Acoustic Bubble S - Dynaflow, Inc

ABSAcousticBubbleSpectrometer Measurement of Bubble Size, Bubble Number & Void Fraction DYNAFLOW, INC.Research & Developmentin Applied SciencesDYNAFLOW, INC.

0121234567890123456789012345The ABS Advantage Very sensitive to bubbles Optically transparent liquid orcontainers are not required Distinguishes bubbles from particles Built on a PC Windows platform with auser-friendly graphical interface Measurements are easily and rapidlyconducted Validated by comparison withmicrophotography Near real-time measurement Cost ations User-friendly systemMaritime – hydrodynamics, propulsor performance,cavitation, power plants, turbomachinery, pumpsBiomedical – blood transfusions, artificial heart valve, bubbles in tissueand blood, decompression sicknessSpace – effects of microgravity on multiphase fluids, reduced pressureeffectsOceanography – atmosphere / ocean interface studies, air entrainment,oxygenation, sound transmission, background noiseMultiphase Flow – valves, pumps, propellers, fluid machinery,industrial & chemical processes, specialty fluids, measurement ofaeration bubbles, boilingEducation / Research – fluid dynamic & cavitation studies, bubble nucleiin water tunnelsEnvironmental – categorizing fish by size, monitoring sewage treatment,mixingABS2

5678901234Acoustic Bubble Spectrometer BackgroundThe ABS Acoustic Bubble Spectrometer is an instrumentdeveloped by DYNAFLOW, INC. to measure bubble size distributionsand void fraction in gas/liquid mixtures and flows. Not only does theABS measure the total “void fraction” (volume fraction of the gasphase in the liquid ), it also measures the distribution of the sizes ofthe bubbles. This difficult-to-measure characteristic proves invaluablein numerous areas. For example the top ten meters of the sea containmany bubbles that greatly affect acoustic properties by scatteringsignals and creating noise. Knowledge of the bubble size distributionisCavitation is the formation of vaporous and gaseous filled bubbles oralso important when modeling chemical or biological processes. The“cavities” in a liquid, typically due to a sudden change in pressure.ABS is also useful in the development of processes that mix a gas in aThis occurs, for example, along the flow path through a pump or overliquid such as waste treatment and the design of aeration systems.a propeller blade. Cavitation can have a dramatic effect on both theperformance and life of such devices. In particular, cavitationAnother application area where the ABS proves invaluable issignificantly affects the performance and operation of propellers,engineered systems that might experience cavitation.hydrofoils and pumps. The collapse of cavitation bubbles cause bothsubstantial physical damage and considerable noise. The distribution ofbubble nuclei strongly influences the propensity for cavitation. This isparticularly important if one wants to experimentally scale a devicesubject to cavitation since it is necessary to properly control and accountfor the existing bubble nuclei distribution in the liquid.Measuring bubble distribution, visually or optically, is painstaking orimpossible when dealing with a large system and a non transparentmedium. Moreover, it becomes increasingly difficult to distinguishsmall particulates from small bubbles. Bubbles, however, are moresensitive to acoustic waves than particulates, hence the ABS can readilypick out small bubbles in a liquid. The ABS also does not requireoptically transparent liquids or containing structures.3DYNAFLOW, INC.

How the ABS System WorksWorksThe ABS is built around a PC Windows platform and includes high-speedcards for signal generation and data acquisition. This high speed operation is required because the signal bursts are very short in order to detectvery small bubbles. Additionally, since the system determines any changesin the speed of sound due to the presence of bubbles, truesimultaneous sampling is mandatory, especially when the distance betweenthe two hydrophones is small.The Raw Signals Display which shows the transmitted and received data.Two hydrophones or transducers are used, one to transmit and the otherto receive a series of short monochromatic bursts of varying frequencies.From these readings, the attenuation and phase speed in the bubbly liquidare determined as a function of the frequency. The data from thesesignals are processed and analyzed with the copyrighted software algorithmsdeveloped by DYNAFLOW. These algorithms employ specialized techniquesto solve the mathematically ill-posed inverse problems, and thus obtain thebubble size distribution and void fraction.Sketch of the ABS Acoustic Bubble Spectrometer Operational SystemIn-line ABS MeasurementsABS4

The Principle of OperationABSThe ABS Acoustic Bubble Spectrometer exploits the fact that bubblesstrongly affect acoustic wave propagation. Bubble size distributionAcousticBubbleSpectrometermeasurements are based on a dispersion relation for sound wavepropagation through a bubbly liquid. Bubbles in the liquid have an effect onthe strength of the signal that reaches the receiver, as well as on the speedat which the signal travels between the emitter and receiver. Bubbles of agiven size have a resonant frequency where effects are the strongest, butwill affect signals of all frequencies. For each burst, the system measuresthe changes in the speed of sound and the signal amplitude. With more bubbles, the speed of sound and the signal’s amplitude decrease further.A multiphase fluid model for sound propagation through bubbly liquidsis combined with a model for the bubble oscillations, including variousdamping modes. The combined model relates the attenuation and phasevelocity of a sound wave to the bubble population or size distribution.These relations produce two ill-posed Fredholm integral equations thatrequire special treatment for their solution, particularly in the presence ofnoise. Novel algorithms developed by DYNAFLOW are used to accuratelysolve these equations using a constrained optimization technique thatimposes a number of physical constraints on the ABS.htm5DYNAFLOW, INC.

Operation of the ABSUser InputMeasurements are easily and rapidly conducted with the aid of a userfriendly Graphical User Interface. All physical, experimental, and analytical parameters are input by the user via dialog boxes initially loadedwith default values. Both raw and processed experimental data can besaved to disk for future use. The signals and results are displayedgraphically by the interface in real time and can also be stored orprinted. The measurement can be started either by clicking on an iconThe Dialog Box to enter Signal Parametersdisplayed on the screen or through use of an external trigger signal tosynchronize with a desired event.A “pure liquid” provides a background reference state. This “no bubble”reference state is used in calculating the bubble size distribution fromdata “with bubbles”. This reference state data set is obtained byconducting an experiment in the absence of bubbles under conditions andsettings otherwise identical to those which will be employed in determining the desired bubble size distribution.The Dialog Box to Enter Physical ParametersDisplayed ResultsResults from the analysis of the experimental data appear on the screenin easy to read displays in the form of plots.Sound Speed Ratio (u c/co) versus frequencyAttenuation Ratio (v) versus frequencyBubble Size Distribution as the number of bubbles per cubiccentimeter versus bubble radius in micronsVoid fraction contributions vs bubble sizeScreen shot of the ABS output graphical user interface showing the sound speed ratio u, theattenuation ratio v and the resulting bubble size distribution.ABS6

ValidationThe bubble distributions obtained from the ABS Acoustic Bubble1210raphy. Bubble populations were generated using electrolysis and air8Number per cm3Spectrometer have been validated by comparison with microphotog-injection through porous tubes. The bubble population obtained usingthe ABS compared very favorably with the results of the microphotography.ABS 30% max airPhoto 30% max airABS 60% max airPhoto 60% max air6420-2020406080100120140160Bubblr Radius, µmAir/WaterMixingTankSensitivity of the bubble size distribution measurements to the amount of injected air7Air Injection6Number per cm 3BubblyFlowMicroporousTubeTransducersWater ShearABS Average 6 tests5Photos Average 132 frames4321TestSection014Bubble ble Radius, µmComparison between ABS measurements and micro video photographyABSMain CharacteristicsData Acquisition5 MS/s. , 10 MS/s.Transducers used :Size rangeFrequency range1cm2 khz10 cm300 khzTypical Bubble sizes2μm500 μmNote that the software and the principle of operation are not dependenton the ranges of instrument operation, which are controlled by the dataacquisition card and the hydrophones used. These can be upgradedTotal Void fraction of 2x10-3without needing to modify the basic bubble size determination software.Comparison of the bubble density distribution between high speed video measurementsand ABS Acoustic Bubble Spectrometer 7DYNAFLOW, INC.

Generation II ABS Acoustic Bubble SpectrometerGeneration II ABS Acoustic Bubble Spectrometer exploits new aquisition cards and PC hardwaredevelopments and significantly improves the ABS performance. It is built on a Windows XP basedPC (either a desktop or a laptop) with high-speed cards for signal generation and data acquisition. Itnow boasts a data acquisition speed up to 10 MS/s for each channel, provides stable signal outputover a wide frequency range, and is being developed to support more than one set of hydrophonesto improve bubble size measurement range.Base System Includes the following: High-speed data aqusition card for signal generation and data acquisition (data samplingrate 5 MS/s per channel). A desktop computer, cables and adapters. First year support and upgrades of the DYNAFLOW proprietary ABS Software to drive thetransducers/hydrophones, and to analyze the data. Two 1/2in square transducers with 15 ft. cablesUpgrades for the system are as follows: Laptop PC Higher Data acquisition speed - 10 MS/s per channel Different size transducers Amplifier Multi-set hydrophones supportGeneration II SpecsBase Systeml Desktop Pentium IV (CPU Only), Windows XPl 5 MHz Data Acquisition Cardl 1/2 inch Transducers (2)l First Year Software Support and UpgradesUpgradeTotal Void fraction of 2x10-3Comparison of the bubble density distributionbetween high speed video measurements andABS Acoustic Bubble Spectrometer l10 MHz Data Acquisition Cardl Notebook PC w/Extension unit for PCIl 64 Bit Dual Core Processor

Side by Side Comparison of the Generation I and Generation II systemsRequirementsGeneration IIGeneration IIAdvantagesGeneration ISampling rate5 MS/s or 10 MS/s foreach channelUp to 2.2 MS/s sharedby two channelsMore accurate resultsespecially for highfrequency signalsResolution12-bit12-bitN/ASignal GenerationVoltage output20V peak-peak. Almost16V peak-peak, voltageconstant voltage outputoutput decreases withover the frequency range frequencyBetter and almostconstant signal outputSignal GenerationFrequency limitCapable of emitting andreceiving signals up to450 kHzCapable of emitting andreceiving signals up to200 kHzHigher frequency signalsEnable detection ofsmaller bubblesGeneration IGeneration IIAdvantagesRequirementsGeneration IIComputerAny computer with onespare PCI slotsComputer must have 2spare ISA and 1 sparePCI slotsCan use virtually anyComputerOperating SystemWindows NT/XPcompatibleWindows NTCompatible with updatedWindows XPPortable SystemAny laptop computerwith a PCMCIA slotSpecial order lunch boxPC is requiredPortable system can usevirtually any laptopDYNAFLOW, INC.10621-J Iron Bridge Road, Jessup, MD 20794-9381 USAPhone: (301) 604-3688, Fax: (301) 604-3689E-mail: info@dynaflow-inc.com URL: http://www.dynaflow-inc.com