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CFD Investigation of Air-Water Test Stand for Three-Stream Airblast Reactor Feed Injector
Abstract
A large-scale parametric air-water test stand (AWTS) study involving more than 40 evaluations was carried out for the purposes of three-stream airblast reactor feed injector optimization; a subset of seven air stream combinations is discussed here. The role of CFD as a supplement to, or a replacement for, air-water testing is of great industrial interest. To this end a set of CFD experiments was carried out to mimic the AWTS study. A compressible geometric reconstruction Volume of Fluid (VOF) method was used to simulate the three-stream interaction. Pressure responses, spray opening characteristics near the feed injector face, and spray distribution were primary measures for both the AWTS and CFD programs. It was found that, over the range of variables studied, there was a partial match between CFD and AWTS results; some measures matched quantitatively, others qualitatively, and some did neither. A self-exciting, pulsatile spray pattern was achieved in CFD and AWTS, and an interesting transition in spray bursting character occurred at moderate inner air flows. Overall, it is shown that the CFD method contained herein can be used to supplement, but not replace, air-water testing for said injector configuration.
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A 3D computational fluid dynamics investigation of particle-induced flow effects and liquid entrainment from an industrial-scale separator has been carried out using the Eulerian-Lagrangian two-way coupled multiphase approach. A differential Reynolds stress model was used to predict the gas phase turbulence field. The dispersed (liquid) phase was present at an intermediate mass loading (0.25) but low volume fraction (0.05). A discrete random walk method was used to track the paths of the liquid droplet releases. It was found that gas phase deformation and turbulence fields were significantly impacted by the presence of the liquid phase; these effects have been parametrically quantified. Substantial enhancement of both the turbulence and the anisotropy of the continuous phase by the liquid phase was demonstrated. It was also found that a large number (&1000) of independent liquid droplet release events were needed to make conclusions about liquid entrainment. Known plant run conditions and entrainment rates validated the numerical method.
A moving-deforming grid study was carried out using a commercial computational fluid dynamics (CFD) solver, FLUENT® 6.2.16. The goal was to quantify the level of mixing of a lower-viscosity additive (at a mass concentration below 10%) into a higher-viscosity process fluid for a large-scale metering gear pump configuration typical in plastics manufacturing. Second-order upwinding and bounded central differencing schemes were used to reduce numerical diffusion. A maximum solver progression rate of 0.0003 revolutions per time step was required for an accurate solution. Fluid properties, additive feed arrangement, pump scale, and pump speed were systematically studied for their effects on mixing. For each additive feed arrangement studied, the additive was fed in individual stream(s) into the pump-intake. Pump intake additive variability, in terms of coefficient of spatial variation (COV), was >300% for all cases. The model indicated that the pump discharge additive COV ranged from 45% for a single centerline additive feed stream to 5.5% for multiple additive feed streams. It was found that viscous heating and thermal/shear-thinning characteristics in the process fluid slightly improved mixing, reducing the outlet COV to 3.2% for the multiple feed-stream case. The outlet COV fell to 2.0% for a half-scale arrangement with similar physics. Lastly, it was found that if the smaller unit's speed were halved, the outlet COV was reduced to 1.5%.
An “in-house” computational fluid dynamics code implementing a Euler-Lagrange approach is extended by incorporating the Euler-Euler (two-fluid model) approach, to improve prediction capabilities of flow and thermal characteristics of turbulent evaporating sprays. The performance of both approaches is assessed by comparing predictions with experimental data for a variety of evaporating-spray test cases. The applicability of the Euler-Lagrange and Euler-Euler approach is established in an isopropyl alcohol–air turbulent flow, in which characteristic droplet quantity predictions are in satisfactory overall agreement with measurements. The evaporating spray characteristics are then predicted under “stabilized cool flame” conditions and the computational results are compared to experimental data for a nonreactive case and a reactive case. In both cases, the velocity and thermal fields are successfully captured by both approaches. Overall, the article demonstrates an approach toward the development of performance-based computational tools.
The second edition of this long-time bestseller provides a framework for designing and understanding sprays for a wide array of engineering applications. The text contains correlations and design tools that can be easily understood and used in relating the design of atomizers to the resulting spray behavior. Written to be accessible to readers with a modest technical background, the emphasis is on application rather than in-depth theory. Numerous examples are provided to serve as starting points for using the information in the book. Overall, this is a thoroughly updated edition that still retains the practical focus and readability of the original work by Arthur Lefebvre.
Experimental and computational (CFD) investigations were conducted on an Airblast Simplex (ABS) nozzle. A Phase Doppler Particle Analyzer (PDPA) was used to study the main characteristics of the nozzle. Water was used as the test liquid with pressure drops of 68.9, 172.3 & 689 kPa, while the air pressure drops were 2.49 & 34.5 kPa. PDPA data were obtained at three different axial distances from the nozzle exit (x = 2.5 (or 7), 17 & 40 mm). In the numerical simulations, the three-dimensional flow field for both air and liquid was computed. The combination of detailed experimental measurements and CFD provided excellent insight into the physics of the atomizer. The three-dimensional CFD simulation showed distinct jets originating the air swirler and secondary flows that cause multiple peaks in number density, volume flux and velocity distributions. The jets suggest why asymmetric distributions are common in sprays. The jetting effect from the swirler vanes emphasizes the importance of swirler and airflow design on the performance of a spray nozzle. The discrete jetting effect is not visually observable, nor is it easily measured using PDPA.
The ABS nozzle examined has several distinct advantages over a pressure swirl atomizer, including, (1) the distinct sheet from the hollow cone spray is dispersed within 17 mm in all cases studied herein, and (2) the spray volume does not change significantly with increasing air or liquid pressure drop. Therefore, careful design of ABS nozzle results in a spray with nearly constant spray volume and SMD over a wide range of Air/Liquid Ratios.
Copyright © 1998 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
A computer model simulating the flow in fuel injection systems and the characteristics of diesel sprays injected from multihole orifice-type injectors has been developed and validated against experimental data. The injection conditions were modeled by solving the wave dynamics in the fuel injection equipment (FIE) using a one-dimensional model The flow in the sac volume was treated as a three-dimensional one for the noncavitating cases, in order to identify the basic flow distribution at the exit of the nozzle holes. For the cavitating cases, a one-dimensional sac volume flow model was developed; correlations giving the discharge coefficient of the injection holes were used in order to predict the quantity of fuel injected. Both inclined and vertical injectors can be modeled, since different flow rates are predicted for the different holes of multihole nozzles, depending on the position of the injection hole relative to the sac volume and the needle seat axis. The injection velocity calculation was based on the effective area (area at the hole exit occupied by liquid due to the presence of cavitating bubbles), which is calculated as a function of the flow conditions in the sac volume, the hole geometric characteristics, and the back pressure. Following the initiation of fuel injection, with droplet velocity, size, hole effective area, and level of turbulence at the nozzle exit calculated from the FIE model, an existing three-dimensional computational fluid dynamics (CFD) diesel spray model was extended and used for the prediction of the spray characteristics. This model is based on the Eulerian-Lagrangian stochastic particle technique; the gas phase is simulated by solving numerically the full Navier-Stokes equations, while the liquid phase is modeled using a Lagrangian particle tracking approximation. Spray submodels were used to represent the various physical phenomena taking place during the spray development. Emphasis was placed on the effect of the nozzle flow on the disintegration of the emerging liquid jet. Different jet atomization models were used in order to predict the spray characteristics at the closest point to the hole exit where experimental data were available. The aerodynamic-induced atomization, the jet turbulence-induced atomization, and a newly developed model for the cavitation-induced atomization were the three mechanisms considered responsible for the disintegration of the liquid jet. The latter mechanism was combined with a correlation representing the radial distribution of the droplets in the spray cone angle. Droplet secondary breakup, droplet collisions, and droplet turbulent dispersion were taken into account as the droplets penetrate into the surrounding gas. Quiescent atmospheric gas conditions have been selected for model validation in order to concentrate on the effect of the nozzle flow on the spray characteristics without the added complications induced by high-density effects or gas motion. The computational results were extensively compared with available experimental data, and confirm that hole cavitation enhances atomization. These results include a typical set of predictions for the flow in the FIE, the velocity, pressure, and turbulence kinetic energy distributions in the sac volume for noncavitating transient flow conditions, the fuel injected per injection hole, and the effective hole area for the one-dimensional sac volume flow model, and the droplet temporal and spatial characteristics (velocity and size) for the spray model Two different injection conditions were examined, corresponding to pump speeds of 600 and 1200 rpm. From the comparison between the two different sets of results, it can be concluded that the combined FIE-spray model is capable of predicting the spray characteristics without a priori knowledge of the flow characteristics in the sac volume and at the exit of the injection holes.
The simulation of the internal flow of a kind of high-pressure swirl injector was carried out with multiphase and cavitation models basing on the movement mesh. The spray was also simulated with the result of the internal flow simulation. The result of spray simulation was compared with experimental results. In addition, the effect on the internal flow which comes from the swirl flutes, the parts for generating the swirl flow, was investigated. It is shown that the amount and configuration of swirl flutes have crucial influence on the swirl velocity and cavitation of the internal flow.
Unsteady flow features of a plant-scale (>1.5 m diameter) cyclone-ejector system have been studied numerically and validated experimentally. Complexity arises from the fact that the transient pressure field within the Lapple cyclone governs the operation of the annular ejector, and vice versa. Eight geometric configurations for improving the system operation were evaluated. Simple geometric changes were shown numerically to make operational improvements while incrementally improving particle collection efficiency. It was also found that compressible, time-dependent CFD results were extremely sensitive to the pressure discretisation approach and to the differential Reynolds Stress pressure strain formulation.
The low frequency response of the spray from a generic air-blast diffusion burner with a design typical of an engine system has been investigated as part of an experimental study to describe the combustion oscillations of aero engine combustors called rumble. The atomization process was separated from the complex instability mechanism of rumble by using sinusoidal forcing of the air mass flow rate without combustion. Pressure drop across the burner and the velocity on the burner exit were found to follow the steady Bernoulli equation. Phase-locked PIV measurements of the forced velocity field of the burner show quasi-steady behavior of the air flow field. The phase-locked spray characteristics were measured for different fuel flow rates. Here again quasi-steady behavior of the atomization process was observed. With combustion, the phase-locked Mie-scattering intensity of the spray cone was found to follow the spray behavior measured in the non-combusting tests. These findings lead to the conclusion that the unsteady droplet SMD mean and amplitude of the air-blast atomizer can be calculated using the steady state atomization correlations with the unsteady burner air velocity.
This work addresses primary atomization modeling, multidimensional spray prediction, and flow characteristics of compound nozzle gasoline injectors. Compound nozzles are designed to improve the gasoline spray quality by increasing turbulence at the injector exit. Under the typical operating conditions of 270-1015 kPa, spray atomization in the compound nozzle gasoline injectors is mainly due to primary atomization where the flow turbulence and the surface tension are the dominant factors. A primary atomization model has been developed to predict the mean droplet size far downstream by taking into account the effect of turbulent intensity at the injector exit. Two multidimensional spray codes, KIVA-2 and STAR-CD, originally developed for high-pressure diesel injection, are employed for the lower-pressure gasoline injection. A separate CFD analysis was performed on the complex internal flows of the compound nozzles to obtain the initial and boundary conditions for the spray codes. The TAB breakup model used in KIVA-2 adequately facilitates the atomization process in the gasoline injection.
The new 'multiscale' model for turbulent flows developed by Wilcox has
been subjected to a continuing series of rigorous applications,
including shock-induced boundary-layer separation, to test its accuracy
in simulating complex flow phenomena. While previous advanced
applications presented by Wilcox have been inconclusive regarding
superiority of the multiscale model over two-equation models, this paper
demonstrates a marked improvement in predictive accuracy for flows which
include boundary-layer separation. As speculated in the original
development of the model, results obtained demonstrate that the model is
superior because it accounts for disalignment of the
Reynolds-stress-tensor and the mean-strain-rate-tensor principal axes.
Effects of surface roughness, mass injection, streamline curvature and
the mixing layer are also analyzed.
We describe the characteristics of a radially spreading unstable liquid sheet in quiescent air via optical measurement techniques
and linear instability theory. A high speed CCD camera system and a complimentary laser refraction method were employed to
measure the intact sheet diameter, unstable wave lengths, wave speed, wave frequency spectrum and spatial wave growth rates.
Linear instability models for thinning, viscous and inviscid liquid sheets, which are available from the literature, allow
for a comparison of experimental data and predicted sheet behaviour. The last section evaluates the differences and similarities
between the current liquid sheet experiment and industrial spray applications such as fuel atomisation via pressure-swirl
nozzles.
The production of a liquid spray can be summarized as the succession of the following three steps; the liquid flow ejection,
the primary breakup mechanism and the secondary breakup mechanism. The intermediate step—the primary breakup mechanism—covers
the early liquid flow deformation down to the production of the first isolated liquid fragments. This step is very important
and requires to be fully understood since it constitutes the link between the flow issuing from the atomizer and the final
spray. This paper reviews the experimental investigations dedicated to this early atomization step. Several situations are
considered: cylindrical liquid jets, flat liquid sheets, air-assisted cylindrical liquid jets and air-assisted flat liquid
sheets. Each fluid stream adopts several atomization regimes according to the operating conditions. These regimes as well
as the significant parameters they depend on are listed. The main instability mechanisms, which control primary breakup processes,
are rather well described. This review points out the internal geometrical nozzle characteristics and internal flow details
that influence the atomization mechanisms. The contributions of these characteristics, which require further investigations
to be fully identified and quantified, are believed to be the main reason of experimental discrepancies and explain a lack
of universal primary breakup regime categorizations.
The 3-D structure of the liquid atomization behavior of an jet flow through a pinhole nozzle is numerically investigated and visualized by a new type of integrated simulation technique. The present computational fluid dynamics (CFD) analysis focuses on the thermodynamic effect on the consecutive breakup of a cryogenic liquid column, the formation of a liquid film, and the generation of droplets in the outlet section of the pinhole nozzle. Utilizing the governing equations for a high-speed turbulent cryogenic jet flow through a pinhole nozzle based on the thermal nonequilibrium LES-VOF model in conjunction with the CSF model, an integrated parallel computation is performed to clarify the detailed atomization process of a high-speed LH2 jet flow through a pinhole nozzle and to acquire data, which is difficult to confirm by experiment, such as atomization length, liquid core shape, droplet-size distribution, spray angle, droplet velocity profiles, and thermal field surrounding the atomizing jet flow. According to the present computation, the cryogenic atomization rate and the LH2 droplets–gas two-phase flow characteristics are found to be controlled by the turbulence perturbation upstream of the pinhole nozzle, hydrodynamic instabilities at the gas–liquid interface and shear stress between the liquid core and the periphery of the LH2 jet. Furthermore, calculation of the effect of cryogenic atomization on the jet thermal field shows that such atomization extensively enhances the thermal diffusion surrounding the LH2 jet flow.
The paper describes a high resolution method (CICSAM) for the accurate capturing of fluid interfaces on meshes of arbitrary topology. It is based on the finite-volume technique and is fully conservative. The motion of the interface is tracked by the solution of a scalar transport equation for a phase-indicator field that is discontinuous at the interface and uniform elsewhere; no explicit interface reconstruction, which is perceived to be difficult to implement on unstructured meshes, is needed. The novelty of the method lies in the adaptive combination of high resolution discretisation schemes which ensure the preservation of the sharpness and shape of the interface while retaining boundedness of the field. The special implicit implementation presented herein makes it applicable to unstructured meshes and an extension to such grids is presented. The method is capable of handling interface rupture and coalescence. The paper outlines the methodology of CICSAM and its validation against academic test cases used to verify its accuracy.
Numerical simulations are carried out to describe the dense zone of a spray where very little information is available, either from experimental or theoretical approaches. Interface tracking is ensured by the level set method and the ghost fluid method (GFM) is used to capture accurately sharp discontinuities for pressure, density and viscosity. The level set method is coupled with the VOF method for mass conservation.The level set–VOF coupling is validated on 2D and 3D test cases. The level set–ghost fluid method is applied to the Rayleigh instability of a liquid jet. Preliminary results are then presented for 3D simulation of the primary break-up of a turbulent liquid jet with the level set–VOF–ghost fluid method.
The paper provides a view of some developments and a perspective on the future role of the Reynolds-averaged Navier-Stokes (RANS) approach in the computation of turbulent flows and heat transfer in competition with large-eddy simulations (LES). It is argued that RANS will further play an important role, especially in industrial and environmental computations, and that the further increase in the computing power will be used more to utilize advanced RANS models to shorten the design and marketing cycle rather than to yield the way to LES. We also discuss some current and future developments in RANS aimed at improving their performance and range of applicability, as well as their potential in hybrid approaches in combination with the LES strategy. Limitations in LES at high Reynolds (Re) and Rayleigh (Ra) number flows and heat transfer are revisited and some hybrid RANS/LES routes are discussed. The potential of very large eddy simulations (VLES) of flows dominated by (pseudo)-deterministic eddy structures, based on transient RANS (T-RANS) and similar approaches, is discussed and illustrated in an example of "ultra-hard" (very high Ra) thermal convection.
A numerical investigation of non-evaporating and evaporating diesel sprays
- S Som
- S Aggarwal
Som, S. and Aggarwal, S.,
2008.
A numerical
investigation of non-evaporating and evaporating diesel sprays.
Proc. ASME International Mechanical Engineering Congress
and Exposition.
Multiscale model for turbulent flows
- C Wilcox
Wilcox, C., 1986. Multiscale model for turbulent flows.
AIAA 24th Aerospace Sciences Meeting. American Institute of
Aeronautics and Astronautics.