Probing and Imaging of Vapor–Water Mixture Properties Inside Partial/Cloud Cavitating Flows

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
Churui Wan ◽  
Benlong Wang ◽  
Qian Wang ◽  
Yongliu Fang ◽  
Hua Liu ◽  
...  
Keyword(s):  
Void Fraction ◽  
Length Distribution ◽  
Vapor Concentration ◽  
Water Mixture ◽  
Cavitating Flows ◽  
Cloud Cavitation ◽  
Cavitation Region ◽  
Impedance Probe ◽  
Density Values ◽  
High Vapor

Experimental results of the void fraction, statistical chord length distribution (CLD), and bubble size distribution (BSD) inside and downstream of hydrodynamic cavities are presented at the laboratory scale. Various cavitating flows have been intensively studied in water tunnels for several decades, but no corresponding quantitative CLD and BSD data were reported. This experimental study is aimed at elaboration of a general approach to measure CLD in typical cavitating flows. Dual-tip electrical impedance probe (dtEIP) is used to measure the void fraction and CLD in different cavitation flows over a flat plate, including both supercavitation and sheet/cloud cavitation. For supercavitating flows, the void fraction of vapor is unity in the major cavity region. In contrast, the maximum void fraction inside the sheet/cloud cavitation region is less than unity in the present studies. The high vapor concentration region is located in the center of the cavity region. Based on the experimental data of CLD, it is found that the mean chord lengths are around 2.9–4.8 mm and 1.9–4.4 mm in the center region and closure region, respectively. The backward converting bubble diameters at the peak of BSD have similar magnitude, with probability density values exceeding 0.2. Empirical parameters of CLD and BSD are obtained for different cavity regions.

scholarly journals Influence of the Cavitation Model on the Simulation of Cloud Cavitation on 2D Foil Section

2008 ◽  
Vol 2008 ◽  
pp. 1-12 ◽  
Author(s):  
S. Frikha ◽  
O. Coutier-Delgosha ◽  
J. A. Astolfi
Keyword(s):  
Void Fraction ◽  
Analytical Study ◽  
Physical Models ◽  
Model Parameters ◽  
Source Terms ◽  
Cavitating Flows ◽  
Cavitation Model ◽  
Cloud Cavitation ◽  
Flow Configurations ◽  
And Behavior

For numerical simulations of cavitating flows, many physical models are currently used. One approach is the void fraction transport equation-based model including source terms for vaporization and condensation processes. Various source terms have been proposed by different researchers. However, they have been tested only in different flow configurations, which make direct comparisons between the results difficult. A comparative study, based on the expression of the source terms as a function of the pressure, is presented in the present paper. This analytical approach demonstrates a large resemblance between the models, and it also clarifies the influence of the model parameters on the vaporization and condensation terms and, therefore, on the cavity shape and behavior. Some of the models were also tested using a 2D CFD code in configurations of cavitation on two-dimensional foil sections. Void fraction distributions and frequency of the cavity oscillations were compared to existing experimental measurements. These numerical results confirm the analytical study.

scholarly journals Bubble Dynamics in a Two-Phase Bubbly Mixture

2010 ◽  
Author(s):  
Arvind Jayaprakash ◽  
Sowmitra Singh ◽  
Georges Chahine
Keyword(s):  
Void Fraction ◽  
High Speed ◽  
Bubble Dynamics ◽  
Bubble Radius ◽  
Discharge Voltage ◽  
Large Bubble ◽  
Water Mixture ◽  
Two Phase ◽  
Mixture Density ◽  
Bubbly Medium

The dynamics of a primary relatively large bubble in a water mixture including very fine bubbles is investigated experimentally and the results are provided to several parallel on-going analytical and numerical approaches. The main/primary bubble is produced by an underwater spark discharge from two concentric electrodes placed in the bubbly medium, which is generated using electrolysis. A grid of thin perpendicular wires is used to generate bubble distributions of varying intensities. The size of the main bubble is controlled by the discharge voltage, the capacitors size, and the pressure imposed in the container. The size and concentration of the fine bubbles can be controlled by the electrolysis voltage, the length, diameter, and type of the wires, and also by the pressure imposed in the container. This enables parametric study of the factors controlling the dynamics of the primary bubble and development of relationships between the bubble characteristic quantities such as maximum bubble radius and bubble period and the characteristics of the surrounding two-phase medium: micro bubble sizes and void fraction. The dynamics of the main bubble and the mixture is observed using high speed video photography. The void fraction/density of the bubbly mixture in the fluid domain is measured as a function of time and space using image analysis of the high speed movies. The interaction between the primary bubble and the bubbly medium is analyzed using both field pressure measurements and high-speed videography. Parameters such as the primary bubble energy and the bubble mixture density (void fraction) are varied, and their effects studied. The experimental data is then compared to simple compressible equations employed for spherical bubbles including a modified Gilmore Equation. Suggestions for improvement of the modeling are then presented.

Transcritical Patterns of Cavitating Flow and Trends of Acoustic Level

2001 ◽  
Vol 123 (4) ◽  
pp. 850-856 ◽  
Author(s):  
Wei Gu ◽  
Yousheng He ◽  
Tianqun Hu
Keyword(s):  
Liquid Flow ◽  
High Speed ◽  
Cavity Flow ◽  
Periodic Flow ◽  
Cavitating Flows ◽  
Cavitation Noise ◽  
Cloud Cavitation ◽  
Three Stages ◽  
Cavity Shedding ◽  
Axisymmetric Bodies

Hydroacoustics of the transcritical cavitating flows on a NACA16012 hydrofoil at a 2/5/8-degree angle of attack and axisymmetric bodies with hemispherical and 45-degree conical headforms were studied, and the process of cloud cavitation shedding was observed by means of high-speed cinegraphy. By expressing the cavitation noise with partial acoustic level, it is found that the development of cavitation noise varies correspondingly with cavitation patterns. The instability of cavitation is a result of cavity-flow interaction, and is mainly affected by the liquid flow rather than by the cavitation bubbles. A periodic flow structure with a large cavitation vortex is observed and found to be responsible for inducing the reentrant-jet and consequent cavitation shedding, and explains the mechanism of periodic cavitation shedding from a new viewpoint. New terms for the three stages, growing, hatching and breaking, are used to describe the process of cavity shedding.

Drag Coefficient and Two-Phase Friction Multiplier on Tube Bundles Subjected to Two-Phase Cross-Flow

2012 ◽  
Vol 135 (1) ◽  
Author(s):  
W. G. Sim ◽  
Njuki W. Mureithi
Keyword(s):  
Drag Coefficient ◽  
Void Fraction ◽  
Euler Number ◽  
Tube Bundle ◽  
Cross Flow ◽  
Water Mixture ◽  
Duct Flow ◽  
Two Phase ◽  
Mass Fluxes ◽  
Wide Range

An approximate analytical model, to predict the drag coefficient on a cylinder and the two-phase Euler number for upward two-phase cross-flow through horizontal bundles, has been developed. To verify the model, two sets of experiments were performed with an air–water mixture for a range of pitch mass fluxes and void fractions. The experiments were undertaken using a rotated triangular (RT) array of cylinders having a pitch-to-diameter ratio of 1.5 and cylinder diameter 38 mm. The void fraction model proposed by Feenstra et al. was used to estimate the void fraction of the flow within the tube bundle. An important variable for drag coefficient estimation is the two-phase friction multiplier. A new drag coefficient model has been developed, based on the single-phase flow Euler number formulation proposed by Zukauskas et al. and the two-phase friction multiplier in duct flow formulated by various researchers. The present model is developed considering the Euler number formulation by Zukauskas et al. as well as existing two-phase friction multiplier models. It is found that Marchaterre's model for two-phase friction multiplier is applicable to air–water mixtures. The analytical results agree reasonably well with experimental drag coefficients and Euler numbers in air–water mixtures for a sufficiently wide range of pitch mass fluxes and qualities. This model will allow researchers to provide analytical estimates of the drag coefficient, which is related to two-phase damping.

Modeling of unsteady structure of sheet/cloud cavitation around a two-dimensional stationary hydrofoil

2015 ◽  
Vol 231 (3) ◽  
pp. 455-469 ◽  
Author(s):  
Feng Hong ◽  
Jianping Yuan ◽  
Banglun Zhou ◽  
Zhong Li
Keyword(s):  
Volume Fraction ◽  
Cavitating Flow ◽  
Wake Flow ◽  
Two Dimensional ◽  
Cavitating Flows ◽  
Cavitation Model ◽  
Ansys Fluent ◽  
Cloud Cavitation ◽  
Lift And Drag ◽  
Multi Phase

Compared to non-cavitating flow, cavitating flow is much complex owing to the numerical difficulties caused by cavity generation and collapse. In the present work, cavitating flow around a two-dimensional Clark-Y hydrofoil is studied numerically with particular emphasis on understanding the cavitation structures and the shedding dynamics. A cavitation model, coupled with the mixture multi-phase approach, and the modified shear stress transport k-ω turbulence model has been developed and implemented in this study to calculate the pressure, velocity, and vapor volume fraction of the hydrofoil. The cavitation model has been implemented in ANSYS FLUENT platform. The hydrofoil has a fixed angle of attack of α = 8° with a Reynolds number of Re = 7.5 × 105. Simulations have been carried out for various cavitation numbers ranging from non-cavitating flows to the cloud cavitation regime. In particular, we compared the lift and drag coefficients, the cavitation dynamics, and the time-averaged velocity with available experimental data. The comparisons between the numerical and experimental results show that the present numerical method is capable to predict the formation, breakup, shedding, and collapse of the sheet/cloud cavity. The periodical formation, shedding, and collapse of sheet/cloud cavity lead to substantial increase in turbulent velocity fluctuations in the cavitation regimes around the hydrofoil and in the wake flow.

An Investigation of the Propagation of Pressure Perturbations in Bubbly Air/Water Flows

1988 ◽  
Vol 110 (2) ◽  
pp. 494-499 ◽  
Author(s):  
A. E. Ruggles ◽  
R. T. Lahey ◽  
D. A. Drew ◽  
H. A. Scarton
Keyword(s):  
Void Fraction ◽  
Fluid Model ◽  
Propagation Speed ◽  
Water Mixture ◽  
Volume Coefficient ◽  
Two Fluid Model ◽  
Pressure Perturbations ◽  
Dispersion And Attenuation ◽  
Range Of Values ◽  
Two Fluid

Dispersion and attenuation was measured for standing waves in a vertical waveguide filled with a bubbly air/water mixture. The propagation speed of pressure pulses was also measured. The data were compared with a two-fluid model for a range of values of the virtual volume coefficient, CVM. The experimentally determined CVM was found to be a function of global void fraction (〈α〉). Moreover it was noted that this CVM was less strongly related to void fraction than those proposed by Zuber (1964) and Van Wijngaarden (1976).

Improving the Scaling of Two-Phase Damping by Local Observations in a Tube Bundle

2010 ◽  
Author(s):  
Sylviane Pascal-Ribot ◽  
Yves Blanchet
Keyword(s):  
Void Fraction ◽  
Chord Length ◽  
Working Fluid ◽  
Damping Factor ◽  
Water Mixture ◽  
Elastic Instability ◽  
Gas Velocity ◽  
Two Phase ◽  
Phase Damping ◽  
Flexible Tube

Experimental data are reported to investigate the dissipation mechanisms that govern two-phase damping and fluid-elastic instability of a single flexible tube in a rigid array. The working fluid is an air-water mixture and the void fraction and interfacial velocity are measured using a bi-optical probe (BOP) positioned upstream of the flexible tube. The present work aims at revisiting the problem of fluid-elastic instability by developping various scaling models of two-phase fluid damping before the onset of instability. For most of the experiments, the measured damping factor was seen to increase with increasing bubble chord length, with decreasing superficial liquid velocity, and with decreasing amplitude of vibration. The Connor’s approach has been generalized to the two-phase flows provided that the reduced velocity is calculated with gas velocity and with mixture density deduced from the local void fraction measured inside the bundle. The collapse of the fluid-elastic data is more satisfactory than when using the Homogeneous Equilibrium Model (HEM). Void fraction, gas velocity, relative velocity, liquid superficial velocity, bubble chord length, vibratory frequency are shown to be relevant parameters to reduce the two-phase damping data. The use of these parameters in non-dimensional numbers such as Capillary number, Reynolds number, pressure ratio, mass ratio leads to helpful observations as well as several promising approaches to the reduction of two-phase damping.

Visualization of Two-Phase Flow Through Microchannel Heat Exchangers

2015 ◽  
Author(s):  
Dhruv C. Hoysall ◽  
Khoudor Keniar ◽  
Srinivas Garimella
Keyword(s):  
Void Fraction ◽  
Two Phase Flow ◽  
Flow Distribution ◽  
Interfacial Area ◽  
High Speed Photography ◽  
Phase Flow ◽  
Water Mixture ◽  
Two Phase ◽  
Microchannel Array ◽  
Flow Through

Multiphase flow phenomena in single micro- and minichannels have been widely studied. Characteristics of two-phase flow through a large array of microchannels are investigated here. An air-water mixture is used to represent the two phases flowing through a microchannel array representative of those employed in practical applications. Flow distribution of the air and water flow across 52 parallel microchannels of 0.3 mm hydraulic diameter is visually investigated using high speed photography. Two microchannel configurations are studied and compared, with mixing features incorporated into the second configuration. Slug and annular flow regimes are observed in the channels. Void fractions and interfacial areas are calculated for each channel from these observations. The flow distribution is tracked at various lengths along the microchannel array sheets. Statistical distributions of void fraction and interfacial area along the microchannel array are measured. The design with mixing features yields improved flow distribution. Void fraction and interfacial area change along the length of the second configuration, indicating a change in fluid distribution among the channels. The void fraction and interfacial area results are used to predict the performance of different microchannel array configurations for heat and mass transfer applications. Results from this study can help inform the design of compact thermal-fluid energy systems.

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