Near-Wall Velocity and Bubble Characteristics in Microbubble Laden Flows

2006 ◽  
Author(s):  
Daniel R. Morse ◽  
Thomas G. Shepard ◽  
James A. Liburdy
Keyword(s):  
Phase Velocity ◽  
Bubble Size ◽  
Fluid Velocity ◽  
Measurement Techniques ◽  
Bubble Velocity ◽  
Wall Region ◽  
Bubble Generation ◽  
Flow Rates ◽  
Piv Measurement ◽  
Near Wall

Microbubble drag reduction has been observed in high Reynolds number turbulent flows. The interaction of microbubbles with the viscous sublayer seems to be of interest. In this study a microchannel on the order of 100 microns was used to simulate the shear rate of the near-wall region of a high Re flow. The interaction of the fluid flow with microbubbles at low void fraction in a microchannel was studied. The microbubble sizes ranged from approximately 10–50 microns. The liquid phase velocities were obtained by PIV measurement techniques. Electrolysis was used to generate bubbles within the channel and microbubble velocities in the flow were determined using separate cross correlation calculations. Simultaneous comparisons are made between the image-averaged bubble velocity and the image-averaged fluid velocity. Image processing techniques were utilized to both remove bubbles and decrease noise in the image. Results are shown comparing the fluid only velocity profile with the two-phase velocity profiles at three flow rates and two bubble generation cases. Results presented include the phase velocity differences, bubble size and bubble separation distances for three flow rates and three different bubble generation levels. It is seen that flow rates within the microchannel significantly reduce the average bubble size.

scholarly journals Investigating a pulsating flow in the smooth channel and at the bifurcation section with regard to the popliteal artery hemodynamics

2021 ◽  
Vol 2119 (1) ◽  
pp. 012020
Author(s):  
V M Molochnikov ◽  
N I Mikheev ◽  
A N Mikheev ◽  
A A Paereliy ◽  
A E Goltsman
Keyword(s):  
Popliteal Artery ◽  
Vortex Flow ◽  
Pulsating Flow ◽  
Experimental Setup ◽  
Wall Region ◽  
Flow Rates ◽  
Turbulent Transition ◽  
Smooth Channel ◽  
Outflow Channels ◽  
Near Wall

Abstract Experimental setup is described. Pulsating flow in a smooth channel, and steady and pulsating flows at a bifurcation section simulating the distal end of an artery anastomosis at different flow rates in the main and outflow channels are studied. Indications of laminar-turbulent transition are observed in the near-wall region of the smooth channel. Mechanisms of turbulization of the near-wall region in the pulsating flow are suggested. Vortex flow structure in the bifurcation section is analyzed.

The Effect of Nozzle Shape and Configuration on Bubble Formation in a Liquid Cross Flow

2012 ◽  
Author(s):  
Mona Hassanzadeh Jobehdar ◽  
Aly H. Gadallah ◽  
Kamran Siddiqui ◽  
Wajid A. Chishty
Keyword(s):  
Liquid Flow ◽  
Bubble Size ◽  
High Speed ◽  
Waste Water Treatment ◽  
Bubble Formation ◽  
Cross Flow ◽  
Bubble Velocity ◽  
Two Dimensional ◽  
Flow Rates ◽  
Nozzle Shape

Gas injection into a liquid cross flow from a nozzle causes bubble formations which have potential applications in industry such as chemical plants, waste water treatment and bio- and nuclear-reactors. The purpose of this study is to experimentally investigate the effects of nozzle shape and configuration with respect to the liquid cross-flow direction, on the bubbly flow characteristics such as bubble formation, detached bubble size and frequency at different gas and liquid flow rates. The experiments were conducted in a Plexiglas two-dimensional rig using a high speed camera. High speed imaging and an image processing algorithm were used to track each individual bubble and to quantify the bubble growth as well as the detachment frequency and the bubble velocity. Back light shadowgraphy which utilizes a low intensity diffuse light source to illuminate the background was used to image bubbles. Nozzles were mounted in the test section which was designed such that the flow in this section has a two-dimensional profile. The results showed that the bubble size increases with an increase in GLR (gas to liquid flow rates ratio). Furthermore, the bubble formations and detached bubble size were strongly influenced by the nozzle shape and configuration.

Experimental Investigation on Near Wall Natural Convection With Fluorescent Nanoparticles

2012 ◽  
Author(s):  
Leping Zhou ◽  
Yaofa Li ◽  
Minami Yoda ◽  
G. P. Peterson
Keyword(s):  
Experimental Investigation ◽  
Diffusion Coefficient ◽  
Natural Convection ◽  
Fluid Velocity ◽  
Fluid Motion ◽  
Brownian Diffusion ◽  
Fluorescent Nanoparticles ◽  
Wall Region ◽  
Heating Wire ◽  
Near Wall

The mechanisms of microscale natural convection are yet to be understood for its fundamental characteristics differing from those at the macroscale. The spatial distribution of the fluid velocity in the near wall region, which is crucial to the mechanisms of microscale natural convection, is especially not well understood. An experimental investigation of the spatial velocity and the Brownian diffusion coefficient in the near wall region of a platinum heating wire, 20 micrometers in diameter, was conducted using a multilayer nano-particle image velocimetry (MnPIV) technique. The in-plane velocity of deionized water in the near wall region between a micro heating wire and a glass wall was measured using MnPIV technique with fluorescently tagged polystyrene nanoparticles, 200 nm in diameter. The results indicate that both the magnitude and the deviation of the fluid motion increases with increasing heat flux. The Brownian diffusion coefficient was also calculated and indicates that the Brownian motion in microscale natural convection is important, especially for nanoscale colloidal tracers. The thermophoresis was found to be negligible, even at low heating powers. To enhance the resolution of the fluid motion in the near wall region, it is necessary to use smaller fluorescent nanoparticles as seed tracers.

Direct numerical simulations of Rayleigh–Bénard convection in water with non-Oberbeck–Boussinesq effects

2019 ◽  
Vol 881 ◽  
pp. 1073-1096 ◽  
Author(s):  
Andreas D. Demou ◽  
Dimokratis G. E. Grigoriadis
Keyword(s):  
Numerical Simulations ◽  
Two Dimensions ◽  
Direct Numerical Simulations ◽  
Wall Region ◽  
Number Range ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard ◽  
Near Wall

Rayleigh–Bénard convection in water is studied by means of direct numerical simulations, taking into account the variation of properties. The simulations considered a three-dimensional (3-D) cavity with a square cross-section and its two-dimensional (2-D) equivalent, covering a Rayleigh number range of $10^{6}\leqslant Ra\leqslant 10^{9}$ and using temperature differences up to 60 K. The main objectives of this study are (i) to investigate and report differences obtained by 2-D and 3-D simulations and (ii) to provide a first appreciation of the non-Oberbeck–Boussinesq (NOB) effects on the near-wall time-averaged and root-mean-squared (r.m.s.) temperature fields. The Nusselt number and the thermal boundary layer thickness exhibit the most pronounced differences when calculated in two dimensions and three dimensions, even though the $Ra$ scaling exponents are similar. These differences are closely related to the modification of the large-scale circulation pattern and become less pronounced when the NOB values are normalised with the respective Oberbeck–Boussinesq (OB) values. It is also demonstrated that NOB effects modify the near-wall temperature statistics, promoting the breaking of the top–bottom symmetry which characterises the OB approximation. The most prominent NOB effect in the near-wall region is the modification of the maximum r.m.s. values of temperature, which are found to increase at the top and decrease at the bottom of the cavity.

The Effects of Adverse Pressure Gradients on Momentum and Thermal Structures in Transitional Boundary Layers: Part 2—Fluctuation Quantities

1996 ◽  
Vol 118 (4) ◽  
pp. 728-736 ◽  
Author(s):  
S. P. Mislevy ◽  
T. Wang
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Transition Region ◽  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Turbulent Shear ◽  
Wall Region ◽  
Pressure Gradients ◽  
Baseline Case ◽  
Near Wall

The effects of adverse pressure gradients on the thermal and momentum characteristics of a heated transitional boundary layer were investigated with free-stream turbulence ranging from 0.3 to 0.6 percent. Boundary layer measurements were conducted for two constant-K cases, K1 = −0.51 × 10−6 and K2 = −1.05 × 10−6. The fluctuation quantities, u′, ν′, t′, the Reynolds shear stress (uν), and the Reynolds heat fluxes (νt and ut) were measured. In general, u′/U∞, ν′/U∞, and νt have higher values across the boundary layer for the adverse pressure-gradient cases than they do for the baseline case (K = 0). The development of ν′ for the adverse pressure gradients was more actively involved than that of the baseline. In the early transition region, the Reynolds shear stress distribution for the K2 case showed a near-wall region of high-turbulent shear generated at Y+ = 7. At stations farther downstream, this near-wall shear reduced in magnitude, while a second region of high-turbulent shear developed at Y+ = 70. For the baseline case, however, the maximum turbulent shear in the transition region was generated at Y+ = 70, and no near-wall high-shear region was seen. Stronger adverse pressure gradients appear to produce more uniform and higher t′ in the near-wall region (Y+ < 20) in both transitional and turbulent boundary layers. The instantaneous velocity signals did not show any clear turbulent/nonturbulent demarcations in the transition region. Increasingly stronger adverse pressure gradients seemed to produce large non turbulent unsteadiness (or instability waves) at a similar magnitude as the turbulent fluctuations such that the production of turbulent spots was obscured. The turbulent spots could not be identified visually or through conventional conditional-sampling schemes. In addition, the streamwise evolution of eddy viscosity, turbulent thermal diffusivity, and Prt, are also presented.

Leakage Flow Investigations on a Through-Wall Crack Related to Thermal Fatigue of Nuclear Power Plant Piping

2017 ◽  
Author(s):  
Stefan Schmid ◽  
Rudi Kulenovic ◽  
Eckart Laurien
Keyword(s):  
Experimental Data ◽  
Nuclear Power ◽  
Numerical Models ◽  
Measurement Techniques ◽  
Experimental Investigations ◽  
Leakage Flow ◽  
Test Rig ◽  
Reduced Pressure ◽  
Flow Rates ◽  
Set Up

For the validation of empirical models to calculate leakage flow rates in through-wall cracks of piping, reliable experimental data are essential. In this context, the Leakage Flow (LF) test rig was built up at the IKE for measurements of leakage flow rates with reduced pressure (maximum 1 MPA) and temperature (maximum 170 °C) compared to real plant conditions. The design of the test rig enables experimental investigations of through-wall cracks with different geometries and orientations by means of circular blank sheets with integrated cracks which are installed in the tubular test section of the test rig. In the paper, the experimental LF set-up and used measurement techniques are explained in detail. Furthermore, first leakage flow measurement results for one through-wall crack geometry and different imposed fluid pressures at ambient temperature conditions are presented and discussed. As an additional aspect the experimental data are used for the determination of the flow resistance of the investigated leak channel. Finally, the experimental results are compared with numerical results of WinLeck calculations to prove specifically in WinLeck implemented numerical models.

Bubble dynamics and oxygen transfer in a hypolimnetic aerator

1998 ◽  
Vol 37 (2) ◽  
pp. 293-300 ◽  
Author(s):  
Vickie L. Burris ◽  
John C. Little
Keyword(s):  
Gas Phase ◽  
Bubble Size ◽  
Oxygen Transfer ◽  
Bubble Dynamics ◽  
Water Flow Rate ◽  
Flow Rates ◽  
Wide Range ◽  
The City ◽  
Close Fit ◽  
Water Supply Reservoirs

A hypolimnetic aerator operating in one of the City of Norfolk's water supply reservoirs was tested. Dissolved oxygen (DO) profiles, water flow rate, and gas-phase holdup were measured over a wide range of applied air flow rates. A model that was developed to predict oxygen transfer in a Speece Cone was modified to conform to the conditions of the hypolimnetic aerator. By varying a single parameter (the initial bubble size) the model was found to provide a close fit to the experimental DO profiles as well as the observed gas-phase holdup. The model was used to show that a doubling in oxygen transfer may be achieved if initial bubble size is reduced from 5 mm to 2.5 mm. Knowing the initial bubble size, it should be possible to predict water velocity by incorporating the effect of momentum. Further work is now underway to test this approach and to examine the possibility of extending this generalized model to cover the range of hypolimnetic aeration and oxygenation devices.

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