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Author: Sina Ghods
Publisher:
ISBN:
Category : Atomization
Languages : en
Pages : 86
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Book Description
The atomization of a liquid jet by a high speed cross-flowing gas has many applications such as gas turbines and augmentors. The mechanisms by which the liquid jet initially breaks up, however, are not well understood. Experimental studies suggest the dependence of spray properties on operating conditions and nozzle geometry. Detailed numerical simulations can offer better understanding of the underlying physical mechanisms that lead to the breakup of the injected liquid jet. In this work, detailed numerical simulation results of turbulent liquid jets injected into turbulent gaseous cross flows for different density ratios is presented. A finite volume, balanced force fractional step flow solver to solve the Navier-Stokes equations is employed and coupled to a Refined Level Set Grid method to follow the phase interface. To enable the simulation of atomization of high density ratio fluids, we ensure discrete consistency between the solution of the conservative momentum equation and the level set based continuity equation by employing the Consistent Rescaled Momentum Transport (CRMT) method. The impact of different inflow jet boundary conditions on different jet properties including jet penetration is analyzed and results are compared to those obtained experimentally by Brown & McDonell(2006). In addition, instability analysis is performed to find the most dominant insta- bility mechanism that causes the liquid jet to breakup. Linear instability analysis is achieved using linear theories for Rayleigh-Taylor and Kelvin- Helmholtz instabilities and non-linear analysis is performed using our flow solver with different inflow jet boundary conditions.
Author: Sina Ghods
Publisher:
ISBN:
Category : Atomization
Languages : en
Pages : 86
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Book Description
The atomization of a liquid jet by a high speed cross-flowing gas has many applications such as gas turbines and augmentors. The mechanisms by which the liquid jet initially breaks up, however, are not well understood. Experimental studies suggest the dependence of spray properties on operating conditions and nozzle geometry. Detailed numerical simulations can offer better understanding of the underlying physical mechanisms that lead to the breakup of the injected liquid jet. In this work, detailed numerical simulation results of turbulent liquid jets injected into turbulent gaseous cross flows for different density ratios is presented. A finite volume, balanced force fractional step flow solver to solve the Navier-Stokes equations is employed and coupled to a Refined Level Set Grid method to follow the phase interface. To enable the simulation of atomization of high density ratio fluids, we ensure discrete consistency between the solution of the conservative momentum equation and the level set based continuity equation by employing the Consistent Rescaled Momentum Transport (CRMT) method. The impact of different inflow jet boundary conditions on different jet properties including jet penetration is analyzed and results are compared to those obtained experimentally by Brown & McDonell(2006). In addition, instability analysis is performed to find the most dominant insta- bility mechanism that causes the liquid jet to breakup. Linear instability analysis is achieved using linear theories for Rayleigh-Taylor and Kelvin- Helmholtz instabilities and non-linear analysis is performed using our flow solver with different inflow jet boundary conditions.
Author: Vincent Henri Marie Le Chenadec
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Energy conversion devices often rely on the combustion of high energy-density liquid fuel to meet weight and volume restrictions. The efficiency of the conversion and the emission of harmful pollutants depend directly on the mixing of the fuel and oxidizer, which itself results from a cascade of mechanisms initiated by the atomization of a coherent liquid stream. Despite existing investigations, the effects of nozzle design, surface tension, turbulence, and cavitation on atomizer performance are still poorly understood. The challenges found in experimental studies caused by the multi-scale and multi-physics aspects of such flows have motivated the development of numerical strategies to simulate the atomization process. In the direct numerical simulation approach considered in this work, the physics of the flow are all solved for directly. In the presence of complex topologies, the underlying equations are stiff, and, therefore, the development and improvement of numerical methods has been an area of active research. Among algorithms that have emerged, sharp interface methods including Volume-of-Fluid and Level Set have been shown to give reasonable performance. Applications of these methods to study primary atomization of turbulent jets have been documented, but the rare convergence studies available show strong grid dependence, suggesting the need for further developments. The present work focuses on two novel developments: an unsplit Volume-of-Fluid interface capturing algorithm designed to improve conservation on arbitrary grids, and a robust flow solver for incompressible Navier-Stokes equations in two-phase flows involving large density ratios. The resulting approach is shown to be stable and second order accurate. The framework is then assessed in a set of validation cases. In particular, the effect of mesh resolution is studied, and the ability of the method to accurately retain under-resolved structures is shown. Finally, computations of a large scale atomizer for varying operating conditions are presented. The excellent computational efficiency of the algorithm motivates the use of such a framework for simulating industrially relevant configurations.
Author: Bejoy Mandumpala devassy
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
In internal combustion engines, the liquid fuel injection is an essential step for the air/fuel mixture preparation and the combustion process. Indeed, the structure of the liquid jet coming out from the injector plays a key role in the proper mixing of the fuel with the gas in the combustion chamber. The present work focuses on the liquid jet atomization phenomena under Diesel engine conditions. Under these conditions, liquid jet morphology includes a separate liquid phase (i.e. a liquid core) and a dispersed liquid phase (i.e. a spray). This manuscript describes the development stages of a new atomization model, for a high speed liquid jet, based on an eulerian two-phase approach. The atomization phenomenon is modeled by defining different surface density equations, for the liquid core and the spray droplets. This new model has been coupled with a turbulent two-phase system of equations of Baer-Nunziato type. The process of ligament breakup and its subsequent breakup into droplets are handled with respect to available experiments and high fidelity numerical simulations. In the dense region of the liquid jet, the atomization is modeled as a dispersion process due to the turbulent stretching of the interface, from the side of liquid in addition to the gas side. Different academic test cases have been performed in order to verify the numerical implementation of the model in the IFP-C3D software. Finally, the model is validated with the recently published DNS results under typical conditions of direct injection Diesel engines.
Author: Grétar Tryggvason
Publisher:
ISBN:
Category : Atomization
Languages : en
Pages : 10
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Book Description
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: Mohsen Behzad Jazi
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Author: Yash Akhil Bhatt
Publisher:
ISBN:
Category : Fluid dynamics
Languages : en
Pages : 118
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Book Description
"Break-up and atomization characteristics of JetA liquid fuel were investigated numerically. The results have been compared to various experimental results to evaluate the accuracy of the numerical model. The CDF code ANSYS-CFX 12.0 was used to carry out the steady state analysis at different time scales. A comparison between the atomization characteristics of a pressure jet atomizer and an air-blast atomizer is shown."--Leaf iii.
Author: Arash Zandian
Publisher:
ISBN: 9781321094527
Category :
Languages : en
Pages : 139
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Book Description
A numerical study of the temporal and spatial instabilities appearing on the interface of an annular liquid jet emerging from an orifice and flowing into a high pressure gas medium has been performed using Direct Numerical Simulation. The purpose of this study is to gain a better insight into the dominant mechanisms in the atomization of annular liquid jets during the start-up portion of the injection. The effects on the growth rate and wavelength of the emerging Kelvin-Helmholtz and Rayleigh-Taylor instabilities of various flow parameters have been investigated: the Reynolds and Weber numbers; fluids properties like gas-to-liquid density and viscosity ratios; and geometrical parameters involved in the problem such as thickness-to-diameter ratio of the liquid sheet. The Reynolds numbers used in this study are in the range from 3,000 to 30,000, and the Weber numbers are in the range of 6,000 up to 150,000. The convergence rate and length of the liquid jet has been also computed and compared for different cases. A characteristic convergence time has been proposed based on the obtained results. Use has been made of an unsteady axisymmetric code with a finite-volume solver of the Navier-Stokes equations for liquid streams and adjacent gas and a level-set method for the liquid/gas interface tracking. Two significant velocity reversals were detected on the axis of symmetry for all flow Reynolds numbers; the one closer to the nozzle exit being attributed to the recirculation zone, and the one farther downstream corresponding to the annular jet collapse on the centerline. The effects of different flow parameters on the location of these velocity reversals are studied. The results indicate that the convergence length and time increase significantly with the gas density and liquid viscosity and decrease with the liquid sheet thickness, while the effects of the gas viscosity and the surface tension are not so considerable. The range of unstable Kelvin-Helmholtz and Rayleigh-Taylor wavelengths have been also studied. The statistical data obtained from the numerical results show that, the average normalized wavelength of the KH instabilities decreases with the Reynolds and Weber numbers and the sheet thickness, and increases with the gas-to-liquid density ratio, and is independent of the viscosity ratio. The wavelength of the KH instabilities were observed to increase in time, except for the very thin liquid sheet, where the average KH wavelength oscillates between two values, indicating occurrence of different sheet breakup cycles. The sheet breakup times and lengths were reported up to the second sheet breakup, and it is shown that the later sheet breakups happen closer to the nozzle exit plane. The RT wavelengths tend to decrease during the start-up period of injection.
Author: Robert Michael Chiodi
Publisher:
ISBN:
Category :
Languages : en
Pages : 180
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Book Description
Multiphase flows are ubiquitous in natural and engineered systems and occur over scales ranging many orders of magnitude, from the movement of the ocean to the flow of blood through a capillary. In each case, the interface between the phases plays a key role in the dynamics of the flow. In this dissertation, I will detail the development of novel numerical methods for accurately and efficiently tracking the interface in simulations of multiphase flows, specifically focusing on flow configurations involving large density ratios, high shear at the interface, and liquid structures that span orders of magnitude in size. One specific instance where this occurs is during primary atomization, where a large liquid structure breaks up into smaller, more stable structures. This important class of flows is present in many different industries that currently employ over 2.5 million people. Enabling more accurate simulation of primary atomization is a central motivation of my work. To this end, I present significant improvements to the state-of-the-art for both level-set and volume of fluid methods. First, in the context of the conservative level-set method, the solution of the reinitialization equation is improved to drastically reduce the amount of error introduced. This reduction in error also leads to better volume conservation properties, especially for the under-resolved structures commonly created during atomization simulations. I then go on to detail improvements to geometric volume of fluid methods, which are capable of discretely conserving phase volumes at any level of resolution. For advecting the phase volumes, I provide a systematic approach to geometric advection on unstructured meshes that leads to significant gains in speed with no sacrifice in solution accuracy. On the topic of interface reconstruction, a novel multi-plane strategy capable of capturing arbitrarily-thin sub-grid scale films is given. To improve the performance of these methods, a general, robust, and efficient polyhedron intersection algorithm has also been developed and will be discussed in detail. In all cases, the work is placed in the context of the current state-of-the-art through direct comparison to published results. A major portion of my work has also involved creating the open-source Interface Reconstruction Library, which provides implementations of modern algorithms used in volume of fluid methods, including those presented in this dissertation. This library, along with the presented methods, has been applied to simulate large-scale, complicated, and realistic atomizing flows. In particular, they have driven the near-field simulation capabilities for an Office of Naval Research Multidisciplinary Research Initiative (MURI) on active spray control, been used to study the dynamics of a compressible liquid jet in cross-flow configuration for the Air Force Research Laboratory, and been employed by Los Alamos National Laboratory to simulate the injection of liquid metal in metal casting simulations.
Author: Mingbo Sun
Publisher:
ISBN: 9789811360268
Category : Aerodynamics, Supersonic
Languages : en
Pages : 284
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Book Description
Based on research into jets in supersonic crossflow carried out by the authors’ team over the past 15 years, this book summarizes and presents many cutting-edge findings and analyses on this subject. It tackles the complicated mixing process of gas jets and atomization process of liquid jets in supersonic crossflow, and studies their physical mechanisms. Advanced experimental and numerical techniques are applied to further readers’ understanding of atomization, mixing, and combustion of fuel jets in supersonic crossflow, which can promote superior fuel injection design in scramjet engines. The book offers a valuable reference guide for all researchers and engineers working on the design of scramjet engines, and will also benefit graduate students majoring in aeronautical and aerospace engineering.
Author:
Publisher:
ISBN:
Category : Electronic books
Languages : en
Pages : 82
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Book Description
A full unsteady Navier Stokes Equation (NSE) solver (known as the laminar model in FLUENT) is used to simulate a confined jet in crossflow (JICF) with Reynolds numbers (Re) in the range 287-2478. Reynolds Averaged Navier Stokes (RANS) equations are used to simulate JICF with Re in the range 4930-20000. Velocity ratios range between 0.84-3.15. Results of using the SST [lower case kappa] -- [lower case omega] or Realizable [lower case kappa] -- [lower case epsilon] are presented. Comparison to experimental data provided by the Air Force Research Lab shows that both the full NSE and RANS equations compare well with experimental data. The SST [lower case kappa] -- [lower case omega] predicts higher velocities than the Realizable model. The full Navier stokes simulation captures the major JICF vortical structures including the Counter-rotating Vortex Pair (CVP), upstream and downstream shear layers, horseshoe vortices, and hanging vortices. Many of these vortical details are lost in the RANS simulation. Two parametric studies on varying density ratio and momentum flux ratio are conducted. Varying density ratio is found to have little to no effect on the jet trajectory in the near-jet region. A lower density ratio leads to a higher Re which increases unsteadiness and turbulence which leads to improved mixing. Increasing momentum flux ratio generally increases jet penetration. However, a JICF with a larger boundary layer can penetrate deeper into the crossflow than a thinner boundary layer JICF even if the momentum flux ratio is smaller. Contributions of this work include providing additional simulation data for JICF at the presented flow conditions as well as validation of two RANS turbulence models and a full Navier Stokes solver.
