Xiongjun Wu

Characterization of the Content of the Cavity Behind a High-Speed Supercavitating Body

A high speed/high flow test facility was designed and implemented to study experimen-tally the supercavitating flow behind a projectile nose in a controlled laboratory setting.The simulated projectile nose was held in position in the flow and the cavity interior wasmade visible by having the walls of the visualization facility “cut through” the supercav-ity. Direct visualization of the cavity interior and measurements of the properties of thecavity contents were made. Transducers were positioned in the test section within thesupercavitation volume to enable measurement of the sound speed and attenuation as afunction of the flow and geometry parameters. These characterized indirectly the contentof the cavity. Photography, high speed videos, and acoustic measurements were used toinvestigate the contents of the cavity. A side sampling cell was also used to sample in realtime the contents of the cavity and measure the properties. Calibration tests conducted inparallel in a vapor cell enabled confirmation that, in absence of air injection, the prop-erties of the supercavity medium match those of a mixture of water vapor and waterdroplets. Such a mixture has a very high sound speed with strong sound attenuation.Injection of air was also found to significantly decrease sound speed and to increasetransmission.

Development of an acoustic instrument for bubble size distribution measurement

Measurement of bubble size distribution and void fraction is of vital importance in many multiphase flow applications. This paper describes an acoustics based device, the ABS Acoustic Bubble Spectrometer®, which can conduct measurements accurately in near real-time in a cost-effective fashion. By propagating short bursts of sound at different frequencies through bubbly medium, it measures frequency dependent attenuations and phase velocities of the acoustic waves and uses them to obtain the bubble size distribution (number of bubbles per size) by solving an inverse problem. Recent developments, both in hardware and software, as well as their validations are presented, these new advancements enable the ABS to measure void fractions up to 3×10−3 with bubble sizes ranging from 10 µm to 3 mm.

Gas Bubble Size Measurements in Liquid Mercury Using an Acoustic Spectrometer

A properly dispersed population of small bubbles can mitigate cavitation damage to aspallation neutron source target. In order to measure such a bubble population, anacoustic device was developed and implemented in a mercury loop at ORNL. The instru-ment generated pulses of various frequencies and measured their acoustic propagation inthe bubbly medium. It then deduced sound speed and attenuation at the various frequen-cies and used an inverse problem solver to provide near real-time measurements of bub-ble size distribution and void fraction. The measurements were then favorably comparedwith an optical method. [DOI: 10.1115/1.4026440]

Development of an acoustics‐based instrument for bubble measurement in liquids

The acoustic bubble spectrometer (ABS) is an acoustics-based device that provides bubble size distribution in a bubbly liquid through measurement at various frequencies of the sound speed and attenuation and solution of an inverse problem. Acoustic bursts of varying frequencies are emitted by one hydrophone and detected by another. A PC and data boards control signal generation, detection, signal processing, inverse problem solution, and results display. Extensive validation experiments were conducted against high speed-video optical measurements. The two methods give very close results for void fractions up to 3e-3, with the ABS possessing the significant advantage of enabling near real-time measurements. The field of application is being expanded to media other than water, and the technique improved to detect larger void fractions, with the help of numerical simulations of non-linear behavior of bubble clouds in acoustic fields.

Modeling Separation and Cavitation Behind a Blunt Body

Cavitation flow behind a blunt body is modeled using a physics-based numerical model of cavitation initiation and transition to larger cavities. The calculations initiate from the dynamics of nuclei, then tracks the dispersed bubble phase with a two-phase viscous model. This solver includes a level set method to model coalescence of the nuclei into large cavities and to track the dynamics of the resulting free surfaces. A transition scheme enables collection of the bubbles into a large cavity and also enables breakup of a large cavity into a bubble cloud. Using this model, simulations are conducted for different flow velocities and corresponding cavitation regimes. When the velocity is relatively small (i.e., large cavitation number), flow separation behind the body results in the shedding of vortices, which capture nuclei in their cores to form elongated vortical cavities. As the flow velocity increases (or as the ambient pressure decreases) the flow evolves into a separated flow with a large cavity behind the body. A reentrant jet may form and move upstream into the cavity towards the body. This jet periodically shears off portions of the cavity volume, resulting in large amounts of bubble clouds. These results are in good qualitative agreements with experimental observations.

Development of a DYNASWIRL ® Phase Separator for Space Applications

®phase separator is able to force even very low void fraction gas out of the liquid into the central core of the vortex, and the extract it from the core. A prototype that was tested on a NASA reduced gravity flight in August 2009 successfully reduced the void fraction down to 10 -8 . Data analysis and flow visualization indicated that a steady vortex core was maintained and that the gas was continuously removed from the vortex core under the reduced-gravity conditions. Development of the phase separator was guided by 3D Numerical simulations using 3DYNAFS

Development of a passive phase separator for space and earth applications

The limited amount of liquids and gases that can be carried to space makes it imperative to recycle and reuse these fluids for extended human operations. During recycling processes gas and liquid phases are often intermixed. In the absence of gravity, separating gases from liquids is challenging due to the absence of buoyancy. This paper describes development of a passive phase separator that is capable of efficiently and reliably separating gas-liquid mixtures of both high and low void fractions in a wide range of flow rates that is applicable to for both space and earth applications.

Development of an Efficient Phase Separator for Space and Ground Applications

The limited amount of liquids and gases that can be carried to space makes it imperative to recycle and reuse these fluids for extended human operations. During recycling processes gas and liquid phases are often intermixed. In the absence of gravity, separating gases from liquids is challenging due to the absence of buoyancy. This paper discusses a phase separator that is capable of efficiently and reliably separating gas-liquid mixtures of both high and low void fractions in a wide range of flow rates that is applicable to reduced and zero gravity environments. The phase separator consists of two concentric cylindrical chambers. The fluid introduced in the space between the two cylinders enters the inner cylinder through tangential slots and generates a high intensity swirling flow. The geometric configuration is selected to make the vortex swirl intense enough to lead to early cavitation which forms a cylindrical vaporous core at the axis even at low flow rates. Taking advantage of swirl and cavitation, the phase separator can force gas out of the liquid into the central core of the vortex even at low void fraction. Gas is extracted from one end of the cylinder axial region and liquid is extracted from the other end. The phase separator has successfully demonstrated its capability to reduce mixture void fractions down to 10 -8 and to accommodate incoming mixture gas volume fractions as high as 35% in both earth and reduced gravity flight tests. The phase separator is on track to be tested by NASA on the International Space Station (ISS). Additionally, the phase separator design exhibits excellent scalability. Phase separators of different dimensions, with inlet liquid flow rates that range from a couple of GPMs to a few tens of GPMs, have been built and tested successfully in the presence and absence of the gravity. Extensive ground experiments have been conducted to study the effects of main design parameters on the performance of the phase separator, such as the length and diameter of the inner cylinder; the size, location, and layout of injection slots and exit orifices, etc., on the swirling flow behavior, and on the gas extraction performance. In parallel, numerical simulations, utilizing a two-phase Navier-Stokes flow solver coupled with bubble dynamics, have been conducted extensively to facilitate the development of the phase separator. These simulations have enabled us to better understand the physics behind the phase separation and provided guideline for system parts optimization. This paper describes our efforts in developing the passive phase separator for both space and ground applications. INTRODUCTION The limits on the amount of liquids and gases that can be carried to space make it imperative to recycle and reuse these fluids. On earth, bubbles in a liquid are easily separable by buoyancy. In microgravity, other external forces, such as a centrifugal force, must be utilized to separate bubbles from liquids. Two categories of centrifugal force separators exist: one uses active rotation by mechanically spinning the tank. This is very efficient but requires a shaft, bearings, and a motor, and a lot of energy. In the second, the tank is fixed, and the rotation is induced by eccentric injection of the mixture (Free Vortex Separator or FVS). These passive separators have no moving mechanical parts, require low power, and have been investigated intensively owing to their simplicity and dependability [1-3]. McQuillen et al. at NASA Glenn Research center have been developing the Cascade Cyclonic Separation Device (CSD-C) since the mid 90’s [4,5]. This separator has