Yuanfeng Zhao
Yunlong Huang2
Zhixia He
Wenjun Zhong- 1Institute for Energy Research, Jiangsu University, Zhenjiang, China
- 2Rong Tong Aero Engine Technology Corporation Limited, Nanjing, China
- 3School of Energy and Power Engineering, Jiangsu University, Zhenjiang, China
This mini-review summarizes the fundamental principle, spray phenomena, and atomization mechanism of air-assisted spray nozzles, and applications in various aerospace propulsion systems, agriculture irrigation, and some promising areas. The structural configurations and atomization mechanisms of air-assisted nozzles are first introduced, followed by an analysis of key design parameters and their influence on atomization performance. The air-assisted atomization process is further characterized through typical flow regimes, vortex identification techniques, and integrated experimental–numerical approaches. Current practical applications are reviewed, along with potential optimization strategies for nozzle design. Future development directions should emphasize integration with smart agriculture technologies to address emerging demands and challenges in agricultural irrigation and atomization systems.
1 Introduction
Air-assisted spray nozzles adopt the strong interactions and aerodynamic forces [1–4] at the gas-liquid interface to enhance the disruption and atomization of the liquid fuel. It has been widely used in various industrial applications, such as aero-engine fuel atomization [1], smart agriculture and precision plant protection [2, 5–8], spray humidification and cooling [9], chemical and food processing [10], and treatment of respiratory diseases [11].
Conventional liquid jet direct injection and atomization, which relies on high-pressure injection of liquids, now faces many application challenges [12–16]. Liquid jets undergo the primarily breakup due to Rayleigh–Taylor and Kelvin–Helmholtz instabilities, and the secondary breakup influenced by aerodynamic forces under turbulent environment. High-quality atomization typically necessitates elevated injection pressures. In contrast, air-assisted atomization technology shows multiple atomization advantages under low-pressure injection conditions, with the core being the use of the kinetic energy of high-speed gas flow rather than the pressure energy of the liquid to break the liquid, and the spray characteristics can be flexibly controlled by adjusting the gas-liquid ratio and nozzle structure [17–20].
Air-assisted spray differs fundamentally from conventional single-jet atomization in both energy source and breakup mechanism: the former enhances shear-induced instability via gas–liquid interaction, while the latter relies primarily on liquid jet instability driven by high liquid kinetic energy. Although the breakup mechanisms of jets in stationary environments have been extensively studied [21–24],the interaction between high-speed gas flow and liquid jets during air-assisted atomization remains more complex and less understood.
The present mini-review mainly focused on the spray phenomena and breakup mechanisms of air-assisted spray nozzles in Section 2 and applications in combustion engines, agriculture irrigation, and some promising areas in Section 3, followed by the conclusion remarking and suggestions for future directions in Section 4.
2 Schematic and spray phenomena of air-assisted spray nozzles
2.1 Schematic and main geometric parameters
Air-assisted nozzles belong to the category of twin-fluid atomizers, characterized by the introduction of high-speed air during liquid ejection to promote efficient atomization via enhanced shear and interfacial instability. The process involves multiple hydrodynamic mechanisms—including Rayleigh–Taylor instability, Kelvin–Helmholtz instability, boundary-layer stripping, and liquid sheet perforation—with the dominant mechanism varying according to nozzle structure, gas-to-liquid velocity ratio, and operating pressure. Air-assisted atomization centers on intense momentum exchange between high-speed air and the liquid phase, accompanied by interfacial instability-driven fragmentation, typically comprising primary and secondary atomization stages. Such nozzles generally consist of liquid and gas passages along with a mixing chamber. Common configurations include internal-mixing and external-mixing types. Based on the gas–liquid mixing location, air-assisted atomization technology can be categorized into internal-mixing and external-mixing types.
As illustrated in Figure 1a, internal-mixing nozzles achieve gas–liquid mixing within the nozzle, often producing finer droplets but being susceptible to clogging. Nozzle performance is governed by geometric, operational, and performance parameters. Geometric parameters—such as liquid and gas orifice diameters, mixing chamber configuration, and gas–liquid impingement angle—determine the fundamental atomization process and efficiency. Among operational parameters, the gas-to-liquid mass flow ratio (GLR) is a critical variable directly influencing atomization energy and droplet size. Gas and liquid pressures and flow rates must be coordinately regulated to optimize performance. Key performance metrics include the Sauter mean diameter (SMD), droplet size distribution span, and spray cone angle [4, 25–27], which collectively reflect the fineness, uniformity, and spatial distribution of the spray.
Figure 1. Schematic of air-assisted nozzle structure and atomization phenomenon. (a) Structure of internal-mixing air-assisted nozzle; (b) structure of external-mixing air-assisted nozzle (adapted with permission from [28] Copyright 2017, by Elsevier Ltd.), (c) AAFI atomization process (adapted with permission from [33] Copyright 2024, The Authors). (d) Different forms of air-assisted spray morphology models for the spindle-shaped spray. (adapted with permission from [34] Copyright 2020, by Elsevier Ltd.).
External-mixing nozzles, as shown in Figure 1b, employ airflow to act directly on the liquid jet or sheet outside the nozzle, offering structural simplicity and greater operational adaptability. Numerous experimental and computational studies have been conducted on various air-assisted nozzle designs [28, 29]. Patel et al. conducted a numerical simulation study on the externally mixed air-assisted electrostatic nozzle for agriculture, while Dai et al. carried out an experimental study on parameters such as droplet size. The primary geometric parameters of external-mix air-assisted nozzles typically include the liquid orifice diameter, air orifice diameter, relative position of the air and liquid outlets, nozzle spacing, and spray angle. In such nozzles, liquid and air interact externally, where momentum exchange occurs to generate droplets, whereas in internal-mix nozzles, gas–liquid mixing and initial breakup take place inside the nozzle. Overall, the external-mix design features a simpler structure and facilitates independent control of air and liquid flow rates, but the droplet size distribution is more susceptible to external airflow conditions. In contrast, the internal-mix configuration enables stronger gas–liquid interaction and finer atomization, though at the cost of greater structural complexity and higher sensitivity to operating conditions.
In agricultural plant protection, external-mixing designs are widely integrated into unmanned aerial vehicles (UAVs) and precision spray systems, enabling low-drift, high-deposition variable-rate application [30]. In external-mixing nozzles, gas interacts with liquid outside the nozzle, where high-speed airflow aerodynamically shears the low-speed liquid jet or sheet. This design offers simplicity, strong anti-clogging capability, and a broad operating range. In aero-engine and gas turbine combustion chambers, internal-mixing nozzles ensure efficient and stable fuel combustion under low-load conditions, though they exhibit greater structural complexity and higher demands for operational stability [2, 31, 32]. Both approaches predominantly utilize gas-phase kinetic energy to drive droplet breakup, substantially reducing liquid pressure requirements.
2.2 Air-assisted atomization phenomena and mechanism
The air-assisted atomization process can be delineated into several distinct stages: liquid enters the nozzle at low pressure and forms an initial jet or sheet through the discharge orifice; high-speed airflow then interacts with the liquid column or film, inducing stretching, oscillation, and gradual breakup under aerodynamic shear; finally, ligaments and larger fragments undergo further disintegration into droplets, forming a spatially distributed spray plume. This process is characterized by sequential fragmentation from macro-scale structures to micro-scale droplets, with dynamics dominated by gas–liquid interaction intensity and interfacial instability. The Air-Assisted Fuel Injector (AAFI) system uniquely stratifies fuel and air within the injector. It is particularly effective in atomizing heavy fuels—which exhibit high viscosity and surface tension—into fine droplets. Figure 1c illustrates the AAFI atomization process: fuel is first metered into a pre-mixing chamber containing compressed air, where primary atomization occurs via air–fuel interaction; the mixture is then discharged through an annular air nozzle into the combustion chamber, where secondary atomization is enhanced by high-velocity airflow, improving fuel dispersion.
In the field of internal combustion engines, atomization mechanisms—particularly fuel jet breakup, droplet size distribution, and combustion characteristics—have been extensively investigated [21–24]. In contrast, research in agricultural engineering remains relatively limited, often focusing on applied aspects such as spray quality and crop deposition efficacy [25–27]. Although both domains share underlying hydrodynamic principles, performance varies significantly across scales, with jet size effects exert considerable influence on droplet formation and dispersion characteristics. These distinctions underscore the importance of cross-disciplinary mechanistic comparisons and experimental studies for optimizing agricultural spray technologies [28–30].
Furthermore, air-assisted atomization in agriculture is evolving toward intelligent systems incorporating variable-rate spraying and target recognition, presenting new challenges related to droplet size distribution and spray penetration. Systematic research on atomization mechanisms, key influencing factors, and modeling techniques is therefore essential for enhancing combustion efficiency, spray accuracy, and the design of optimized spraying equipment with cross-domain applicability [2, 31–34].
2.3 Effects of vortex formation on air-assisted atomization
Wu et al. [34] experimentally identified two distinct spray patterns of AAFI as illustrated in Figure 1d: the spindle-shaped spray and anchor-shaped spray. Spindle-shaped sprays typically occur with moderate back pressure, and a balanced air–fuel momentum exchange, resulting in elongated and symmetric sprays. In contrast, anchor-shaped sprays often appear under higher back pressure, where strong recirculation zones anchor the spray root, enhancing fuel–air mixing but potentially causing atomization instability. Furthermore, the nozzle structure also exerts a certain influence on the spray pattern. Both patterns exhibit similar near-field spray structures. The key distinction manifests in far-field spray tip morphology. The spindle-shaped spray results from relatively higher injection velocity coupled with intensive droplet breakup and evaporation, which induces low-pressure zones at the spray leading edge and ultimately leads to spray tip collapse. The anchor-type spray is predominantly governed by its leading-edge dual-vortex-ring structure, which originates from shear layer instability induced by droplet slippage. The shear layer subsequently propagates along the spray periphery, accumulates near the spray tip, then undergoes lateral displacement before finally swirling back into the spray, evolving into a vortex ring configuration.
The influence of vortices on jet breakup has been widely recognized as a critical factor in atomization processes [35]. Vortex structures generated during jet–air interactions not only enhance shear forces at the gas–liquid interface but also induce strong fluctuations in local velocity and pressure fields, thereby accelerating surface wave growth and liquid ligament formation [36, 37]. Large-scale vortices promote primary breakup by destabilizing the liquid column, while smaller-scale vortical motions contribute to the secondary disintegration of ligaments and droplets, ultimately leading to a finer and more uniform spray [24, 37, 38]. Consequently, the evolution and control of vortex dynamics are considered key mechanisms governing atomization efficiency and spray quality. The above-mentioned formation of these vortex rings creates high-pressure zones on both sides near the vortex core, thereby compressing the liquid spray to form hollow vortex rings. In air-assisted sprays, this phenomenon is more pronounced due to the initial presence of abundant microbubbles that rupture distally, facilitating droplet ejection and vortex ring generation at relatively lower spray velocities. This demonstrates strong coupling between air and liquid phase dynamics [34].
Quantifying vortex strength is therefore essential for understanding and optimizing spray dynamics. Common approaches include the evaluation of vorticity, defined as the curl of the velocity field, which provides a direct measure of local rotational intensity. The magnitude of vorticity, often obtained through particle image velocimetry (PIV) or computational fluid dynamics (CFD), is widely employed to characterize the strength of vortical regions. Circulation, expressed as the line integral of velocity around a closed contour, serves as a global indicator of vortex intensity [35–37]. In addition, advanced identification methods such as the Q-criterion or λ2-criterion allow for precise recognition and assessment of coherent vortex structures.
Geometric optimization, including the use of swirlers or tailored nozzle exits, can significantly affect vortex strength and stability. Operational parameters, such as the air-to-liquid ratio and injection pressure, provide further control over the flow field. Moreover, the integration of flow-guiding elements or micro-structures enables fine-tuned manipulation of local vortices [39, 40]. In the future, intelligent control systems that couple real-time sensing with dynamic flow adjustment are expected to offer advanced regulation of vortex behavior, thereby achieving superior atomization performance [41].
3 Applications of air-assisted spray nozzles
3.1 Small-scale air-assisted nozzles in energy-related engines
The outlet diameter of Small-scale air-assisted nozzles typically ranges from 1 to 8 mm, and they are capable of spraying droplets with a Sauter Mean Diameter of 5–50 μm [42]. In power engineering applications such as internal combustion engines, research on air-assisted nozzles emphasizes transient fuel jet breakup and combustion efficiency improvement. Advanced diagnostic techniques, including laser-induced fluorescence (LIF) and particle image velocimetry (PIV), are widely employed to investigate the physical properties of air-assisted sprays. These methods enable high-precision visualization and measurement of key parameters such as droplet size, velocity field, and spatial distribution. Phase Doppler particle analyzers (PDPA) and laser diffraction particle sizing instruments are commonly utilized to evaluate atomization performance under varying injection pressures, flow conditions, and nozzle geometries [43–61].
Figure 2a presents an air-assisted direct injection (AADI) system designed to mitigate knock in high-compression-ratio elliptical rotary engine. The AADI system demonstrated effective knock suppression, improved thermal efficiency, and reduced heat transfer losses compared to baseline configurations, highlighting its potential for enhancing anti-knock performance and overall energy efficiency.
Figure 2. Applications of air-assisted nozzles. (a) Simulated and experimental spray morphologies at different moments of elliptical rotor engine (adapted with permission from [57]Copyright 2024, by Elsevier Ltd.), (b) Air-assisted Spraying Device for Dust Suppression on Tunneling Machine and Vector Diagram of the spray field (adapted with permission from [62]Copyright 2025, by Elsevier Ltd.), (c) Air-assisted atomization humidifier (adapted with permission from [63] Copyright 2025, by Elsevier Ltd.), (d) Compressed air injects into mixing chamber filled with water, (adapted with permission from [64]Copyright 2023, by Elsevier Ltd.).
3.2 Large-scale air-assisted nozzles in agricultural irrigation
The application of air-assisted atomization in agricultural engineering goes far beyond “spraying”, and its core value lies in achieving precise, efficient and environmentally friendly plant protection operations, which are manifested in significantly reducing drift pollution [62–64], improving canopy penetration and uniform deposition [65, 66], adapting to low-volume spraying, and achieving precise variable spraying [67], etc. Large-scale air-assisted nozzles range from 10 to 50 mm [20], generating droplets of 80–450 µm, depending on airflow and liquid properties [18]. A large number of experimental studies using techniques such as laser particle size analyzer, high-speed photography and particle image velocity measurement have revealed the effects of nozzle structure, gas-liquid ratio and spray pressure on droplet size distribution, droplet deposition efficiency and drift behavior [68, 69]. In terms of numerical simulation, Euler-Lagrange coupling, VOF and large eddy simulation (LES) methods are widely used in agricultural engineering to study gas-liquid interactions. The simulation in agricultural engineering focuses on the transport and deposition distribution of droplets in the canopy [2, 65–67, 70].
Air-assisted spray technology has been widely used for pest and disease control in orchard crops [8, 69, 71, 72] and is gradually expanding to areas such as between facility farmers [29, 73–76]. Air-assisted spraying has been widely integrated into a variety of agricultural spraying equipment [77]. Figure 2b presents an air-assisted spraying system specifically engineered for tunnel boring equipment. This system utilizes the Coanda effect to improve atomization performance and increase spray projection distance. By optimizing air and water pressure parameters, the system achieves substantial enhancements in dust suppression efficiency, with total coal dust and respirable dust suppression rates exceeding those of conventional systems by more than 30%.
In recent years, agricultural drones based on air-assisted spraying, orchard air-assisted sprayers [20, 78, 79], high-gap boom sprayers [30, 71] have been widely used, and intelligent devices have been continuously developed, significantly improving the efficiency and uniformity of pesticide application. At the same time, by combining methods such as CFD simulation [80, 81] and response surface optimization [82], researchers have proposed a variety of improvement schemes in terms of spray Angle, wind speed control, droplet size regulation, etc. [2, 5, 6, 62, 83–85], effectively reducing the risk of pesticide drift and improving pesticide utilization. Some intelligent spray systems also integrate lidar, ultrasonic sensors or machine vision modules [86–90] to achieve real-time perception of crop canopy structure and adaptive control of spray volume, further promoting the development of variable spray and targeted application technology. Also, the integration of unmanned aerial vehicle platforms and air-assisted systems has become a hot application direction [91, 92]. Drones equipped with air-assisted nozzles can achieve uniform atomization and precise control while hovering, showing unique advantages in areas such as mountain orchards that are difficult to cover with traditional machinery. Ahmed et al. [90, 91] noted that although unmanned aerial vehicles have been widely used in crop management, monitoring, seeding and pesticide spraying, they still face many challenges when used as spray platforms. Unlike conventional drones, spray drones need to perform tasks in dynamic payloads and complex environments, which poses higher requirements for real-time obstacle detection, path planning, and collision avoidance capabilities.
3.3 Future designs for air-assisted nozzles
In recent years, machine learning and intelligent optimization algorithms have provided new ideas for nozzle design [90, 91]. On the one hand, machine learning models based on experimental data and numerical simulation results (such as random forests, support vector machines, deep neural networks) can quickly predict spray particle size distribution, spray Angle and deposition effect, significantly reducing the experimental costs required for the development of traditional nozzles [63, 82, 93]. On the other hand, methods such as genetic algorithms, multi-objective optimization, and reinforcement learning have been applied to nozzle structure and flow field design to achieve comprehensive optimization of nozzle geometry, gas-liquid ratio, and gas flow path [94, 95].
Air-assisted atomization technology has been successfully implemented in the refrigeration industry. Figure 2c presents the Air-Assisted Atomization and Humidification (AAAH) technology designed to address humidification and cooling challenges in high-power fuel cell systems. The results indicate that AAAH significantly enhances the cathode inlet relative humidity and water recovery rate while reducing thermal load. Figure 2d investigates the sensitivity of air-assisted atomizers in outdoor snowmakers under varying operational conditions. The findings reveal that the spray cone angle initially increases and then decreases with rising air-to-water pressure ratio, while the Sauter Mean Diameter (SMD) continuously declines. Additionally, the study demonstrates that atomizers employing an independent air-water injection mode effectively prevent clogging and improve flow capacity.
4 Discussions and concluding remarks
Air-assisted atomizing nozzles demonstrate broad application potential in agricultural and power engineering due to their ability to achieve efficient atomization under low-pressure conditions. However, several key scientific challenges remain in nozzle optimization design. Central among these is a deeper understanding of atomization mechanisms, particularly the role of high-speed gas flow in perturbing and disrupting the liquid jet interface. While jet instability in stationary environments has been well studied, the interaction between high-speed gas flow and liquid jets in air-assisted nozzles remains more complex and less understood. The characteristics of vortex dynamics and their effects on droplet size distribution, spray uniformity, and drift behavior have not been fully elucidated, impeding scientifically guided nozzle optimization and flow field control. Future research should prioritize integrated experimental and numerical approaches to unravel the multi-scale mechanisms of gas–liquid interaction, which will be crucial for advancing nozzle design and application.
Additive manufacturing, such as 3D printing, enables the fabrication of complex internal geometries and microstructures that are difficult to achieve with conventional machining, thereby improving air–liquid mixing efficiency and spray performance [96, 97]. It also allows for rapid prototyping and customization, accelerating design iteration while reducing costs. In parallel, lightweight composite materials, including carbon fiber and ceramic-based composites, provide significant advantages in terms of weight reduction, durability, and resistance to wear and corrosion. Moreover, functional enhancements such as hydrophobicity or self-cleaning surfaces can be integrated through advanced material processing [98].
Looking forward, the evolution of smart agriculture imposes increasing demands on spray and atomization technologies. Enhancing pesticide and water use efficiency is essential to achieve precise chemical application and water-saving irrigation. Concurrently, controlling spray drift, ensuring environmental safety, and adapting to complex field wind conditions present ongoing challenges. Air-assisted atomizing nozzles must balance fine droplet generation with anti-drift performance and target deposition efficiency. Future developments should emphasize integration with smart sensing, machine learning, and automatic control technologies to enable dynamic, adaptive regulation of spraying systems.
Author contributions
YZ: Writing – original draft, Investigation. YH: Writing – review and editing, Investigation. ZH: Writing – review and editing, Conceptualization, Supervision. WZ: Writing – review and editing, Resources.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. National Natural Science Foundation of China (No.52376113).
Conflict of interest
Author YH was employed by Rong Tong Aero Engine Technology Corporation Limited.
The remaining 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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Keywords: air-assisted injector, atomization, nozzle design, droplet size, vortex dynamics
Citation: Zhao Y, Huang Y, He Z and Zhong W (2025) Mini-review: air-assisted spray nozzles and applications. Front. Phys. 13:1705908. doi: 10.3389/fphy.2025.1705908
Received: 15 September 2025; Accepted: 30 September 2025;
Published: 13 October 2025.
Edited by:
Xi Xia, Shanghai Jiao Tong University, ChinaReviewed by:
Xuesong Li, Shanghai Jiao Tong University, ChinaYanzhi Zhang, Dalian University of Technology, China
Copyright © 2025 Zhao, Huang, He and Zhong. 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: Zhixia He, enhoZUB1anMuZWR1LmNu