Patent Application
20120118395
Cavitation is the process of the formation of vapor, void, or bubbles due to pressure changes in a liquid flow. The motion, and eventual collapse of the bubbles can cause local pressure changes in the liquid, which can be transmitted to a target surface either in the form of a shock wave, or by micro-jets, depending on the bubble-surface distance. Pressure greater than 100,000 psi has been measured in a shock wave resonating from cavitating bubbles. It is generally understood that the cycle of formation and collapse of the bubbles that occurs, often at a high frequency, can generate dynamic stress on the surfaces of objects; ultimately the dynamic stress can contribute to the fatigue of the target surface, including micro-cracks that form and coalesce on the surface, eventually leading to material removal known as cavitation damage.
Cavitation occurs in many high speed fluid flows, for example, near the propellers of ships, pumps, turbines, and in pipelines, leading to cavitation damage on the surfaces of these objects. Cavitation damage can lead to premature failure in civil structures and industrial plants, resulting in increased maintenance cost and lost production. It can occur over a wide spectrum, from diesel engines to nuclear power plants, from marine vehicles to spacecraft, from simple industrial pumps to hydro, gas, and steam turbines. Cavitation can be observed not only in liquids but also in gaseous or plasma phases. Plasma cavitation is known to be less pronounced but, in effect, is not much different than that of cavitation in water. The potential of plasma cavitation-induced damage on spacecraft, including satellites, in addition to the potential radiation damage, must be considered for the proper lifetime estimate of spacecraft traveling through the magnetosphere. Another case in which it is desirable to study the effects of cavitation is in connection with the use of the Spallation Neutron Source (“SNS”) accelerator. The SNS accelerator provides a mega-Watt short-pulsed proton beam on a mercury target to generate what may be the world's brightest neutron beam. Such a power level represents approximately a 5-10 fold increase over the power level of previous pulsed proton accelerators. Power increases to 2-3 MW have been considered in the SNS power upgrade project. Complications associated with the required remote maintenance of the mercury target make understanding it especially critical for operators to understand the lifetime of the target in order to avoid unplanned target replacements. For the SNS short pulse delivery on a liquid mercury target, cavitation damage is one of the major concerns in the target handling. Thus, a better understanding of cavitation damage on the target surface can increase the ability to understand when the target needs to be replaced.
The phenomenon of cavitation has proved to be difficult to study in the laboratory because situations that lead to cavitation involve turbulent flow and a very rapid phase change—on the order of microseconds. Due to insufficiency of relevant and reliable data, modeling and ensuing prediction and prevention of premature failure due to cavitation remains a difficult task.
Currently there are two ASTM standards available for cavitation damage study, including: (1) using a vibratory apparatus (ASTM G32-03) or (2) by liquid jet (ASTM G134-95). The vibratory apparatus test is an ultrasonic method, in which the tested sample is either set in a glass jar or positioned at the tip of an acoustic horn and vibrated at high frequency (20 kHz) while immersed in a liquid. This test method has been shown to produce cavitation damage on the surface of a specimen. Although the mechanism for generating fluid cavitation in this method differs significantly from that occurring in flowing systems and hydraulic machines, the nature of the material damage mechanism is considered to be similar. The vibratory test method therefore offers a small-scale, relatively simple and controllable test that can be used to compare the cavitation erosion resistance of different materials. There are, however, a number of disadvantages associated with the vibratory test. For example, the vibratory apparatus generates heat during operation at the desired 20 kHz frequency; as a result, the local test temperature can be highly variable. This temperature effect may have significant impact on the test results. In addition, as a result of the large amount of kinetic energy deposited onto the test specimen, the induced vibration on the test sample may further create other material failure mechanisms that are not directly associated with the cavitation damage process. Therefore, such testing may well overestimate the cavitation damage for a target cavitation environment. The liquid jet method utilizes a significantly focused and direct high water jet pressure (˜2K to 3K psi) directed toward a target surface in an attempt to recreate a cavitation event. Unfortunately, this method is known to have significant kinetic energy and is not likely to resemble most of the cavitation damage events.
In addition, a channel method has also been used to study cavitation damage. This technique requires a large scale test instrument for cavitation generation; an example is the US Navy Large Cavitation Chanel (LCC) which is a variable pressure, low turbulence, low acoustic noise, re-circulating, vertical plane, stainless steel, 1.4 million gallon cavitation tunnel. This test is capable of creating cavitation events, and cavitation damage, similar to real world applications. The main disadvantage of this test is that the instrument size is huge and requires a large facility to accommodate the test channel and the associated pumps. In addition, the test circuits require modification for each test type. This is not an easy task due to the large and complex set-up and significantly increases the operation effort and test cost. Thus, researchers and manufacturers are continually searching for an apparatus and method for generating cavitation and cavitation damage in a small scale, portable, and easily reproducible environment.
The present invention provides a compact, portable, repetitive pressure-pulse system, using laser induced shock wave, to accurately mimic cavitation.
In one embodiment, the developed system utilizes a pulse laser to generate pressure waves within a closed-cell container. The container may include a body having a wall defining an internal chamber. The wall includes a window extending through the wall from the internal chamber to an area outside the body. A specimen holder positions the specimen within the internal chamber so that the surface of the specimen is facing the internal chamber. A cavitation media is disposed inside the internal chamber and in contact with the surface of the specimen. The pulse laser is disposed in said area outside the body, such that the laser is operable to provide a laser beam through the window and into the cavitation media. The laser may generate a shock wave within the cavitation media, which resonates from the focal point of the laser through the interior chamber and creates cavitation damage on the surface of the test specimen.
In one embodiment, the laser generates a short duration laser pulse, and the cavitation media is deionized water. The interior surface of the chamber may have a predetermined shape to enable the shock waves to propagate within the chamber in a desired manner.
In one embodiment, a wave collector may be positioned within the chamber. The wave collector defines a secondary chamber, and includes a wave collector window that extends into the secondary chamber. The wave collector window is generally aligned with the window in the cavitation unit such that the laser beam can be directed into the window, through the cavitation chamber and through the wave collector window and into the secondary chamber. The test specimen may be positioned within the wave collector. The secondary chamber may also include a predetermined interior surface shape. For instance, the interior surface may be concave elliptical shaped or concave spherical shaped.
The present invention also provides a method for generating cavitation, including the steps of: (a) providing a cavitation media; (b) providing a test specimen proximate to the cavitation media; and directing a laser beam into the cavitation media to initiate laser optical breakdown in the cavitation media, producing a shock wave propagating through the media and creating cavitation damage on a surface of the test specimen.
The short pulse duration of the pulse laser, normally in a 5-20 ns range, is more efficient in generating shock waves compared to the stationary wave pattern generated by ultrasonic devices. The developed novel system have many distinctive advantages for cavitation research; in particular: (i) it may have no external driver (pump) or moving part; (ii) the relatively small design can avoid the lengthy and large water channel loop required in conventional approaches; (iii) it can be tailored to multiple design variables to realistically mimic cavitation environments such as high temperature, high pressure, and corrosive media; (iv) it can have low testing time and cost. The present apparatus provides an expedient tool and methodology to benchmark cavitation damage, estimate structural material lifetime, and assist the development of optimized advanced system approaches or materials to mitigate cavitation damage.
Before the embodiments of the invention are explained in detail, it is to be understood that the invention is not limited to the details of operation or to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention may be implemented in various other embodiments and of being practiced or being carried out in alternative ways not expressly disclosed herein. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items and equivalents thereof. Further, enumeration may be used in the description of various embodiments. Unless otherwise expressly stated, the use of enumeration should not be construed as limiting the invention to any specific order or number of components. Nor should the use of enumeration be construed as excluding from the scope of the invention any additional steps or components that might be combined with or into the enumerated steps or components.
A system for generating cavitation and cavitation damage on a test sample is shown in
The top end disk 22 of the chamber 12 may include a transparent quartz window 29 for laser passage and the front end disk 24 may include a glass viewport 31 to enable viewing of the interior of the chamber 12. The left 26 and the right 28 end disks may include fitting nozzles 30 connected to water tubing 32. The bottom end disk 34 may be used as a platform to support a test specimen 33. The back end disk may simply seal its respective opening 16, or may include one or more of a variety of additional test components or sensors. Each end disk may be removably attached over its respective opening 16 by a conventional attachment means, such as one or more bolts 23 extending through the disk and into the side of the cavitation chamber 12.
In one embodiment, a laser source (not shown) is positioned outside the chamber near the top end disk 22. The laser is generally positioned such that it emits a laser beam 11 that is passed through the quartz window 29 in the top disk 22 and focused inside the chamber 12 at a desired distance above the test specimen 33. The quartz window 29 can be configured to cooperate with the position of the laser, such that the quartz window focuses the laser beam 11 at the desired position.
A variety of different types of lasers may be used in connection with the present invention. In one embodiment, the laser may be a pulsed neodymium-doped yttrium aluminum garnet (“Nd:YAG”) laser generated by a Raytheon brand SS-500 Laser System, which can generate maximum energy of 50 Joules per pulse. The wave length of this laser is 1.06 μm. The pulse length of this laser is approximately 5 to 7 msec, while the laser power, pulse frequency, and the total number of pulses may be varied by a user as controllable parameters.
A variety of media types may also be used within the chamber 12 for creating the cavitation flow. In one embodiment, the cavitation media is deionized water. For cooling purposes, the water may be circulated through the cavitation chamber 12 by an electrical pump (not shown), such as an Aquarius 300, Oase-Pumpen, produced in Germany. The pump may be immersed in a stainless steel reservoir (also not shown). As shown in
The test specimen 33 may be a sample of any desired material to be tested. In the embodiment shown in
The chamber 12 may additionally be equipped with a temperature measurement device, a temperature control device, a pressure measurement device and a microscope. In one embodiment, the temperature measurement device may be a thermocouple embedded in the cavitation chamber to monitor and record the temperature of the media in the vicinity of the test sample surface. In one embodiment, the temperature control device may include a solid-state heater and cooler (thermoelectric devices) in the bottom 34 and back disks or in other disks. In another embodiment, the temperature control device may include a water bath to control the temperature of the circulating media; or, in yet another embodiment, the cavitation chamber may be equipped with heating and cooling coils to control the temperature of the media within the chamber 12. The pressure measurement device may include a piezoelectric pressure transducer embedded in the cavitation chamber to monitor and record the pressure in the vicinity of the test sample surface. The microscope, with imaging and recording capabilities, can be attached to the viewport 31 on the front 24 disk to aid in monitoring the cavitation phenomenon.
In operation, when a high-power laser pulse is focused into the chamber, and into the liquid media, it initiates a laser optical breakdown of the water media, creating plasma in the focal volume of the water. The associated thermal expansion of the focal volume generates a series of shock waves within the water. When the propagating shock waves are bounced back by a solid surface (i.e., the interior surface of the chamber 12 or the surface of the test specimen 33), cavitation can occur due to the local pressure change. Thus, by controlling the intensity, spatial frequency, and the repetition rate of the laser pulse, one can generate repetitive pressure pulses within the solid boundary at the desired frequency, duration, and intensity and remotely deliver them to the tested surface to create cavitation damage on the selected material samples.
Cavitation experiments were conducted using the apparatus shown in
The aluminum samples were tested for 1000 sec using 7 ms pulse length, 5 Hz pulse frequency, a PFN of 60 (corresponding to 3.5 kW), and a defocus length of 0.5 inch. Air bubbles and sparks were observed during cavitation testing. The laser-illuminated area appeared white in color after the experiment. In contrast to stainless steel samples, when tested, the polished aluminum samples showed different behavior under the pulsed laser, such as that much fewer visible bubbles appeared on the targeted area of the sample surface. However, when testing an unpolished aluminum sample surface, numerous bubbles, sparks, and dark colored regions similar to that observed on stainless steel samples re-appeared. Based on these phenomenological observation and measurements of temperature and laser power absorption, this may be caused by the combination of non-focused transmitted laser heating and pulsed laser-plasma induced thermal cycle fatigue at the targeted surface area.
During the experiments, it was observed that a large amount of laser input energy was absorbed at the laser's focal point, in addition to the laser passing through the media. The rapid media volume change (or media evaporation) at focal point associated with laser plasma provided significant pressure rise to the surrounding media boundary and in turn caused shock waves and pressure waves to propagate throughout the media, including toward the targeted sample surface. Two populations of bubbles were formed during the testing: (1) at the laser focal region due to laser plasma, and (2) near the solid/media interface due to tensile rarefaction wave rebound from the solid boundary (sample surface). In addition, a fraction of non-focused laser beam also was transmitted through the water media and reached the sample surface. This provided heat input directly onto the sample surface. Based on the observed cavitation events it was noted that: 1) the amount of laser absorbed by the sample (or interacting with the solid boundary of the media) depended on the sample surface conditions, and 2) the temperature on the sample surface is dictated by the laser source, media flow-rate and the media's temperature, and the thermal conductivity of the sample material. It was noted that during the cavitation experiments, laser-plasma cavitation and laser illumination work synergistically to damage the surface of the target. At an early stage, cavitation induced pits and indentation were easily identified; while as the testing proceeded, surface crack populations also increased accordingly, which may indicate laser heating associated damage.
An alternative embodiment of the cavitation chamber 12 is shown in
In one embodiment, the purpose of the wave collector 40 is to confine the thermal shock induced pressure waves in a small volume (i.e., the interior of the wave collector) near the test sample surface. The wave collector 40 may be made of aluminum, although alternative materials may be used. In one embodiment, the wave collector 40 is a cylindrical tube with a predetermined and well-defined inner-surface topography. In one embodiment, shown in
Regardless of the internal shape, the wave collector may include at least one opening 35 aligned with the laser beam 11, such that the laser beam can be directed through the quartz window 29, the chamber 12, and into the wave collector 40 through the opening 35. When using the spherical wave collector, the laser beam 11 is focused at the spherical center 54. Thus, any shock wave 56 that travels away from the test specimen 33 is reflected back to the specimen 33 by the wave collector inner wall 50. For the case of the ellipsoidal wave collector, the laser beam 11 is focused on one of the two foci 58 of the defining ellipse, while the surface of the test specimen 33 is placed at the other focus 62. Since, in theory, any wave 60 emitted from one focal point of an ellipsoid will gather at the other focus, the ellipsoidal wave collector not only reflects the shock waves 60 travelling away from the test specimen 33, but also concentrates the shock waves 60 onto the specimen surface due to its position near the focal point 62. In order to mount the test sample 33 within the wave collector, a custom sample holder may be used. In one embodiment, the test sample 33 is mounted on top of the side end disk 27, between generally parallel internal walls 64, 66 of the wave collector 40.
In an experiment conducted using the wave collector embodiment, pressure was measured at the surface of the test specimen 33 with the ellipsoidal wave collector as well as without any wave collector. The laser pulse length was fixed at 7 ms, the pulse frequency was fixed at 5 Hz, and the power factor number was fixed at 60 (3.5 kW). It was observed that the pressure measured with the wave collector 40 was approximately 15 times higher than the pressure measured without the wave collector 40.
Tests were conducted with the wave collector embodiment using stainless steel and aluminum test samples. Stainless steel samples were tested using this embodiment with the ellipsoidal wave collector. The laser pulse length was 7 ms, frequency was 5 Hz, and the output power was 5.5 kW (PFN=70). The focus depth was 0.25 inch. Indents or pits were observed on the surface of the test specimen, which are clear evidences of cavitation damage. These damage features were very similar to those observed on stainless steel samples subject to vibratory cavitation.
Polished aluminum samples were also tested using the wave collector embodiment. Similar to the stainless steel samples, no evidence of direct laser heating was observed (unlike the tests performed without the wave collector). Indentation and pits were observed similar to those seen on the tested stainless steel samples and very similar to the damaged surface of aluminum samples subject to flow cavitation.
Another embodiment of the present invention is shown in
Another embodiment of the present invention is shown in
Another embodiment for generating a cavitation event and resulting cavitation damage is shown in
Another embodiment of the present invention is shown in
The above description is that of current embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. This disclosure is presented for illustrative purposes and should not be interpreted as an exhaustive description of all embodiments of the invention or to limit the scope of the claims to the specific elements illustrated or described in connection with these embodiments. For example, and without limitation, any individual element(s) of the described invention may be replaced by alternative elements that provide substantially similar functionality or otherwise provide adequate operation. This includes, for example, presently known alternative elements, such as those that might be currently known to one skilled in the art, and alternative elements that may be developed in the future, such as those that one skilled in the art might, upon development, recognize as an alternative. Further, the disclosed embodiments include a plurality of features that are described in concert and that might cooperatively provide a collection of benefits. Directional terms, such as “vertical,” “horizontal,” “top,” “bottom,” “upper,” “lower,” “inner,” “inwardly,” “outer” and “outwardly,” are used to assist in describing the invention based on the orientation of the embodiments shown in the illustrations. The use of directional terms should not be interpreted to limit the invention to packages of any specific orientation(s). The present invention is not limited to only those embodiments that include all of these features or that provide all of the stated benefits, except to the extent otherwise expressly set forth in the issued claims. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.
This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
