Author: Sai Saran Kandati
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Author: Robert E. Childs
Publisher:
ISBN:
Category :
Languages : en
Pages : 17

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The growth instabilities on the interface between a liquid jet and its gaseous environment is an important mechanism in spray atomization, and it is the subject of the work reported herein. Numerical simulations based on the Navier-Stokes equations were used to model liquid/gas interface flows. An algorithm was developed for solving the unsteady Navier-Stokes equations for incompressible fluid with a discontinuity in density and with surface tensions and its accuracy was demonstrated. In flows representative of round pressure-atomized jets and pressure-swirl atomizers, nonuniform mean velocity distributions resulting from viscous boundary layers were found to have a significant effect on instability growth. In a round jet, the inclusion of a boundary layer-like velocity profile significantly reduced the growth rate of small wavelength instabilities. The velocity profile had a much greater effect than surface tension on the initial atomization process for the flow parameters considered. A good estimate of initial fuel droplet size was obtained by considering boundary layer effects but disregarding surface tension. In a flow representative of the fuel issuing from a pressure-swirl nozzle, nonuniformity of the velocity profile was found to increase the growth rate of a disturbance mode which is directly responsible for spray breakup. Keywords: Fuel sprays, Computational fluid dynamics. (MJM/AW).

Author: Grétar Tryggvason
Publisher:
ISBN:
Category : Atomization
Languages : en
Pages : 10

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Atomization of liquid fuels is studied by numerical simulations. The Navier-Stokes equations are solved by a finite difference/front tracking technique that allows resolution of inertia! and viscous forces as well as the inclusion of surface tension at the deformable boundary between the fuel and the air. The secondary breakup of drops has been examined by extensive axisymmetric simulation of four systems: Impulsive and gradual disturbances for two different density ratios (1.15 and 10). At low density ratios. the density disappears as an independent control parameter and we have shown that the low density results apply to density ratios as high as two if we rescale time using the Boussinesq approximation. In addition to full simulations where the Navier-Stokes equations are solved a few inviscid simulations have also been done for the small density ratio case to isolate the effect of viscosity. The breakup of a planar interface has been examined. The presence of surface tension leads to the generation of fingers of interpenetrating fluids. In the limit of a small density ratio the evolution is symmetric, but for large density stratification the large amplitude stage consists of narrow fingers of the denser fluid penetrating into the less denser one. The dependency of the density difference is explained in terms of the advection of interfacial vorticity by the density weighted mean velocity.

Author: National Aeronautics and Space Administration (NASA)
Publisher: Createspace Independent Publishing Platform
ISBN: 9781722127954
Category :
Languages : en
Pages : 56

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The tracking of free surfaces between liquid and gas phases and analysis of the interfacial phenomena between the two during the atomization and breakup process of a liquid fuel jet is modeled. Numerical modeling of liquid-jet atomization requires the resolution of different conservation equations. Detailed formulation and validation are presented for the confined dam broken problem, the water surface problem, the single droplet problem, a jet breakup problem, and the liquid column instability problem. Seung, S. P. and Chen, C. P. and Ziebarth, John P. Unspecified Center...

Author: Arpit R. Agarwal
Publisher:
ISBN:
Category :
Languages : en
Pages : 89

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Liquid sprays appear in a wide range of engineering systems, for example, internal combustion engines, irrigation sprays, printing, food processing, and others. The spray formation process, i.e., the process that converts the injected liquid into a cloud of fine droplets, is also known as atomization. This process is a multi-dimensional, multi-scale, turbulent process, with complex topology of the interface. Fully resolved atomization computations are challenging and computationally expensive. Therefore, most engineering studies, where this process is completely unresolved, depend on lower-order models to describe relevant physics. In the present study, however, we leverage accurate numerical methods along with high spatio-temporal resolution simulations to revisit atomization theory. High fidelity simulations of atomization have only recently become feasible. We have access to spatially resolved data about the liquid and gas distribution along with the velocity field. We are using this data towards developing a better understanding of the atomization mechanisms. A better understanding of the underlying physics ultimately leads to better engineering models. We also investigate the numerical methods themselves, specifically the surface tension computation. Accurate representation of surface tension depends on the accurate computation of local curvature. This continues to a weakness in the popular simulation methods. This work tries to identify promising curvature schemes in the context of complex flow problems. The current work is presented as three chapters of this document: 1. A closer look at linear stability theory in primary atomization modeling Here we look at dominant breakup models. These models are based on the idea that interfacial instability leads to the primary atomization. Here, the underlying assumptions in this theory are outlined and the extent of their validity is established. It is then examined whether these most violent perturbations are actually responsible for the fragmentation of the jet or if there is some other mechanism leading to the breakup. A main finding from the work shows that while the most unstable modes are captured in the simulations and agree with theoretical predictions that inform the present models, these modes are not directly responsible for fragmenting the liquid core or causing primary atomization. Their action is limited to breaking up the surface of the jet, while the liquid core of the jet remains intact for another 20 jet diameters downstream. 2. Effect of internal nozzle flow on primary atomization In this part, we study the physics of internal nozzle flow. The focus is on the effect of nozzle asymmetries and imperfections on primary atomization. This is done by adopting three representative geometries, namely two scans of a real injector nozzle, and a canonical configuration with purely external flow. We find that primary atomization is sensitive to internal nozzle flow; small changes to the nozzle geometry (O(1[mu]m)) affect bulk atomization characteristics (O(1000[mu]m)). Here we explore the underlying mechanism for the same. We find that in spite of more pronounced atomization for the rougher geometry, the magnitude of the turbulent liquid kinetic energy is roughly the same as the smoother geometry. This highlights the important role of mean-field quantities, in particular, non-axial velocity components, in precipitating primary atomization. 3. Evaluating Surface Tension Schemes with Respect to High-Fidelity Atomization Simulations In complex multi-phase flows like atomization, surface tension is expected to play a vital role in some of the dynamics. Accurate representation of the surface tension force is challenging as it directly tied to the accuracy of the interface curvature calculation. Here, we consider two aspects, first, we analyze three different curvature computation schemes for two-phase flow simulations, and then we evaluate the level of influence these differences in the numerical schemes have on problems of practical interest. The differences in the accuracy of the three methods is first analyzed through simpler static and dynamic test cases that are common in numerical methods literature. We find that the signed distance-based computation of curvature performs significantly better in these test problems. However, this difference plays a small role in more complex simulations like the retraction of a liquid column. A key finding here is that increasing complexity of the curvature scheme may only lead to marginally better performance in realistic flow problems.

Author: Yong Yi
Publisher:
ISBN:
Category :
Languages : en
Pages : 296

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Author:
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ISBN:
Category :
Languages : en
Pages : 0

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The secondary breakup of liquid fuel drops was studied by numerical simulations. The Navier-Stokes equations were solved by a finite difference/front tracking technique that included inertia, viscous forces, and surface tension at the deformable boundary between the fuel and the air. The breakup of drops accelerated impulsively as well as by a constant body force was studied by axisymmetric simulations for two different density ratios (1.15 and 10). The low density ratio results can be used for other density ratios by simple rescaling of time. It was shown that the drops break up in different modes, depending on the relative strength of surface tension versus inertia. The modes are similar to those found experimentally for drops in air at atmospheric pressure and breakup maps constructed from the computational results show similar, transitions. There are, however, some differences. Bag breakup is, for example, not found for impulsively accelerated drops in the low density ration limit. Computations of the heat transfer of drops that are breaking up shows a rapid increase, and the drops often reach the ambient temperature before breakup is completed. Three-dimensional simulations show that while drops undergoing breakup remain axisymmetric initially, eventually they are unstable to three-dimensional disturbances.

Author: Warren Bryan Tauber
Publisher:
ISBN:
Category :
Languages : en
Pages : 222

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Author: Sang-Ku Chang
Publisher:
ISBN:
Category :
Languages : en
Pages : 546

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Author: Robert E. Childs
Publisher:
ISBN:
Category :
Languages : en
Pages : 6

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The objective of the work was the investigation of fluid dynamic and surface tension instabilities that effect the breakup of fuel jets in the atomization process. Computational and analytical methods were used. Initial work involved the evaluation of numerical methods for flows with density discontinuities. It was discovered that many commonly used methods are poorly suited to predicting two phase flows. However, a suitable method was found, and it was programmed in a Navier-Stokes prediction code. The code has passed some accuracy tests, and others are ongoing. Predictions of simple one and two phase flows were conducted as part of these tests. A method for computing surface tension effects was included in the prediction algorithm, but further work is ongoing to make the method more robust. A inviscid stability analysis of an expanding-radius liquid tubular jet was performed. It was found that no new instability mechanisms are introduced by the rate of change of radius. A viscous stability analysis of a shear layer with a discontinuity in density was initiated and will be continued. Keywords: Liquid jets, Mathematical models.