Experimental investigation of conical bubble structure and acoustic flow structure in ultrasonic field

Accepted Manuscript Experimental Investigation of Conical Bubble Structure and Acoustic Flow Structure in Ultrasonic Field Xiaojian Ma, Biao Huang, Guoyu Wang, Mindi Zhang PII: DOI: Reference:

S1350-4177(16)30169-9 http://dx.doi.org/10.1016/j.ultsonch.2016.05.027 ULTSON 3237

To appear in:

Ultrasonics Sonochemistry

Received Date: Revised Date: Accepted Date:

27 November 2015 11 April 2016 18 May 2016

Please cite this article as: X. Ma, B. Huang, G. Wang, M. Zhang, Experimental Investigation of Conical Bubble Structure and Acoustic Flow Structure in Ultrasonic Field, Ultrasonics Sonochemistry (2016), doi: http://dx.doi.org/ 10.1016/j.ultsonch.2016.05.027

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Manuscript Number: ULTSON-D-15-00752R2 Title: Experimental Investigation of Conical Bubble Structure and Acoustic Flow Structure in Ultrasonic Field Article Type: Full Length Article Keywords: Conical bubble structure, bubble motion, acoustic flow field, high-speed camera, PIV Corresponding Author: Prof. Biao Huang ([email protected]) Corresponding Author's Institution: School of Mechanical and Vehicular Engineering, Beijing Institute of Technology, Beijing 100081, China. First Author: Xiaojian Ma Order of Authors: Xiaojian Ma; Biao Huang; Guoyu Wang; Mindi Zhang Co-Author's Institution: School of Mechanical and Vehicular Engineering, Beijing Institute of Technology, Beijing 100081, China. Abstract: The objective of this paper is to investigate the transient conical bubble structure (CBS) and acoustic flow structure in ultrasonic field. In the experiment, the high-speed video and particle image velocimetry (PIV) techniques are used to measure the acoustic cavitation patterns, as well as the flow velocity and vorticity fields. Results are presented for a high power ultrasound with a frequency of 18 kHz, and the range of the input power is from 50W to 250W. The results of the experiment show the input power significantly affects the structures of CBS, with the increase of input power, the cavity region of CBS and the velocity of bubbles increase evidently. For the transient motion of bubbles on radiating surface, two different types could be classified, namely the formation, aggregation and coalescence of cavitation bubbles, and the aggregation, shrink, expansion and collapse of bubble cluster. Furthermore, the thickness of turbulent boundary layer near the sonotrode region is found to be much thicker, and the turbulent intensities are much higher for relatively higher input power. The vorticity distribution is prominently affected by the spatial position and input power.

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1. Introduction The commercial value of high intensity ultrasound energy has caused wide attention in many fields. The pressure change induced by propagation of ultrasonic waves can generate a mass of the micro bubbles in liquid. When bubbles expand and collapse close to the boundary, the jetting is generated which is able to remove particles from the surface, namely the local surface cleaning [1]. Additionally, the extreme condition of high temperature (up to 10,000K) and high pressure (up to 1,000atm) [2] caused by the collapse of cavitation bubbles can be also employed in various industries, such as wastewater treatment [3], extraction [4], chemical reaction [5], heat transfer [6], changes in morphology of the biological cells [7]. However, the implementation of the ultrasonic cavitation is still limited due to the obstacles, such as broad-band noise [8], serious cavitation erosion [9], and low mixing coefficient [10]. To further solve the problems and optimize the ultrasound application, more detailed and comprehensive investigation of the cavitation bubble pattern and acoustic flow structure should be conducted. The early studies on the ultrasonic cavitation pattern has been reported that cavitation bubbles can self-organize into special cavitation structures (i.e. conical bubble structure (CBS), acoustic Lichtenberg figure, streamers, smokers, starfish, bows, webs, rings and jellyfish) [11-13]. Among the various ultrasonic cavitation structures, the studies on CBS generated by sonotrode have made significant progress in understanding and modeling complex multiphase flow. In recent years, a number of theoretical analysis and numerical simulations on comprehensive knowledge of mechanism of cavitation bubbles motion in ultrasonic field have been carried out [14-16]. To further obtain much effective data and improve the understanding of the CBS, various experimental studies were conducted. Moussatov et al. [17] used the digital photo camera to obtain the CBS images at 20.7 kHz. They showed the CBS is composed of moving 2

bubbles which undergo attractive and repulsive Bjerknes forces caused by high acoustic pressure gradients and strongly nonlinear oscillations of cavitation bubbles, and the change of the sonotrode diameter and acoustic intensity will result in significantly change of CBS. Mandroyan et al. [18] employed laser tomography technique to examine the modification of CBS by the presence of an electrode at 20 and 40 kHz. They illuminated that the presence of the electrode largely influences the bubble cavitation zones. Bai et al. [19] conducted the measurements of the CBS in the magnitude and the direction of acoustic radiation forces by the tailing method. They demonstrated the different structures of CBS result from distinct vibration modes. Especially, Luo et al. [20] and Bai et al. [12] employed high-speed camera to investigate the bubble layer pattern and bubble motion on the radiating surface. The results showed bubbly branch-like structure and bubble layer are thicker with the increase of the input power. Furthermore, some large bubbles in bubble layer align themselves along a line and numerous small bubbles are attracted by large bubble. Those visualization techniques mentioned above show some information about the CBS pattern, but not to allow fluid field to be quantitatively researched. Recent years, particle image velocimetry (PIV) technique have been used by the researchers to measure acoustic flow field. Frenkel et al. [21] used PIV to investigate flow fields induced by ultrasound. They found a positive linear relationship between the ultrasonic intensity and the peak velocity. Chouvellon et al. [22] used PIV to observe the effect of the power, the water height, and the fluid viscosity on velocity of acoustic flow. Especially, velocity was found to increase in a nonlinear way with the electric power. Layman et al. [23] employed the synchronized PIV and IR thermograph to investigate the correlation between temperature and acoustic flow field at 20 kHz. They illuminated that a relatively small change in viscosity of flow can alter the heat generation. 3

Mandroyan et al. [24] used PIV to measure the acoustic flow velocities with the presence of the electrode at 20 and 40 kHz. They found the presence of the electrode is an obstacle to the axial flow. And they also found the maximal velocity is higher for the smallest horn diameter. Although cavitation structure and acoustic flow field have received much attention in the past years, the complex structure and flow field of CBS are still not well understood, and hence additional researches are still needed. In this paper, a high-speed video camera is used to capature the characteristics of CBS patterns. The PIV technique is used to determine the velocity of acoustic flow fields. The objectives of the present study are to (1) improve the understanding of the patterns of CBS and the transient bubble motion via combined different input power case, (2) investigate characteristics of acoustic flow structure in capturing the time-average velocity and vorticity field versus different input power. 2. Experimental Setup In present experiment, high power ultrasound is produced by ultrasonic processor (Institute of Acoustics, Chinese Academy of Science, China) with a frequency of 18 kHz, and the range of the input power is from 50W to 250W, which is measured by an electrical power meter. An electronic circuit consisting of variable inductances and capacitors allows the adjustment the impedance of processor and sonotrode. The sonotrode has a plane radiating face whose diameter d is 20 mm and oscillated as a piston with simple harmonic motion. Fig. 1 shows schematic diagram depicting the experimental setup for investigating conical bubble structure under the sonotrode. The experimental setup consists of the ultrasonic cavitation devices, the imaging and illumination system. The sonotrode is submerged into a water tank (320.0 mm × 240.0 mm × 200.0 mm), whose bottom is made by the absorbent material to reduce the acoustic wave reflection. The distance between the sonotrode and the bottom of the 4

water tank is 8.3d (about two times that of the wave length in water). To obtain better cavitation phenomena, fresh tap water with many nuclei is used in present experiment so as to reduce the cavitation threshold. Compared to that in pure water, the similar results can be obtained in tap water but with more cavitation bubbles [19]. The temperature of water in the tank is maintained at 25℃. Furthermore, a high-speed camera (HG-LE, by Redlake) is used to capture cavitation phenomena at framing rates as high as 100,000 frames per second (fps). A lower recording speed of 3000 fps and 5000 fps is adopted to maintain the desired spatial resolution in present study. The shooting angle of the high-speed camera can be adjusted for a better photographic effect. A piece of glass is placed between the camera and the light source in order to form a better light distribution in the background. Moreover, the PIV is used to measure the acoustic flow fields and its arrangement is shown in Fig. 2. The light source is a dual head, Nd: YAG laser where the beam has been expanded to a 1mm wide sheet and 532 nm wave lengths (green spectrum). For the actual data acquisition, the CCD-cameras used here are with a resolution of 1024×1024 pixels. The water is seeded with special fluorescent 70µm ceramic micro spheres as “tracer particles”, which can avoid precipitating and maintain floating effectively. The commercial PIV-software Dynamics Studio is used to process the velocity vector fields with the interrogation areas of 32×32 pixels and 50% overlap in general. In order to remove the erroneous vectors, multiple filters have been employed in the PIV post-process by specifying the relative tolerance. 3. Results and Discussions 3.1 The effect of input power on CBS pattern Fig. 3 presents the typical conical bubble structure (CBS) under four different input power of Pin=50W, 100W, 200W, and 250W, respectively. It can be observed that an inverted cone-like 5

bubble structure is generated under the sonotrode, and the shape of the cavity zone is significantly different at distinct input power. In order to compare the characteristics of CBS caused by different sonotrodes, the parameter I is defined that the ratio of the active power delivered to the sonotrode divided by the radiating surface area. If the power losses in the sonotrode are neglected, I is the average acoustic intensity at the radiating surface. Fig. 4(a) gives schematic interpretation about definition of length and diameter of CBS and Fig. 4(b) illuminates the corresponding dimensionless variable (denoting length or diameter of CBS divided by d) compared with the result in the reference [17]. The error bars mark the fluctuation span of length and diameter of CBS. As observed, similar trends can be observed between the current results and pervious experimental measurement (Moussatov et al, 2003) under different input power: with the increase of input power, the diameter of CBS on the radiating surface increases evidently, whereas the length of CBS from the top to the endpoint of the cone becomes shorter. Fig. 5 shows the time evolution of the normalized cavity area. An example of a typical cavitation pattern observed at Pin= 100W is presented in Fig. 5(a), which includes both the original CBS visualization illuminated by high-speed camera and the schematic interpretations drawn by an in-house feature-recognition software package [25]. Fig. 5(b) presents the time evolution of the normalized cavity area under different input power. Compared to the cavity area for relatively lower power (Pin=50 W and Pin=100 W), the cavity area seems more unstable and has larger oscillating amplitude as time changes for relatively higher power (Pin=200W and Pin=250W). Overall, the increase of input power results in substantial increase in the cavity area. The trend of unstable and oscillating cavity area at different input power may be consistent with the report about broad-band noise by Price et al. [26] that the acoustic emission increases with

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the increasing input power. This is mainly because temporal fluctuation of the cavity area may result in the broad-band noise [8]. Fig. 6 shows the typical motion of the cavitation bubble under different input power. It is calculated by the optical flow technique which is an image-based method for computing the flow field. An assumption is proposed on the continuity equation, resulting in optical flow energy of the form [27]:

E (u x , u y ) = ∫ It + div( Ii u x , u y ))2 d Ω + α R(ux , u y ) Ω

(1)

where Ω is the spatial domain, Ii is the image intensity, R(ux, uy) is an appropriate regularization scheme, and α > 0 is the regularization parameter. As observed, the straight lines represent the calculated vectors of cavitation bubbles taking 0.33ms and the small dots are the terminal points for bubble motion. When input power is 50 W and 100 W, cavitation bubbles can be observed to move toward a fixed point on the symmetry axis away from the sonotrode, thus forming the CBS. However, when input power is 200W and 250W, some cavitation bubbles cannot focus at the fixed point, diffuse significantly and continue to move downstream, though mass cavitation bubbles still form a rough cone-like structure. For the velocity of bubble motion, with increase of the input power at range from 50W to 250W, the velocity of cavitation bubbles has an apparent increase, as shown in Fig. 7. The error bars mark the fluctuation span of velocity. Those observations are different with the previous reports by Yasui et al. [28] that some bubbles move toward the horn tip due to the secondary Bjerknes force acting from the bubbles near the horn tip. 3.2 Transient motion of bubbles on radiating surface Fig. 8(a) shows bubble layer on radiating surface for different input power from side view and Fig. 8(b) gives schematic presentation of bubble layer. When the input power is 50W, several 7

separate large cavitation bubbles (the diameter dl= 0.1mm ~ 0.3mm) and numerous small cavitation bubbles (ds<< 0.1mm) are adsorbed on the radiating surface. With the input power increasing to 100W, more cavitation bubbles on radiating surface appear and begin to form thin and incomplete bubble layer. Increasing the input power to 200W stretches the bubbly area, raises the bubble density and enlarges the bubbles size. When input power is 250W, the radiating surface is more and more densely covered with bubble clusters composed of many small cavitation bubbles. Meanwhile, the spatial bubble distribution looks more or less inhomogeneous and the bubble layer becomes thicker with increase of the input power. This phenomenon may account for the variable diameter and length of CBS at different input power as mentioned in section 3.1. The increasing thickness of bubble layer may change the pressure distribution and focus at shorter distance at the axis [29]. The acoustic field which has been changed by the focusing action makes CBS transformation [30] i.e. shortening the length and enlarging the diameter of CBS. In order to gain a detail understanding of the transient motion of the cavitation bubbles on the radiating surface, Fig. 9 and Fig. 10 illuminate two different types of cavitation bubble motion, respectively. Fig. 9(a) shows transient motion of the cavitation bubbles at 50W and Fig. 9(b) gives schematic presentation of cavitation bubble motion. As observed, a branch-like structure pattern is generated by vibrating surface, which is composed of several moving large cavitation bubbles, such as bubble A ~ D, and numerous small cavitation bubbles. Those filamentary branches on the radiating surface are relatively stationary in a certain period. Especially, when t = 0ms, the small cavitation bubbles align themselves, move in the form of a certain route, and form the large cavitation bubble A, B, C and D, respectively. Then those four large cavitation bubbles move to the center of the radiating surface at t = 0.2ms, and they reach and aggregate at 8

the center of the radiating surface when t = 0.4ms. Eventually, all of them coalesce into large cavitation bubble E at t = 0.6ms. In the process, bubbles motion attracting each other and coalescing can be explained quite well by secondary Bjerknes force in the condition that bubbles oscillate in phase [31]. Due to the high density distribution of bubbles, the separation distance between cavitation bubbles is very small. Arising secondary Bjerknes forces between neighboring bubbles cause them to merge in the acoustic field. Fig. 10 (a) shows the transient behaviors of the bubble cluster at 250W and Fig. 10(b) gives the schematic presentation of behaviors of bubble cluster to further highlight the process. The bubble cluster is composed of many small cavitation bubbles, which is shown inside the white dotted circle, when the input power is 250W. As observed, the position of bubble cluster trapped on the periphery of radiating surface remains stationary, and the bubble cluster undergoes an oscillating process: coalescence, shrink, expansion, and collapse. When t = 0ms, numerous small cavitation bubbles aggregate and coalesce into a bubble cluster. Then the diameter of bubble cluster shrinks into about 1mm at t = 0.2ms. When t = 0.4ms, bubble cluster grows up and forms a bubbly sphere whose diameter is about 1.5mm. Finally the spherical bubble cluster transiently collapses and generates a high density distribution of small cavitation bubbles at t = 0.6ms. 3.3 Acoustic flow structure In the experiments, the acoustic flow fields are measured by PIV images, corresponding to a mean flow displacement of 2% of the view field (80.0 mm × 60.0 mm) between captures. Based on the measured instantaneous velocity fields, the 100 instantaneous PIV image pairs are captured to minimize the measurement uncertainties and the interval between the image pairs is 400µs.

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It is well known that cavitation bubbles have a profound effect on the flow structure [32, 33]. Hence,

to

better

understand

the

effect

of

unsteady cavitation

structure on

the

production/alteration of the velocity field, the spatial distribution between CBS and acoustic flow structure under different input power is provided in Fig. 11. Here, x/d denotes the dimensionless radial distance from the central axis of the sonotrode, y/d is the dimensionless axial distance from the sonotrode, and red dotted triangle represents CBS (data from Fig. 4). It could be observed that the highest velocity of flow structure is presented inside CBS under the sonotrode at each input power, which indicates the velocity distribution of acoustic flow structure is closely related to the presence of cavitation bubble structure. To quantitatively investigate the effect of unsteady cavitation structure on the production/ alteration of the velocity field, the turbulence intensity It, an essential parameter to measure mixing intensity for industrial demand, is investigated. Fig. 12 shows average amplitudes of the turbulence intensity along the central axis of sonotrode (x/d = 0) for different input power. Here, turbulence intensity It is defined as:

I t = u '2 + v '2

(2)

and 1 n u' = ∑ (ui − u ) n − 1 i =1

2

2

v '2 =

1 n ∑ (vi − v) n − 1 i =1

(3)

2

(4)

where u’ and v’ are the horizontal and vertical components of the turbulent velocity fluctuations;

u and v are the horizontal and vertical components of the mean velocity; n is the total of data samples. When the input power is 50W, the turbulent fluctuations are very limited and are 10

confined to a very thin boundary layer near the sonotrode. Enlarging the input power to 100W and 200W increases the thickness of turbulent layer evidently. For input power Pin=250W, turbulence intensity reaches a largest scale value of 0.9m/s and the turbulent boundary layer increases to 0.75d near the sonotrode. The increasing trend of the turbulent boundary layer thickness is represented by the dotted line in Fig. 12. The bubble layer on the radiating surface, discussed in Section 3.2, are much thicker for Pin = 200W and 250W, which leads to a much thicker turbulent boundary layer compared to Pin= 50W and 100W. Fig. 13 shows comparisons of the out-of-plane (z-component) vorticity contours for different input power are shown. Here, ω z is defined as:

ω z = ∂v / ∂x − ∂u / ∂y

(5)

As expected, a pair of symmetrical vortical zones, with positive and negative direction respectively, is formed near the sonotrode. High levels of vorticity are observed around the cavity boundary for different input power. The vortical zones behave with obvious stage characteristics with increase of the input power. Firstly, the vortical zones becoming much longer with the increase of input power, and extend downstream. Secondly, the position where the vortex is formed, indicates the inherent relation between vortex and the cavitation area. To further understand the spatial distribution of the vorticity field, the variations in averaged vorticity with different monitoring locations for ultrasound power of 250W are presented in Fig .14(a). Five monitoring locations are chosen, along the central axis of the sonotrode at 0.5d intervals, namely y = 0.5d, d, 1.5d, 2d, and 2.5d. As observed, the highest vorticity is observed for regions closer to the sonotrode (y=0.5d), which gives the absolute value of 175s-1. However, the absolute value of vorticity decreased to almost 50s-1 near the bottom of the tank (y=2.5d). The position where direction of the vorticity changes occurs under the central sonotrode and the 11

absolute vorticity decreases to 0s-1. Moreover, the region beside the sonotrode can be hardly affected by the acoustic flow structure. Fig. 14(b) presents the vorticity profiles corresponding to the monitoring location of y=0.5d under different input power. As observed, the absolute value of vorticity increases from 25s-1 to 175s-1 as the input power increases from 50W to 250W. Therefore, the vorticity under different input power is prominently affected by input power. According to the PIV results, the absolute value of vorticity with various input power has the maximum vorticity of 175s-1 in the system. Besides, the absolute vorticity at the highest power (250W) is almost 9 times greater than that of the lowest power (50W) which is about 20s-1. 4. Conclusions Experimental studies are presented for conical bubble structure (CBS) in the ultrasonic field under different input power. High-speed videos of the evolution of the cavitation structures and PIV measurements of the velocity and vorticity fields are used to investigate the flow structure. The primary findings include: （1）

The results of the experiment show that different input power results in distinct

characteristics of CBS. With increase of the input power, the diameter of CBS on the radiating surface increases evidently, but the length of CBS from the top to the endpoint of the cone becomes shorter. The cavity area enlarges evidently with increase of the input power. For the velocity of bubble motion in CBS, with increase of the input power at range from 50W to 250W, the velocity of cavitation bubbles significantly increases (approximately 0.2~3.5m/s). （2）

Input power induces different bubble layer and bubble motion on the radiating surface.

The increasing input power results in thicker bubble layer, which may change the pressure 12

distribution, shorten the length, and enlarge the diameter of CBS. In addition, two different modes of bubble motion are found, namely, formation, aggregation and coalescence of cavitation bubbles, and aggregation, shrink, expansion, and collapse of bubble cluster. （3）

Cavitation structure contributes to the acoustic flow distribution. Unsteady cavitation

structure is an important source of velocity production and modification, which leads to significant increase in the turbulence level and turbulent boundary layer thickness. Furthermore, the vorticity distribution is prominently affected by the spatial position and input power. Acknowledgments The authors gratefully acknowledge the support by the National Natural Science Foundation of China (Grant Nos. 51306020 and 51479002), and the Open Foundation of State key Laboratory of Hydraulic and Mountain River Engineering (Sichuan University, China). References [1] C.D. Ohl, M. Arora, R. Dijkink, Surface cleaning from laser-induced cavitation bubbles, Appl. Phys. Lett. 89 (2006) 074102. [2] B. Sajjadi, A.A.A. Raman, S. Ibrahim, Influence of ultrasound power on acoustic streaming and micro-bubbles formations in a low frequency sono-reactor: Mathematical and 3D computational simulation, Ultrason. Sonochem. 24(2015) 193-203. [3] N.N. Mahamuni, Y.G. Adewuyi, Advanced oxidation processes (AOPs) involving ultrasound for waste water treatment: A review with emphasis on cost estimation, Ultrason. Sonochem. 17 (2010) 990-1003.

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Figure legends Fig. 1. Layout of high-speed video camera system

Fig. 2. Arrangement of the PIV system

Fig. 3. CBS under different input power. Frame rate: 3000fps.

Fig. 4(a) Schematic interpretation about definition of length and diameter of CBS; (b) dimensionless variable (length or diameter of CBS divided by d) against input power compared with the result in the reference [17]. The error bars mark the fluctuating span of length and diameter of CBS.

Fig. 5. (a) Typical CBS visualization and schematic interpretation, and (b) the time evolution of the normalized cavity area under different input power.

Fig. 6. Vectors of the cavitation bubbles under different input power. The straight lines are the calculated vectors of cavitation bubbles taking 0.33ms and the small dots are the terminal points for bubble motion.

Fig. 7. The velocity of cavitation bubbles under different input power. The error bars mark the fluctuation span of velocity.

Fig. 8. Bubble layer on radiating surface for different input power with frame rate of 5000fps.

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Fig. 9. (a) Consecutive images about motion of the cavitation bubbles on radiating surface at 50W with frame rate of 5000fps, and (b) the schematic presentation of cavitation bubble motion.

Fig. 10. (a) Consecutive images about the coalescence, shrink, expansion and collapse of bubble cluster on radiating surface at 250W with frame rate of 5000fps, and (b) the schematic presentation of behaviors of bubble cluster.

Fig. 11. Spatial distribution of CBS and acoustic flow structure under different input power.

Fig. 12. Average amplitudes of the turbulence intensity at x/d = 0 along the central axis of sonotrode for different input power. The dotted line represents the turbulent boundary layer thickness.

Fig. 13. Average z-vorticity fields and streamlines under different input power.

Fig. 14. Distribution of vorticity magnitude (a) at different monitoring location under input power of 250W, and (b) under different input power at monitoring location of y=0.5d.

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Figure.2.

Figure.3.

Figure.6.

Figure.1.

Figure.4.

Figure.5.

Figure.7.

Figure.8.

Figure.9.

Figure.10.

Figure.11.

Figure.12.

Figure.13.

Figure.14.

The effects of input power on conical bubble structure are investigated by experiments. Two different modes of transient bubble motion are clarified. The ultrasonic cavitation structure effects to the acoustic flow distribution.

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