Acoustic waves in multifractional bubbly liquids

High Temperature - The propagation of acoustic waves in multifractional mixtures of a liquid with vapor-gas and gas bubbles of different sizes and different compositions with phase transitions is...     

Acoustic waves in two-fraction bubble liquid with phase transformations

The propagation of acoustic waves in two-fraction mixtures of liquid with vapor-gas and gas bubbles of different sizes and compositions with phase transformations has been studied. A system of the differential equations of the motion of the mixture is presented, and the dispersion relation is deduced. Two local maxima in the frequency dependence of the attenuation coefficient for the case of the two-fraction mixture of water with vapor-gas bubbles of air and helium bubbles are revealed. The evolution of the weak pulsed perturbations of the pressure in this mixture was calculated numerically. It was established that the substitution of part of the vapor-gas bubbles in the monodisperse bubble mixture with phase transitions for inert gas bubbles can lead to both a decrease and an increase in the attenuation coefficient in the low-frequency region depending on the sort of gas.     

Optical observations of acoustical radiation force effects on individual air bubbles

Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 microm to 79 microm was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 microm. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position.     

Visualization of bubble behavior and bubble diameter correlation for NH3–H2O bubble absorption

The objectives of this paper are to visualize the bubble behavior for an ammonia–water absorption process, and to study the effect of key parameters on ammonia–water bubble absorption performance. The orifice diameter, orifice number, liquid concentration and vapor velocity are considered as the key parameters. The departing bubbles tend to be spherical for surface tension dominant flow, and the bubbles tend to be hemispherical for inertial force dominant flow. A transition vapor Reynolds number is observed at a balance condition of internal absorption potential (by the concentration difference) and external absorption potential (by the vapor inlet mass flow rate). As the liquid concentration increases, the transition Reynolds number and the initial bubble diameter increase. The initial bubble diameter increases with an increase of the orifice diameter while it is not significantly affected by the number of orifices. Residence time of bubbles increases with an increase in the initial bubble diameter and the liquid concentration. This study presents a correlation of initial bubble diameter with ±20% error band. The correlation can be used to calculate the interfacial area in the design of ammonia-water bubble absorber.     

The flow of liquid past a plate with boiling and injection of gas

A classical model boundary layer problem is considered for the flow of liquid past a plate in view of injection of a vapor-gas mixture from its surface. The obtained self-similar solutions enable one to estimate the typical values of thickness of the vapor-gas layer, the value of heat-transfer coefficient as a function of temperature of liquid, intensity of injection and composition of mixture being injected, and the velocity of flow past the plate. In addition, the problem is considered of reducing the hydrodynamic drag owing to vapor and vapor-gas “lubrication” because of boiling of liquid and injection of vapor-gas mixture from the plate surface. The possibility is analyzed of the emergence of vapor film due to viscous friction forces in the case where the liquid is in the vicinity of the boiling point.     

Oxidative Hydrolysis in Water Vapor-Air Phase of CsI Radioaerosols Produced by CsI Sublimation from Metallic Surface

Behavior of CsI radioaerosols produced by CsI sublimation from a platinum support in argon, air, and water vapor-air mixture was studied. During 10-12 min of the vaporization at 900-1570 K, CsI radioaerosols undergo oxidative hydrolysis with atmospheric oxygen and water vapor to form CsOH aerosols and I₂. The cesium-to-iodine ratio determined in various fractions shows that oxidation of CsI in argon is minimal and is caused by the presence of oxygen and water traces. Oxidative hydrolysis of CsI strongly increases with increasing water vapor content in the vapor-gas flow. The degree of oxidative hydrolysis of CsI in the gas flow depends not only on the content of water vapor and oxygen but also on the initial CsI/O₂ molar ratio.     

Limit Superheating of Stretched Liquid

Investigation of explosive vaporization with pulse heating of an extended liquid in a pressure range of from–10 tо +0.1 МPа revealed regimes when bubbles occur by the homogeneous fluctuation nucleation mechanism. These regimes are distinguished by a great phase transformation rate and a nucleation concentration at the moment of the attainment of a certain liquid temperature. This metastable liquid temperature is found to be the highest and is estimated by coordinates of the liquid–vapor spinodal. The kinetics of cavitation and boiling is investigated by disturbance of the heat flux of a miniature, fast-heated wire sensor. The extended liquid pressure near the sensor was set by the method of the positive pressure pulse inversion. Experimental results are generalized with application of explosive vaporization theory for the impact boiling regime at positive pressures.     

Experimental investigation of the thermal conductivity of vapor-gas mixtures with low vapor concentration

We give the results of an experimental investigation of the thermal conductivity of vapor-gas mixtures with low vapor concentration.     

Thermal processes in compression of a vapor bubble in liquid hydrocarbon based on the homobaric model

The mathematical model of a single homobaric vapor bubble, which is based on the assumption that the pressure is uniform in the vapor volume, is used for studying bubble collapse in liquid hydrocarbon-octane. A goal of this work was determination of the initial conditions under which the significant warming—up of the vapor and liquid is possible. Certain peculiarities inherent in hydrocarbon bubbles are revealed. The limits of applicability of basic assumptions of the homobaric model are considered.     

Interaction of the acoustic signal with motionless discretely layered medium containing a layer of bubbly liquid

The dynamics of the pulse perturbation of the low-amplitude pressure in the motionless discretely layered medium containing a layer of liquid with polydisperse gas bubbles has been studied theoretically. Theoretical method basics of the calculation of the acoustic signal distortion during the diagnostics of multilayer samples containing a layer of bubbly liquid are presented. It is shown that specific dispersion and dissipative properties of the layer of bubbly liquid can affect considerably the dynamics of the acoustic signal in the multilayer medium as a function of the main frequency of the signal. The theoretical models of the dynamics of multiphase media can be verified using this method. It was established that it is possible to use this theory for the calculation of the acoustic signal distortion at its interaction with multilayer objects containing the layer of bubbly liquid.     

Heat transfer associated with vapor condensation from vapor-gas mixtures

Reports results of a theoretical analysis and experimental investigation of heat transfer accompanying vapor condensation from vapor-gas mixtures. Gives a dimensionless equation for estimating the surface necessary for vapor condensation in the presence of an inert gas.     

Method for measuring nitrogen and helium concentration in fuel components

An approach is considered for organizing indirect measurements of nitrogen and helium in liquid fuel components based on creating an equilibrium distribution between vapor-gas and liquid phases, determination of temperature, pressure, and thermal conductivity of the equilibrium vapor phase, followed by calculation using Henry constants. Translated from Izmeritel’naya Tekhnika, No. 4, pp. 65–67, April, 2009.     

Effect of the phase transformations on acoustics of a mixture of gas with vapor,droplets, and solid particles

The propagation of acoustic waves in the two-fraction mixtures of gas with vapor, droplets, and solid particles of different materials and sizes with phase transformations has been studied. Nonstationary and nonequilibrium effects of the interphase exchange of the impulse, mass, and heat have been taken into account. A system of the differential equations of the motion of the mixture has been presented, and the dispersion relation has been deduced. The high- and low-frequency asymptotics of the attenuation coefficient have been obtained and analyzed. The effect of the heat and mass exchange on dispersion and dissipation of acoustic waves in the two-fraction mixtures of gas with vapor, droplet, and solid particles has been studied.     

Mechanisms of contrast agent destruction

Various applications of contrast-assisted ultrasound, including blood vessel detection, perfusion estimation, and drug delivery, require controlled destruction of contrast agent microbubbles. The lifetime of a bubble depends on properties of the bubble shell, the gas core, and the acoustic waveform impinging on the bubble. Three mechanisms of microbubble destruction are considered: fragmentation, acoustically driven diffusion, and static diffusion. Fragmentation is responsible for rapid destruction of contrast agents on a time scale of microseconds. The primary characteristics of fragmentation are a very large expansion and subsequent contraction, resulting in instability of the bubble. Optical studies using a novel pulsed-laser optical system show the expansion and contraction of ultrasound contrast agent microbubbles with the ratio of maximum diameter to minimum diameter greater than 10. Fragmentation is dependent on the transmission pressure, occurring in over 55% of bubbles insonified with a peak negative transmission pressure of 2.4 MPa and in less than 10% of bubbles insonified with a peak negative transmission pressure of 0.8 MPa. The echo received from a bubble decorrelates significantly within two pulses when the bubble is fragmented, creating an opportunity for rapid detection of bubbles via a decorrelation-based analysis. Preliminary findings with a mouse tumor model verify the occurrence of fragmentation in vivo. A much slower mechanism of bubble destruction is diffusion, which is driven by both a concentration gradient between the concentration of gas in the bubble compared with the concentration of gas in the liquid, as well as convective effects of motion of the gas-liquid interface. The rate of diffusion increases during insonation, because of acoustically driven diffusion, producing changes in diameter on the time scale of the acoustic pulse length, thus, on the order of microseconds. Gas bubbles diffuse while they are not being insonified, termed static diffusion. An air bubble with initial diameter of 2 microns in water at 37 degrees C is predicted to fully dissolve within 25 ms. Clinical ultrasound contrast agents are often designed with a high molecular weight core in an attempt to decrease the diffusion rate. C3F8 and C4F10 gas bubbles of the same size are predicted to fully dissolve within 400 ms and 4000 ms, respectively. Optical experiments involving gas diffusion of a contrast agent support the theoretical predictions; however, shelled agents diffuse at a much slower rate without insonation, on the order of minutes to hours. Shell properties play a significant role in the rate of static diffusion by blocking the gas-liquid interface and decreasing the transport of gas into the surrounding liquid. Static diffusion decreases the diameter of albumin-shelled agents to a greater extent than lipid-shelled agents after insonation.     

The collapse of micro cavitation bubbles

The inertia dominated collapse of micro cavitation bubbles in a viscous liquid with surface tension is investigated. In addition to the well known dramatic collapse mode, a smooth decrease of the void cavity is also possible due to the viscous damping. This effect is especially important for micro bubbles although the importance of surface tension increases strongly with decreasing initial radius. The collapse discussion can be performed easily without large numerical efforts by means of an analytical collapse criterion formulated by Bogoyavlenskiy. The results presented are relevant for cavitation theory and particular helpful for applications in microfluidic systems.     

Ultrasonic contrast agent shell rupture detected by inertial cavitation and rebound signals

Determining the rupture pressure threshold of ultrasound contrast agent microbubbles has significant applications for contrast imaging, development of therapeutic agents, and evaluation of potential bioeffects. Using a passive cavitation detector, this work evaluates rupture based on acoustic emissions from single, encapsulated, gas-filled microbubbles. Sinusoidal ultrasound pulses were transmitted into weak solutions of Optison at different center frequencies (0.9, 2.8, and 4.6 MHz), pulse durations (three, five, and seven cycles of the center frequencies), and peak rarefactional pressures (0.07 to 5.39 MPa). Pulse repetition frequency was 10 Hz. Signals detected with a 13-MHz, center-frequency transducer revealed postexcitation acoustic emissions (between 1 and 5 micros after excitation) with broadband spectral content. The observed acoustic emissions were consistent with the acoustic signature that would be anticipated from inertial collapse followed by "rebounds" when a microbubble ruptures and thus generates daughter/free bubbles that grow and collapse. The peak rarefactional pressure threshold for detection of these emissions increased with frequency (e.g., 0.53, 0.87, and 0.99 MPa for 0.9, 2.8, and 4.6 MHz, respectively; five-cycle pulse duration) and decreased with pulse duration. The emissions identified in this work were separated from the excitation in time and spectral content, and provide a novel determination of microbubble shell rupture.     

Heterogeneous Cavitation in Liquid Helium 4 Near a Glass Plate

We have used a focused acoustic wave to study cavitation, i. e. the nucleation of bubbles, in liquid helium 4 near a clean glass plate. From the reflectance of light at the glass/helium interface, we measured the amplitude of the acoustic wave in the focal region and the nucleation pressure. From an analysis of the transmitted light we also measured the nucleation probability. We observed three different regimes with different statistics and threshold pressures in the range 0 to –3 bar, significantly less negative than for homogeneous cavitation.     

Visual Observation of a Sound-Induced Bubble in Liquid 3He–4He Mixtures

We report visual observation of a sound-induced bubble in superfluid ³He–⁴He liquid mixtures using a high-speed camera at a rate of 1 msec/frame. The experiments were performed in the ³He dilute phase of phase-separated mixtures at 300 mK. The resonant frequency of the piezoelectric transducer was 9.36 MHz and the diameter of the active electrode was about 4 mm. When an acoustic wave pulse of sufficient magnitude was applied to the dilute phase from the transducer under saturated vapor pressure, a single bubble was nucleated on the active area. The bubble expanded almost spherically on the transducer, as it reached maximum size, it started shrinking, detached from the transducer, and collapsed. We also investigated the motion of the bubble in mixtures with a ³He concentration of 25% at 750 mK. In this case, the bubble grew elliptically on the transducer and detached from it without much change in shape.     

Acoustic characterization of microbubble dynamics in laser-induced optical breakdown

A real-time acoustic technique to characterize microbubbles produced by laser-induced optical breakdown (LIOB) in water was developed. Femtosecond laser pulses are focused just inside the surface of a small liquid tank. A tightly focused, high frequency, single-element ultrasonic transducer is positioned so its focus coincides axially and laterally with this laser focus. When optical breakdown occurs, a bubble forms and a pressure wave is emitted (i.e., acoustic emission). In addition to this acoustic signal, the microbubble is actively probed with pulse-echo measurements from the same transducer. After the bubble forms, received pulse-echo signals have an extra pulse, describing the bubble location and providing a measure of axial bubble size. Wavefield plots of successive recordings illustrate the generation, growth, and collapse of cavitation bubbles due to optical breakdown. These same plots also can be used to quantify LIOB thresholds.