Session A18: Jets I

Liquid jet stability through elastic planar nozzles

Jet breakup lengths have been extensively studied for a multitude of nozzle characteristics and external stimulants, yet jets issuing from deformable, elastic nozzles have not been considered. In this study we take the enduring topic of jet breakup into a new realm by introducing nozzles that passively deform when exposed to liquid flow by making an approximately 500 micron orifice in thin sheets. We perform the experiments with nozzles of varying hardness and thickness, starting with a rigid BeCu nozzle, and continuing with shore hardness 70A, 65A, 35A and 20A. We observe nozzle dilation scales well with Reynolds number and that softer nozzles experiences greater dilation, as expected. We introduce a modification to linear stability theory to describe the break-up length of deformable nozzles to account for the dilation, a scaling which works best for our stiffer nozzles. The three softest materials provide the most stable jets through the range of flow rates in which they can operate before failure. For all nozzles, breakup is highly variable with time and jet velocity.   

Electrospraying of binary-mixture of water-alcohol

There is still debate surrounding the electrohydrodynamic pulverization of water in a cone-jet mode in the air at atmospheric pressure. Indeed, researchers working on nanoelectrospray ionization mass spectrometry are aware that deionized water can be electrosprayed in cone-jet mode. The explanation given is that if liquids with sufficiently high surface tension are electro-atomized, corona discharges could form before the formation of the cone jet because the threshold value of the electric field required to form a Taylor cone-jet, which increases with the surface tension of the liquid-gas atmosphere, can become larger than the electrical breakdown threshold of the air. Spraying in cone-jet mode is preferred due to the generation of the mono-dispersed droplet which has applications in many fields such i.e. printing, coating, heat transfer, etc. Water electrospraying is very important for the application of heat transfer since it has the best-known thermophysical properties. Therefore, in this study, high throughput electrospraying of water is investigated utilising a binary fluid made of water and alcohol. In contrast to a straightforward nozzle, a hemispherical capped nozzle is employed to achieve a high flow rate. No cone jet mode is observed for water while a binary mixture of water-ethanol can be electrosprayed at the maximum flow rate of 90 ml/hr for the mixture of 40 % ethanol by volume in water. This can be attributed to the fact that alcohol reduces the surface tension of the water, thus water can be electrospray in the air atmosphere once surface tension is reached below a certain value.

    

Key role of void fraction in bubble clouds created by plunging liquid jet impacts

A jet plunging in a pool of same, or different liquid, entrains air and often forms a bubble cloud. This ubiquitous phenomenon is widely encountered in nature, like breaking waves in water bodies, and industrial applications like reducing foam formation in chemical processes. Through a simple force balance model, Guyot et al. (PRL 124, 194503 (2020)) recently elucidated the key role of void fraction on the bubble cloud depth and thereby,  introduced two general scenarios, namely, inertial and buoyancy-dominated bubble clouds. Here, we further explore the influence of void fraction using novel lab–scale experiments with injectors ranging from 1.2 to 10 mm in diameter, wherein the jet fall height is varied up to one meter in order to control the air entrainment process via the impact velocity and jet morphology. In particular, we carefully measure void fraction in bubble clouds using optical probes. We first show that, for a given impact velocity, bubble cloud depth decreases with an increase in jet fall height, due to air-intake augmentation during free-fall and impact. Furthermore, we demonstrate a transition from inertial to buoyant clouds at a critical impact velocity, based on jet diameter and void fraction. New scaling laws on the cloud void fraction are also proposed.

    

Sheet's getting real: dynamics and fragmentation in complex fluid sheets created by impinging jets

We encounter fragmentation in fluid sheets whenever we visit a waterfall or hold our thumb over the end of a hose. Because of their 2D geometry, these sheets present a unique opportunity to explore material instabilities. The vast majority of research into the behavior of fluid sheets has centered around Newtonian fluids; we are working toward an analogous understanding of complex fluid sheets. In complex fluids, the viscosity depends on the applied stress; among them are ketchup, fresh concrete, and mucus. Thus, understanding how and why these fluids fragment is important across fields as diverse as food science, civil engineering, and biology. In our experiments, we generate the sheets via the collision of two liquid jets and film their dynamics using high-speed photography. Our findings indicate that quickly-expanding sheets (created by faster jets) are less stable than slowly flowing sheets, and that higher viscosities generate thicker and more stable sheets. Fragmentation can be categorized into different regimes based on jet velocity, jet diameter, and the fluid's rheological properties. We quantify the observed instabilities by examining the size, shape, and thickness of each sheet, as well as the rate of any hole formation and wavelength of instabilities along the rim.   

Vortex breakdown in non-premixed swirling jet flames

Numerical simulations are used to assess the swirl-induced stabilization of low-Mach-number non-premixed axisymmetric swirling jet flames at moderate Reynolds number. Previous experimental investigations have shown that the formation of bubble vortex breakdown, an aerodynamic recirculation region that forms for sufficiently large values of the swirl number S, reduces the velocity at the flame base, and may lead to complex transitions in the structure of the flame. A critical value for the onset of bubble breakdown (S^*_B) is first identified for the isothermal flow and Burke-Schumann flames with infinitely fast chemistry, assuming typical conditions for methane combustion with air. The transition S^*_B is found to be relatively constant as the jet fuel-feed mass fraction Y_F,j is varied. A single-step finite-rate reaction is then considered, leading to flames that lift off the injector for moderate values of the Damköhler number (Da). Increasing values in the prescribed time-dependent inflow swirl S(t) results in increased entrainment and reduced liftoff heights, stabilizing the flame closer to the injector. The onset of bubble vortex breakdown at S^*_B produces a compact flame with enhanced recirculation of hot combustion products.   

Near field entrainment characteristics and recirculation zones in a swirling liquid jet

Entrainment and mixing are desirable in many industrial applications involving chemical reactors and combustion chambers. Here, we explored the near-field entrainment characteristics in a swirling liquid jet injected into an ambient gaseous phase. We have carried out three-dimensional numerical simulations using the open-source code Gerris for a wide range of swirl numbers 0.5≤S≤1.55 and  Reynolds number range of 50≤Re≤300. The main objectives of the present work are to understand the near field entrainment characteristics of the swirling jet and the development of recirculation zones in the ambient air. The results indicate that, the entrainment is enhanced at higher swirl strength for a constant Reynolds number. The maximum entrainment has been found to occur at the shear layer between the swirling liquid jet and ambient air. Also, multiple recirculation zones develops outside the liquid vortex core which facilitates better mixing in the flow. It is also observed that the ambient air experiences a toroidal recirculation in the flow field, which leads to shearing and breakup of the liquid-air interface. Furthermore, we have also computed the entrainment coefficient (C_e)  which varies from 0 to 0.15. These entrainment coefficients are comparable to the entrainment coefficients obtained for thermal plumes but lesser than the entrainment coefficients observed for free turbulent jets and plunging jets.   

Ultrasonic Jet Penetration During Streaming and Cavitation in Liquid

In the present work we investigate ultrasonic streaming with and without the cavitation for various fluids, using a validated numerical model and scaling analysis. The present study distinctly identified the two regimes, namely streaming and cavitation dominant regimes, and presented a plausible correlation for ultrasonic streaming penetration depth in liquid metals. A scaling analysis is presented to identify the dependency of streaming velocity on the operational parameters and thermophysical properties of the melts, which extends to incorporate high levels of input power and cavitation that are well beyond the scope of the available scaling relations. The scaling and numerical predictions are validated with in-house experimental measurements of the flow field and penetration depth. The present work provides a scaling analysis-based correlation that is applicable for a wide range of operational parameters and liquid metals. The present work finds applications in uniform distribution of nano/micro sized particle in Metal Matric Composites, and microstructure refinement in metallic systems.

    

A Numerical Study on Mixing Characteristics of Underexpanded Sonic Twin Jets

Twin jets, such as aircraft systems and combustors, find use in many engineering and practical applications. The study of supersonic flow at the exit of twin nozzles is of great interest to scientists and engineers to comprehend the jet-to-jet interaction and flow physics. The present numerical study investigates the mean flow field and mixing characteristics of underexpanded sonic twin jets using Reynolds-Averaged Navier-Stokes (RANS) equations based SST K-ω turbulence model. The numerical simulations are performed under various nozzle pressure ratios (NPR) ranging from 2 to 6; the NPR is defined as the stagnation pressure upstream of the nozzle to the back pressure downstream of the nozzle. The simulated results are validated with the qualitative and quantitative experimental results. Good overall agreements regarding shock cell structures, velocity decay and radial velocity profiles can be observed. However, the shear layer thickness determined from the present turbulence model showed a wider spread, hence slightly over-estimating the jet mixing behavior. In addition, the results show that the length of the supersonic core increases with NPR till 4. The increase in the core is attributed to the mutual interaction between the twin jets, resulting in a reduced entrainment rate.

    

Vortical structures of a round jet formed bifurcating flows using synthetic jets

Bifurcating flows are formed in a jet and are called side jets. It has been known that velocity fluctuation is periodic and high and vortex rings significantly deforms at the potential core in a round jet. Although the effect of helical modes appears in the natural transition, it is not essential for the formation of side jets. To reveal vortical structures in a round jet with side jets, bifurcating flows were formed in a round air jet using synthetic jets. By three synthetic jets generated at the outlet of a nozzle, three bifurcating flows form at the end of potential core and their starting positions are fixed. It was found a bifurcating flow are formed at the mountain part of a wavy deformed vortex ring and between a pair of streamwise vortices, and the bifurcating flow is induced flows by rotations of three vortices. In this research, since the growth process of streamwise vortices was unknown in the experiments, a numerical simulation of the controlled jet was performed. The bifurcating flows are induced by developed streamwise vortex pairs in the braid region at the upper side of a vortex ring. The vortex pairs grow from the inside of the vortex ring in the downstream to the outside of the successive vortex ring in the upstream. The vortex pairs go upstream in the jet.