Penetration of Circular and Elliptical Liquid Jets into Gaseous Crossflow: A Combined Theoretical and Numerical Study

A combined theoretical and numerical study of liquid jet deformation discharged perpendicularly into a subsonic transverse gas flow is carried out. Near-field trajectory of the jet is determined from an analytical approach for momentum flux ratios up to 100. Force balance on liquid element is analyzed in free stream direction assuming that surface tension and viscous forces are small compared to the aerodynamic force acting on the liquid column. Mass shedding from jet surface and liquid evaporation are neglected; therefore, the jet cross-sectional area and the jet velocity are invariable. A logarithmic correlation for the trajectory of elliptical liquid jets is proposed that takes into account the liquid to gas momentum ratio and drag coefficient. The changes in freestream properties and the gas velocity are incorporated in terms of the drag coefficient. In the numerical part, the drag coefficients of elliptical profiles with various aspect ratios are formulated based on the gas Reynolds number using a two dimensional model. The trajectories of elliptical jets with various aspect ratios are calculated based on the obtained drag coefficients. It is shown that the jets with lower aspect ratios penetrate more into the crossflow. Furthermore, the deformation of a circular liquid jet subject to a gaseous crossflow is simulated using a three dimensional model. Volume of Fluid method is employed to capture the interface between the two phases and the first moment of closure is used to model Reynolds stresses in Reynolds Averaged Navier-Stokes equations. The deformations of the jet cross-section as the jet penetrates into the crossflow are illustrated. It is shown that the model is capable of resolving the Counter-rotating Vortex Pair (CVP) formed downstream of the jet

Data-Driven Modelling of Multiphase Flow Systems

Dynamical systems specifically in the field of fluid mechanics are composed of underlying complicated governing phenomena originated from nonlinearities and instabilities. Encountered with the challenge of analyzing vast amount of data, the concept of reduced order modelling (ROM) was emerged to map the high resolution spatio-temporal data onto a low-dimensional space using the most prominent embedded features. This dissertation considers two ROM techniques of proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) applied to liquid injection systems. These approaches have been widely used to tackle the challenges of analyzing spatio-temporal coherence of dynamical systems. Despite the numerous works implementing POD and DMD, there has been a lack of physical meaning for the modes generated by them. An interpretation of POD and DMD modes is provided in this thesis by the recognition of dominating features. The main focus will be primitively on benchmark problems to validate the efficacy of the methods and consequently to the liquid jets exposed to air crossflows in a hierarchical scheme. A grasp of the prominent spatial structures and their corresponding leading dynamic frequencies will be provided through the analysis of POD and DMD frequency spectra. Effects of several different factors such as the gaseous Weber number, liquid-gas momentum flux ratio and the injector aspect ratio are investigated in this study. Finally, the power of ROM techniques to create features for machine-learnt classifiers that are sufficient for categorization of sundry types of flow regimes is investigated in a supervised manner. These classifiers are opted from a range of classical machine learning algorithms like support vector machines (SVM) and random forest (RF) that have been extensively employed for classification tasks in the recent years. The best combination of reduced order models with the machine learning algorithms are presented

Aerodynamic primary breakup at the surface of nonturbulent round liquid jets in crossflow

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76258/1/AIAA-1998-716-957.pd

Primary breakup of liquid sheets in crossflow and liquid jets in gaseous coflow

This thesis discusses the breakup of liquid sheets under the influence of crossflow and the breakup of micro-liquid jets in coflow air. The breakup of liquid jets and sheets is important for many applications, e.g., agricultural sprays, fuel atomization, and spray coatings, nasal spray delivery, among others. The aerodynamic effects on the liquid breakup process are studied experimentally to identify the breakup regime transitions and droplet sizes. Two different setups were used to investigate experimentally the breakup process in both the crossflow and coflow configurations. A subsonic wind tunnel was used to simulate the crosswind effects on flat fan nozzle spray. Two flat fan nozzles were mounted at the ceiling of the test section of the wind tunnel. Rotameters, pitot tube, and inclined manometer were used to measure the flowrates and velocities, respectively. A micro coflow injector was constructed at the new product development center from two beveled needles (16G and 30G) one inside the other and were used to study breakup of micro liquid jet in with and without the presence of coflowing air.A high-speed digital imaging technique was used in both setups to capture different breakup regimes and to measure the breakup regime transitions, the location of the end of the liquid core, the breakup time, the location, and the size of droplets at the onset of breakup and the spray trajectory. The effects of spray angle for flat fan nozzle spray and the aerodynamic effects on the spray outcomes are presented in this thesis. The results were correlated using phenomenological analyses. For liquid sheets in crossflow, the aerodynamic effects were responsible for initiating bag breakup and reducing the droplet sizes. which are susceptible to drift. This would be problematic for spraying herbicides in windy conditions. For the breakup of micro liquid jets in gaseous coflow the aerodynamic effects resulted in accelerating the breakup process and reducing the droplet sizes. The results show that the presence of air resulted in a large reduction of the droplet sizes compared with pressure atomization only. The effects of reducing the injector size on different measurements were obvious when compared with previous studies of larger coflowing injectors

Biodiesel Spray Characterization: A Combined Numerical and Experimental Analysis

A fundamental study on the characteristics of biodiesel spray is performed and compared with diesel spray at the same condition. In this vein, the liquid jet in cross flow problem is applied to compare the spray penetration depth, droplet size distribution, spatial flux distribution and breakup study of biodiesel, diesel and their blends. Both experimental and numerical analyses have been performed to shed more light on the physics of atomization of liquid jets in cross flow. In the experimental part, shadowgraph technique and image processing have been used in order to capture the penetration of the spray. In addition, droplet size measurement and spatial flux distribution are found by using Phase Doppler Particle Analysis (PDPA). The experimental study shows less penetration depth for biodiesel in comparison with diesel. On the other hand, the droplets’ mass flux distribution with biodiesel is less in the vicinity of the windward side of the spray. In the numerical part of this work, the near field of the injection is simulated using Volume of Fluid (VOF) coupled with the Large Eddy Simulation (LES) turbulence model. As a result a considerable dissimilarity has been found between the breakup regime of biodiesel and diesel. Namely, at Weber numbers of above 40, where the breakup regime of most liquids including diesel occurs in atomization mode, the breakup regime of biodiesel is bag breakup. The main cause of this behavior can be attributed to the remarkably higher viscosity of biodiesel compared with many conventional fuels. The geometry of the orifice can play an important role in controlling the atomization parameters. In this vein, elliptical jets with various aspect ratios between 1 (circular) and 3.85 is performed for several Weber numbers, ranging from 15 to 330. The elliptical jet is first characterized in free air in order to find its capillary behavior in Rayleigh instability regime. The axis-switching phenomenon and breakup length of the jets are the important parameters characterized in this research. Second, the elliptical jets in crossflow are simulated to find differences from the circular orifices in terms of penetration depth, surface waves and breakup length. The simulations of elliptical jets in crossflow were performed with relative gas–liquid Weber numbers of 11, 18 and 30, which cover the bag and multimode primary breakup regime in crossflow. The results show remarkable changes in liquid shapes before disintegration for different aspect ratios

Deformation, wave phenomena, and breakup outcomes of round nonturbulent liquid jets in uniform gaseous crossflow

Scope and Method of Study: An experimental and computational research is performed to study the deformation and breakup of round nonturbulent liquid jets in uniform gaseous crossflow. Pulsed photography and shadowgraphy in conjunction with high-speed imaging were used to study the wave phenomena and the droplets properties/transport dynamics of a nonturbulent liquid jet injected into a uniform crossflow within the bag breakup regime. The computational study extended the previous two-dimensional study by adding the third dimension, allowing the wave properties to be modeled. The computational simulation employed the Volume of Fluid (VOF) formulation of FLUENT, and was run on a 3-processors parallel Linux cluster and P4 desktops. The validated, time-accurate, CFD simulation analyzes the surface properties of the liquid jets within the column, bag, and shear breakup regimes by considering the effects of surface tension, liquid viscosity, and crossflow Weber number at large liquid/gas density ratios (>500) and small Ohnesorge numbers (<0.1).Findings and Conclusions: Present experimental results show that the column waves along the liquid jet are attributed to Rayleigh-Taylor instabilities and the nodes layout per bag affected the breakup mechanisms of the bags. Three distinctive sizes of droplets were produced in the bag breakup regime. The size of bag-droplets normalized by the nozzle exit diameter was constant. The different trajectories for bag- and node-droplets suggested that separation of bag- and node-droplets is possible. The computational results included jet deformations, jet cross-sectional area, jet velocity, wake velocity defect, wake width, and wavelengths of column and surface waves. Present computational results yielded a similarity solution for the inner wake region. In bag breakup, the lower pressure along the sides of the jet pulled the liquid away from both the upwind and downwind surfaces of the liquid cross-section. In shear breakup, the flattened upwind surface pushed the liquid towards the two sides of the jet. In bag breakup, the flow field inside the liquid jet consisted of a counter-rotating vortex pair that was not observed in column and shear breakup. Finally, Phenomenological analyses were effective to understand the conditions for breakup regime transitions

Digital holographic diagnostics of near-injector region

Scope and Method of Study: The primary breakup of near-injector region of liquid jets is of interest motivated by its application to gas turbine fuel injectors, diesel fuel injectors, industrial cleaning, medical spray, and inkjet printers, among others. The dense spray region near the injector is optically obscure for Phase Doppler Interferometer. Moreover, two-dimensional methods, e.g. shadowgraphy, have limited depth-of-field that renders them impractical for measuring droplet sizes and velocities of three-dimensional spray structure. The main objective of this study is to investigate the dense spray near the injector region of liquid jets using digital holography. The digital microscopic holography (DMH) was used for drop size and velocity measurements of the breakup of aerated liquid jets. Digital microscopic holography (DMH) is similar to digital inline holography (DIH) except that no lens is used to collimate the object beam. Two Nd:YAG lasers were used to generate two independent laser pulses, and the laser beams were expanded with an objective lens and a spatial filter. This eliminates two lenses from the typical optical path used for in-line holography, which results in a much cleaner hologram recording.Findings and Conclusions: Using a commercial grade CCD for the DMH, the cost of CCD sensor needed for recording holograms could be reduced. The dense spray region of aerated liquid jets in crossflow was investigated using the DMH (test condition: 1mm jet diameter, 8% GLR, and qo=0.74). The spray structure of aerated liquid jets was obtained by patching several high resolution holograms. Droplet velocities in three dimensions were measured by tracking their displacements in the streamwise and cross-stream direction and by tracking the change in the plane of focus in the spanwise direction. The distributions of the streamwise and cross-stream velocities were uniform in the near-injector region and could be characterized by the mass-average velocity except for very small and very large droplets. Double view DMH reduced the uncertainty of spatial measurements in the spanwise direction

Numerical Modeling of Suspension and Particle Transport in Thermal Spray Processes

Fine microstructured coatings have attracted many attentions in recent years due to various unique properties such as remarkable wear resistance, enhanced catalytic behavior, and superior thermal insulation. Suspension thermal sprays have been shown to be viable techniques in generating this kind of coatings. In these techniques, suspension which is a combination of a base liquid and fine solid particles is injected into a high-temperature high-velocity jets. After suspension breakup, the evaporation/combustion of base liquid becomes dominant. Then, the remained particles are accelerated and heated up by the gas flow and are deposited on a substrate which results in the generation of dense and well-adhered coatings. Suspension thermal spraying is very complex and many fields such as turbulent flow, multiphase flow, compressible flow, combustion, atomization, suspension properties, and plasma physics are involved in the mentioned technique. In addition, many parameters and mechanisms in this technique are still unknowns. Therefore, both numerical and experimental studies should be performed to obtain a comprehensive understanding of various phenomena in suspension thermal spraying and to improve the coating quality. The main goal of this study is the numerical modeling of suspension thermal sprays. An Eulerian-Lagrangian approach with two-way coupling assumption is presented and suspension droplet evolution in the atmospheric plasma spraying and high velocity oxygen fuel spraying techniques is investigated. In this model, suspension is considered as a multi-component mixture and a predefined droplet distribution is injected into the jet. In this approach, the breakup process is simulated using Taylor Analogy Breakup (TAB), and Kelvin-Helmholtz Rayleigh-Taylor (KHRT) breakup models. After breakup process is complete, the liquid component of suspension droplet evaporates/burns, and the particles/agglomerates are tracked in the domain. In general, the effects of suspension injection velocity, suspension properties, suspension injector location, standoff distance, substrate shape, and gas properties on the coating characteristics can be investigated by this approach. For example, in the case of radial injection of suspension into a plasma plume, it is illustrated that if particles move close to the jet centerline, particle velocity and temperature as well as probability of particle impact on the substrate will increase. The mentioned Eulerian-Lagrangian approach revealed that the breakup phenomenon mainly controls the droplets/particles trajectories, temperatures and velocities. However, the typical TAB and KHRT models ignore liquid/suspension column deformation, and need experimental calibration. To study the breakup process in more details, the effect of nonuniform gaseous crossflow and liquid column perturbations on the primary breakup of liquid jets are investigated. A coupled level set and volume-of-fluid method together with the large eddy simulation turbulence model are used to study the behavior of nonturbulent liquid jets in nonuniform crossflows. It is shown that liquid penetration height is significantly affected by the crossflow nonuniformity. In addition, to investigate the effects of liquid column perturbations on the breakup process, experimental studies are performed using shadowgraphy technique. General correlations for the penetration height, the column breakup point, and the onset of surface breakup are presented. It is found that the liquid column perturbations result in formation of large ligaments very close to the liquid and gas flows interaction point. These ligaments control the droplet size distribution and have significant effects on particle in-flight behavior, and coatings quality. The results of these studies can be used to estimate the spray trajectory in suspension plasma spray process, and to improve the accuracy of TAB and KHRT breakup models

Tomographic shadowgraphy of swirled non-reactive spray injection in a generic aero engine burner under realistic operating conditions

This contribution describes the application of tomographic shadowgraphy to measure instantaneous velocities of droplets undergoing airblast-atomization in the non-reactive flow of a generic aero engine burner model at Weber numbers of Weaero = 36