Numerical Simulation of Condensation of Natural Fog Aerosol under Acoustic Wave Action

Chang Liu, Yun Zhao, Zhangfu Tian, Hefeng Zhou
2020 Aerosol and Air Quality Research  
10 Acoustic condensation has proved its excellent performance in the dissipation of suspended fine 11 particles. It is also promising for artificial fog dissipation, but the current research is still lacking. 12 This paper sums up the mathematical model of acoustic condensation. Considering the physical 13 properties of droplets, the improved root mean square (IRMS) acoustic condensation kernel 14 function is proposed, which is compared with the analytic solution to verify the correctness. 15
more » ... rough the method of numerical simulation, different condensation effects of the different 16 condensation mechanisms are compared. Study the effect of acoustic frequency, sound pressure 17 level and initial concentration on the condensation of natural fog aerosol under the action of 18 sound waves. The results show that the droplet condensation efficiency can be improved by 19 acoustic effect significantly; The effect of acoustic frequency on droplet condensation is 20 nonlinear, and there is an optimal frequency of 600 Hz. Besides, high-frequency sound wave has 21 a better effect on small particles' condensation, while low-frequency sound wave has a better 22 effect on large particles' condensation. The sound pressure level and initial particle 23 concentration is positively correlated with the condensation efficiency, and there is a critical 24 particle size. After the acoustic condensation, the concentration of particles larger than the critical 25 particle size increases, and decreases conversely. 26 Fog weather has caused a great negative impact on people's daily life, it not only does harm 31 to people's health, but also makes visibility seriously reduced, result in great safety risks on 32 people's traffic travel. Traditional artificial fog dissipation can be divided into two specific 33 operations: artificial warm fog dissipation and artificial cool fog dissipation (Gao et al., 2010). 34 However, the traditional fog dissipation method is highly energy-consuming and corrosive, which 35 is easy to cause secondary pollution, does more harm than good. Acoustic wave fog dissipation 36 refers to use acoustic wave to radiate a fog field, so that droplets contact and collide with each 37 other, condense into larger particles, achieve the effect of fog dissipation. Compared with the 38 traditional fog dissipation method, Acoustic wave fog dissipation is a more promising 39 technology. 40 In 1931, Patterson and Cawood (1931) first discovered acoustic agglomeration in the 41 laboratory. In 1936, the Faraday Society in London held a meeting to study the feasibility of 42 using sound waves for fog removal. During World War II, acoustic agglomeration technology 43 was used in airports-defogging (Chou, 1980) to improve visibility over runways and airports. In 44 1963, Zhang et al. (1963) conducted a preliminary experimental study on the dispersing of water 45 mist by sound waves. 'Rain gushes after lightning' is known as a sound-induced condensation 46 acceleration phenomenon in nature. In 1964, Moore et al. (1964) recorded, measured, discussed 47 and analyzed this phenomenon, and concluded that sound waves played an auxiliary role in 48 promoting cloud condensation, while the discharge phenomenon of lightning promoted the 49 A C C E P T E D M A N U S C R I P T 3 mutual attraction between charged droplets, which was the main cause of this phenomenon. In 50 1965, Mednikov (1965) published the first book on acoustic agglomeration, summarized the 51 previous research work of scholars in various countries, firstly established a systematic theory to 52 explain the phenomenon of acoustic agglomeration, put forward the famous agglomeration 53 mechanism (Orthokinetic Interaction Mechanism). The concept of agglomeration volume was 54 introduced which refered to the area around a large particle that can agglomerate with a small 55 particle, if the small particles appear in the agglomeration volume of a large particle, the collision 56 and agglomeration can occur due to the difference of the entrainment coefficient between the two 57 particles and the velocity difference. In 1979, Shaw and Tu (1979) adopted two acoustic sources, 58 a low-frequency electro-acoustic horn (1~3 kHz) and a high-frequency horn (10~20kHz), to 59 conduct acoustic agglomeration experiments on monodisperse aerosols, and found that the 60 agglomeration effect was better at low frequencies, while the acoustic attenuation was more 61 serious at high frequencies. Rajendran et al. (1979) compared the agglomeration effect of aerosol 62 under the flow and static conditions, and found that the agglomeration effect was reduced under 63 the flow condition. He believed that the reason was the turbulence generated by the sound wave 64 was suppressed by the airflow, so that the agglomeration caused by turbulence was reduced. In 65 1995, Caperan et al. (1995) studied the entrainment effect of ultrasonic field on ethylene glycol 66 aerosol. The results show that sound condensation is affected by the amplitude of sound wave and 67 is proportional to the quadratic square of the amplitude of sound wave. 68 Into the 21st century, Acoustic wave condensation technology has been widely used in 69 suspension particle elimination(Hoffmann, 2000; Liu et al, 2009; Fan et al, 2013). In 2002, Hou 70 A C C E P T E D M A N U S C R I P T 4 et al. (2002) investigated the dissipation effect of low-frequency sound wave (< 50Hz) on water 71 fog, and qualitatively concluded that lower frequency and higher sound intensity are conducive to 72 the dissipation of water fog. In 2013, Peng (2013) carried out a study on the mechanism of oil 73 mist condensation by ultrasonic wave through numerical simulation. In the simulation results, the 74 condensation effect of particles becomes better with the increase of temperature, and worse with 75 the increase of pressure. The increase of ultrasonic frequency is not conducive to the 76 condensation of particles. The coagulation effect is the best when the ultrasonic frequency is at 20 77 kHz. As the coagulation effect decreases monotonously with the increase of the acoustic 78 frequency in the simulation process, 20 kHz is not the optimal frequency, and whether the 79 ultrasonic wave is a favorable frequency band for acoustic coagulation also remains to be 80 considered. Sahinoglu st al. (2013) studied the condensation effect of ultrasonic condensation on 81 bubbles in liquid, and proposed a visualization method to describe the molecular trajectory of the 82 aggregation process. In 2015, Tamara and Svetlana (2015) applied the acoustic condensation 83 method to clouds and discussed the theoretical basis and feasibility of acoustic method to enhance 84 rainfall. In 2018, Yu (2018) carried on the simulation and experiment of the ultrasonic 85 condensation process of water mist, obtained that standing wave sound field is more likely to 86 cause hydromechanical condensation between particles 87 At present, there are a lot of researches on the emission reduction of fine particle, the 88 theoretical and experimental research on condensation of water mist (droplet) also has a certain 89 basis, However, there are few studies on acoustic wave condensation for fog aerosol, especially 90 under natural conditions. Fog drops have larger particle size, wider distribution spectrum, and 91 A C C E P T E D M A N U S C R I P T 5 different physical and chemical properties. At present, the known condensation mechanisms 92 include orthokinetic interaction mechanism, acoustic wake mechanism, mutual radiation 93 mechanism, gravity sedimentation mechanism and Brownian motion mechanism. Among them, 94 the orthokinetic interaction mechanism is the most important condensation mechanism (Liu, 2009; 95 Song, 1990). In the existing literature, most of the simulation of condensation process only uses 96 one or two mechanisms for model establishment (Hoffmann and Koopmann, 1996; Hoffmann, 97 1997; Gonzalez et al, 2000; Cleckler et al, 2012), but in the actual condensation process, five 98 mechanisms exist simultaneously, interact with each other and depend on each other(Wu, 2014). 99 In order to make the acoustic condensation technology economically applied in the field of 100 defogging, it is necessary to study the influence of each parameter in the acoustic condensation 101 process on the condensation effect, find out the "optimal operating parameters", make it work 102 under the best conditions and reduce energy consumption, so that the acoustic condensation 103 technology may become a practical technology. In the existing literature, the frequency range of 104 acoustic waves adopted by researchers is very large, ranging from as low as 20 Hz to as high as 105 30 kHz (Zhang et al, 1963; Hou et al, 2002; Yu, 2018) . In these studies, various acoustic 106 frequencies all produce a certain condensation effect, so the study of the optimal frequency is still 107 controversial. 108 In this paper, the present condensation kernel functions is fused and improved, a more 109 practical mathematical model is proposed, and each condensation influence parameter is 110 discussed, finally the optimal condensation parameters based on the natural fog condition are 111 obtained. 116 The agglomeration process of fine particles can be simplified as Fig. 1 , which shows the 117 particle motion and condensation situation within a sound wave period in order from (a) to (e). 118 119 A C C E P T E D M A N U S C R I P T 36 concentration decreases with sound effect, above critical size, particle concentration inecreases 645 with sound effect, this is because small particles condense into larger particles, largre particles 646 settle, finally achieve the effect of fog dissipation. Speed of condensation of small particles into 647 larger particles is faster than sedimention speed of large particles. From the overall (Fig. 5. (b) ), 648 the number of large particles is rising, the total concentration is still falling. 649 650 CONCLUSIONS 651 652 This paper proposes the IRMS condensation kernel function, considers orthokinetic 653 interaction mechanism, acoustic wake effect mechanism, mutual radiation mechanism, gravity 654 sedimentation mechanism and Brownian motion mechanism, and fuses them through the method 655 of the root mean square, so that the simulation performance has been improved and more in line 656 with reality. In the simulation process, the measured data of natural fog is selected as the research 657 object, which makes up for the lack of simulation data of natural fog condensation. Through the 658 control variable method, the effects of the sound wave frequency, sound pressure level and initial 659 droplet concentration on the sound wave condensation are studied. First of all, the effects of 660 frequency on condensation efficiency is not linear, there is a optimal frequency 600 Hz, and 661 particle sizes has different sensitivity to different frequency, high frequency has good effect on 662 small particles, and low frequency has good effect on large particles. The particle size distribution 663 diagram is bimodal after the sound wave; Secondly, the sound pressure level is positively 664 correlated with the efficiency of condensation. The higher the sound pressure level, the better the 665 condensation effect. In view of the high energy consumption and strong vibration characteristics
doi:10.4209/aaqr.2020.06.0361 fatcat:pnfddvktcrg6fjs7b3cbrrcqsa