Thermal-EHD contacts lubricated with oil/refrigerant solutions: a new cavitation modeling approach based on refrigerant solubility

A first cavitation modeling with thermal effects for oil/refrigerant solutions lubricated ElastoHydroDynamic (EHD) point contacts is reported in this work. The solubility of the oil/refrigerant system is introduced into the Generalized Reynolds equation coupled with the elasticity equation and the energy conservation equation. The numerical results show a very good agreement with the published experimental results concerning film thickness prediction. Moreover, the present model describes the cavitation region on a physical basis. A discussion with other cavitation models from the literature is proposed. It puts into light the necessity of taking into account the solubility of the refrigerant into oil for such problems. Compared to pure oil, oil/refrigerant solutions can potentially reduce the amount of liquid oil for the next contact due to its higher cavitation intensity.

Variation of lubricant distribution across the radial direction in a journal bearing

A Computational Fluid Dynamics (CFD) analysis of two-phase flow was used to obtain the distribution of lubricant in a journal bearing, including inlet tube and groove. It was found that for an incomplete starting film, the oil spread-length varies along the groove depth and film thickness. The magnitude of variation was found to be independent of the inlet mass flow rate. Numerical simulations of the proposed model show that in the cavitation region the streamlets do not fill the entire film thickness. The present numerical model agrees with experimental observations.

Mechanism of EDM Intensification at Ultrasound Application

This paper considers some theoretical provisions on the impact ultrasonic mechanical vibrations have on the throughput of an electroerosive piercing of small-diameter holes. The approximate estimates confirm the hypothesis that the cumulative jets mechanism makes the greatest contribution to the intensification of a multiphase medium flow in the interelectrode gap. A model is proposed for a periodic localization of the cavitation region in the bottom part of the annular side gap. It allows explaining the occurrence of a multiphase medium flow during hole processing.

Study of the relationship of cavitation erosion activity and cavitation noise intensity

The study of the erosion activity of cavitation is of considerable interest for clarifying the mechanism of the effect of cavitation on biological tissues and cells. This paper proposes an improved technique for assessing the erosion activity of acoustic cavitation. The results of testing this technique in relation to the problem of studying the distribution of erosion activity in the cavitation region, generated by a radiator with a rod waveguide, are presented. The experiments were carried out using a submersible emitter with a resonant frequency of 32 kHz. It was found that erosion activity rapidly decreases with distance from the emitter and depends on the distance to the emitter L as 1/L3 when the diameter of the emitter is less than or of the order of the wavelength in the used liquid. It was shown that there is a correlation between the erosion activity of cavitation and the readings of the cavitometer with the output signal being the integral intensity of the highfrequency component of the cavitation noise in the frequency range up to 10 MHz. Piezoelectric sensors were used to register cavitation noise. In particular, in liquids characterized by a higher level of erosion activity, the output signal of the cavitometer is also higher. In this case, the readings of the cavitometer change depending on the distance to the radiator as 1/L. Based on the data obtained, a method is proposed for assessing the erosion activity of cavitation by the magnitude of the intensity of cavitation noise in a cube. It is shown that this parameter is linearly related to the results of measurements of the erosional activity of cavitation. The results obtained will be used in the development of a specialized cavitometer designed to assess the erosion activity of cavitation during in vitro studies of the effect of ultrasound on cells.

Assessment of Cavitation Models for Compressible Flows Inside a Nozzle

This study assessed two cavitation models for compressible cavitating flows within a single hole nozzle. The models evaluated were SS (Schnerr and Sauer) and ZGB (Zwart-Gerber-Belamri) using realizable k-epsilon turbulent model, which was found to be the most appropriate model to use for this flow. The liquid compressibility was modeled using the Tait equation, and the vapor compressibility was modeled using the ideal gas law. Compressible flow simulation results showed that the SS model failed to capture the flow physics with a weak agreement with experimental data, while the ZGB model predicted the flow much better. Modeling vapor compressibility improved the distribution of the cavitating vapor across the nozzle with an increase in vapor volume compared to that of the incompressible assumption, particularly in the core region which resulted in a much better quantitative and qualitative agreement with the experimental data. The results also showed the prediction of a normal shockwave downstream of the cavitation region where the local flow transforms from supersonic to subsonic because of an increase in the local pressure.

Large Eddy Simulations of In-Nozzle Cavitation Phenomena for Cold Fuel Injection

The present work investigates the in-nozzle phenomenology of cold fuel injections. Large Eddy Simulations (LES) were performed using a 3D model of a step nozzle injector with water and iso-octane serving as working fluids and the examined cases spanning across a range of temperatures that is relevant to an engine’s start-up operation. The aim is to shed light on the influence exerted by temperature on the in-nozzle cavitation mechanism, which in turn affects the primary atomization and the structure of the downstream emerging spray. Results suggest that a decrease in the injected fuel’s temperature induces a reduction of the nozzle’s void fraction and a shrinkage in the streamwise length of the cavitation region. This suggests that the size and intensity of the hydrodynamic cavitation features tend to become suppressed in cold conditions. The phenomenon appears to be driven by the temperature dependence of the injected fluid’s thermophysical properties, primarily the vapour pressure, with lower values hindering phase change.

Combined method for acoustic cavitation research

At present, the field of applications of powerful ultrasound is expanding intensively, and the improvement of equipment and technological processes continues. With that, the key factor in the effect of ultrasonic vibrations on processes in liquids and liquid-like media is cavitation, i.e. the phenomenon of formation, pulsation and collapse of gas microbubbles under the influence of variable pressure. The widespread introduction of promising ultrasound technologies is largely constrained by the fact that the patterns of cavitation generation are not well understood, and the data known in the literature are contradictory and are characterized by low reproducibility. This paper describes an innovative method for studying ultrasonic cavitation. In order to increase the reproducibility of the results and the reliability of the conclusions about the correlation of various cavitation effects, it is proposed to register simultaneously the parameters characterizing these effects. An installation designed to implement this method has been developed and tested. The installation provides the ability to register the full output signal of the hydrophone, the intensity of the glow generated in the cavitation region - sound luminescence, the cavitation noise spectrum and its individual components. Technical characteristics of the installation allow you to adjust the rate of development of the cavitation region by varying the duration and period of the ultrasound pulses. It is possible to conduct experiments both in low-frequency (LF) and high-frequency (HF) fields as well as in interacting HF and LF ultrasonic fields. During the testing of the installation, the results were obtained that are of considerable interest from the point of view of refining the ideas about the mechanism for generating cavitation effects. It was found that preliminary treatment of the liquid in an ultrasonic field with the aim of its degassing for 15–20 min provides a significant increase in the reproducibility of measurements, especially for liquids with a high gas content. Based on a comparison of the time dependences of the signals of the cavitation sensor and the photomultiplier output, the characteristic stages of the development of the cavitation region are distinguished, which differ in the dynamics of the development of the cavitation region and in the composition of the cavitation noise spectra recorded.

Study on the Common Rail Type Injector Nozzle Design Based on the Nozzle Flow Model

In this paper, a nozzle flow model was used to design an injector nozzle and obtain initial spray conditions for the dimethyl ether (DME) common rail-injection system. In order to deliver the same amount of energy as that provided by diesel at a low injection pressure of 50 MPa, the injector for DME needs nozzle holes with larger diameters and a higher SAC volume for the same injection duration. In addition, the needle lift and needle seat diameter should be increased to maintain a minimum flow area ratio. Although the vapour pressure and maximum injection pressure of DME are lower than those of diesel, the nozzle in a DME system showed higher discharge coefficients and effective nozzle exit diameters for the same injection duration owing to low kinematic viscosity. However, because the maximum injection pressure in DME is lower than that with diesel, and the length of the cavitation region is narrower.

Electric Discharge in Liquids under the Effect of Vibration

In this study, plasma discharge in liquids at intensive ultrasonic field above the cavitation threshold has been proven to be of great interest for initiation of various physical and chemical processes. The feature of arc discharge in liquid media is the localization of plasma region near the electrodes and “falling” form of volt-ampere characteristics. In the region of intensive cavitation, the fraction of gas-vapor component in the liquid exists, therefore it can be assumed that the electric breakdown in the cavitation region should become easier, which can result in the initiation of various forms of discharges.

Identification of Cavitation Instabilities on a Three-Bladed Inducer by Means of Strain Gages

This paper reports the experimental characterization of the cavitation-induced flow instabilities of a high head three-bladed inducer at design condition detected simultaneously by means of piezoelectric pressure transducers located at different axial and azimuthal stations on the casing of the pump and strain gages mounted on the pressure side of each blade. The simultaneous analysis performed in the stationary and rotating frames, supported by high-speed movies, suggests that the mechanism of formation of the detected subsynchronous rotating cavitation resembles that of modal stall in compressors, being the cavitation region the source of compliance. In fact, at decreasing cavitation number, a strong attached cavitation, developed preferentially on one blade and capable of completely surrounding and unloading the following blade, starts to be destabilized as consequence of the progressive intensification of a modal oscillation. The developed complex instability, consisting in a strong oscillation of the attached cavitation, is detected in both the frames as the simultaneous presence of a subsynchronous rotating cavitation and a cavitation surge.