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Towards the optimization of a pulsatile three-stream coaxial airblast injector
Abstract and Figures
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 characterization and 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 simulations was carried out to complement the AWTS study. 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 somewhat of a match between CFD and AWTS results. A self-exciting, pulsatile spray pattern was achieved in CFD and AWTS (frequencies between 75 and 600Hz), and an interesting transition in spray bursting character occurred at moderate inner air flows. The oscillatory flow pattern mimics prior work in terms of the energy of the fluctuations, but the fact that the present fluctuations occur at an order of magnitude lower frequency is apparently related to the comparatively low gas/liquid momentum ratio in the current study. 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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... Designs typically involve an "assisting" gas on the outside with the liquid fuel on the inside, including the efficient "flow blurring" design implemented in [15] for disintegrating algae oil. Some designs entail co-flowing gas streams on both the inside and the outside of the liquid sheet, called "airblast", and those also are known to exhibit pulsations [16][17][18][19][20][21][22][23][24][25][26]. ...
... The mesh size gradient is a consequence of early studies giving very preliminary estimates of droplet size, as well as learnings from [16][17][18][19][20][21][22][23][24]. The azimuthal boundaries of the 1/32 nd wedge are treated with periodic planes, because atomization is not axisymmetric. ...
... The azimuthal boundaries of the 1/32 nd wedge are treated with periodic planes, because atomization is not axisymmetric. The risks of using a reduced order model are discussed in [16][17][18][19][20][21][22][23][24], where it was found that the Sauter mean diameter (SMD) was affected only about 10% by the incorporation of 11.25° wedge boundaries. That risk is acceptable in this scoping effort. ...
... [5], Impinging Jet [6], and Pre-Filming [7]. Some designs swirl the liquid, some the gas, and some incorporate 34 external acoustic excitations. ...
... Given the low , the model was treated as isothermal at ambient conditions to match the biofuel 205 experiment. These methods have been extensively validated and used in the past [7,21,[47][48][49][50] . ...
... In many real field applications such as airblast nozzle(injector) the pulsating conditions are observed. Strasser [5] investigated the characteristics of threestream co-axial airblust nozzle in the experiments and computations with frequencies between 75 Hz and 600 Hz which is used to generate an atomized fuel stream for a large-scale reactor. On the basis of frequencies, three distinct flow patterns was observed. ...
In the present study, a computation study is performed to investigate the effect of imposed oscillation of nozzle pressure ratio (NPR) on the flow structure in a two-dimensional, axisymmetric supersonic converging nozzle. In this study, the underexpanded flow conditions are considered which are dominated by diamond shock-cell structure. The computational results are well validated with the available experimental measurements. The flow is initially computed to be fully developed and then oscillations are imposed. NPR is increased from 1.6 to 2.6 and then decreased again to 1.6 and thus completes a cycle. Results showed that the external flow structure of the nozzle is dependent on the process of change of pressure ratio during the oscillation. Distinct flow structures are observed during increasing and decreasing processes of the change of pressure ratio even when the nozzle is at the same NPR. Irreversible behaviors in the locations of jet centreline axis and off-axis as well as expansion, compression and neutral zones, are observed at the same NPRs during this oscillation. Further, the effect of oscillation frequency is explored on this irreversible behavior at 100 Hz, 200 Hz, 500 Hz and 1000 Hz frequencies.
We reveal mechanisms driving pre-filming wave formation of non-Newtonian banana puree inside a twin-fluid atomizer at a steam-puree mass ratio of 2.7%. Waves with a high blockage ratio form periodically at a frequency of 1000 Hz, where the collapse of one wave corresponds to the formation of another (i.e., no wave train). Wave formation and collapse occur at very regular intervals, while instabilities result in distinctly unique waves each cycle. The average wave angle and wavelength are 50{degree sign} and 0.7 nozzle diameters, respectively. Kelvin-Helmholtz instability (KHI) dominates during wave formation, while pressure effects dominate during wave collapse. Annular injection of the puree into the steam channel provides a wave pool, allowing KHI to deform the surface; then, steam shear and acceleration from decreased flow area lift the newly formed wave. The onset of flow separation appears to occur as the waves' rounded geometry transitions to a more pointed shape. Steam compression caused by wave sheltering increases pressure and temperature on the windward side of the wave, forcing both pressure and temperature to cycle with wave frequency. Wave growth peaks at the nozzle exit, at which point the pressure build-up overcomes inertia and surface tension to collapse and disintegrate the wave. Truncation of wave life by pressure build-up and shear-induced puree viscosity reduction is a prominent feature of the system, and steam turbulence does not contribute significantly to wave formation. The wave birth-death process creates bulk system pulsation, which in turn affects wave formation.
Characterization of viscous, non-Newtonian atomization by means of internal waves is presented for a twin-fluid injector. Atomization of such fluids is challenging, especially at low gas-liquid mass ratios (GLRs). This paper details mechanisms that enhance their disintegration in a "wave-augmented atomization" process. The working fluid, banana puree, is shear-thinning and described by the Herschel-Bulkley model. Unlike a conventional airblast injector, an annular flow of banana puree is injected into a core steam flow, encouraging regular puree waves to form inside the nozzle. A pulsing flow develops with three distinct stages: stretch, bulge, and burst leading to an annular puree sheet stretching down from the nozzle exit. Rayleigh-Taylor instabilities and viscosity gradients destabilize the surface. During wave collapse, the puree sheet bulges radially outward and ruptures violently in a radial burst. Near-nozzle dynamics propagate axially as periodic fluctuations in Sauter mean diameter occur in a wave pattern. Numerical simulations reveal three atomization mechanisms that are a direct result of wave formation: 1) wave impact momentum, 2) pressure buildup, and 3) droplet breakaway. The first two are the forces that exploit puree sheet irregularities to drive rupture. The third occurs as rising waves penetrate the central steam flow; steam shear strips droplets off, and more droplets break away as the wave collapses and partially disintegrates. Waves collapse into the puree sheet with a radial momentum flux of 1.7 × 10 ⁵ kg/m-s ² , and wave-induced pressure buildup creates a large pressure gradient across the puree sheet prior to bursting.
The focus of the present paper is to understand the large scale unsteadiness in the atomization of swirl coaxial jets and correlate it with the downstream spray statistics. A gas-centered swirl coaxial atomizer, in which a gaseous jet fragments an annular swirling liquid sheet, displays pulsating flow at certain momentum flux ratios (MFRs). These conditions were chosen to study the unsteady dynamics in the spray formation. Various zones of the spray such as near orifice region, primary atomization zone and far field spray were diagnosed using high-speed shadowgraphy technique. Proper orthogonal decomposition was employed on the time-resolved spray images to understand various unsteady modes. Three modes observed prominently were identified as large scale unsteady structures viz. axisymmetric pulsating mode, explosive mode and swirling mode. The pulsating mode was found to be more dominant in the pulsation regime, whereas the other modes gained significance at higher MFRs when the pulsation regime ceases to exist. Fourier analysis of temporal coefficients pertaining to pulsating mode showed a definitive frequency. The analysis of liquid shedding rate was found to be in synchronization with the pulsating mode which shows the upstream influence on periodic atomization. Spatio-temporal measurements of droplet sizes were carried out far from the atomizer. The temporal variation in the droplet number density and Sauter Mean Diameter depicted unsteady behavior; however, there is no preferred frequency in the power spectrum. This clearly indicates that the far field spray loses its memory of upstream periodic atomization in the pulsation regime of swirl coaxial atomizer.
Medical inhalers have been used for the treatment of a wide range of respiratory diseases including the COVID-19 pandemic. In this study, we investigate the annular liquid jet breakup with a coaxial supersonic gas jet using Large Eddy Simulations. This contributes a further understanding and improvement of medical inhaler designs. The liquid is sucked in by the low pressure as a result of the high velocity gas jet and breaks up due to the interaction with the gas jet. This type of spray nozzle configuration is commonly used in medical inhalers. Two different gas nozzle diameters are studied. The simulated liquid structure is compared with experimental results. The gas jet pressure and radial velocity of the liquid are found to be coupled and the interaction between them plays an important role in formation of the liquid structure. The effect of gas nozzle diameter on flow rate, mean radius of the liquid, mean radial velocity as well as its oscillation behavior has been investigated. The power development of dominate frequencies of averaged radial liquid velocity along the flow direction is plotted. The growth of the instabilities can be observed from these figures.
Fuel injection characteristics, in particular the atomization and penetration of the fuel droplets, are known to affect emission and particulate formation in diesel engines. It is also well established that the primary atomization process is induced by aerodynamics in the near nozzle region, as well as cavitation and turbulence from the injector nozzle. However, most breakup models used to simulate the primary breakup process in diesel engines only consider the aerodynamically induced breakup. In this paper, the standard breakup models in Diesel Engine modeling code called “CONVERGE” are examined in constant volume spray chamber geometry using the available spray data. Since non-evaporating sprays provide a more stringent test for spray models, the x-ray data from Advanced Photon Source is used for detailed validation of the primary breakup model, especially in the region very close to the nozzle. Extensive validation of the spray models is performed under evaporating conditions using liquid length and spray penetration data. Good agreement is observed for global spray characteristics. However, the breakup model could not reproduce some of the experimental trends reported in literature thus identifying the need for a more comprehensive primary breakup model. An attempt is made to statically couple the internal nozzle flow with spray simulations, and examine the effect of nozzle orifice geometry on spray penetration.
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
In this paper the Navier-stokes equations for a single liquid slug have been solved in order to predict the circulation patterns within the slug. Surface tension effects on the air-water interface have been investigated by solving the Young–Laplace equation. The calculated interface shape has been utilized to define the liquid slug geometry at the front and tail interfaces of the slug. Then the effects of the surface tension on the hydrodynamics of the two-phase slug flow have been compared to those where no surface tension forces exist. The importance of the complex flow field features in the vicinity of the two interfaces has been investigated by defining a non-dimensional form of the wall shear stress. The latter quantity has been formulated based on non-dimensional parameters in order to define a general Moody friction factor for typical two-phase slug flows in microchannels. Moreover, the hydrodynamics of slug flow formation has been examined using computational fluid dynamics (CFD). The volume-of-fluid (VOF) method has been applied to monitor the growth of the instability at the air-water interface. The lengths of the slugs have been correlated to the pressure fluctuations in the mixing region of the air and water streams at an axisymmetric T-junction. The main frequencies of the pressure fluctuations have been investigated using the Fast Fourier Transform (FFT) method.
We report in this work the evolution of a physically-based drop size-distribution of atomized drops coupling the Maximum Entropy Formalism (MEF) and the Monte Carlo method. The atomization is performed using a Spray On Demand (SOD) print-head which exploits ultrasonic generation via a Faraday instability. The physically-based distribution is a result of the coupling of a MEF specific formulation and a general Gamma distribution. The prediction of the drop size distribution of the new device is performed. The dynamic model which prediction capability is fairly good is shown to be sensitive to operating conditions, design parameters and physico-chemical properties of the fluid. In order to achieve the drop size-distribution evolution, we solve the distribution equation, reformulated via the mass flow algorithm, using a convergent Monte Carlo Method able to predict coalescence of sprayed droplets.
A new mathematical model is developed for calculating droplet break-up frequency based on both drag and turbulence induced fragmentation stresses. The droplet break-up model is introduced into a CFD methodology that is based on the Eulerian–Eulerian approach. The CFD solver couples the population balance equation along with the Navier–Stokes equations for predicting the droplets diameter. Finally, preliminary results using this CFD model are presented for the case of a coaxial airblast atomizer and a good agreement with the experimental data is achieved.
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.
We consider the stability of a multi-layer plane Poiseuille flow of two Bingham fluids. It is shown that this two-fluid flow is frequently more stable than the equivalent flow of either fluid alone. This phenomenon of super-stability results only when the yield stress of the fluid next to the channel wall is larger than that of the fluid in the centre of the channel, which need not have a yield stress. Our result is in direct contrast to the stability of analogous flows of purely viscous generalised Newtonian fluids, for which short wavelength interfacial instabilities can be found at relatively low Reynolds numbers. The results imply the existence of parameter regimes where visco-plastic lubrication is possible, permitting transport of an inelastic generalised Newtonian fluid in the centre of a channel, lubricated at the walls by a visco-plastic fluid, travelling in a stable laminar flow at higher flow rates than would be possible for the single fluid alone.