A method for controlling and maintaining the thickness of a liquid layer during ultrasonic spraying

This work is described a method for indirectly controlling the height of the film of sprayed material on the oscillating surface of ultrasonic atomizer by changing the resonant frequency of the ultrasonic oscillator. The relevance of the development of this method is due to the need for the development and widespread use of the ultrasonic spraying method to solve the most pressing problems of modern industry. In this regard, there is a need to establish the dependences of the spraying performance on the height of the film of sprayed fluid on the oscillating plane of the atomizer and to create, based on the identified dependencies, a method for controlling and maintaining the necessary and sufficient height of the fluid film, the spraying of which will provide the best dispersion characteristics of the generated aerosol at a given spraying performance. As a result of the research, it was shown that in the operating range of film height of the sprayed fluid, the change in the resonant frequency can reach 100 Hz, which is sufficient to control the film height. This made it possible for the first time to develop a method for automatically controlling the ultrasonic spraying process, ensuring the maintenance of optimal modes of ultrasonic exposure (amplitude of vibrations of the spray surface) and the thickness of the sprayed fluid height.


Introduction
Atomization is the process of dispersing liquids into small droplets [1], which is widely used in various fields, including the combustion of fossil fuels [2], processing of raw materials, spray drying, and the production of powders (by the reaction of an aerosol with the medium into which the atomization is performed) [3], spray cooling, fuel combustion, inkjet printing [4].There are many liquid atomization methods, such as two-fluid atomization [5], electrostatic atomization [6], pressure atomization [7], and ultrasonic atomization [8,9].Compared with other methods, ultrasonic atomization has unique advantages such as high atomization uniformity, high material utilization rate and high economic performance, and therefore has been widely used in industry in recent years [10], which has stimulated research in the field of ultrasonic atomization process control.
The ultrasonic method for producing aerosols is widely used in the field of processing raw materials such as spray drying (production of dry extracts of medicines, production of powders by reacting the aerosol with the medium into which the spraying is carried out).In the field of coating, such as spraying conductive films in the production of sensors, spraying coatings on coronary stents in their production.In medicine, this is obtaining an aerosol of medicines (inhalers), spraying disinfectants.
With ultrasonic atomization, liquid is dispersed into droplets in a very short time under the influence of high-frequency vibrations (f>20 kHz) [11].This process was first studied by Wood and Loomis in 1927.It was found that ultrasonic atomization is, in fact, a process of strong deformation of a liquid under the influence of external vibration.Faraday first began studying forced vibrations of liquids in 1831.He placed various liquids in a vertically oscillating cylindrical container and observed the appearance of standing capillary waves on the surface of the liquid.It was experimentally established, for the emergence of capillary waves, a film of fluid on the oscillating surface must have a certain thickness, which depends on the resonant frequency of oscillations and the characteristics of the dispersible fluid.It was found that the fluid on the spraying surface should have a thickness not exceeding half the wavelength of ultrasonic vibrations in sprayed liquid [12].
Later studies allowed to identify the values of the optimal thickness of the fluid on the spray surface.With this thickness, the maximum spray productivity and the best dispersed characteristics of the drops formed are ensured (the minimum spread relative to the average value).It was established that for Newtonian liquids this thickness is height by h≈0.1 mm [13].In the case of an uncontrolled increase and a decrease in the thickness of the film on a spraying surface, the process of formation of drops stops.
In this regard, the implementation of the process of ultrasound spray of fluid is impossible without continuous control and maintaining the optimal value of the film thickness on the oscillating surface.The proposed, developed and studying method for the implementation of the spraying process with continuous control of the thickness of the sprayed film is aimed at solving this problem.In practice, the ultrasonic atomization process is implemented using sprayers, which consist of an ultrasonic oscillatory system (4), an electronic generator for powering the oscillatory system (5) and a spray liquid supply system (6).An ultrasonic oscillatory system converts vibrations using piezoelectric elements the electrical voltage feeding it into the mechanical vibrations of the working end.The ultrasonic generator ensures the conversion of industrial network energy into the energy of electrical vibrations with a frequency corresponding to the resonant frequency of the ultrasonic oscillatory system.In addition, the electronic generator ensures stabilization of the amplitude of mechanical vibrations (due to changes in the output voltage) and automatic adjustment of the frequency of the output voltage in accordance with changes in the parameters of the ultrasonic vibrating system.

Method of ultrasonic atomization of liquid in a layer
The supply system ensures a stable and uniform supply fluid on the working plane of the UOS with a given performance.As a rule, liquid is supplied to the working surface of the ultrasonic oscillatory system through a longitudinal internal channel located in the center of the ultrasonic oscillatory system.Under the influence of mechanical vibrations, the supplied liquid spreads over the spray surface, forming a film on which standing capillary waves appear.The possibility of formation and amplitude of capillary waves are determined by the amplitude of vibrations of the spray surface and the thickness of the liquid film.When a sufficient amplitude of capillary waves is reached (when the force acting on the "crest" of the wave exceeds the forces of surface tension of the liquid), a drop detaches from the wave crest.Deviation of the height of the fluid film covering the spray surface from the optimal meaning, upward or downward, leads to a decrease in the amplitude of capillary waves, and, consequently, the spraying performance.
In a steady-state ultrasonic atomization process, the height of the fluid film on the surface of the atomizer will be determined by the difference between the performance of liquid supply to the spray surface and the performance of atomization of this liquid due to capillary waves.Since the spraying performance depends on the amplitude of vibrations of the spray surface, by changing the amplitude of vibrations of the spray surface it is possible to regulate the height of the film of sprayed fluid (by changing the rate of moisture removal from the surface of the fluid).

Method of controlling the thickness of the liquid layer
Direct measurement of the height fluid film during dispersion is possible only in laboratory conditions.It requires expensive optical equipment, and is difficult to automation (in particular, due to a high degree of transparency of materials).Therefore, a method is needed to assess the height of the spray layer on the oscillating surface according to indirect signs.
From an acoustic point of view, a film of fluid on an oscillating surface is an acoustic load and affects the resonant characteristics of the all UOS.The oscillatory system during the impact process can act as a primary converter [14].There are methods [15] that allow one to evaluate have an effect on of acoustic load on the resonant parameters of an oscillating system.
The principle of electro-mechanical analogies allows us to consider the piezoelectric transducer of an ultrasonic oscillatory system in the form of an equivalent electrical circuit.The electrical equivalent circuit of the ultrasonic oscillatory system is shown in Figure 2   Analyzing the electrical circuit of the piezoelectric transducer (Fig. 2), we determine that its resonant frequency depends on the values of the reactive elements Lm and Cm.Since a liquid located on the surface of the UOS can be considered an acoustic load based on the method of electro-mechanical analogies, it will "change" the Lm element.Based on the foregoing, it can be assumed that the presence (and volume) of the fluid on the working surface of the vibrating system will affect its resonant frequency.
Thus, by measuring the deviation of the resonant frequency of UOS during the spraying of the fluid, control can be ensured on the surface of the vibrating system of the amount of fluid, i.e.Determine the thickness of the film optimal for the implementation of the process of ultrasound spray.For the practical implementation of this control method, it is initially necessary to establish a relationship between the thickness of the liquid film on the surface of the spray and a change in the resonant frequency of UOS.To solve this problem, a specialized stand has been developed and manufactured.

Research stand
Figure 3 shows a stand for conducting experimental studies.The wall includes an ultrasonic oscillating system (2), a measuring module for obtaining amplitude-frequency characteristics (3), connected to a personal computer (4), and a dispenser for supplying sprayed liquid (1).
1 -Liquid supply system, 2 -Spray system, 3 -Measuring unit, 4 -Computer To ensure the possibility of maintaining a film of liquid of arbitrary thickness on the spray surface, the oscillating system was located with the spray end up.In addition, the spray ends were made replaceable.The spray tip is cup-shaped to hold the required volume of liquid.Figure 4 shows a drawing and photograph of one of the test spray tips.Table 1 shows their main dimensions.The test liquid is supplied into the instrument cavity through a syringe dispenser, which allows you to measure the required amount of liquid.Studying the piezoelectric transducer circuit (Fig. 2) made it possible to determine that the resonant frequency is influenced by the added mass (the mass of liquid at the spray end of the piezoelectric transducer).Therefore, the dependences of the frequency of the piezoelectric transducer on the mass of the sprayed liquid at the resonant frequency of the UOS were initially plotted.Subsequently, taking into account the density and area of the spraying surface, you can determine the volume and thickness of the film of sprayed fluid.To establish the dependence between the variation resonant frequency piezoelectric transducer and the spray fluid mass, the following research algorithm was proposed: 1. Obtaining a frequency response "dry" UOS; 2. Add model fluid to the working surface of the UOS; 3. Obtaining a frequency response of UOS; 4. Repetition of points from 2 to 4 until the cavity of the spraying end will not be filled; 5. Determination for the received frequency frequencies of UOS for each fluid volume; 6. Determination of the relationship of a variation in the resonance frequency piezoelectric transducer on the spray fluid mass in the form of a difference in the obtained resonant frequencies and the own resonant frequency of the UOS.During the studies, water, ethanol and glycerin were used.Hereinafter they are called model liquids.Their main physical parameters of which are given in Table 2.The choice is due to the need to study liquids with significantly different physical properties.

Discussion
Since their temperature affects the resonant frequency of mechanical systems, to reduce the error temperature, each series of experiments was carried out at the same temperature of the oscillatory system.Changes in the resonance frequencies of ultrasound spray systems from the weight of fluid on the surface of the spraying are indicated in Figure 5. Graphs, Figure 5. show the presence of the dependence of the resonant frequency of the oscillatory system on the mass of the liquid at its spraying ending.Of the dependencies (Figure 5), it is clear that the resonant frequency of the oscillatory system is close to linear and is poorly harvested by the effect of properties (viscosity, surface tension coefficient) itself.Since the attached mass affects the frequency element of the equivalent scheme, it can be assumed to increase this influence with an increase in the working frequency of the oscillatory system.
In Figure 6 shows graphs of changes in the resonant frequency from the mass of the liquid when the UOS at a frequency of 44kHz.Comparison of the dependencies in Figure 5 and Figure 6 shows that increasing the frequency of the UOS increases the sensitivity of the proposed method for indirectly monitoring the height of the fluid film.

Determination of layer thickness
Since the determining factor in the quality of the ultrasonic spraying is a film of sprayed fluid, or rather its thickness, it is necessary to flow from the mass of the liquid to its volume, which will allow knowing the area of spraying ending to obtain the thickness of the formed film.
In Figure 7 schedules of the dependence of the shift the resonant frequency of the UOS with the spraying ending No. 1 from the volume of the fluids on its surface are presented.From Figure 7 it can be concluded that the dependence of the shift the frequency of the piezoelectric transducer resonant on the volume of the fluid on the surface of the spraying is varied for various fluids.Since a linear dependence of a resonance frequency shift on the fluid weight was previously revealed, it can be assumed that the coefficient connecting these dependencies will be the density of the fluid.

1ethanol, 2water, 3propanetriol
To exclude the need to set the density of sprayed fluid, preliminary testing of the sample can be carried out to determine the connection of the volume of the fluid at the spray end with a shift the frequency of the piezoelectric transducer resonant.The volume sprayed located on the surface of the spraying can be calculated, knowing the volume of the test sample corresponding to the change in the resonant frequency and the current change in the resonant frequency according to expression 1: Vm=(ΔF*Vtst)/(ΔFtst), (1) where Vm fluid sprayed volume, ∆F is shift the frequency of the piezoelectric transducer resonant, Vtst -test volume, ∆Ftst is shift the frequency of the piezoelectric transducer resonant from test.
Knowing the volume of the fluid on the surface of the spraying end and the area of the working surface of the spraying end we find the middle height of the fluid, using the expression 2.
h=Vm/S (2) where h is the middle height of the fluid, Vm is the measured volume, S the spray tool area.In the measured range of film thicknesses liquid, the shift the frequency of the piezoelectric transducer resonant can reach 100 Hz, which is sufficient to control the film thickness (sensitivity up to 1 Hz per 1 µm thickness).

Algorithm for controlling the spraying process
As it was said at the beginning of the article, to ensure the stability of the aerosol production process, it is necessary to maintain the optimal height of the fluids on the spray tool.We found that the film thickness can be determined by changing the resonant frequency of the ultrasonic oscillatory system.Figure 8 shows a block diagram of an algorithm for controlling the spraying process based on maintaining an optimal liquid height on the flatness tip.The block algorithm, the diagram of which is presented in Figure 8, is based on recording the shift frequency of the piezoelectric transducer resonant of the spray UOS during the spraying process.After turning on the device, while the spray end of the UOS is "dry", the natural frequency of the oscillating system is measured.After this, using a dispenser (which must be controlled centrally with an ultrasonic generator), a test volume of liquid is insert on the spray end and the natural frequency is measured.Based on the shift of the natural frequency from the test fluid, the amount of liquid injected and the area of the spray end, the coefficients for the flowing liquid are determined, determining the relationship between the resonant frequency and the height of the fluids film on the surface.After starting the spraying process, the device maintains the optimal (set for the current sprayed liquid) the height of the liquid film, when the fluid flow rate changes, the algorithm changes the vibrations amplitude of the working tool, maintaining the thickness of the film.
The proposed algorithm for the implementation of monitoring and control has a disadvantage associated with the dependence of the frequency of the ultrasonic sensor on the ambient temperature.Therefore, to implement a continuous spraying process, it is necessary to ensure thermal stabilization of the ultrasonic vibrating system or (which is more preferable) to introduce an adjustment to the natural resonant frequency of the ultrasonic vibrating system depending on its actual temperature.
Figure 9 shows a block diagram of a spraying installation that implements the proposed algorithm for controlling the spraying process.
1-ultrasonic oscillatory system, 2-spray liquid supply system, 3-measuring unit, 4-matching unit, 5-control microcontroller, 6-power stage, 7-controlled master oscillator, 8-controlled power source.The spraying installation consists of a spray system (1), an electronic ultrasonic generator including power cascades (6), measurement units (3) and matching (4), a controlled master oscillator (7), and a microcontroller (5) that implements the entire control algorithm generator and the spraying process itself.A dispenser ( 2) is used to supply the sprayed material during the spraying process and in test mode.The entire control algorithm (Figure 8) is implemented in a control microcontroller, which, based on information received from the measuring unit through a controlled power source, changes the amplitude of oscillations of the spray tip. Figure 10 shows a photograph of an experimental ultrasonic sprayer that implements a method for controlling the film thickness during the spraying process.Its main features are presented in Table 3. Due to the use of a spraying process control algorithm with automatic thickness maintenance, it was possible to automate the spraying process.Maintaining the middle height of the fluid film at an optimal level allows productivity to vary widely.

Conclusion
А method of monitoring of the height of the fluid on the working surface of the ultrasonic sprayer was proposed and developed, by a measurements shift frequency UOS.The possibility of practical implementation of control is confirmed by the results of research, which made it possible to establish that in the working range of the thickness of the film of sprayed liquids, a change in the resonance frequency can reach 100 Hz, which is sufficient to control the thickness of the film.
It is shown that the density of the liquid is the main physical factor that must be taken into account when determining the proportionality coefficient between height the film of liquid on the oscillating spray surface and the shift frequency of the piezoelectric transducer resonant.
This made it possible for the first time to develop a method for automatically controlling the ultrasonic spraying process, ensuring the maintenance of optimal modes of ultrasonic exposure (amplitude of vibrations of the spray surface) and the fluid height.The practical implementation of the proposed method made it possible to develop and manufacture an ultrasonic atomizer with automatic maintenance of the fluid height.The atomizer ensures the formation of liquid droplets with a productivity of up to 15 ml/s at an ultrasonic frequency of 22 kHz.
[16].Lm equivalent mass, Cm reciprocal value of the stiffness of the material (compliance), Rp losses, Rs acoustic radiation, C electrical capacitance of the piezoelectric rings, Rd losses in the piezomaterial

spray ending No. 1 spray ending No. 2 Figure 5 .
Figure 5. Dependence resonant frequency of UOS on the mass liquid for various spray ends, at frequency of the oscillating system is 22 kHz.spray ending No. 3

Figure 6 .
Figure 6.Dependence resonant frequency of the ultrasonic piezoelectric system depending on the mass liquid for various spray ends, at frequency of UOS 44 kHz.

Figure 7 .
Figure 7.The shift the frequency of the piezoelectric transducer resonant depending on the liquid volume.

Figure 8 .
Figure 8. Block diagram of the spraying process control algorithm.

Figure 9 .
Figure 9. Structural diagram of the atomizer with automatic maintenance of the sprayed fluid height.

Figure 10 .
Figure 10.Ultrasonic sprayer that automatically maintains the thickness of the sprayed fluid height.

Table 1 .
Characteristics of replaceable spray tips.

Table 2 .
Properties of model liquids.