Classification of regimes determining ultrasonic cavitation erosion in solid particle suspensions
Introduction
Cavitation was first described and investigated by Barnaby [1] and Parsons [2] in their analysis of the failed sea trial of an 1885 fast battleship, for which they found cavitation erosion (CE) to be responsible. Since those early investigations, researchers in numerous fields have examined cavitation and the damage it causes to hydraulic structures and machines, ship propellers, artificial heart valves, and the horn-type sonochemical reactors used for wastewater treatment [3], [4], [5], [6], [7]. For instance, the eroded tips of poorly-maintained horns often fail to vibrate within the desired frequency range, thus attenuating energy transmission or even overloading the ultrasonic generators. Frequent horn-tip replacements also increase operating costs, posing a real challenge to the scale-up of sonochemistry [8]. In general, cavitation involves asymmetrical bubble collapse near an extended solid surface that generates intense shock waves and micro-jets, causing significant damage to the surface. The cavitation process can be complicated by the inclusion of additives (e.g. polymers, microorganisms, gas bubbles, solid particles, and corrosive chemicals [9], [10], [11]) in an ambient liquid medium such as industrial wastewater. These additives may interact with the cavitation bubbles and alter the physical or chemical properties of the liquid, thus influencing bubble nucleation, growth, and collapse. Much research in recent years has focused on the synergistic effects of cavitation bubbles and solid particles on the erosive wear of metals in inert liquids [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]; however, opinions remain divided on the hypothesized role the solid particles play in the erosive wear caused by cavitation.
Early investigators assumed that the rough surfaces of solid particles would provide additional nucleation sites for the formation of cavitation bubbles, a hypothesis that was supported by the enhancement of sonochemical yields in the presence of insoluble solids [24], [25]. Over the past two decades, the microscopic interactions between a solid particle and an adjacent (or attached) collapsing bubble were visualized with the aid of high-speed photography [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. Photographic observations have shown that a particle, during bubble collapse or rebound, can be significantly accelerated before striking a solid surface. In this impingement process, the particle trajectories can range from perpendicular to grazing angles, which causes more damage to the abraded surface than the rolling contact of particles driven solely by turbulent flow. Moreover, it was envisaged that particles in sufficient quantities could modify a liquid's macroscopic properties such as viscosity [36], [37], [38], thus shielding the solid surface from the destructive effects of impinging particles or micro-jets [39].
The demonstrated damaging yet protective actions of solid particles during cavitation complicate the full characterization of CE, as well as its prediction and prevention. Accordingly, CE in particle suspensions (CEP) has been dealt with in three, apparently distinct, fields that consider different mechanisms under various conditions: (a) bubble nucleation and bubble dynamics, (b) fluid and particle mechanics, and (c) the rheology of solids suspensions. Experiments conducted and conclusions drawn within these fields often ignore one another, and limited evidence is available to show when and how research findings in these fields might be reconciled or overlap.
Among numerous factors that can influence CEP, the suspended particle properties such as shape, hardness, density, size (particle diameter dp), and concentration (volume fraction φp) have all been the subjects of considerable research interest over the past decade. Chen et al. [19] revealed that irregularly shaped micro-particles (5 μm, 0.08 vol%) could make indentations in the solid surface and produce a higher surface roughness than spherical micro-particles, although the particle shape had little to do with the number of collapsing bubbles. Their team also observed a reduction in the degree of material loss for small spherical SiO2 particles with increasing particle size (0.3–23.34 μm) [20]; however, this result was limited to very dilute suspensions. Hu and Zheng [21] reported an increase in mass loss with the φp of large sharp-edged sands (152 μm) varying from 1.2 to 4 vol%, despite a reduction in the mass loss at a low φp (0.2–1.2 vol%) and a slight decrease in the roughness when φp exceeded 2 vol%. Wu and Gou [22] recognized a clear distinction between small (26–43 μm) and large (63–531 μm) river-sand particles (1–3.3 vol%) with respect to their aggravating or alleviating effects on material removal, a result that was further validated by Wang et al. [23]. Although these findings have attempted to consider the role of solid particle size and concentration in the characterization of CEP, limited work has been conducted to clearly identify the relationship between CEP and either the particle concentration or size (or both) while excluding the potential influences of other factors, particularly the particle shape.
The purpose of the research presented herein is to classify the regimes that determine CEP via vibratory tests, and to describe and examine the underlying mechanisms that lead to erosion aggravation or alleviation. Based upon orthogonally designed experiments, we investigate the effects of the concentration and size of monodisperse spherical silica particles on CEP, and then analyze the experimental results with several parameters characterizing different regimes.
Section snippets
CEP test facilities and procedure
Vibratory CEP tests were performed with a piezoelectric device (VCY–1500, Shanghai Y&Y Sonic) operating at 20 ± 0.5 kHz and 1000 ± 50 W (Fig. 1a). The replaceable specimen threaded into the horn tip was axially oscillated with a peak-to-peak amplitude of 50 ± 2.5 μm. The test surface of the specimen was immersed to a depth of 2 mm in an agitated solid particle suspension in a cylindrical Perspex vessel (R = 100 mm) that was filled to a height of H = 100 mm. The suspension temperature in the
Results
The measured mass loss was divided by the density of the carbon steel to obtain the volume loss, which, in turn, was divided by the specimen surface area to determine the mean depth of erosion. Material removal from the specimens increased with the exposure time for suspensions of small, medium, or large silica particles (10, 60, or 100 μm) at low, medium, or high concentrations (0.5, 5, or 20 vol%) in a manner similar to that of specimens in particle-free transformer oil (Fig. 3). When φp was
Effect of nuclei addition
Introducing solid particles into a liquid initially free of solids influences the nucleation processes, commonly classed as homogeneous or heterogeneous. Heterogeneous nucleation is much easier to achieve because ultrapure liquids are rarely encountered in nature, which, if any, have an extremely high cavitation threshold. Heterogeneous nucleation forms at preferential sites such as phase boundaries and the solid surfaces of foreign impurities, particularly when there are microscopic crevices on
Conclusions
Based on the results of ultrasonic CEP tests, we drafted a conceptual diagram (Fig. 15) that takes particle size (10–100 μm) and concentration (0.5–20 vol%) into account to determine whether and why the addition of solid particles alleviates or aggravates erosion. The main findings are as follows:
- (a)
For silt-sized particles (10–50 μm) with φp > 1 vol%, adding solid particles leads to less CEP than in the particle-free case due to an increase in the suspension viscosity, which increases with an
CRediT authorship contribution statement
Kunpeng Su: Conceptualization, Investigation, Formal analysis, Writing - original draft. Jianhua Wu: Funding acquisition, Supervision, Writing - review & editing. Dingkang Xia: Visualization, Methodology, Data curation, Formal analysis.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51779081); the Fundamental Research Funds for the Central Universities, China (Grant No. 2019B70914); and the Postgraduate Research & Practice Innovation Program of Jiangsu Province, China (Grant No. SJKY19_0482).
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