Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R25: Multiphase Flows: Bubbly Flows, Cavitation and Ventilation |
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Chair: Gretar Tryggvason, University of Notre Dame Room: 304 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R25.00001: DNS and Modeling of Turbulent Gas-Liquid Channel Flows Gretar Tryggvason, Ming Ma, Jiacai Lu DNS studies of gas-liquid flows in vertical turbulent channels are presented. Results from a simulation of a pressure driven turbulent channel flow with a friction Reynolds number of 500 where a large number of bubbles of different sizes are injected at time zero, shows that small bubbles quickly migrate to the wall, but the flow takes much longer to adjust to the new bubble distribution. The evolution of turbulent statistic and the void fraction distribution is examined, including area concentration and the components of the area tensor. Another series of simulations of bubbles injected into turbulent channel flow, where the bubbles are allowed to coalesce and break apart, is also presented. For high enough surface tension all the bubbles coalesce into one large slug, but as the surface tension is reduced, large enough bubbles break up and the flow eventually reaches an approximate equilibrium where coalescence is matched by breakups. The resulting state generally contains bubbles with a distribution of sizes. The various quantities characterizing the flow are followed over time and their dependency of the flow parameters examined. Preliminary attempts to model the flow using a set of averaged equations, using closure relations derived from the DNS data are discussed. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R25.00002: Nucleation of Super-Critical Carbon Dioxide in a Venturi Nozzle Dorrin Jarrahbashi, Sandeep Pidaparti, Devesh Ranjan The supercritical carbon dioxide (S-CO$_{2}$) Brayton cycle combines the primary advantages of the ideal Brayton and Rankine cycles by utilizing CO$_{2}$ above its critical pressure. In addition to single phase and small back work ratios, supercritical fluids offer other advantages, e.g. heat transfer augmentation and low specific volume. Pressure reduction at the entrance of the compressor may cause homogenous nucleation, vapor production, and collapse of bubbles due to operation near the saturation conditions. Transient behavior of the flow after nucleation may cause serious issues in operation of the cycle and affect the materials used in design. The flow of S-CO$_{2}$ through a venturi nozzle near the critical point has been studied. A transient compressible 3D Navier-Stokes solver, coupled with continuity, and energy equation has been used. Developed FIT libraries based on a piecewise biquintic spline interpolation of Helmholtz energy have been integrated with OpenFOAM to model S-CO$_{2}$ properties. The mass fraction of vapor created in the venturi has been calculated using homogeneous equilibrium model (HEM). The flow conditions that lead to nucleation have been investigated. The sensitivity of nucleation to the inlet pressure and temperature, flow rate, and venturi profile has been shown. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R25.00003: Abstract Withdrawn
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Tuesday, November 24, 2015 1:29PM - 1:42PM |
R25.00004: Investigation of Gas Holdup in a Vibrating Bubble Column Shahrouz Mohagheghian, Brian Elbing Synthetic fuels are part of the solution to the world's energy crisis and climate change. Liquefaction of coal during the Fischer-Tropsch process in a bubble column reactor (BCR) is a key step in production of synthetic fuel. It is known from the 1960's that vibration improves mass transfer in bubble column. The current study experimentally investigates the effect that vibration frequency and amplitude has on gas holdup and bubble size distribution within a bubble column. Air (disperse phase) was injected into water (continuous phase) through a needle shape injector near the bottom of the column, which was open to atmospheric pressure. The air volumetric flow rate was measured with a variable area flow meter. Vibrations were generated with a custom-made shaker table, which oscillated the entire column with independently specified amplitude and frequency (0-30 Hz). Geometric dependencies can be investigated with four cast acrylic columns with aspect ratios ranging from 4.36 to 24, and injector needle internal diameters between 0.32 and 1.59 mm. The gas holdup within the column was measured with a flow visualization system, and a PIV system was used to measure phase velocities. Preliminary results for the non-vibrating and vibrating cases will be presented. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R25.00005: Turbulent hydraulic jumps: Effect of Weber number and Reynolds number on air entrainment and micro-bubble generation Milad Mortazavi, Ali Mani Air entrainment in breaking waves is a ubiquitous and complex phenomenon. It is the main source of air transfer from atmosphere to the oceans. Furthermore, air entrainment due to ship-induced waves contributes to bubbly flows in ship wakes and also affect their performance. In this study, we consider a turbulent hydraulic jump as a canonical setting to investigate air entrainment due to turbulence-wave interactions. The flow has an inlet Froude number of 2.0, while three different Weber numbers (We $=$ 1820, 729, 292), and two different Reynolds numbers (Re $=$ 11000, 5500) based on the inlet height and inlet velocity are investigated. Air entrainment is shown to be very sensitive to the We number, while Re number has a minor effect. Wave breaking and interface collisions are significantly reduced in the low Weber number cases. As a result, micro-bubble generation is significantly reduced with decreasing Weber number. Vortex shedding events are observed to emerge at the toe of the jump in all of the cases. For high Weber number regimes, shedding of vortices is accompanied by engulfment of air pockets into the jump in a periodic manner, while for lower Webber number regimes such events are significantly suppressed. Reynolds number is shown to have a negligible effect on the air entrainment, wave breaking and micro-bubble generation, contrary to the previous assumptions in other studies. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R25.00006: A Study of the Influence of Numerical Diffusion on Gas-Solid Flow Predictions in Fluidized Beds Ronak Ghandriz, Reza Sheikhi In this work, an investigation is made of the influence of numerical diffusion on the accuracy of gas-solid flow predictions in fluidized beds. This is an important issue particularly in bubbling fluidized beds since numerical error greatly affects the dynamics of bubbles and their associated mixing process. A bed of coal (classified as Geldart A) is considered which becomes fluidized as the velocity of nitrogen stream into the reactor is gradually increased. The fluidization process is simulated using various numerical schemes as well as grid resolutions. Simulations involve Eulerian-Eulerian two-phase flow modeling approach and results are compared with experimental data. It is shown that higher order schemes equipped with flux limiter give favorable prediction of bubble and particle dynamics and hence, the mixing process within the reactor. The excessive numerical diffusion associated with lower order schemes results in unrealistic prediction of bubble shapes and bed height. Comparison is also made of computational efficiency of various schemes. It is shown that the Monotonized Central scheme with down wind factor results in the shortest simulation time because of its efficient parallelization on distributed memory platforms. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R25.00007: Effect of cavitation in high-pressure direct injection Bahman Aboulhasanzadeh, Eric Johnsen As we move toward higher pressures for Gasoline Direct Injection and Diesel Direct Injection, cavitation has become an important issue. To better understand the effect of cavitation on the nozzle flow and primary atomization, we use a high-order accurate Discontinuous Galerkin approach using multi-GPU parallelism to simulate the compressible flow inside and outside the nozzle. Phase change is included using the six-equations model. We investigate the effect of nozzle geometry on cavitation inside the injector and on primary atomization outside the nozzle. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R25.00008: Measurements of Jet Effect on a Ventilated Cavity Ivan Kirschner, Michael Moeny, Michael Krane, Michael Kinzel An experimental study was performed to evaluate some of the claims of Paryshev (2006) regarding changes to ventilated cavity behavior caused by the interaction of a jet with the cavity closure region. The experiments, conducted in the 1.22m diameter Garfield Thomas Water Tunnel, were performed for a 0.0222 EDD to tunnel diameter ratio, Fr = 14.5 and 26.2. The model consisted of a converging-section nozzle mounted to the base of a 27.9mm 37$^{\circ}$ cone cavitator placed on the tunnel centerline at the end of a 138.4mm long streamlined strut. A ventilated cavity was formed over the model. Then an air jet, issuing from a converging nozzle, was initiated. Changes to cavity behavior were quantified in terms of cavitation number, thrust-to-drag ratio, and stagnation pressure ratio at the jet nozzle. The results show that, while the overall trends predicted by Paryshev were observed, the data did not fully collapse, suggesting that many of the effects neglected by Paryshev's model have measureable effect. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R25.00009: LES of turbulent cavitation Aswin Gnanaskandan, Krishnan Mahesh Large Eddy Simulation is employed to study two unsteady turbulent cavitating flows:~cyclic cavitation over a cylinder and sheet to cloud cavitation over a wedge. A homogeneous mixture model is used to treat the mixture of water and water vapor as a compressible fluid. The governing equations are solved using a novel predictor-corrector method (Gnanaskandan and Mahesh, Int. Journal of Multiphase Flow, 2015, 70:22--34). Cavitating flow over a cylinder at Reynolds number (based on cylinder diameter and free stream velocity) Re $=$ 3900 and cavitation number $\sigma \quad =$ 1.0 is simulated and the wake characteristics are compared to the single-phase results at the same Reynolds number. It is observed that cavitation suppresses turbulence in the near wake and delays three-dimensional breakdown of the vortices. The role of cavitation-induced vorticity dilatation in suppressing vortex shedding frequency is discussed. Next, cavitating flow over a wedge at Re $=$ 200,000 (based on wedge height and inlet velocity) and $\sigma \quad =$ 2.1 is presented. The mean void fraction profiles obtained are compared to experiment and good agreement is obtained. Cavity auto--oscillation is observed, where the sheet cavity breaks up into a cloud cavity periodically.~The Strouhal number corresponding to auto-oscillation also agrees well with the experiment. The process of transition from sheet to cloud cavitation will be discussed. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R25.00010: Cavitation dynamics on a NACA0015 hydrofoil using time resolved X-ray densitometry Harish Ganesh, Juliana Wu, Steven Ceccio Recent investigations of partial cavitation have shown that the transition from stable to shedding cavities can be related to the presence of both propagating bubbly shocks and re-entrant liquid jets originating in the cavity closure region. In the present study, formation of sheet cavitation and its transition to periodically shedding cavities is studied on a NACA0015 hydrofoil in a recirculating water tunnel at different attack angles. Using high-speed videos and time resolved X-ray densitometry, the instantaneous void fraction flow fields are obtained to identify the principal mechanism responsible for transition from stable to shedding cavities over a range of attack angles and cavitation numbers. The role of attack angle is of particular interest, since is it related to the pressure gradient at cavity enclosure, and can lead to the formation of stronger reentrant flows. The relative importance of reentrant liquid flow and bubbly shock wave propagation will be discussed [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R25.00011: X-ray densitometry based void fraction flow field measurements of cavitating flow in the wake of a circular cylinder Tiezhi Sun, Harish Ganesh, Steven Ceccio At sufficiently low cavitation number, the wake vortices behind bluff objects will cavitate. The presence of developed cavitation can alter the underlying vortical flow. In this study, cavitation dynamics in the wake of a circular cylinder is examined in order to determine the relationship between the void fraction in the cavity wake and the resulting modification to the flow compared to the non-cavitating flow. Cavitation in the wake of a cylinder is investigated using high-speed video cameras and cinematographic X-ray densitometry. Using synchronized top and side views from high-speed video cameras, the morphology and extent of the cavities forming on the wake of the circular cylinder is studied for a range of cavitation numbers, at a Reynolds number of 1x10$^{\mathrm{-5}}$, which lies at the transition region between sub-critical to critical regime of wake transitions. The time resolved and mean X-ray densitometry based void fraction of the spanwise and plan view averaged flow field will be related to the vortex dynamics in an attempt to understand the role of vapor production in the observed dynamics. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R25.00012: On the relationship between air entrainment, internal flows and closure mechanism in a ventilated supercavity Ashish Karn, Roger Arndt, Jiarong Hong An understanding of underlying physics behind ventilation demand is critical for the operation of underwater vehicles based on ventilated supercavitation for a number of reasons viz. gas entrainment requirements for cavity formation and sustenance. The prior studies on the ventilation demand have reported that the gas entrainment requirement to form a supercavity is substantially larger than that needed to sustain it. This phenomenon, known as ventilation hysteresis, is particularly important from the viewpoint of reduction in gas requirements. However, little physical insights into this phenomenon has yet been provided. In this study, systematic investigations are conducted into ventilation hysteresis with respect to the formation and collapse behaviors of ventilated supercavities. It is suggested that the supercavity formation process is driven by bubble coalescence, whereas its collapse is related to the pressure difference across the supercavity interface at its rear portion. Further, we examine the relationship between ventilation hysteresis, supercavity closures and air entrainment requirements for supercavity formation and sustenance under steady and unsteady flow conditions. These observations are directly related to the internal flows inside the supercavity. [Preview Abstract] |
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