Theoretical analysis of heat and mass transfer in swirl atomizers

In this study, theoretical analyses have been performed to investigate the effects of atomizer construction and controlled pressure difference of swirl atomizers. The analysis of fluid field in the swirl chamber is governed by mass/energy conservation rules; in the region outside the nozzle, the analysis of oscillation of liquid sheet is based on Squire’s expression for the amplitude growth rate. With some physical assumptions of control volume, initial values and model correlation, analytical results make it possible to predict film thickness, velocity distribution, spray cone angle and droplet size directly. The distribution of velocity profile and boundary layer thickness in the swirl chamber have been established with the aid of MATLAB. Based on the results we obtained, we here propose the change of individual design parameter and its corresponding flow number to optimize the performance of swirl atomizers.     

Some features of spray breakup in effervescent atomizers

The near orifice spray breakup at low GLR (gas to liquid ratio by mass) values in an effervescent atomizer is studied experimentally using water as a simulant and air as atomizing gas. From the visualizations, the near orifice spray structures are classified into three modes: discrete bubble explosions, continuous bubble explosions and annular conical spray. The breakup of the spray is quantified in terms of the mean bubble bursting distance from the orifice. The parametric study indicates that the mean bubble bursting distance mainly depends on airflow rate, jet diameter and mixture velocity. It is also observed that the jet diameter has a dominant effect on the bubble bursting distance when compared to mixture velocity at a given airflow rate. The mean bubble bursting distance is shown to be governed by a nondimensional two-phase flow number consisting of all the aforementioned parameters. The location of bubble bursting is found to be highly unsteady spatially, which is influenced by flow dynamics inside the injector. It is proposed that this unsteadiness in jet breakup length is a consequence of varying degree of bubble expansion caused due to the intermittent occurrence of single phase and two-phase flow inside the orifice.     

Characterization of fan spray atomizers through numerical simulation

The present paper focuses on the mathematical modeling of industrial fan spray atomizers. The two-phase flow taking place inside the nozzle’s tip and the exterior region near the outlet of three different industrial nozzle designs has been modeled and simulated. As a result, valuable information has been obtained regarding the influence of the inner geometry on the flow and also the formation and development of the liquid sheet. Characteristic magnitudes such as the discharge coefficient and the liquid sheet thickness factor have been obtained and validated through experimental measurements. The accumulation of liquid at the border of fan-shaped liquid sheets, also known as rim, has been studied in the analyzed designs, revealing the presence of a tangential velocity component in the liquid sheet and a relationship between the incoming flow rate of the rim and the angle of the liquid sheet. The dependence of the results on turbulence modeling has also been analyzed, drawing interesting conclusions regarding their influence on the liquid sheet mean flow characteristics and on the surrounding gas. Thus, the mathematical model developed has been proved to be a useful tool for nozzle manufacturers; it provides the most important characteristic parameters of the liquid sheet formed given certain nozzle geometry and, additionally, those data necessary to carry out studies of instability, breakup and atomization of the liquid sheet.     

Experimental investigation of swirl atomizer spray defections

Introducing, distinguishing and correcting the observed defections in swirl-pressurized atomizers, applicable in cruise missiles, are the main purpose of this experimental work. Twenty sets of twelve reversed engineering atomizers infected with various defections which cannot perform correctly were considered. A set of eight original atomizers without any defections is used to not only identify and illuminate the defections, but also to provide and approve of the atomizers’ performance. In this work, over 350 tests performed for both two sets mostly repeated tests to identify the problems and make the corrections to perform as well as the original ones.     

Theoretical and experimental investigations on the formation of air core in a swirl spray atomizing nozzle

Theoretical and experimental studies have been made to investigate the variations of air core diameter, the most important hydrodynamic picture inside a swirl nozzle, with the pertinent guiding parameters like injection condition expressed as the Reynolds number at inlet to the nozzle and the geometrical dimensions of the nozzle, namely, the length and diameter of the swirl chamber, angle of spin chamber and the orifice diameter. The theoretical relations have been established through an approximated analytical solution of the hydrodynamics of flow of a viscous incompressible fluid in a swirl nozzle. A series of experiments have been carried out to support and compare the theoretical results. Finally, it has been recognized that for any nozzle, the air core diameter becomes a direct function of Reynolds number Re _i at inlet to the nozzle only at its lower range and then remains constant. Amongst the nozzle geometrics, the ratio of orifice to swirl chamber diameter D ₂/D ₁ has got the most predominant effect on the air core diameter. An increase in the ratio of orifice to swirl chamber diameter D ₂/D ₁, and in the spin chamber angle 2 and a decrease in the swirl chamber length to diameter ratio L ₁/D ₁ increase the ratio of air core to orifice diameter and vice versa.Nomenclature A _E Area of tangential inlet ports of the nozzle - A ₂ Area of the orifice - a Air core radius - D ₁ Swirl chamber diameter - D ₂ Orifice diameter - d ₂ Air core diameter - E A nondimensional parameter defined by equation (14) - E _R A nondimensional parameter defined by equation (33) - L ₁ Length of the swirl chamber - P Static pressure - P _b Back pressure of the nozzle - Q Volume flow rate - R Radius vector or the longitudinal co-ordinate with respect to spherical co-ordinate system (figure 3) - R ₁ Radius of the swirl chamber - R ₂ Radius of the orifice - Re _i Reynolds number at inlet to the nozzle - R _z Radius of the nozzle at any section - r Radial distance from the nozzle axis - U Longitudinal component of velocity with respect to spherical co-ordinate system (figure 3) - V Component of velocity in the axial plane perpendicular to R as defined in (figure 3) - V _r Radial velocity component - V _z Axial velocity component - V _Ø Tangential velocity component - Average tangential velocity at inlet to the nozzle - w Component of velocity perpendicular to axial plane with respect to the spherical co-ordinate as defined in figure 3 - z Distance along the nozzle axis from its inlet plane - Half of the spin chamber angle - Boundary layer thickness - ₂ Boundary layer thickness at the orifice - Angle which a radius vector according to the system of spherical coordinates (figure 3) makes with the nozzle axis - Dynamic viscosity - Kinematic viscosity - Density - Ø Running co-ordinate in the azimuthal direction with respect to the cylindrical polar co-ordinate system as shown in figure 3 - Circulation constant     

Study on the characteristic of the spray angle in pressure swirl spray atomisation

Based on the suggested atomisation theory for the swirl spray conical film, the formula for the spray angle characteristic of pressure swirl spray atomisation θ=tg^-12·(1-φ) is derived from the relation of acting forces in swirl spray.The spray angle characteristics of swirl spray are worked out with various formulas and compared with actual test data. The results show that the derived formulas for spray angle in this article agree comparatively well with the results from experiments, and that the expressions are simple. They are of definite value in practice.     

In-cylinder swirl formation process in a four-valve diesel engine

An experimental study is presented on the steady flow in a four-valve diesel engine rig by using hot-wire anemometry. Through the analysis of the in-cylinder three-dimensional flow field of a four-valve diesel engine, the in-cylinder swirl formation process at various valve lift was investigated. A new criterion is proposed for predicting the emergence of a stable-swirl formation interface, based on swirl angular momentum flux. A stable-swirl formation interface exists when the main swirl angular momentum flux is nearly equal to the in-cylinder air total angular momentum flux. Received: 13 March 2000/Accepted: 7 February 2001     

LDA/PDA measurements of instantaneous characteristics in high pressure fuel injection and swirl spray

 Transient dynamics of two injection flows, upstream and downstream a swirl injector, are investigated. Capillary n-heptane pipe flow is measured using laser Doppler anemometer to obtain instantaneous time series of centerline velocity and to reconstruct series of instantaneous and integrated flow rates and pressure gradient. A collimated laser sheet and a high-speed video camera visualize injected spray flow. Finally, the phase Doppler anemometer measurements are introduced to analyze instantaneous patterns of droplets velocity-size and number density into fuel spray. All measurements are employed at similar temporal resolution close to 30 μs. Results indicate that both flows are strongly time-dependent and well correlated in time-phases. Initial transitions are completed by 100 μs. Opening or closing of the injector valve affects both flows as strong delta oscillation causes spray penetration dynamics and a post injection effect. A combination of intrusive laser-based techniques allows indication of the basic injection and spraying characteristics need to optimize high-pressure fuel injectors and combustion late injection mode at a high speed. Received: 19 December 1998/Accepted: 13 August 1998     

Large Eddy Simulation of the spray formation in confinements

The particle and powder properties produced within spray drying processes are influenced by various unsteady transport phenomena in the dispersed multiphase spray flow in a confined spray chamber. In this context differently scaled spray structures in a confined spray environment have been analyzed in experiments and numerical simulations. The experimental investigations have been carried out with Particle-Image-Velocimetry to determine the velocity of the gas and the discrete phase. Large-Eddy-Simulations have been set up to predict the transient behaviour of the spray process and have given more insight into the sensitivity of the spray flow structures in dependency from the spray chamber design.     

Theoretical and experimental studies on the coefficient of discharge and spray cone angle of a swirl spray pressure nozzle using a power-law non-Newtonian fluid

An extension of earlier work is made in the present paper to determine both theoretically and experimentally the coefficient of discharge and spray cone angle of a swirl nozzle using a time-independent purely viscous power-law non-Newtonian fluid. The theoretical predictions are made through an approximate analytical solution of the hydrodynamics of flow inside the nozzle. Experiments are carried out with aqueous solutions of CMC (carboxymethyl cellulose sodium salt) powder of various concentrations as the working fluids. The rheological properties of the working fluids are established by a capillary tube viscometer. From both the theoretical and experimental analyses, the pertinent independent input parameters are recognised as the generalised Reynolds number at inlet to the nozzle Re_Gi, the flow behaviour index of the fluid n, length-to-diameter ratio of the swirl chamber L₁/D₁, spin chamber angle 2α and the orifice-to-swirl-chamber-diameter ratio D₁/D₁. Although the theory predicts the correct qualitative trend in all cases, it does not agree well with the experimental results. Therefore, on the basis of the theoretical results, emperical relationships between nozzle characteristics and input parameters heve been established. Finally it is recognised that, regarding the injection conditions and fluid properties, the generalised Reynolds number at nozzle inlet Re_Gi and the flow behaviour index n have inverse and direct effects, respectively, on the coefficient of discharge, but have a negligible influence on the spray cone angle. Amongst the nozzle geometries, an increase in the values of D₂/D₁ and 2α or a decrease in the value of L₁/D₁ decrease the coefficient of discharge and increase the spray cone angle.     

Experiments on the formation of a recirculation zone in swirling coaxial jets

 The near flow field of coaxial air jets, with swirl in the outer one, was studied using flow visualization and hot-wire anemometry. The flow is sensitive to both the swirl number and the mass flow ratio between the outer and inner jets. A necessary condition for the formation of an internal recirculation zone (IRZ) is that the swirl number must exceed a minimum value which depends on the mass flow ratio. Spectral analysis of the velocity fluctuations indicates that the formation of an IRZ in the present flow does not appear to be related to the growth of convective flow instabilities. Analysis of the flow visualization and X-wire data indicates that the vorticity dynamics model for vortex breakdown proposed by Brown and Lopez [J Fluid Mech (1990) 222: 553] provides a plausible mechanism for the formation of an IRZ in this flow. Received: 14 June 1999/Accepted: 7 December 1999     

Correlation of swirl number for a radial-type swirl generator

An experimental investigation was undertaken to derive a new correlation for the swirl number of a radial-type swirl generator under various Reynolds numbers and various vane angle conditions. A radial-type swirl generator with 16 rotary guide vanes was used to generate an annular swirling jet flow. The Reynolds numbers ranged from 60 to 6000, and the vane angles from 0° to 56°. Quantitative measurements for the velocities were made by using an optical method of laser-Doppler anemometry (LDA). Three-component velocity profiles of axial, radial, and azimuthal components at the swirling jet exit were measured for various flow conditions. A flow visualization method using smoke-wire and still photography was also applied to observe the flow patterns of the recirculation region behind the circular bluff body. Under low Reynolds number conditions, the swirl strength was found to be strongly dependent on the Reynolds number as well as on the guide vane angle. Based on the experimental results, a modified swirl number S is derived to characterize the swirling flow, which is useful for the design of a swirl generator.     

Adaptive collision meshing and satellite droplet formation in spray simulations

Improved numerical methods and physical models have been applied to droplet collision modeling. Numerically, an adaptive collision mesh method is developed to calculate collision rate. This method produces a collision mesh that is independent of the gas phase mesh and adaptively refined according to local parcel number density. An existing model describing the satellite droplet formation during the collision process is improved to reflect the experimental findings that the satellite droplets are much smaller than the parent droplets. The adaptive collision mesh and the improved satellite model have been used to simulate three impinging spray experiments. The model was able to qualitatively predict the occurrence of small satellite drops and bi-modal post-collision drop size distributions. The effect of the collision mesh and the satellite droplet model on a high-speed non-evaporating diesel spray is also assessed.     

Drop size spectra in sprays from pressure-swirl atomizers

A study on the characterization of sprays from Newtonian liquids produced by pressure-swirl atomizers is presented. The global drop size spectra of the sprays are measured with phase-Doppler anemometry, and global mean drop sizes are derived as moments of the spectra for varying atomizer geometry, liquid flow rate, and physical properties of the liquids. Dimensional analysis provides a correlation for the non-dimensional global Sauter mean diameter. A relationship between the global Sauter mean drop size and the global drop size RMS is established. A method is developed for predicting the global drop size spectra in the sprays, using easily accessible experimental input parameters. The basis for the function defining the spectrum is a gamma distribution, which is known from the literature as physically relevant for ligament-mediated sprays.     

Experimental evaluation of pneumatic atomizers by hot film

The spray characteristics of pneumatic atomizers are examined using hot film anemometry. Measurements of drop size distribution, concentration profile and drop frequency are carried out over wide ranges of relative position between air peripheral jet exit and the concentric water nozzle exit. The objective is to clarify the role of turbulence in producing a wide spectrum of particle sizes and numbers in the spray. The output signal from hot film contained multispikes with different amplitudes and thickness which has been correlated to the drop number density and the droplet size through the use of probability density function. Generally it is found that an increase of the guide relative position enhances the mixing process and produces finer spray. This is attributed to the utilization of high wave number eddies in shortening the liquid jet break up length.

---

Versuchsauswertung eines pneumatischen Pulverisators durch einen Heißfilm

Zusammenfassung Die Spraykennzeichen eines pneumatischen Pulverisators sind mit der Heißfilmanemometrie untersucht worden. Es wurden Messungen über die Tropfengrößenverteilung, Konzentrationsprofile und Tropffrequenz über weite Bereiche der relativen Position von äußerem Luftstromausgang und dem konzentrischen Wasserdüsenausgang durchgeführt. Das Ziel ist die Funktionsklärung der Turbulenzen in der Produktion von einem weiten Spektrum der Teilchengrößen und Teilchenzahlen im Spray. Das Ausgangssignal vom Heißfilm beinhaltet mehrere Impulsspitzen mit verschiedenen Amplituden und Breiten. Diese wurden für die Tropfenzahldichte und Tröpfchengröße mit der Wahrscheinlichkeitsdichte-Funktion berechnet. Im allgemeinen wurde herausgefunden, daß ein Erhöhen der relativen Position des Führungsrohres den Mischprozeß verbessert und einen feineren Spray ergibt. Dies ist darauf zurückzuführen, daß hochfrequente Wirbel die Abbruchlänge des Flüssigkeitsstroms verkürzen.

     

Low-pressure twin-fluid atomization: Effect of mixing process on spray formation

The present work comparably examines four different twin-fluid atomizers operated under the same operating conditions. Spray formation was examined by several approaches. The internal flow pattern was estimated using a simplified analytical approach, and the results were supported by the observation of the liquid discharge in the near-nozzle region. A high-speed back illumination was used for visualisation of the primary breakup. In the region of fully developed spray, the dynamics of droplets was studied using a phase-Doppler analyser (PDA). The information obtained from all methods was then correlated. Results show that the spray formation process depends mainly on the internal design of twin-fluid atomizer at low gas to liquid ratios (GLR). The amount of gas influences the character of the internal two-phase flow, a mechanism of the liquid breakup, droplet dynamics and a resulting drop size distribution. Differences among the atomizers are reduced with the increase in GLR. Moreover, it was shown that a certain mixing process can inherently create the annular internal flow which generates a stable spray characterized by relatively low mean droplet size.     

Liquid flow in a simplex swirl nozzle

Pressure-swirl nozzles are widely used in applications such as combustion, painting, air-conditioning, and fire suppression. Understanding the effects of nozzle geometry and inlet flow conditions on liquid film thickness, discharge coefficient and spray angle is very important in nozzle design. The nozzle-internal flow is two-phase with a secondary flow which makes its detailed analysis rather complex. In the current work, the flow field inside a pressure-swirl nozzle is studied theoretically. Using the integral momentum method, the growth of the boundary layer from the nozzle entry to the orifice exit is investigated and the velocity through the boundary layer and the main body of the swirling liquid is calculated. A numerical modeling and a series of experiments have also been performed to validate the theoretical results. The effect of various geometrical parameters is studied and results are compared for viscous and inviscid cases. In addition, the condition in which the centrifugal force of the swirling flow overcomes the viscous force and induces an air core is predicted. The theoretical analysis discussed in this paper provides better criteria for the design and the performance analysis of nozzles.     

Nonlinear modeling of drop size distributions produced by pressure-swirl atomizers

An axisymmetric boundary element method (BEM) has been developed to simulate atomization processes in a pressure-swirl atomizer. Annular ligaments are pinched from the parent sheet and presumed to breakup via the linear stability model due to Ponstein. Corrections to Ponstein’s result are used to predict satellite droplet sizes formed during this process. The implementation provides a first-principles capability to simulate drop size distributions for low viscosity fluids. Results show reasonable agreement with measured droplet size distributions and the predicted SMD is 30–40% smaller than experiment. The model predicts a large number of very small droplets that cannot typically be resolved in an experimental observation of the spray. A quasi-3-D spray visualization is presented by tracking droplets in a Lagrangian fashion from their formation point within the ring-shaped ligaments. A complete simulation is provided for a case generating over 80,000 drops.