Single-phase mixing through a narrow gap

Mixing through narrow gaps connecting adjacent flow paths is an important mass and heat transfer process for many thermo-hydraulic applications. Such flows are considered balanced when the inlet flow speeds of adjacent subchannels are matched. In the present work, experimental observations are presented for balanced and unbalanced flows including the mixing coefficients and flow visualization within the gap. The large coherent structures are identified, with frequency in general agreement with those reported by previous investigators. To utilize Proper Orthogonal Decomposition (POD) for the discrete data yielded by PIV, we employ method of Singular Value Decomposition (SVD). The bulk of the mixing is attributed to the dominant modes and demonstrate that mixing rates estimated from velocity measurements are in fair agreement with mixing coefficients based on tracer concentration measurements

Application of X-ray micro-computed tomography on high-speed cavitating diesel fuel flows

The flow inside a purpose built enlarged single-orifice nozzle replica is quantified using time-averaged X-ray micro-computed tomography (micro-CT) and high-speed shadowgraphy. Results have been obtained at Reynolds and cavitation numbers similar to those of real-size injectors. Good agreement for the cavitation extent inside the orifice is found between the micro-CT and the corresponding temporal mean 2D cavitation image, as captured by the high-speed camera. However, the internal 3D structure of the developing cavitation cloud reveals a hollow vapour cloud ring formed at the hole entrance and extending only at the lower part of the hole due to the asymmetric flow entry. Moreover, the cavitation volume fraction exhibits a significant gradient along the orifice volume. The cavitation number and the needle valve lift seem to be the most influential operating parameters, while the Reynolds number seems to have only small effect for the range of values tested. Overall, the study demonstrates that use of micro-CT can be a reliable tool for cavitation in nozzle orifices operating under nominal steady-state conditions

Experimentally validated x-ray image simulations of 50 μm x-ray PIV tracer particles

We evaluate Beer-Lambert (BL) ray-tracing and Monte Carlo N-Particle (MCNP) photon tracking simulations for prediction and comparison of x-ray imaging system performance. These simulation tools can aid the methodical design of laboratory-scale x-ray particle image velocimetry (XPIV) experiments and tracer particles by predicting image quality. Particle image signal-to-noise ratio (SNR) is used as the metric of system performance. Simulated and experiment data of hollow, silver-coated, glass sphere tracer particles (AGSF-33) are compared. As predicted by the simulations, the AGSF-33 particles are visible with a SNR greater than unity in 100 ms exposure time images, demonstrating their potential as x-ray PIV or particle tracking velocimetry (XPTV) tracers. The BL approach predicts the image contrast, is computationally inexpensive, and enables the exploration of a vast parameter space for system design. MCNP simulations, on the other hand, predict experiment images slightly more accurately, but are more than an order of magnitude more computationally expensive than BL simulations. For most practical XPIV system design applications, the higher computational expense of MCNP is likely not justified by the modest accuracy improvement compared to BL

Effect of superhydrophobic surfaces on bubble column flow dynamics

Bubbly flow in bubble column reactors promotes mixing necessary for many chemical processes. We show that if superhydrophobic-coated material is introduced into a bubble column, there can be a substantial difference in gas holdup and earlier initiation of churn-turbulent flow which can alter larger-scale mixing without a need to change the superficial gas velocity. Addition of superhydrophobic surface can also cause bubbles to (directly or indirectly assisted by the surface) escape faster to the free surface resulting in a reduced void fraction (i.e., reduced gas holdup). As the flow becomes optically opaque at few percent gas phase volume fraction, we utilize two dual plane wire mesh sensors to obtain velocity profiles and bubble size distributions, in addition to the traditional pressure and level based gas holdup measurements to calculate average phase fraction. Additionally, a custom build photon-counting dual energy threshold X-ray computed tomography system is employed to get a higher resolution measurement of the time average phase fraction non-intrusively. We report satisfactory agreement between these techniques with differences arising for understood reasons, and use the insight thus yielded to discuss the effect of superhydrophobic surfaces on bubble column flow dynamics

Experimental Methods for the Study of Hydrodynamic Cavitation

A review of traditional and novel experimental methods for the investigation of hydrodynamic cavitation is presented. The importance of water quality is discussed, along with its characterization and management. Methods for the direct and indirect experimental determination of cavitation inception are presented. Along with traditional optical visualization, methods of measuring developed cavitation are described, including point and surface electrical probes, optical bubble probes, acoustic measurements, and indirect measurements of noise and vibration. Recent developments in the use of ionizing radiation as a means to visualize cavitating flows are also discussed

Effect of coherent structures on particle transport and deposition from a cough

The transport of droplets expelled when coughing is of critical importance for understanding and preventing airborne disease transmission. However, the cough flow structure is complex, and numerous simplifications are often made to the initial flow condition in laboratory and numerical studies. We aim to challenge some of these assumptions, through particle transport experiments from a highly repeatable cough generator with a realistic oral cavity. In the present study the simultaneous transport and deposition of 22–27 μm, 45–53 μm, and 180–212 μm particles was investigated. Hot-wire measurements of the flow field showed the formation of two parallel jets on either side of the tongue, which produced a free shear layer downstream. Quantitative deposition measurements show that particle deposition patterns are strongly dependent on the formed shear layer for the smaller particles but not the largest size. The effect of circulation was characterized using a modified Stokes number of the form given by Davila and Hunt (2001). The scaling analysis and quantitative data both support the conclusion that while the 180–212 μm particles are not strongly affected, the transport and deposition of the smaller particles are influenced by orifice geometry. These results demonstrate that particle transport from a cough, for which most released droplets are below 22 μm, is a strong function of coherent flow structures present in the 3D flow field

On multi-point gas injection to form an air layer for frictional drag reduction

Air layer drag reduction has been shown to be a feasible drag reducing technique at the laboratory and at full ship scales. In most studies, the air layers have been generated via gas injection from two-dimensional spanwise slots. However, given ship's structural considerations, it would be preferable to use discrete holes. The present study expands on the work on single orifice gas injection to multi-hole injection. When compared with slot injection, multi-point injection lead to a reduced range of gas fluxes that formed an air layer. Gas injected from a series of discrete holes can exhibit complex flow patterns, including roll-up into the core of liquid vortices that form as part of the process of injecting gas into the liquid boundary layer. The finite span and length of the model utilized for the present experiments was modest. It remains to be shown if a larger model with similar scaled up geometry (and with more beanwise holes) would enable the formation of a stable air layer with a gas flux per unit span that is similar to that required for slot injection. Nevertheless, the results presented here illustrate the complexity associated with gas injection through multiple perforations in a hull

The dynamics of partial cavity formation, shedding and the influence of dissolved and injected non-condensable gas

In the present study, the experimental set-up of Ganesh et al. (J. Fluid Mech., vol. 802, 2016, pp. 37-78) is used to examine the dynamics of a shedding cavity by examining the vapour production rate of the natural cavity and determining how minimal injection of non-condensable gas can substantially alter the vapour production rate, the resulting cavity flow and the cavity shedding process. The influence of the dissolved gas content on the shedding natural cavity flow is also examined. High-speed visual imaging and cinemagraphic X-ray densitometry were used to observe the void fraction dynamics of the cavity flow. Non-condensable gas is injected across the span of the cavity flow at two locations: Immediately downstream of the cavity detachment location at the apex of the wedge or further downstream into mid-cavity. The gas injected near the apex is found to increase the pressure near the suction peak, which resulted in the suppression of vapour formation. Hence, the injection of gas could result in a substantial net reduction in the overall cavity void fraction. Injection at the mid-cavity did less to suppress the vapour production and resulted in less significant modification of both the mean cavity pressure and net volume fraction. Changes in the cavity void fraction, in turn, altered the dynamics of the bubbly shock formation. Variation of the dissolved gas content alone (i.e. without injection) did not significantly change the cavity dynamics

Interaction of a Compressible Bubbly Flow with an Obstacle Placed Within a Shedding Partial Cavity

Bubbly shocks act as the dominant mechanism of shedding of partial cavities under certain conditions. The compressible nature of low void fraction regions in a partial cavity can provide conditions necessary for the existence of such bubbly shocks. Using X-ray densitometry as a flow visualisation mechanism, the dynamics of bubbly shock fronts with and without the presence of an obstacle are presented. Different flow features associated with the shock dynamics are identified and the associated flow physics explained

Jet impingement on the underside of a superhydrophobic surface

Water patch topology and momentum loss resulting from a jet impacting on the underside of a flat plate with varied hydrophobicity were studied. The jet's Reynolds and Froude numbers ranged from 3700 to 31 000, and from 1 to 23, respectively. Hence effects of gravity were expected to be non-negligible, and data suggest that this is the case for hydrophobic surfaces. The interplay of gravity, surface tension, inertia and viscosity resulted in two distinct behaviours. On hydrophilic surfaces, water spread uniformly. Friction reduced momentum, and led to accumulation at the edges of the patch until gravity overcame surface tension and produced droplets. On hydrophobic surfaces, two rims formed, enclosing a thin laminar film. The patch shape was mostly determined by the balance of kinetic and surface energy. Dewetting occurred in most cases when the two rims merged, but for a narrow parameter, range water detached soon after impact and formed a type of skewed water bell. The transition in detachment topology was predicted reasonably by a simple model considering whether an attached or detached rim minimizes energy. Due to promotion of dewetting by gravity, the water patch area decreased compared to that reported in previous studies, which considered jet impingement on vertical surfaces and tops of horizontal surfaces. Owing to the application that motivated this study, the streamwise force on the plate was also measured. On hydrophobic surfaces the reduction in force correlated with the reduction in water patch area. Water patch area and momentum loss were both found to scale best with the contact-angle-modified Weber number