Cavitation Dynamics at Sub-Millimeter Scale

This paper is focused on the dynamics of cloud cavitation at Sub-millimeter scales, between 0.1 and 10 mm. A simple flow configuration consisting in a small diameter jet impacting a wall at 90° and then flowing radially between two plates is used. Cavitation is obtained in the gap between the two plates, due to the local flow acceleration. The scale of the setup was changed by varying separately the gap between the two plates, and the nozzle diameter. Observations were performed from the transparent bottom plate, using a high-speed camera and intense illumination, for various Reynolds and cavitation numbers. The analysis is focused on the variations of the cavity length and the Strouhal number based on the characteristic frequency of the flow instability

Observing the thermodynamic effects in cavitating flow by IR thermography

When dealing with liquid flows, where operating temperature gets close to the liquid critical temperature, cavitation cannot be assumed as an isothermal phenomenon. Due to the relatively high density of vapor, the thermodynamic effect (decrease of temperature in the bulk liquid due to latent heat flow) becomes considerable and should not be neglected. For applications like pumping cryogenic fuel and oxidizer in liquid propulsion space launchers, consideration of the thermodynamic effect is essential - consequently the physical understanding of the phenomenon and its direct experimental observation has a great value. This study presents temperature measurements in a cavitating flow on a simple convergent-divergent constriction by infrared (IR) thermography. Developed cavitating flow of hot water (-100 °C) was evaluated by high-speed IR thermography and compared with conventional high-speed visualization, at different operating conditions with the velocity range at the nozzle throat between 9.6 and 20.6 m/s and inlet pressure range between 143 and 263 kPa. Temperature depression near the nozzle throat - near the leading edge of cavitation was measured in a range up to [vartriangle]T = 0.5 K. This confirms the presence of the thermodynamic effects by cavitation phenomenon and it is in agreement with its theory. In the study, average temperature fields, fields of temperature standard deviation and time-resolved temperatures, are presented and discussed. In addition, statistical analysis between temperature drop and cavitation flow characteristics is shown

Cavitation dynamics in water at elevated temperatures and in liquid nitrogen at an ultrasonic horn tip

Understanding and predicting thermodynamic effects is crucial when the critical point temperature is close to the operating temperature of the fluid, like in cryogenics. Due to the extreme difficulties of experimental investigation, predicting of thermodynamic effects in cavitation often bases on data in liquids other than cryogenics. Most often used surrogate liquids are hot water or certain refrigerants, which are selected by a single fluid property, most commonly by the thermodynamic parameter ∑. The paper presents a systematic study of the cavitation dynamics in water at 20 °C, 40 °C, 60 °C, 80 °C and 100 °C and in addition in liquid nitrogen (LN2). Cavitation dynamics on a 4.8 mm (tip diameter) ultrasonic horn tip, which oscillated at 20 kHz was investigated by high-speed visualization at 300,000 frames per second (fps). Simultaneously acoustic emissions were recorded by a high frequency pressure transducer. Measurements were performed under variation of the acoustic power in a closed, insulated vessel, where pressure could be optionally set. The main purpose of the presented investigation is to determine whether hot water can act as a surrogate liquid to cryogenics. The results may implicate the future investigations and development of a new generation of rocket engines, which also feature the possibility of re-ignition while in orbit – understanding and predicting of cavitation behaviour is becoming a crucial part at the (liquid oxygen – LOX and liquid hydrogen – LH2) turbo-pump design

Cavitation bubble interaction with a rigid spherical particle on a microscale

Cavitation bubble collapse close to a submerged sphere on a microscale is investigated numerically using a finite volume method in order to determine the likelihood of previously suspected mechanical effects to cause bacterial cell damage, such as impact of a high speed water jet, propagation of bubble emitted shock waves, shear loads, and thermal loads. A grid convergence study and validation of the employed axisymmetric numerical model against the Gilmore’s equation is performed for a case of a single microbubble collapse due to a sudden ambient pressure increase. Numerical simulations of bubble-sphere interaction corresponding to different values of nondimensional bubble-sphere standoff distance δ and their size ratio ε are carried out. The obtained results show vastly different bubble collapse dynamics across the considered parameter space, from the development of a fast thin annular jet towards the sphere to an almost spherical bubble collapse. Although some similarities in bubble shape progression to previous studies on larger bubbles exist, it can be noticed that bubble jetting is much less likely to occur on the considered scale due to the cushioning effects of surface tension on the intensity of the collapse. Overall, the results show that the mechanical loads on a spherical particle tend to increase with a sphere-bubble size ratio ε, and decrease with their distance δ. Additionally, the results are discussed with respect to bacteria eradication by hydrodynamic cavitation. Potentially harmful mechanical effects of bubble-sphere interaction on a micro scale are identified, namely the collapse-induced shear loads with peaks of a few megapascals and propagation of bubble emitted shock waves, which could cause spatially highly variable compressive loads with peaks of a few hundred megapascals and gradients of 100 MPa/μm

A novel rotation generator of hydrodynamic cavitation for the fibeillation of long conifer fibers in paper production

Refining of cellulose pulp is a critical step in obtaining high quality paper characteristics, however, this process is slow and costly especially for refining longer conifer fibers which are the preferred source for high quality paper production and give the paper its strength. In this study, we have applied a novel rotation generator of hydrodynamic cavitation for refining conifer rich pulp samples. Our results show that the device is capable of generating intense shear forces and multiple zones of developed cavitation and is successful in increasing the drainage rate of high consistency pulp (3%). The paper produced by means of the obtained pulp has higher quality because of its higher tensile index (50.5 kN m kg-1) and burst index (3 kPa m2 g-1). These physical parameters were sufficient for newsprint paper and other paper/board quality manufacture. In addition, this laboratory scale rotation generator proved to be economically efficient in comparison to the routinely employed laboratory beaters. To our knowledge, this is the first example of using hydrodynamic cavitation for the refinement of softwood fiber pulp of standard industrial consistencies (3%)

Numerical insight into the Kelvin-Helmholtz instability appearance in cavitating flow

Recently the development of Kelvin-Helmholtz instability in cavitating flow in Venturi microchannels was discovered. Its importance is not negligible, as it destabilizes the shear layer and promotes instabilities and turbulent eddies formation in the vapor region, having low density and momentum. In the present paper, we give a very brief summary of the experimental findings and in the following, we use a computational fluid dynamics (CFD) study to peek deeper into the onset of the Kelvin-Helmholtz instability and its effect on the dynamics of the cavitation cloud shedding. Finally, it is shown that Kelvin-Helmholtz instability is beside the re-entrant jet and the condensation shock wave the third mechanism of cavitation cloud shedding in Venturi microchannels. The shedding process is quasi-periodic

Cavitation erosion in liquid nitrogen

Thermodynamic effects in cavitation become significant only when the critical-point temperature is close to the operating temperature of the fluid, as in the case of cryogenic fluids. Therefore, the understanding and the prediction of the cavitation effects in such cases is crucial in many applications - for example the turbopumps for liquid hydrogen (LH2) and oxygen (LOX) in space launcher engines. The new generation of rocket engines will also feature the possibility of re-ignition while in orbit and prolonged period of operationhence cavitation erosion is becoming an issue at the design stage of the turbo-pumps. In the study, we show measurements of cavitation erosion in liquid nitrogen (LN2), where cavitation was generated by an ultrasonic transducer. The damage was evaluated on aluminium samples. Special care was given to accurate setting of the operation point - especially the operating pressure, which defines the size of cavitation. We show that it is less aggressive than cavitation in water and that its aggressiveness cannot be described by a single fluid property (for example the most commonly used Brennen\u27s thermodynamic parameter Σ), but by a combination of several (viscosity, density, vapor pressure, surface tension, thermodynamic parameter) - in the present paper we addressed this point by a simple bubble dynamics model with consideration of the thermodynamic effect to qualitatively predict the results of the measurements. Finally, we also compared performance of several other engineering materials

Vpliv hidrodinamske kavitacije na pripravo vodne raztopine detergent za pranje tekstila

Washing machines are one of the most energy and water demanding domestic appliances. Over years, a significant effort of the scientific community has been invested into making laundry more "sustainable". Nevertheless, the preparation of detergent solution has been entirely overlooked step of laundering. The preparation of aqueous detergent solutions in the currently available washing machines takes up to 10 minutes. In this work, we propose a design of a special rotary hydrodynamic cavitation generator, which would impact this process. New detergent dissolution rates have been experimentally tested on the laboratory model washing machine using the designed cavitation generator. The dissolution rates have been determined from the measurements of the undissolved detergent after the specific time of treatment. Additionally, the influence of hydrodynamic cavitation on that process has been isolated and investigated. To do so, two flow regimes have been established: the regime with cavitation present and the regime where cavitation was not present. In order to evaluate cavitational intensity, pressure oscillations inside cavitation generator have been recorded. Results indicate that cavitation significantly increases the detergent dissolution rates. In the cavitation flow regime, more than 80 % of the detergent is dissolved in approximately 10 seconds. With no cavitation present, about 150 seconds are needed to dissolve the same amount of the detergent. Intensification of the process can be attributed to mechanical effects of cavitation. This research shows that use of the cavitation generators in the washing machines could lead to shorter washing programs and henceforth potential water and energy savings.Pralni stroji so eni izmed največjih porabnikov energije in vode v gospodinjstvih. V zadnjih letih je znanstvena skupnost vložila veliko truda v trajnostni razvoj, tudi na področju pranja tekstila. Kljub temu pa je priprava raztopine vode in detergenta popolnoma prezrt proces s strani raziskovalcev. Priprava pralne raztopine v pralnih strojih, ki so trenutno na voljo na trgu, lahko traja do 10 minut. Avtorji raziskave menimo, da bi z izkoriščanjem hidrodinamske kavitacije lahko ta proces močno izboljšali. Pojem kavitacija označuje nastanek parnih mehurčkov v kapljevini, njihovo aktivnost in prehod nazaj v kapljevito stanje. Pojavi se zaradi lokalnega zmanjšanja tlaka, pri čemer ostane temperatura medija približno nespremenjena. Kavitacija je vedno bolj uveljavljena metoda v procesni in kemijski industriji, saj se ob kolapsih kavitacijskih mehurčkov vzpostavijo ekstremne razmere. Te razmere omogočajo izvedbo procesov, za katere so sicer potrebne enormne količine energije

Cavitation bubble interaction with compliant structures on a microscale

Numerous studies have already shown that the process of cavitation can be successfully used for water treatment and eradication of bacteria. However, most of the relevant studies are being conducted on a macro scale, so the understanding of the processes at a fundamental level remains poor. In attempt to further elucidate the process of cavitation-assisted water treatment on a scale of a single bubble, the present paper numerically addresses interaction between a collapsing microbubble and a nearby compliant structure, that mechanically and structurally resembles a bacterial cell. A fluid–structure interaction methodology is employed, where compressible multiphase flow is considered and the bacterial cell wall is modeled as a multi-layered shell structure. Simulations are performed for two selected model structures, each resembling the main structural features of Gram-negative and Gram-positive bacterial cell envelopes. The contribution of two independent dimensionless geometric parameters is investigated, namely the bubble-cell distance δ and their size ratio ς. Three characteristic modes of bubble collapse dynamics and four modes of spatiotemporal occurrence of peak local stresses in the bacterial cell membrane are identified throughout the parameter space considered. The former range from the development of a weak and thin jet away from the cell to spherical bubble collapses. The results show that local stresses arising from bubble-induced loads can exceed poration thresholds of cell membranes and that bacterial cell damage could be explained solely by mechanical effects in absence of thermal and chemical ones. Based on this, the damage potential of a single microbubble for bacteria eradication is estimated, showing a higher resistance of the Gram-positive model organism to the nearby bubble collapse. Microstreaming is identified as the primary mechanical mechanism of bacterial cell damage, which in certain cases may be enhanced by the occurrence of shock waves during bubble collapse. The results are also discussed in the scope of bacteria eradication by cavitation treatment on a macro scale, where processes of hydrodynamic and ultrasonic cavitation are being employed

Liposome destruction by a collapsing cavitation microbubble: a numerical study

Hydrodynamic cavitation poses as a promising new method for wastewater treatment as it has been shown to be able to eradicate bacteria, inactivate viruses, and destroy other biological structures, such as liposomes. Although engineers are already commercializing devices that employ cavitation, we are still not able to answer the fundamental question: What exactly are the damaging mechanisms of hydrodynamic cavitation in various ap-plications? In this light, the present paper numerically addresses the interaction between a single cavitation microbubble and a nearby lipid vesicle of a similar size. A coupled fluid-structure interaction model is employed, from which three critical modes of vesicle deformation are identified and temporally placed in relation to their corresponding driving mechanisms: (a) unilateral stretching at the waist of the liposome during the first bubble collapse and subsequent shock wave propagation, (b) local wrinkling at the tip until the bubble rebounds, and (c) bilateral stretching at the tip of the liposome during the phase of a second bubble contraction. Here, unilateral and bilateral stretching refer to the local in-plane extension of the bilayer in one and both principal directions, respectively. Results are discussed with respect to critical dimensionless distance for vesicle poration and rupture. Liposomes with initially equilibrated envelopes are not expected to be structurally compromised in cases with (delta)>1.0, when a nearby collapsing bubble is not in their direct contact. However, the critical dimensionless distance for the case of an envelope with pre-existing pores is identified at (delta)=1.9. Additionally, the influence of liposome-bubble size ratio is addressed, from which a higher potential of larger bubbles for causing stretching- induced liposome destruction can be identified