IDENTIFYING AND MONITORING THE ROLES OF CAVITATION IN HEATING FROM HIGH-INTENSITY FOCUSED ULTRASOUND

For high-intensity focused ultrasound (HIFU) to continue to gain acceptance for cancer treatment it is necessary to understand how the applied ultrasound interacts with gas trapped in the tissue. The presence of bubbles in the target location have been thought to be responsible for shielding the incoming pressure and increasing local heat deposition due to the bubble dynamics. We lack adequate tools for monitoring the cavitation process, due to both limited visualization methods and understanding of the underlying physics. The goal of this project was to elucidate the role of inertial cavitation in HIFU exposures in the hope of applying noise diagnostics to monitor cavitation activity and control HIFU-induced cavitation in a beneficial manner. A number of approaches were taken to understand the relationship between inertial cavitation signals, bubble heating, and bubble shielding in agar-graphite tissue phantoms. Passive cavitation detection (PCD) techniques were employed to detect inertial bubble collapses while the temperature was monitored with an embedded thermocouple. Results indicate that the broadband noise amplitude is correlated to bubble-enhanced heating. Monitoring inertial cavitation at multiple positions throughout the focal region demonstrated that bubble activity increased prefocally as it diminished near the focus. Lowering the HIFU duty cycle had the effect of maintaining a more or less constant cavitation signal, suggesting the shielding effect diminished when the bubbles had a chance to dissolve during the HIFU off-time. Modeling the effect of increasing the ambient temperature showed that bubbles do not collapse as violently at higher temperatures due to increased vapor pressure inside the bubble. Our conclusion is that inertial cavitation heating is less effective at higher temperatures and bubble shielding is involved in shifting energy deposition at the focus. The use of a diagnostic ultrasound imaging system as a PCD array was explored. Filtering out the scattered harmonics from the received RF signals resulted in a spatially- resolved inertial cavitation signal, while the amplitude of the harmonics showed a correlation with temperatures approaching the onset of boiling. The result is a new tool for detecting a broader spectrum of bubble activity and thus enhancing HIFU treatment visualization and feedback.Gordon Center for Subsurface Sensing and Imaging Systems via NSF ERC Award Number EEC-9986821 and the U.S. Army, award number DAMD17-02-2-0014

Identifying bubble occurance during pool boiling employing acoustic emission technique

This paper reports the results of a study for the early detection of bubble formation during the boiling process using acoustic emission. The feasibility of using AE technology to detect and monitor early bubble formation during pool boiling is assessed, and the results show that AE technology is an affective tool for this purpose. There is a clear correlation between the AE signal levels and height of the water level above the heated surface during the boiling process. The different types of heated fluid influence AE energy levels during the bubble formation process. Statistically, it was found that the best AE parameters to indicate bubble formation were AE-RMS, AE-Energy and AE-Amplitude

Pump Cavitation Severity Evaluation Using Accelerometers and Dynamic Pressure Transducers after Installation

Lectur

Experimental investigation of bubble activity at an early stage using te acoustic emission technique in two-phase flow systems.

This thesis presents an experimental investigation and identifies the feasibility of the use of AE technology to detect and monitor both early stage bubble occurrence and throughout the boiling process. The research programme also included monitoring of bubble formation/collapse phenomena in ball and globe valves using AE techniques. It was demonstrated that an AE piezoelectric sensor can detect pressure pulses associated with bubble occurrence during pool boiling and cavitation in flow through valves. For the pool boiling test, a dedicated test-rig was used to diagnose and monitor bubble formation. It was concluded that bubble occurrence is detectable with AE techniques and there is a clear relationship between increasing AE levels and bubble formation during the boiling process. For the valve tests, a purpose-built test-rig was used to monitor and detect cavitation phenomena with various flow rates and different valve opening percentages. It was shown that AE will detect incipient cavitation and that there is a clear correlation between AE signal levels and the flow rate through the ball and globe valves at a constant opening percentage. This investigation successfully demonstrated that AE monitoring is capable of early diagnosis and monitoring of bubble formation phenomena in boiling processes and valves. This research developed a methodology and prototype framework for using the AE technique for detection and diagnosis of early bubble formation and collapse, allowing cavitation development to be tracked, and maintenance activity to be planned to maximise equipment life and minimise downtime.PhD in Energy and Powe

HIGH INTENSITY FOCUSED ULTRASOUND AND OXYGEN LOAD NANOBUBBLES: TWO DIFFERENT APPROCHES FOR CANCER TREATMENT

The study of applications based on the use of ultrasound in medicine and biology for therapeutic purposes is under strong development at international level and joins the notoriously well-established and widespread use of diagnostic applications [1]. In the past few years, High Intensity Focused Ultrasound (HIFU) has developed from a scientific curiosity to an accepted therapeutic modality. HIFU is a non invasive technique for the treatment of various types of cancer, as well as non-malignant pathologies, by inducing localized hyperthermia that causes necrosis of the tissue. Beside HIFU technology, other innovative therapeutic modalities to treat cancer are emerging. Among them, an extremely innovative technique is represented by oxygen loaded nanobubbles (OLNs): gas cavities confined by an appropriately functionalized coating. This is an oxygenating drugs aimed at re-oxygenation of cancerous tissue. Oxygen deficiency, in fact, is the main hallmark of cancerous solid tumors and a major factor limiting the effectiveness of radiotherapy. In this work, these two approaches to treat tumours are under study from a metrological point of view. In particular, a complete characterization of an HIFU fields regarding power, pressure and temperature is provided while oxygen load nanobubbles are synthesized, characterized and applied in in vitro and in vivo experiments

Kavitaatioeroosion karakterisointi Francis-turbiinin juoksupyörän materiaalille

In this study, the cavitation erosion resistance was characterized for a steel type used in Francis turbine runner blades. The goal of the study was to define the cavitation erosion rate of the runner blade steel and to compare the results to those of previously studied materials. The previously studied materials were aluminium alloy 7075, nickel aluminium bronze alloy C95400 and stainless steels A2205 and 304L. The material was tested in a cavitation tunnel in which sheet and cloud cavitation appears in the test section. Acoustic emission signal was measured from the sample experiencing cavitation erosion. Cavitation erosion can be divided into four distinct stages; the incubation period, the acceleration period, the deceleration period and the steady state period. Each of these periods is connected to the material response of a sample in a cavitation field. The material response is linked to the stage of work hardening and material cavity formation, as cavitation modifies the material surface. The cavitation tunnel used in this study circulates water and it has variable upstream and downstream pressures. The test section is a radially diverging channel in which cavitation inception occurs in the beginning of the radial section and cavitation closure occurs several millimeters downstream. The sample was cylindrical with one face experiencing cavitation. The cavitation erosion evolution was measured with a contact profilometer. The volume loss and the maximum depth of penetration were calculated from the surface profiles and they were compared to results from other materials. The acoustic emission results were compared with the erosion evolution. Material characteristics were obtained by elemental analysis, by macroscopic compression tests, by nanoindentation tests and by split Hopkinson pressure bar tests. The results show that the studied steel is as resistant to cavitation erosion as aluminium alloy 7075 and less resistant to cavitation erosion than nickel aluminium bronze alloy C95400 and stainless steels A2205 and 304L. The reason for the lower resistance compared to the stainless steels is corrosion, lower quality level or both. The voltage root mean square value of the acoustic emission signal reduced with increasing cumulative erosion time. Acoustic emission monitoring was found to be a potential method in estimating cavitation erosion evolution in hydraulic machines.Tässä työssä tutkittiin Francis-turbiinin juoksupyörän siivessä käytettävän terästyypin kavitaatiokestävyyttä. Työn tavoitteena oli tutkia materiaalin eroosionopeutta sekä verrata sitä aiemmin tutkittuihin materiaaleihin. Aiemmin tutkitut materiaalit olivat alumiiniseos 7075, nikkeli alumiini pronssiseos C95400 sekä ruostumattomat teräkset A2205 ja 304L. Materiaalia tutkittiin kavitaatiotunnelissa joka tuottaa taso- sekä pilvimäistä kavitaatiota testausosiossaan. Näytekappaleesta mitattiin akustista emissiota kun se oli kavitaatioeroosion vaikutuksen alaisena. Kavitaatioeroosiolla on neljä toisistaan erotettavaa vaihetta: alkuvaihe, kiihtymisvaihe, hidastumisvaihe sekä tasaisen eroosionopeuden vaihe. Jokainen vaihe on yhteydessä materiaalin kavitaatiovasteeseen. Tämä vaste on yhteydessä materiaalin muokkauslujittumisen sekä onkaloiden syntymisen tasoihin, koska kavitaatio muokkaa materiaalin pintaa. Tutkimuksessa käytetty kavitaatiotunneli kierrättää vettä ja sen ylä- sekä alavirran paineita voidaan muuttaa. Testausosio on radiaalisesti laajeneva kanava, jossa kavitaatio syntyy radiaalisen osion alussa ja romahtaa muutaman millimetrin päässä alavirtaan. Näytekappale oli sylinterimäinen ja sen yksi sivu altistui kavitaatioeroosiolle. Kavitaatioeroosion eteneminen mitattiin kontaktiprofilometrillä. Pintaprofiileista laskettiin tilavuushäviö sekä suurin eroosiosyvyys ja näitä tuloksia verrattiin toisten materiaalien vastaaviin tuloksiin. Akustisen emission mittaustuloksia verrattiin eroosion kehittymiseen. Materiaalin ominaisuudet selvitettiin alkuaineanalyysillä, makroskooppisilla puristustesteillä, nanoindentaatiolla sekä split Hopkinson pressure bar -testeillä. Tulokset osoittavat, että tutkittu teräslaatu on kavitaatiokestävyydeltään yhtä hyvä kuin alumiiniseos 7075, mutta huonompi kuin nikkeli alumiini pronssiseos C95400 tai ruostumattomat teräkset A2205 sekä 304L. Syy ruostumatonta terästä heikompaan kestävyyteen on joko korroosiossa, matalammassa laatutasossa tai molemmissa. Akustisen emission signaalin ulostulojännitteen neliöllisen keskiarvon kokonaiskeskiarvon havaittiin laskevan kavitaatioeroosion edetessä. Tämän havainnon vuoksi akustisen emission arvioitiin olevan potentiaalinen menetelmä hydraulisten koneiden kavitaatioeroosion vaiheiden tunnistamiseen

Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives

We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency