Experiments in Fluids最新文献

The evolution of the neutralization reaction in a stationary droplet of cylindrical shape, extracting a surfactant from the surrounding mixture in a vertical Hele-Shaw cell, was studied experimentally. In our experiment, we used a new method of the simultaneous visualization of the refractive index field of light and the distribution of acidity levels in the solutions of source reactants and reaction products within the droplet. To carry out this approach, we utilized a Fizeau interferometer and added a pH indicator to the droplet before the experiment. A digital video camera recorded the resulting interference pattern with the superimposed color distribution created by solutions with different acidity levels. The study was conducted on a system of liquids where the chemical potentials would be equal when the concentration of the extracted reactant in the droplet was much higher than in the surrounding environment. Two variants of reaction realization—with and without Marangoni convection development—were considered. We determined the structures of flows and concentration fields in the droplet and its neighborhood and traced their evolution. Also, we evaluated the characteristic times of the extraction process depending on the initial reactant concentrations and droplet sizes. It was found that the resulting Marangoni convection had an oscillatory character and continued after the reaction completion. As expected, the formation of the capillary motion intensified the progression of chemical reaction inside the droplet.

Flows over cavities are relevant to many branches of engineering and are known to be a source of instabilities, high-pressure disturbances, and large recirculating regions, leading to excessive pressure loads. In this paper, we study the dynamical behavior of a 6.44:1 length-to-depth transitional cavity flow (i.e., where the shear layer partly enters the cavity) with wall proximity and lateral apertures. Mitigation of pressure loads is investigated through steady blowing upstream of the cavity’s leading edge. Concurrent pressure and particle image velocimetry (PIV) measurements along with companion unsteady numerical simulations have been performed to identify the mechanisms underlying the flow dynamics of both baseline and controlled cases. Experiments are reproduced numerically using the Improved Delayed Detached Eddy Simulations (IDDES) approach with shear stress transport eddy viscosity model ((k-omega) SST) at a Reynolds number of (Re=2.8 times 10^5). Results underline that steady blowing changes the flow drastically upstream of the cavity by thickening the boundary layer and reducing the flow rate passing the cavity. The controlled flow transforms the dynamics of the cavity shear layer, impacting the inner cavity flow, and leads to a significant reduction of the pressure loads. This mitigation is associated with a strong reduction in turbulent momentum at the shear layers interface.

A method for analyzing liquid ligaments of a textural atomization process is presented in this article for the case of a rocket engine type-assisted atomization process under combustion. The operating point positions the atomization process in the fiber-type regime carrying an intense textural atomization process. Multiscale in nature, the method based on image analysis associates a scale distribution with a family of ligaments, this distribution being sensitive to the number, size and shape of these ligaments. The quality of scale distributions measured by image analysis depends on the spatial resolution and the precision of area measurements of surfaces with curved boundaries but described by square pixels. Part of the work consisted of improving the method for measuring scale distributions by using a sub-pixel image analysis technique and refining the surface area measurement method. Another part directed the multiscale analysis toward the estimation of the diameter distributions of the blobs that characterize the large-scale deformation of the ligaments. The analysis describes the atomization process at a level of detail never reached. For instance, assuming that the blobs are drops in formation, the estimated diameter distribution (bimodal in the case examined here) and the number of these drops are evaluated as a function of the distance from the injector. This information indicates where the process is most intense and where it stops. Furthermore, these diameter distributions receive a mathematical expression whose parameters report clear evolutions with the distance from the injector. This shows the possibility of elaborating mathematical models appropriate for textural atomization mechanisms.

In nature as well as in industrial applications, turbulent mixing is ubiquitous. In most cases, these are different fluids with different physical properties (density and/or viscosity). Moreover, all important changes such as mass, momentum and scalar fluxes occur across the turbulent/non-turbulent interface, a thin and sharp layer that separates the turbulent core from the irrotational surrounding fluid. In this paper, we present statistics conditioned on the instantaneous interface position in the very near field of a variable viscosity jet to study the birth and growth of turbulence. The simultaneous scalar concentration and velocity fields are obtained from planar laser-induced fluorescence, where the images undergo an original correction and normalization process, and stereo-particle image velocimetry, respectively. We show that the turbulence is much more advanced in the variable viscosity flow (VVF), which exhibits some features that are visible much later in the constant viscosity flow (CVF). Furthermore, this study reveals a change in the nature of the mixing process between VVF and CVF, which needs to be further investigated.

This review focuses on flow sensors in microfluidics specific to on-chip detection inside a microchannel. These sensors are distinct from external, off-chip flow sensors that are often associated with microfluidics. We explore the various mechanisms and physical principles involved in their working and compare their pros and cons. We consider the working principles that can be used for sensing at the microscale and prepare a typical designer’s perspective with respect to flow sensors that can be integrated on a microfluidic chip. Developing an accurate on-chip flow sensor would enable autonomous flow control leading to advancements in point-of-care applications of microfluidics. We also highlight some of the challenges that have kept researchers at bay from developing an all-weather on-chip flow sensor for microfluidics. Also included is a brief discussion on the relevant applications of on-chip flow sensors including preventive healthcare, drug development, and microreactors. This review should give an impetus to development of better and larger variety of on-chip flow sensors.

A thorough understanding of media tightness in automotive electronics is crucial for ensuring more reliable and compact product designs, ultimately improving product quality. Concerning the fundamental characteristics of fluid leakage issues, the dynamic wetting and penetration behavior on small scales is of special interest and importance. In this work, four T-shaped microchannels with one inlet and two outlets are experimentally investigated in terms of contact angle dynamics and interface movement over time, generating novel insight into the wetting mechanisms and fluid distribution. With a main channel width of 1 mm, a crevice width of (w = {0.3},hbox {mm}, {0.4},hbox {mm}) and a rounding edge radius of (r = {0.1},hbox {mm}, {0.2},hbox {mm}), the geometrical effects on the fluid penetration depth in the crevice and the interface edge pinning effect are analyzed quantitatively using an automated image processing procedure. It is found that the measured dynamic contact angles in all parts can be well described by molecular kinetic theory using local contact line velocities, even with local surface effects and abrupt geometry changes. Moreover, a smaller crevice width, a sharper edge and a larger flow velocity tend to enhance the interface pinning effect and prevent fluid penetration into the crevice. The rounding radius has a more significant effect on the interface pinning compared with crevice width. The experimental data and image processing algorithm are made publicly available.

When producing metal-oxide nanoparticles via flame spray pyrolysis, precursor-laden droplets are ignited and undergo thermally induced disintegration, called ‘puffing’ and ‘micro-explosion’. In a manner that is not fully understood, these processes are associated with the formation of dispersed phases inside the droplets. This work aims at visualizing the interior of precursor-laden burning single droplets via diffuse back illumination and microscopic high-speed imaging. Solutions containing iron(III) nitrate nonahydrate (INN) and tin(II) 2-ethylhexanoate (Sn-EH) were dispersed into single droplets of sub-100 μm diameter that were ignited by passing through a heated coil. At low precursor concentration, 50% of the INN-laden droplets indicate a gas bubble of about 5 μm diameter in the center of the droplet. The bubble persists for several hundred microseconds at a similar size. In almost all of these cases, the bubble expands at some point and the droplet ends up in a micro-explosion. In some of these instances, the droplet’s surface shows spatial brightness modulations, i.e., surface undulations, indicating the formation of a viscous shell. With increasing INN concentration, the fraction of droplets showing surface undulations, gas bubbles, and micro-explosions drastically decreases. This may be associated with a more rigid viscous shell and reduced mobility of bubbles. Bright incandescent streaks originating from the disrupting INN-laden droplets, may indicate sub-micrometer droplets or particles from within the droplets or formed in the gas phase. In contrast, Sn-EH-laden droplets show very fast disruptions, typically less than 10 μs from first visible deformation to ejection of secondary droplets. Bubbles and surface undulations were not observed.

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