inserted into the water jet [65, 91, 92] has been shown to be an effective way of improving the uniformity of water distribution from irrigation sprinklers.
For applications in high-thrust rocket engines [, 93] and opposed-piston compression ignition engines (CIE) [94, 95], liquid fuel atomization and subsequent spray combustion would be substantially enhanced owing to the promoted droplet collision probability (Figure 2f). The impingement is a direct and efficient way to promote the atomization of heavy oil or bio-oil [96, 97]. In addition, for the pintle injector [, , 66, 98] utilized in the applications of variable-thrust rocket engines, the spray cone is formed by the radial jet flow impinging the axial annular film flow, which has a simple structure, continuous flow regulation, and stable combustion superior to other variable-thrust methods.
The phenomenon of jet–jet impingement in agricultural irrigation should not be essentially different from that in combustion systems once the dynamic similarities are satisfied with approximately dimensional parameters. For example, jet–jet/film breakup is generally controlled by two important parameters: the Weber number ( is the liquid density, the jet diameter, the relative velocity, and the surface tension coefficient) measures the relative importance of the jet impact inertia compared to the surface tension; the Ohnesorge number ( is the liquid viscosity) represents the relative importance of the viscous force to impact inertia and surface tension. A larger jet diameter in agricultural irrigation generally leads to a larger or a smaller which is approximately equivalent to an increase of jet impact velocity or a decrease of liquid viscosity promoting the development of jet instabilities and subsequent jet breakup. However, the dispersed droplet size in agricultural irrigation would be larger than that in combustion systems owing to its initial sufficiently large jet diameter.
In addition, the internal cavitation [99, 100] of various nozzles is also a significant factor influencing the flow rate and external jet atomization in agriculture [101]; this is the same in combustion systems that influence the combustion emission characteristics [102, 103] based on dual-fuel direct injection [103].
5 Discussion and concluding remarks
For jet–jet impingement at given fluid property and impact angle, the most direct and straightforward way to enhance atomization is to increase the impingement velocity. However, in practical applications of fuel injection in engines, achieving higher injection velocities usually requires a sufficiently large injection pressure drop which negatively impacts economic efficiency and implementation feasibility owing to the difficulty of creating such a large injection pressure drop. In addition, for variable-thrust engines at low throttling levels, the injection velocities cannot be sufficiently high. Thus, alternative injection strategies—such as swirl injection or off-center impingement-to-break symmetry—could be employed to attain desirable spray enhancement.
For jet–film impingement, decreasing film thickness leads to worse spray characteristics with increased spray angle and enhanced nonuniformity of droplet distribution—indicating that the atomization is dominated by the local effective impact between jet and film. Jet–jet impingement generally has better atomization than jet–film impingement owing to sufficient impact; however, the jet–film injection element is still widely and successfully used in pintle injectors in variable-thrust rocket engines because its mixing characteristics between fuel and oxidizer is better than jet–jet impingement, although atomization plays a secondary role in combustion when the combustor is sufficient large for complete combustion.
Jet–jet/film impingement for atomization enhancement has been widely applied in agriculture and propulsion systems. The most crucial factor causing liquid jet atomization is generating and magnifying the non-uniformity induced by such as hydrodynamic instabilities, local-strain-rate-dependent non-Newtonian fluid, gelled propellants, or exerted in an external electromagnetic field. In addition, it is essential to fully exploit interactions between the external flow field and droplets, thereby regulating energy transfer between hydrodynamic instabilities and external vortical structures. This enables precise control over the spatial distribution and droplet size of atomization characteristics.
The roles of various hydrodynamic instabilities and possible competition among the impact inertia, surface tension, and viscosity of various liquid jets in affecting jet–jet/film breakup and atomization characteristics are strongly coupled so that it is difficult to obtain a general design principle by focusing on only one parameter at a time. Fortunately, artificial intelligence (AI) techniques and machine learning algorithms in agriculture [104–107] could be very powerful for jet/film breakup prediction models by importing various experimental data to address the prediction bottleneck of traditional physical models in strongly nonlinear, multi-scale scenarios; this merits extensive future study.
Statements
Author contributions
CH: Conceptualization, Formal Analysis, Writing – review and editing, Writing – original draft. ZF: Writing – review and editing, Investigation, Writing – original draft. ZZ: Project administration, Supervision, Writing – review and editing. ZH: Writing – review and editing, Funding acquisition, Supervision.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. The authors acknowledge the Open Foundation of National Key Laboratory of Multi-perch Vehicle Propulsion Systems (QDXT-WY-202407-11) from the Beijing Institute of Technology.
Acknowledgments
The authors acknowledge the Open Foundation of National Key Laboratory of Multi-perch Vehicle Propulsion Systems (QDXT-WY-202407-11) from the Beijing Institute of Technology.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
jet-jet/film impingement, atomization, vortex dynamics, spray injector, vortex diagnose
Citation
He C, Feng Z, Zhang Z and He Z (2025) Applications of jet–jet/film impingement for atomization enhancement. Front. Phys. 13:1674248. doi: 10.3389/fphy.2025.1674248
Received
27 July 2025
Accepted
13 August 2025
Published
08 September 2025
Volume
13 - 2025
Edited by
Xi Xia, Shanghai Jiao Tong University, China
Reviewed by
Hongliang Luo, Harbin Engineering University, China
Kun Wu, Institute of Mechanics (CAS), China
Updates
Copyright
© 2025 He, Feng, Zhang and He.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Zehao Feng, fengzehao@ujs.edu.cn
Disclaimer
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