Paper ID ICLASS SURFACE WAVE PROPAGATION AND BREAKUP IN PLANAR LIQUID SHEETS OF PREFILMING AIRBLAST ATOMISERS

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1 ICLASS-2006 Aug.27-Sept.1, 2006, Kyoto, Japan Paper ID ICLASS SURFACE WAVE PROPAGATION AND BREAKUP IN PLANAR LIQUID SHEETS OF PREFILMING AIRBLAST ATOMISERS Umesh Bhayaraju 1 and Christoph Hassa 2 1 Institute of Propulsion Technology, DLR e.v-german Aerospace Centre, Umesh.Bhayaraju@dlr.de 2 Institute of Propulsion Technology, DLR e.v-german Aerospace Centre, Christoph.Hassa@dlr.de ABSTRACT The liquid sheet breakup mechanism on a prefilming surface plays an important role in understanding the fuel placement in aeroengine combustors. Detailed knowledge of near-field and far-field characteristics of the spray is necessary for validation of spray combustion models incorporating airblast atomisers. An experimental approach is adopted in the present study to investigate the liquid sheet breakup mechanism on a prefilming surface and the characterisation of the spray. The liquid sheet breakup mechanism is studied by High-Speed Flow visualisation. Characterisation of the spray is carried out by PDA measurements. An effort is made to measure the near-field velocities of large structures with Particle Image Velocimetry. The sheet thickness on the prefilming surface is measured. The visualisation studies show that the liquid sheet breakup on the prefilming surface is different from the liquid sheet breakup without prefilmer surface. The PDA Measurements show that the relationship between Weber number and global SMD follows a power law. Application of PIV shows some promise in the acquisition of velocity data in the atomiser near-field. Keywords: Prefilming Surface, Liquid Film, Surface Waves, Wave Plunging, Surface Stripping, PIV, PDA 1. INTRODUCTION Airblast atomisers are used in gas turbine combustors as they produce fine sprays and also ensure thorough mixing of fuel and air [1]. The most commonly employed are prefilming airblast atomisers in which the fuel is spread into an annular liquid sheet on a swirler cup or on a cylindrical surface, called prefilming surface, before high velocity air streams interact with the liquid sheet. The atomisation quality of the spray in the near-field and far-field of the airblast atomiser plays an influential role in the formation of NOx emissions and development of combustion instabilities. Hence it is very essential to study the breakup of liquid sheets, the behaviour of ligaments in the near-field and general characteristics of the spray under the elevated operating conditions that exist in gas turbine combustors. However, research on breakup of liquid sheets on a prefilming surface and their atomisation characteristics has drawn little attention. So far, the available literature is on liquid sheets without prefilming surface. Hagerty and Shea [2] by experimental and analytical studies attributed the breakup of liquid sheets to formation of small amplitude perturbations that grow exponentially on the surface of the liquid sheet. Chigier [3] and Stapper, et al. [4] described the breakup mechanism of planar liquid sheets. Several regimes like cellular breakup and stretched streamwise ligament breakup are discussed. They attributed the breakup regimes to static pressure and relative velocity of air to liquid. Linear and non-linear models are developed to predict the liquid sheet behaviour [5]. Based on experimental data and analytical models, several correlations have been developed to predict the atomisation characteristics for prefilming airblast atomisers [1]. The correlations seem to be valid only for specific applications. They also assume, that the behaviour of liquid sheets on prefilmer surfaces is similar to liquid sheets without prefilming surface. In order to understand the liquid sheet behaviour on a prefilming surface, Berthoumieu et al. [6] carried out a preliminary investigation of the liquid sheet breakup on a prefilming surface. Umesh et al. [7] have further studied the breakup of liquid sheets by comprehensive flow visualisation studies. It is observed from the studies that the liquid mass strips from the surface of the liquid sheet as it propagates on a prefilming surface at high ambient pressure and velocity conditions (surface stripping phenomena). As a continuation of the work, new observations have been recorded at different operating conditions and it is now realised that the breakup of liquid sheets on a prefilming surface follows a mechanism that is different from non prefilming atomisers. This is explained in the present paper. An experimental approach is adopted to investigate the liquid sheet breakup mechanism on a prefilming surface. The liquid sheet breakup mechanism is studied by Background shadowgraphy and High-Speed imaging. PIV processing is carried out for near-field velocity flowfield of ligament structures which is of importance for validation of spray combustion codes incorporating airblast atomizers. The characterisation of the spray in the far-field is carried out by PDA measurements. 2. EXPERIMENTAL SETUP 2.1 Test Setup The experiments are conducted at the LPP (Lean Premixed Prevaporiser Channel) test facility at DLR, Köln-Porz, Germany. The LPP test facility is designed to carry out research on two-phase non-reacting flows at high pressures and temperatures simulating near real gas turbine inlet conditions to the combustor. The test facility can be operated at a maximum static pressure of 20 bar and static temperature of 850 K. The air is supplied from a reservoir at 60 bar to the test facility. The air is split into two lines providing preheated air and cooling air to the LPP test

2 section. A detailed description of the test rig can be found in [8]. The frame of the LPP test section is made from stainless steel. The schematic of the test section is shown in Fig. 1. primary air inlet secondary air quartz channel fuel inlet atomiser Figure 1. Test section It has two glass channels providing optical access from three sides to the test section. The primary glass channel is made up of quartz glass. The length of the quartz channel is 200 mm long with a cross section of 40 x 40 mm and 5 mm thickness. The preheated air enters the quartz channel. The secondary cooling channel surrounding the primary quartz channel has pressure windows on three sides and a stainless steel test section frame on the fourth side for introduction of atomisers and pressure transducers. The atomiser is mounted at 30 mm from the entrance of the test section. 2.2 Test Atomisers The atomisers are designed to conduct basic studies on liquid sheet atomisation of prefilming airblast atomiser. In practical atomisers the liquid sheet is cylindrical.. However, in the experimental studies presented here, the cylindrical liquid sheet is approximated to a 2-D planar flat sheet for ease of visualisation of liquid sheet breakup and for spray characterisation. Fig. 2 shows the schematic of the two injectors designed for the current studies. A transparent injector, Fig. 2(a), is designed to perform flow visualisation experiments. The angle of attack of air on the liquid film is 5 and 7. The span of the liquid sheet is 18 mm. The second injector, Fig. 2(b), is designed to conduct PDA measurements. In this case the angle of attack is 0 and the span of the liquid sheet is 12 mm. For both atomisers, the length of the prefilming surface is 4 mm and the thickness of the injector slit is 300 microns. y sonic hole z 2.3 Flow Visualisation Setup Flow visualisation is performed to observe the liquid sheet breakup on a prefilming surface and to observe the growth of ligaments in the near field. A schematic of the flow visualisation setup for High-speed imaging is shown in Fig. 3. A photographic flash lamp, Bowen s 1500 model, is used for background illumination. The flash duration of the lamp is approx. 800 milliseconds. A diffuser sheet is used to create a uniform background. A high speed intensified camera, the Ultra 8 model from DRS Hadland Ltd, is used to capture the images. The camera captures 8 sequential images with a minimum of 1000 frames per second and a maximum of 1,000,000 frames per second. The exposure time can be varied independently from 1 millisecond to 10 nanoseconds. The CCD chip has a 2048 x 2048 pixel resolution which is split into equal flash lamp diffuser high speed camera Figure 3. High-speed imaging - Background illumination setup imaging areas giving 520 x 520 pixels per frame. Images are captured from two views to observe the liquid sheet. 2.4 PIV Processing Consecutive images acquired during high speed imaging are processed by the PIV software (PIVTEC software developed by Chris Willert) at different operating conditions. The velocity of surface waves on the liquid sheet and the velocity field of ligaments and droplets in the near-field are calculated. Image processing: In the images acquired, the liquid structures are black, due to background illumination. The cross-correlation PIV algorithm searches for high intensity pixel areas in the images and recognises them as particles. For this reason, the acquired images are inverted on the prefilming surface (a) slit edge x z flo (b) Figure 2. Test atomisers All dimensions are in mm image 001 image 002 Figure 4. PIV processing, displacement vectors representing the movement of surface wave

3 intensity scale to make the structures appear in high intensity. Further processing steps included: passing the images through a 3x3 median filter, setting a dynamic threshold to remove CCD chip noise, high pass filtering with a Gaussian weighting function to increase spatial frequency and finally anti-aliasing smoothing to adjust for the background noise. Evaluation: The processed images are evaluated with an interrogation window size of 64 x 32 pixels with 50 % overlap between adjacent interrogation windows. This gives a resolution of 32 x 16 pixels for the image plane. A multigrid cross-correlation algorithm is used with an initial sampling window of 128 x 128 pixels. The advantage of this algorithm is that the movements of large structures are recognised as the algorithm first interrogates on a bigger window (128 x 128) and subsequently reduces the window size to the initially set interrogation window (64 x 32). Furthermore, the peak of the correlation also occupies several pixels, hence the peak detection was carried out by detecting the centre of mass. Fig. 4 shows the processing and evaluation of the images. A total of images are processed at each axial location and operating condition to obtain the velocity contours of the liquid structures. The region in the images covers approximately 8 x 8 mm; therefore several sets of image sequences are acquired along the flow direction. Due to the noise in the images, accurate estimation of the velocities was possible for particles greater than 50 microns even though the minimum particle size that can be measured is 16 microns. Since, in the near-field, greater quantity of mass flux is expected to be in the particle range greater than 50 microns, estimating the velocity of the particles is expected to give a first approximation of the flow field in the near-field. Also it has to be noted that the velocity is biased towards higher particle size classes. 2.5 Sheet Thickness Measurements Measurement of film thickness at realistic conditions is a difficult task mainly due to the small thickness of the liquid film and its corrugated surface. The thickness of the liquid sheet is 300 μm at the entrance of the atomiser slit. Also, the liquid sheet thickness has to be measured in the middle of the span of the liquid sheet for accurate measurements. The span of the liquid sheet is 18 mm and hence creates difficulties in optical access when measuring from the side view. In the present measurements, the liquid sheet thickness is measured at the edge of the liquid sheet. There is always spillover at the edge of the liquid sheet and hence the liquid sheet thickness measured at the side is always underestimated. The side view images acquired are used to measure the liquid sheet thickness. The liquid sheet thickness is characterised by the maximum and minimum thickness it can achieve from a sequence of images. The maximum s and minimum s from 8 sequences are averaged at one operating condition. 2.6 PDA Measurements A 2-D standard PDA setup, Dantec Inc., is used for measurement of droplet diameters and velocity flow fields. A green beam, λ = nm, and a blue beam, λ = 488 nm, from an Argon ion laser of Coherent make, are used for measuring two components of velocity of the droplets. A schematic of the setup is shown in Fig. 5. The spray in the current study is dense with high droplet concentration. Hence, the transmitting optics, from Dantec Inc, model 9060x0321, with two beam expanders of 1.85 and 1.98 are used together along with converging lens of 310 mm focal length to form a measurement volume with Gaussian beam diameter of 35 µm thus increasing the spatial resolution. The beam separation is 25 mm. The receiving optics are positioned at α = 62 which is very close to Brewster angle (ψ = 65 for kerosene). A total of samples is acquired at each measurement point. Due to the high density of the spray, the measurements are acquired 90 mm downstream of the even though atomisation is complete at around 10 mm from the. A total of 175 measuring points are acquired in a plane with a stepwidth of 1 mm. The validation levels are set at +2 db. The liquid volume flux is calculated by a volume flux algorithm developed in-house, which works on burst lengths for the determination of the detection volume [9]. 3. RESULTS 62 Figure 5. 2d PDA setup 3.1 Surface Wave propagation and Wave plunging on a Liquid sheet The stream of air that interacts with the surface of the liquid sheet produces different breakup regimes at various operating conditions, which is not observed for liquid sheets without prefilming surface reported in the literature. As the liquid sheet is exposed to the air stream, small amplitude waves are formed on the liquid surface. These are formed due to the local pressure difference between the liquid and air as explained in [10]. As they propagate, they quickly grow in amplitude, forming into a larger wave, termed as surface wave. Fig. 6 shows the formation of surface waves. surface waves Fig. 6. Surface waves The typical wavelengths of these waves are around 1-2 mm. Usually waves with such wavelengths are referred in the literature as capillary waves where surface tension plays a dominant role. However in the present case, the

4 propagation of these surface waves is effected by surface tension and viscous forces; the initial length scales of the liquid sheet being 300 μm. Fig. 7 (a, b) shows a sequence of images of surface wave propagation from two different views. It can be observed that as the wave propagates the amplitude increases. The amplitude of the surface wave is determined by the balance between surface tension forces, local viscous forces and differential pressure head between air and liquid. Due to increased momentum exchange between air and surface wave, the wave speed and amplitude of the top section of the wave increases, traveling at higher velocity than lower section of the wave, as shown in the schematic Fig 7 (e). Image 01 flow Image 02 surface wave t image 01 surface wave plunging. Even though the wave plunges, it does not break away from the surface wave, as surface tension forces are strong. In Fig 7(a) and (b), the images 3 & 4 show the wave plunging. At this Weber number, the wave speed is around 3 m/s. The plunged wave travels at twice the speed of the surface wave. These velocities are measured from the consequtive images. Also an absence of small disturbances formed due to imperfections in the atomiser can be observed in Fig 7(a), indicating that the phenomenon is more fundamental. The liquid sheet is intact until the edge of the prefilming surface. The disintegration of the liquid sheet at the edge of the prefilming surface is similar to the liquid sheet without prefilming surface as explained in [7]. As the Weber number increases, the wave speed increases and the surface wave propagates at very high velocities, stretching the surface of the liquid, inducing surface stresses. These are balanced by the surface tension. At limiting conditions, when the stresses are very dominant, the surface tension forces accumulate liquid mass to balance the induced stresses and ligament like structures are formed. Fig. 8 shows the formation of ligament like structures in the streamwise direction. The thickness and spacing of the ligaments depend on wave speed, surface tension and thickness of the liquid film. However the ligaments do not break up into individual ligaments. image 02 Image 03 wave plunging image 03 Figure 8.Formation of ligaments (P=2bar, V air =60m/s) Image 04 (a) front view plunged wave image 04 (b) side view σ σ τ u (e) τ μ y (c) (d) Figure 7. Phenomenon of wave plunging at low Weber numbers. (We = 32, t = 300μm) At limiting conditions, depending on the wave speed, the amplitude reaches a limiting value and the wave plunges ahead, similar to gravity waves [10]. This is termed wave 3.2 Surface Stripping from the Liquid film At high Weber numbers, due to increasing activity of momentum transport between surface wave and air, the wave reaches the limiting amplitude very close to the atomiser slit. Also, it has to be noted, as shown in the schematic of fig. 9 (c), that there is a high velocity gradient in the liquid sheet because the top section of the liquid sheet is propagating with the wave speed whereas the lower section is propagating by the velocity induced by the growth of boundary layer. To some extent the induced stresses are balanced by the viscous forces. However, if the momentum transport between air and surface wave is strong enough to overcome internal viscous forces and external surface tension forces, the surface wave breaks rather than plunges and liquid mass strips from the surface. This phenomenon is called surface stripping. Fig 9 (a, b) shows the surface wave breakup and stripped mass lump acceleration. The dark region in the circled area is a typical mass lump being stripped from the liquid surface. The size of the lumps can be around 300 μm. The typical velocity of the mass is around 20 m/s at the indicated operating

5 condition. From the visualisation experiments, it is observed that the surface stripping phenomenon starts around We = 100. image 01 t image 01 Also, the thickness of the liquid sheet decreases as the mass is stripped from the liquid surface. In fact, one can observe the formation of layers of liquid sheet on the prefilming surface propagating at different velocities, Fig.9(d). As the liquid sheet approaches the prefilming edge, more mass is stripped from the liquid surface. This is important, as it is well known, that the thickness of the liquid film at the edge has a dominant effect on the atomisation. At the, the air streams act from both sides of the liquid sheet and the liquid sheet breaks in a similar fashion as without prefilming surface. However, the effect of the can play a crucial role. Fig. 10 shows the liquid sheet breakup at the. As can be observed, at low Weber numbers even though the liquid sheet is intact till the edge of the prefilming surface breakup, it reorients at the edge and the breakup is very chaotic. image 02 image 02 image 03 image 03 We =32 We =380 Figure 10. Effect of image 04 u a (a) u l (c) image 04 (b) image 01 image 02 (d) Figure 9. Phenomena of Surface Stripping at high Weber numbers (We= 380) 3.3 Sheet Thickness Measurements For atomisation models dealing with prefilming atomisers, it is essential to know the temporal thickness and wavyness of the liquid sheet and the amount of mass stripped from the liquid surface. These parameters are very dominant factors for evaporation of the fuel film. As the liquid sheet is wavy and propagating, the amplitude of the wave increases and reaches a maximum. Also it is observed that a minimum thickness of the liquid sheet formed on the prefilming surface is unaffected by the aerodynamic forces. Hence the liquid sheet is defined by the maximum and minimum thickness it can achieve as shown in Fig. 11(a). Figure 11(b) shows a typical propagation of a liquid sheet on the prefilming surface at We = 32. It can be observed that the liquid sheet has maximum and minimum amplitudes. The measured average maximum amplitude is ~ 600 μm which is almost twice the actual film thickness at the atomiser slit. The minimum amplitude is ~ 100 μm. Since the liquid sheet behaves like a surface wave, the increase in amplitude has to be balanced by a decrease in amplitude to preserve mass balance. As the Weber number increases, due to surface stripping a defined amount of mass is lost and the thickness of the liquid sheet decreases. Hence, the maximum amplitude the liquid sheet can attain also decreases. Fig 11(b) shows the typical wavy nature of the liquid sheet at We = 285. The dotted line indicates the liquid film. The dark regions above the dotted line indicate the mass ejected due to surface stripping. The average maximum amplitude at this operating condition is ~ 378 μm and the average minimum amplitude is ~90 μm. Also at this operating condition calculated from images, liquid lumps stripped from the surface are in the order of ~

6 300 μm in diameter. The amount of mass that is stripped from the surface can be approximated accordingly. h min h max (a) (a) in mm 560 μm 190 μm (b) We= (b) We=32 Pressure=2bar, Velocity= 30 m/s Figure 12. velocity flow field of the liquid structures close to the in mm (c) We=285 Figure 11. Maximum and minimum amplitude of the liquid sheet 3.4 PIV of near-field Velocities When the combustion flame front is close to the atomiser, the velocity and size of the particles viz. ligaments and non-spherical particles in the near-field determine the combustion characteristics. They are also important for spray combustion codes. There are several constraints with PDA measurements (dense spray, non-spherical particles) very close to the atomiser under realistic operating conditions, and hence there is a need to find an alternative ways of estimating the size and velocity of these particles. The PIV technique is demonstrated in the present case as a promising methodology by which one can measure the velocity of particles and a way has to be determined to estimate the particles sizes based on area of the particles by image processing. Fig. 12(a) shows the propagation of liquid sheet and breakup into ligaments and droplets at low Weber number, We = 32. At this operating condition, the ligaments exist until 15 mm from the atomiser edge. Further downstream the ligaments break into large droplets. The PIV processing of the images as described in earlier sections gives a good estimation of the flow field of the particles, Fig. 12(b). The velocity of the surface wave on the liquid sheet is around 3-5 m/s and the ligament structures are around 5-10 m/s (a) We=95 Pressure=6bar, Velocity= 30 m/s (b) We=150 Pressure=2bar, Velocity= 60 m/s (c) We=380 Pressure = 6bar, Velocity= 60 m/s Figure 13. velocity flow field of the liquid structures close to the

7 SMD (μm) The breakup of ligaments into large droplets accelerates the large droplets that attains a velocity of around 20 m/s at 20 mm. The different shades in the contour above represent different stages of the breakup. Fig. 13 shows the velocity fields at different Weber numbers. As can be observed in the figure, the length scales of the breakup decrease as the Weber number is increased, indicating the typical behaviour of atomisation. For comparison, the magnitude of the velocity contours is set at 30 m/s, even though at higher Weber numbers the particles accelerate and disintegrate to reach a terminal velocity. 3.5 PDA Measurements The global SMD is calculated by mass weighted averaging of local SMD s over an area, Y= (-6, 6) and X=(-12,12), in a plane at 90mm away from the injector giving a unique SMD for an operating condition. Fig. 14 shows the effect of pressure and velocity on global SMD at a liquid sheet velocity of 1m/s at the entrance of the prefilmer slit. As can be observed, an increase in velocity of air decreases the SMD. The static pressure of air has a similar effect on the SMD indicating that the velocity and pressure are inversely related to SMD. However, the effect of velocity on SMD is much higher than the effect of pressure. This is due to high momentum flux ratios V=40m/s V=90m/s V=60m/s Pressure (bar) Figure 14. Variation of global SMD with pressure and velocity. The combined effect of static pressure and velocity of air is represented by the Weber number (We = ρv 2 air t/σ). Fig. 15 shows the effect of the Weber number variation on the global SMD of the spray. As the Weber number increases, the global SMD decreases. It can also be observed that the relationship between SMD and Weber number follows a power law. This could be due to the atomisation behaviour of the liquid sheet. Regression analysis is performed by assuming a power law relationship between SMD and Weber number. This demonstrates that SMD α We However, the proportionality does not take into account the effect of liquid velocity and geometrical parameters, i.e. prefilmer length. The effect of surface stripping from the liquid surface and the effect of are observed from the Local SMD profiles across the centre line of the span of the liquid sheet (y=0 mm, z=90 mm). Fig. 16 shows the effect of Weber number on local SMDs. SMD y = We R 2 = Weber No Figure 15. Variation of SMD with Weber number. As can be observed in the figure, at low Weber number, We= 56, the SMD profile is symmetric and droplet diameters are relatively bigger at the boundaries of the spray than in the middle. As the Weber number, increases to We 128, the local SMD on side of surface of the liquid sheet, negative x direction, decreases very rapidly indicating the initiation of liquid mass stripping from the surface of the liquid sheet. Whereas local SMD on the side of the prefilmer plate, in positive x direction, follows a similar trend as for lower Weber number. This is because, the liquid at the turns due to Coanda effect causing mass accumulation at the edge and hence producing larger droplets. As the Weber number is increased further to 190, the liquid sheet starts thinning as it propagates over the prefilmer surface and loses more mass from the surface of the liquid sheet. The thinning of liquid sheet, reduces the effect of and hence the local SMD decreases on both sides of the liquid sheet. At high Weber numbers, We = 380, as the liquid mass stripping reaches a limiting value, the SMD decreases on both sides equally. At very high Weber numbers, We > 500, the SMD decreases more on the side of the prefilmer plate rather than on the side of the liquid surface. In the present case, the is very thin, < 50 microns. However, if the edge is thick the SMD on the side may not reach very low values due to stagnation of liquid at the edge of the prefilmer by coada effect. The present PDA data is in conformity with the theory discussed in the earlier sections of the paper. Further studies are carried to see the effect of liquid flux on the global SMD. Fig. 17 shows the effect of liquid mass flow rate on global SMD. It can be observed that as the liquid mass flow rate increases the global SMD increases and reaches a peak value. This could be due to high level of surface stripping at lower mass flow rates. However as liquid mass flow rate increases, the relative amount of liquid mass that strips off from the surface is less and hence the global SMD increases

8 and reaches a constant value. The velocity of the liquid sheet at this condition is 1 m/s. 70 and pressure increases global SMD of the spray decrases. The relationship between SMD and Weber number follows a power law. The local SMD profiles show predominant effect of surface stripping and the and at various operating conditions. local SMD (μm) ACKNOWLEDGEMENTS The authors wish to thank Prof. Tropea, Institut für Strömungsmechanik, TU Darmstadt for lending the high speed camera. We also thank Chris Willert, for providing us with the PIV software. 6. NOMENCLATURE SMD (μm) x (mm) Figure 16. Effect of Weber number on local SMD We=56 We=128 We=190 We=380 We=570 We= mass flow rate in gm/s Figure 17. Effect of liquid mass flow rate on global SMD (We=250) 4. CONCLUSIONS The propagation of liquid sheets on the prefilmer surface follows a different mechanism than the propagation of liquid sheets without prefilming surface. At low Weber numbers, surface waves are formed on the liquid sheet which behave similar to gravity waves. The surface waves travel at ~3-5 m/s. At high Weber numbers, the surface of the liquid sheet is stripped and mass is ejected. Sheet thickness measurements reveal the amount of mass stripped from the surface. PIV analysis of high speed images shows that velocity measurements of large structures in the near-field of the atomiser are attainable. More efforts have to be made to enhance the technique for both size and velocity measurements. PDA droplet size measurements show that as velocity h min Minimum amplitude of wave[µm] hmax Maximum amplitude of wave [µm] SMD Sauter mean diameter [µm] t Thickness of the atomiser slit [µm] u Velocity of liquid [m/s] V Velocity of air [m/s] ψ Brewster angle [degrees] 7. REFERENCES 1. Lefebvre, A. H., Atomisation and Sprays, Hemisphere Publishing Corp., pp , Hagerty, W. W., Shea, J. F., A study of the stability of Plane Fluid Sheets. Journal of Applied Mechanics, , December Chigier, N. A., The Physics of Atomisation, Plenary Lecture, ICLASS-91, MD, U.S.A, July Stapper, B. E., Sowa, W. A and Samuelson, G. S., An experimental study of the effects of liquid properties on the breakup of a two-dimensional liquid sheets, Journal of Eng. for Gas Turbines and Power, Vol. 114, pp , January Mehring, C and Sirignano, W. A., Capillary Stability of Modulated Swirling Liquid Sheets, Atomisation and Sprays, 14, , Berthoumeiu, P., Carentz, H., Experimental study of a thin planar liquid sheet disintegration, 8 th Annual meeting of ILASS (America), CA, USA, Jul Bhayaraju, U., Giuliani, F. and Hassa, C., A study of planar liquid sheet breakup of prefilming airblast atomisers at high ambient air pressures, 20 th ILASS Europe, 5 th -7 th Sept Brandt, M,. Gugel, K. O., and Hassa, C., Experimental investigation of the liquid fuel evaporation in a premix duct for lean premixed and prevapourised combustion., J. of Eng. For Gas turbines Power Vol. 119, pp , Behrendt, T., Hassa, C., Investigation of the Spray Dynamics of Aeroengine Fuel Injectors under Atmospheric and Simulated Pressure Conditions, AGARD-Conference Proceedings 598 on Advanced Non-Intrusive Instrumentation for Propulsion Engines, May Kenyon, E. K. and Sheres, D., Einstein s Gravity Wave Method applied to a Two-Layer Fluid with Current Shear, Physics Essays, 10(1), pp , 1997.