- Original Article
- Open Access
Influence of Self-excited Vibrating Cavity Structure on Droplet Diameter Characteristics of Twin-fluid Nozzle
© The Author(s) 2018
- Received: 16 June 2017
- Accepted: 14 August 2018
- Published: 23 August 2018
It is a great challenge to find effective atomizing technology for reducing industrial pollution; the twin-fluid atomizing nozzle has drawn great attention in this field recently. Current studies on twin-fluid nozzles mainly focus on droplet breakup and single droplet characteristics. Research relating to the influences of structural parameters on the droplet diameter characteristics in the flow field is scarcely available. In this paper, the influence of a self-excited vibrating cavity structure on droplet diameter characteristics was investigated. Twin-fluid atomizing tests were performed by a self-built open atomizing test bench, which was based on a phase Doppler particle analyzer (PDPA). The atomizing flow field of the twin-fluid nozzle with a self-excited vibrating cavity and its absence were tested and analyzed. Then the atomizing flow field of the twin-fluid nozzle with different self-excited vibrating cavity structures was investigated. The experimental results show that the structural parameters of the self-excited vibrating cavity had a great effect on the breakup of large droplets. The Sauter mean diameter (SMD) increased with the increase of orifice diameter or orifice depth. Moreover, a smaller orifice diameter or orifice depth was beneficial to enhancing the turbulence around the outlet of nozzle and decreasing the SMD. The atomizing performance was better when the orifice diameter was 2.0 mm or the orifice depth was 1.5 mm. Furthermore, the SMD increased first and then decreased with the increase of the distance between the nozzle outlet and self-excited vibrating cavity, and the SMD of more than half the atomizing flow field was under 35 μm when the distance was 5.0 mm. In addition, with the increase of axial and radial distance from the nozzle outlet, the SMD and arithmetic mean diameter (AMD) tend to increase. The research results provide some design parameters for the twin-fluid nozzle, and the experimental results could serve as a beneficial supplement to the twin-fluid nozzle study.
- Atomizing nozzle
- Sauter mean diameter
- Arithmetic mean diameter
- Self-excited vibrating cavity
- Phase Doppler particle analyzer
With the increasing consumption of fossil fuels and the heavy emissions of dust from industry, serious environmental and health problems have drawn great attention from the public in recent years [1, 2]. Micron and sub-micron droplets have made significant contributions to reducing exposure to fine particles. Atomization and cavitation are the two important approaches to generating droplets [3–5]. Owing to the smaller droplets size, larger coverage of droplets, and wider applicability, the excellent performance of atomizing technology has motivated intensive studies in many fields [6–9]. However, the spray process of a twin-fluid nozzle involves multiphase coupling, dynamic changing, evaporation, and condensation, which have great influence on the stability and uniformity of droplets in the spray field and the atomizing efficiency of the twin-fluid nozzle. Therefore, it would be a great accomplishment if the fine droplet mists could be generated steadily through changing the structure of twin-fluid nozzle.
Many efforts have been made by scholars around the world to improve the atomizing performance of spray nozzles in the past decades, such as designing nozzle structures [10, 11], innovating atomizing methods [12, 13], and optimizing operating conditions [14, 15]. Presently, it is believed that fine and uniform droplet mists can be obtained easily by the twin-fluid nozzle atomizing technology combined with external forces, and this was demonstrated to be a promising method [12, 16].
Electrostatically charged and acoustic waves vibration are the two main incentive approaches, which provide a new mentality for the improvement of the nozzle atomizing performance. In fact, as early as 1984, Law et al.  proposed the measuring of charged droplets by laser Doppler anemometry. Furthermore, they studied the trajectories of droplets in an electric field, which has profound consequences for the electrostatic crop spraying of foliar tips. Wang et al. [16, 18] analyzed the charged twin-fluid spray structure, the droplet diameter, the axial jet velocity, and the turbulent fluctuation intensity in an electrostatic charged spray by particle image velocimetry (PIV) and a phase Doppler particle analyzer (PDPA). Shi et al.  investigated the spray angles under a stable cone-jet mode in an electro-spraying experiment under single and combined electric fields. Moreover, they declared that both electrode parameters and space charges have a significant impact on the electric field distribution. However, safety of the charged droplets and electrostatically charged process are difficult to ensure in some places, such as coal mines, high concentration dust areas.
Based on the acoustic waves vibration technology, Grant  applied ultrasonic gas atomization to an injection spray. Narayanan et al. [21, 22], ascertained the effects of the vibrating cavity structure on frequency and amplitude characteristics through contrasting experiments. In addition, Zu et al.  researched the characteristics of air vibration in a Hartmann resonance cavity with/without actuators. The result indicated that actuators enhance the strength and onset process of oscillating remarkably. Ruan et al.  investigated the effects of geometric parameters on the flow and acoustic field characteristics of an ultrasonic atomizer by experimentation and numerical simulation. However, most of these studies mainly focused on the characteristics of acoustics and air flow fields, but have not taken liquid into consideration.
Rajan et al.  pointed out that finer droplets with high sphericity and uniform size distribution could be achieved by ultrasonic energy. Compared to the conventional nozzles, the atomizing process induced by ultrasound had a lower energy consumption. Narayanan et al.  investigated the effects of Hartmann cavity acoustics on the droplets atomization by experimentation. The results revealed that the existence of a sound field led to the droplets undergoing large deformation and becoming irregular in shape. Ferreira et al.  experimentally investigated the influence of different structural parameters and flow conditions on the droplets size of a twin-fluid nozzle with an internal mixing chamber. Meanwhile, the main parameters of optimum results had been determined. Li et al.  researched the effects of jetting distance and impacts of the collision wall angle on the outlet velocity based on the computational fluid dynamics (CFD) numerical simulation technology. Liu et al.  studied the influence of air pressure on droplet diameter, atomization angle and water consumption of a modified ultrasonic nozzle. According to the achievements of former researchers, it can be considered that fine droplet mists and uniform droplet size distribution can be achieved in an acoustic field, which could be generated by a vibrating cavity structure of the twin-fluid nozzle. Acoustic field characteristics of twin-fluid nozzles have been mainly investigated in recent years. As we know, droplet diameter characteristics are the important indexes of twin-fluid atomizing performance, but investigation of the influence of vibrating cavity structure on that is reported rarely.
Exploring the influence of a vibrating cavity structure on droplet diameter characteristics is beneficial for recognizing the atomizing principles, which is a critical factor to improve the atomizing performance of the twin-fluid nozzle. On the basis of previous studies [29–31], by using an open atomizing test bench which is based on a PDPA, this study is mainly focused on the influence of a self-excited vibrating cavity structure on the droplet diameter characteristics of a twin-fluid nozzle.
2.1 Experimental Set-up and Measurement Technique
A phase Doppler particle analyzer (Dantec) was used for droplet diameter measurement. The PDPA performed real-time measurements with high precision (0.5%) and stability, and the measuring range of diameters was 0.5–13000 μm. The PDPA was comprised of an Argon-ion laser, a beam separator, a transmitter, a receiver, an analyzer, and a computer. The laser was produced by the Argon-ion laser and transferred from beam separator to transmitter. The data collection focal point was generated by the laser from the transmitter, and the receiver collected the real-time signals and transmitted them to the BSA analyzer for data analysis. Results could be displayed through the computer and the location of the focal point would be adjusted by the 3D traverse.
2.2 Test Nozzle Specification
2.3 Testing and Analytic Methods
All twin-fluid atomizing tests were conducted in a closed environment. In order to keep the testing environment steady, the temperature of the environment was maintained at 24 ± 1 °C, the relative humidity was maintained between 45%‒50%, and the atmospheric pressure was about 102.6 kPa. In addition, the viscosity and surface tension of the water were 0.9142 mPa·s and 0.0721 N/m, respectively.
The air compressor and water pump were opened at the beginning of test, and then the operating parameters adjusted as required. For ensuring that the atomizing flow field was steady, the data collecting system would be started five seconds after the nozzle began to spray. The time of testing was sustained for 30 s, then the data would be recorded and processed. In order to ensure the accuracy and repeatability of results, all the recorded data were the average of three replicates.
3.1 Influence of the Self-excited Vibrating Cavity and Its Absence on Droplet Diameter Characteristics
3.1.1 SMD Spatial Distribution Analysis
Compared to the SMD spatial distribution of the twin-fluid atomizing nozzle without a self-excited vibrating cavity, it can be concluded that the SMD was smaller and the coverage of droplets was bigger for the nozzle with a self-excited vibrating cavity. The strong collision of droplets between the nozzle outlet and vibrating cavity lead to the increase of the droplets’ radial velocity, which caused the coverage of droplets to increase remarkably. As seen, the SMD near the nozzle outlet without a self-excited vibrating cavity was about two times larger than that with a self-excited vibrating cavity. This result is due to the acoustic field which was generated by the intense oscillations of the self-excited vibrating cavity. Furthermore, this phenomenon can prove that the acoustics are beneficial to enhancing the atomization. Narayanan  had a similar discovery in the research of atomization in the acoustic field of a Hartmann whistle.
3.1.2 Comparative Analysis of SMD and AMD
3.2 Influence of the Orifice Diameter on Droplet Diameter Characteristics
3.2.1 SMD Spatial Distribution Analysis
It is clear that the SMD of droplets increased with the increase of the orifice diameter of the self-excited vibrating cavity. The atomizing performance was superior when the orifice diameters were 1.0 mm and 1.5 mm, and the SMD of more than half the atomizing flow field was under 35 μm. With the orifice diameter continuing to increase to 2.5 mm, the SMD increased slowly and the atomizing performance became worse. This is due to the decrease of acoustic field frequency, because Ruan et al.  reported that acoustic field frequency showed a decreasing trend with the increase of the orifice diameter of the vibrating cavity. Furthermore, the high acoustic field frequency had a great effect on improving the turbulence of the fluid field which was beneficial for decreasing the droplet diameter. The results reveal that a better SMD spatial distribution was obtained when the self-excited vibrating cavity had a smaller orifice diameter.
3.2.2 Comparative Analysis of SMD and AMD
3.3 Influence of the Orifice Depth on Droplet Diameter Characteristics
3.3.1 SMD Spatial Distribution Analysis
The SMD changed slowly when the orifice depth increased from 2.0 mm to 2.5 mm, and the SMD spatial distribution was similar. Furthermore, the influence of the acoustic field frequency decrease on the SMD shows that a weakening trend can be indicated when the orifice depth was above 2.0 mm. This result reveals that the SMD of droplets was finer when the orifice depth of the self-excited vibrating cavity was smaller.
3.3.2 Comparative Analysis of SMD and AMD
3.4 Influence of the Distance between the Nozzle Outlet and Self-excited Vibrating Cavity on Droplet Diameter Characteristics
3.4.1 SMD Spatial Distribution Analysis
From the above analysis, it is known that the structural changes of the self-excited vibrating cavity have great influence on the droplet diameter by influencing the acoustic field frequency. Ruan and Narayanan [24, 26] had a similar discovery; that the acoustic field frequency showed a large oscillation, and the frequency decreased first and then increased with the increase of S. Therefore, a good, consistent relationship between the SMD decrease and the acoustic field frequency increase is presented.
3.4.2 Comparative Analysis of SMD and AMD
A self-excited vibrating cavity was verified to be an effective key for improving the atomizing performance of a twin-fluid nozzle. The structural parameters of the self-excited vibrating cavity had a great effect on the breakup of large droplets.
The SMD increased with the increase of orifice diameter or orifice depth of the self-excited vibrating cavity. A good, consistent relationship between the SMD decrease and the acoustic field frequency increase was presented. A smaller orifice diameter or orifice depth is beneficial to enhancing the turbulence around the outlet of nozzle and decreasing the SMD. The atomizing performance was better when the orifice diameter was 2.0 mm or the orifice depth was 1.5 mm.
SMD increased first and then decreased with the increase of the distance between the nozzle outlet and the self-excited vibrating cavity. A superior atomizing performance was obtained when the distance was 5.0 mm, and the SMD of more than half the atomizing flow field was under 35 μm.
With axial and radial distance increase, the SMD and AMD tended to increase. Meanwhile, the SMD and AMD near the nozzle outlet zone were smaller than those in other zones. High acoustic field frequency could exacerbate the breakup of droplets, and the distribution of droplet diameter become wider, which caused an obvious difference between the SMD and AMD.
D-RG was in charge of the whole trial; BC wrote the manuscript; S-FW and J-HZ assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.
Bo Chen, born in 1990, is currently a PhD candidate at School of Mechanical Engineering, Yanshan University, China. His main research interests include fluid machinery and multiphase flow.
Diao-Rong Gao, born in 1962, is currently a professor and a PhD candidate supervisor at School of Mechanical Engineering, Yanshan University, China. He received his PhD degree from Yanshan University, China, in 2001. His research interests include CFD, PIV, heavy machinery fluid transmission and control and new types of fluid components and devices.
Shao-Feng Wu, born in 1987, is currently a lecturer at School of Mechanical Engineering, Hangzhou Dianzi University, China. He received his PhD degree from Yanshan University, China, in 2017. His research interests include fluid transmission and control and fluid machinery.
Jian-Hua Zhao, born in 1983, is currently an associate professor at School of Mechanical Engineering, Yanshan University, China. He received his PhD degree from Yanshan University, China, in 2013. His research interests include the simulation and analysis of hydrostatic bearing.
The authors declare that they have no competing interests.
Supported by National Natural Science Foundation of China (Grant No. 51705445), Hebei Provincial Natural Science Foundation of China, (Grant No. E2016203324), and Open Foundation of the State Key Laboratory of Fluid Power and Mechatronic Systems of China (Grant No. GZKF-201714).
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