Parametric Experiments on Coaxial Airblast Jet Atomization

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THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS.n345 E. 47 St., New York, N.Y. 117 9-GT-81 r+ The Society shall not be responsible for statements or opinions advanced in papers or In dis ^.

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS.n345 E. 47 St., New York, N.Y GT-81 r+ The Society shall not be responsible for statements or opinions advanced in papers or In dis ^. cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. M Discussion is printed only if the paper is published in an ASME Journal. Papers are available ^^L from ASME for fifteen months after the meeting. Printed in USA. Copyright 199 by ASME Parametric Experiments on Coaxial Airblast Jet Atomization ZOLTAN FARAGO Institute for Chemical Propulsion and Chemical Engineering German Aerospace Research Establishment (DLR) D-711 Lampoldshausen, Federal Republic of Germany and NORMAN CHIGIER Department of Mechanical Engineering Carnegie Mellon University Pittsburgh, PA 15213, USA ABSTRACT Experiments using high speed, high magnification, and high contrast photography on airblast coaxial atomizers were carried out to study the wave characteristics of liquid surfaces, ligament breakup, and droplet formation. Liquid flow rate was changed from 4 to 5 kg/h, corresponding to a velocity range of 1.5 to 18 m/s, and a Reynolds number range of 14 to 18. Air flow rate was varied from 8 to 7 kg/h, corresponding to a velocity range of 22 to 18 m/s, and a Reynolds number range of 13 to 15. Tube wall thicknesses of 145 and 32 microns were used. Under different flow conditions, different jet instabilities (capillary, helical and Kelvin-Helmholtz) and different dominant mechanisms of ligament formation were observed. One of the most surprising experimental results is that, under certain flow conditions, the coaxial round liquid jet, surrounded by an axisymmetric annular air stream, forms a flat curling liquid sheet. This liquid sheet breaks into droplet clouds with a frequency of a few thousand Hertz and emits strong oscillations and fluctuating, highly non-axisymmetric vibrations. NOMENCLATURE u W water mean velocity [m/s], u W = m / pa Da air mean velocity [m/s], U a = 2 Pd yn / P di coaxial tube inner diameter [m] dh air gap hydraulic diameter [m] v im, kinematic viscosity of water, vw = 1 cst = 1-6 m2/s Va kinematic viscosity of air, Va = * 1-5 m2/s Rew Reynolds Number of water, Rew = uw (di / v W) Rea Reynolds Number of air, Re a = ua (dh / Va) u th potential velocity, uw = 2 p total gauge pressure of the fluid [Pa] S density of the fluid [kg/m3] Cd discharge coefficient, Cd = u/uth A flow area [m2] Pdyn dynamic pressure of the fluid [Pa] m mass flow rate [kg/s] M mass flow rate [kg/h] Subscript w = water, a = air, th = theoretical, i = inner, h = hydraulic, dyn = dynamic INTRODUCTION Experimental data of spray characteristics have long been recognized as extremely important for calculations of spray flames and pre-vaporized liquid fuel combustion. The interest in atomization data is, therefore, very high in any organization dealing with combustion. Accordingly, several institutions, many of them coordinated by NASA (Gross, 1987), are working on liquid jet atomization. Different investigations of atomization and mixing studies are also being carried out. The most thorough reviews of earlier work have been prepared by Giffen and Muraszew (Giffen and Muraszew, 1953), Abramovich (Abramovich, 1955)), and more recently by Chigier (Chigier, 1981), Ferrenberg (Ferrenberg, 1985), Cheremisinoff (Cheremisinoff, 1986), and Lefebvre (Lefebvre, 1989). Nevertheless, the state of modeling atomization is not sufficiently advanced to provide accurate prediction. The physics of drop formation is only qualitatively understood. Even the current calculations on the liquid breakup region and average drop size are based mostly on average flow parameters and dimensionless numbers. There is a large number of important variables and relevant dimensionless numbers, including the details of the design of the nozzle, the jet velocity and turbulence, and the physical properties of both the liquid and the gas, which influence atomization. Recently, new and promising results are being reported of numerical simulations of primary atomization in liquid jets. The effects on undulation, wave growth and surface oscillation on ligament breakup and drop formation were the starting point in Dombrowski's model of the aerodynamic instability in thin liquid sheets (Dombrowski and Fraser, 1954; Dombrowski and Johns, 1963; Fraser et al., 1962). Current studies on liquid breakup are also based on jet surface stability analysis using linear (Reitz and Bracco, 1982) or approximate nonlinear-models Przekwas et al., 1989) or utilizing the vortex dynamics approach (Sirignano et al., 1989). These numerical analyses require experimental verification. At present, photography is one of the most effective techniques for visualizing and analyzing the onset of surface waves, wave growth, ligament formation, and hence for the experimental verification of atomization models. In the experiments of this investigation, water was used as the liquid to be atomized. In the air 'Presented at the Gas Turbine and Aeroengine Congress and Exposition June 11-14, 199 Brussels, Belgium

assisted coaxial atomizers, air flow simulated the effect of the 16 gaseous flow on the liquid jet atomization. 1.36 EXPERIMENTAL PROCEDURE A schematic diagram of the test section is shown in Fig. 1. Water is stored in a pressurized tank with a capacity of 1 liters.

2 assisted coaxial atomizers, air flow simulated the effect of the 16 gaseous flow on the liquid jet atomization EXPERIMENTAL PROCEDURE A schematic diagram of the test section is shown in Fig. 1. Water is stored in a pressurized tank with a capacity of 1 liters. The driving force for the water supply is pressurized air. In contrast to any water pump, this kind of water supply gives a pulsation- and vibration-free water flow and a very accurate repeatability for the experiments. Reducing any pulsation or vibration of the flow is necessary for this experiment to avoid triggering surface wave formation caused by flow disturbances upstream of the exit orifices. The highest possible water pressure which could be achieved with this equipment was about 6 kpa if no air was used to assist the atomization. However, when the maximum air flow rate was used for the atomization, the maximum inlet water pressure dropped to about 5 kpa gauge pressure. The inlet pressure of the atomizing air could be varied from zero to 25 kpa, the latter being connected to an air mass flow rate of 7 kg/h or an air velocity of 18 m/s. Pressure ceuge^j Pressure Regulator, Chmgeable Inrcr Tube Changeable Outer Tube Screen Q5 4- (:,enter Positions for lend Tube ( An valve F WWateruized J'., Hou se Air z, r, phi Positio,ter for Test Nozzle AW Spark Light Controllable Exhaust Axial Positioner for Central Tube 1 G ce" ' z- phi z- phi positioner _^ t Wet. Fig. 2 Sketch of the test nozzle (dimensions are in mm) $ t Fig. 1 Schematic diagram of the test section A variable spray exhaust was used both to protect the optical equipment from the spray and to guarantee stable surrounding conditions for the atomization. In preliminary tests, it was found that an environmental air velocity into the exhaust of about.5 m/s is necessary to protect the spray surroundings from unintentional air movements caused by opening doors or room heating. The exhaust system and the test atomizer were connected to the same threedirectional (z-r-phi) positioner. Therefore, even when moving the nozzle to highlight different parts of the atomization process, the axial distance between atomizer outlet and exhaust inlet was always kept constant (12 mm). The accuracy of the positioner is plusminus 1 microns in the axial and radial (z,r) directions and.1 degree in the rotation. A Canon F1 camera with f = 2 mm macro lens and different extension tubes and bellows was used for photography. The light source was an Electro-Optics EG&G 549 Microflash System with.5 microsecond flash duration having 5 x 1 7 beam candlepower light intensity and approximately daylight quality (EG&G, 1984). Figure 2 shows the sketch of the coaxial atomizer used in this investigation. The reduction of the air tube diameter at the end produces a flat air velocity profile at the outlet. The lengths of all central tubes are 55 mm. Fig. 3 is an enlargement of the nozzle exit. EXPERIMENTAL RESULTS AND DISCUSSION Flow Conditions When the liquid Reynolds number is below 6, the flow conditions are laminar as shown in Fig. 4. The Reynolds number.25 range of 6 < Re < 1 is the transition region. Here, an increase of the Reynolds number is connected with a decrease of the discharge coefficient. The flow is turbulent when the Reynolds number is above 1. This very high Reynolds number for the laminar-turbulent-transition indicates that the water inlet into the e U $16. Fig. 3 Sketch of the nozzle exit (dimensions are in mm) laminar 8 turbulent / transition 1 21 Reynolds Number Reµ. Fig. 4 Discharge Coefficient for Water as a Fusion of the Reynolds Number 2

nozzle is uniform and oscillation-free and there is no vibration in the experimental system. Figure 5 indicates that the air flow is turbulent if the air Reynolds numbers is above 1.

3 nozzle is uniform and oscillation-free and there is no vibration in the experimental system. Figure 5 indicates that the air flow is turbulent if the air Reynolds numbers is above 1. For the air assisted atomization in the present experiments, the air flow conditions were always turbulent. P Reynolds Number Rea Fig. 5 Discharge Coefficient of the Atomization Air as a Function of the Reynolds Number Photographic Experiments The photographs presented in Figs. 6 through 9 were selected from about 15 photographs. Figure 6 shows the water jets with the nozzle exit velocities between 1.1 and 18.2 m/s, and a coaxial air stream with 48.5 m/s velocity surrounding the water jet. Figure 6A shows a typical cylindrical liquid jet transforming into a thin water sheet which curls up forming a shallow half-bowl or ladle. Because of the higher liquid flow rate, Fig. 6B shows the onset of the ladle formation further downstream in comparison to Fig. 6A. Figure 6C shows that the laminar jet starts becoming wavy and unstable at an axial distance of about 1D. The length of the disturbance-free liquid cylinder increases with increasing Reynolds number up to Re = 25. Then, with increasing water velocity, the length of the disturbance-free liquid cylinder is reduced (Fig. 6D) until the appearance of turbulence at the nozzle exit (Fig. 6E). At higher water velocities, the effect of air velocity, which is 48.5 m/s, on the water surface shape becomes negligible (Figs. 6F, 6G, and 6H). Figure 6G shows bone-shaped coherent liquid structures with a unit length of about 2.5D to 3D. Figure 7 shows the central water jet surrounded by an air stream of 64.8 m/s velocity. The length of the undisturbed cylindrical jet increases with increasipg water mass flow rate until Re = 5 as shown in Figs. 7A through 7D. Figures 7A, 7B, and 7C show the liquid ladle formation described in Fig. 6A. The different sizes of the partly burst curling liquid sheets are randomly due to the different triggering time of thq spark source. Though the water flow conditions are the same, Fig. 7E shows a laminar liquid jet downstream of the nozzle exit unlike the turbulent jets of Fig. 6E. The effect of air flow on the atomization is strong for a wide mass flow range as evidenced in Figs. 7A through 7G. However, the effect of the air flow on the liquid jet surface is insignificant at the highest water mass flow rate (see Figs. 7H and 6H). This can be explained by the mass flow rate ratios. A comparison of Figs. 7H and 6A, where the relative velocities at the water-air interface are nearly the same, but the mass flow rate ratios are different, shows a significant influence of mass flow rate ratios on the surface conditions. The disintegration of the round liquid jet in the coaxial air stream with m/s velocity is shown in Fig. 8. This air stream affects the atomization considerably even at the highest water mass flow rate of this study as shown in Fig. 8. The same effect that caused the ladle or half-bowl formation of Figs. 6A, 7A, 7B, and 7C now leads to a cloud of liquid ligaments and drops with a shape similar to the cupping of the palm of a hand as shown in Figs. 8A and 8B. Figures 8B and 8C show the jet disintegration at identical air and water flow conditions. These two photographs show the cupped-palm shaped spray from the "side view" (Fig. 8B) and "front view" (Fig. 8C) with respect to the camera position. Figure 8D shows a spray cloud front view, like 8C, which has a higher water mass flow rate. In Figs. 6A, 7A, 7B, 7C, 8A, 8B, 8C, and 8D, the whole cylindrical jet transforms into a stretched membrane with the shape of a shallow ladle. Some of the surface waves form similar but small scale curling sheets. Figs. 8C through 8H contain several small scale ladles, although it is more difficult to observe them because of their smaller size. (For example in Fig. 8G: right hand side, axial distance 3D; left hand side, axial distance 4D; left hand side, axial distance 4.5D; etc.) Fig. 8H is an enlargement of the boxed area in Fig. 8G. The water and air flow conditions in Fig. 9 are the same as in Fig. 6A i.e., where the nozzle exit velocity is 1.1 m/s for water flow and 45.8 m/s for air flow. This figure shows examples of the curling sheet formation and of the ladle history. At the top of Fig. 9A, parts of the bursting upper ladle can be seen. The intact handle of the upper ladle pulls the liquid cylinder to the right hand side out of the main stream direction. The air pressure at the wind-side and the under-pressure at the wind-shadow-side of the water cylinder result in a hook formation of the liquid jet. At the upper part of the hook, the air begins to form a stretched liquid membrane. This is the onset of the lower ladle formation. A fully developed ladle can be seen in Fig. 9B. The handle of this half bowl pulls the intact liquid jet out of the axial direction. In Fig. 9C, the upper ladle reaches a critical size at which the force resulting from the air shear is higher than the force resulting from the surface tension, and the ladle begins to burst into ligaments and drops. The largest liquid accumulation is where the handle of the upper ladle is connected to the lower ladle. This part of the disintegrating jet forms the largest drops and ligaments. These ligaments break to smaller ligaments and drops according to their critical Weber number (Hinze, 1955) showing typical half bowl shapes already investigated by Lane (Lane, 1951) and Simmons (Simmons, 1979). Such cups of liquid membranes can be seen in Figs. 9D (right hand side, axial distance 11D) and 9E (left hand side, axial distance 9D). Figure 9F is an enlargement of the boxed area in Fig. 9E. As shown in the different photographs of Fig. 9, relatively large drops are also formed from the frame of the ladle. The smallest drops are generated from the bursting membrane. The laminar or semiturbulent liquid jet forms large-scale coherent structures with a length of about 2.5 D as shown in some of the photographs (6E through 6H). At very low water velocities (laminar liquid jet) and at an air to liquid mass flow rate ratio of about 2 to 5, the coherent liquid structures form flat curling sheets which break into drops with a wide size range. Figure 9 shows those events. The diameter of the largest drop is about.5 to 1 jet diameter. If the size of the largest drop is over-critical (the Weber number is larger than the critical Weber number), this drop breaks into smaller droplets via a new curling sheet formation (Fig. 9D through 9F). The time difference between the formation of the curling liquid sheet and its bursting into drops and ligaments decreases with increasing air to water mass flow rate ratio.

4 A B C D a = 1.1 m/s u` = 1.5 m/s u = 3.2 m/s u = 4.5 rn/s Ni = 2.9 kg/h Ni = 4 kg/h M = 8.5 kg/h Ni = 12 kg/h p = 5.62 kpa p = 7.3 kpa p = 21.1 kpa p = 35.2 kpa Re = 197 Re = 1456 Re = 311 Re = 437 E F G H u = 6.56 m/s u = 9.65 rn/s u = rn/s u = 18.2 m/s Ni = 17.5 kg/h M = 25.5 kg/h M = 34 kg/h M = 48.5 kg/h p = 7.3 kpa p = 14.6 kpa p = 281 kpa p = 562 kpa Re = 637 Re = 928 Re = 1238 Re = 1765 Fig. 6 Water Jet with Coaxial Air Flow Air Velocity = 45.8 m/s, Air Reynolds Number = 2613, Air Mass Flow Rate = 17 kg/h The scale on each photograph equals 1 mm. u, M p and Re refer to water. 4

5 .,. ^, f Wit' A u = 1.1 m/s 141 =2.9 kg/h p = 4.92 kpa Re = 197 u = 1.5 n /s M =4 kg/h p = 7.3 kpa Re = 1496 Li = 2.62 m/s M =74 kg/h p = 14.6 kpa Re = 2544 i =4.5 W., M = 12 kg/h p = 35.2 kpa Re = 437 U = 6.56 m/s u = 9.56 nvs u = m/s U = 18.2 m/s M = kg/h Ivl = 25.5 kg/h M = 34 kg/h M = 48.5 kg/h p = 7.3 kpa p = 14.6 kpa p = 281 kpa p = 562 kpa Re = 637 Re = 928 Re = 1238 Re = 1765 Fig. 7 Water Jet with Coaxial Air Flow Air Velocity = 64.8 m/s, Air Reynolds Number = 3697, Air Mass Flow Rate = 24 kg/h The scale on each photograph equals 1 mm. u, M p and Re refer to water. 5

6 A B.--- C D. -. U = 1.5 m/s ii = 2.62 m/s it = 4.5 m/s M = 4 kg/h. M = 7.4 kg/h M = 12 kg/h p = 7.3 kpa p = 14.6 kpa p = 35.2 kpa Re = 1496 Re = 2544 Re = 437 E F G - u = 6.56 m/s U = 9.56 m/s ii = m/s M = 17.5 kg/h M = 25.5 kg/h M = 34 kg/h p = 7.3 kpa p = 14.6 kpa p = 281 kpa. Re = 637 Re = 928 Re = 1283 H' enlargement from Fig. 8G Fig. 8 Water Jet with Coaxial Air Flow Air Velocity = m/s, Air Reynolds Number = 7388, Air Mass Flow Rate = 48 kgfh The scale on each photograph equals 1 mm. u, M,p and Re refer to water. 6

7 AB C ç. a v R. y J. EF enlargement G from Fig. 9E,. H CONCLUSIONS The results presented here are based on the analysis of more than 15 photographs of the liquid jets for different nozzle geometries at various water and air flow conditions. However, only a small number of the photographs (Figs. 6 through 9) were selected to describe the mechanisms of round liquid jet disintegration for coaxial nozzles. From the photographic evidence presented, the key events of the round liquid jet disintegration into drops seems to be very similar to those of the liquid plane sheet disintegration. The round jet forms rolled up flat sheets which then break up into ligaments and drops. In the transition region from laminar to turbulent flow, the growing waves on the liquid jet surface form themselves into curling sheets. With increasing water turbulence and constant air to water mass flow rate ratios, the number of the surface waves forming curling sheets increases, but their liquid content decreases. This leads to a smaller size of curling sheets and droplets. The photographs examined for this presentation show nonaxisymmetric liquid shapes and non-axisymmetric spray clouds during the atomization process, although long time exposure photographs show that the spray is axisymmetric in the long time average. Fig. 9 Water Jet Transformation from a Cylinder into a Curling Sheet Water Flow Conditions: u = 1.1 m/s, 1VI = 2.9 kg/h, Re = 197 Air Flow Conditions: u = 48.5 m/s, M = 17 kg/h, Re = 2613 The scale on each photograph equals 1 mm. 7 A large number of the photographs indicate that the mean velocity distribution in the emerging liquid jet determines the character of the surface wave formation at the liquid-air-interface. For better understanding of this phenomena, measurements of the liquid mean velocities are needed. REFERENCES Abramovich, G. N., "Prikladnaja Gazovaja Dynamika" (Applied Gas Dynamic), Moscow, Cheremisinoff, N. P. (Ed.), "Gas-Liquid Flows", Encyclopedia of Fluid Mechanics, Vol. 3, Gulf Pub!. Co., Houston, Texas, Chigier, N., "Energy, Combustion and Environment", McGraw Hill, Dombrowski, N. and Fraser, R. P., "A Photographic Investigation into the Disintegration of Liquid Sheets", Philos. Trans.. R. Soc. London Ser. A. Math. Phys. Sci., Vol 247, No. 924, Dombrowski, N. and Johns, W. R., "The Aerodynamic Instability and Disintegration of Viscous Liquid Sheets" Chem. Eng. Sci., Vol. 18, 1963.

8 EG&G ELECTRO-OPTICS, Operation Manual # B-3113A,35 Congress, Salem, MA 197, Ferrenberg, A. (Ed.), "Atomization and Mixing Study", NASA contractor report No. NAS8-3454, Dec Fraser, R. P., Eisenklam, P., Dombrowski, N. and Hasson, D., "Drop Formation from Rapidly Moving Sheets, AIChE J. Vol. 8, No. 5, 1962; cited in (Lefebvre, 1989). Giffen, E., and Muraszew, A., "The Atomisation of Liquid Fuels", J. Wiley & Sons, New York, Gross, C., "JANNAF Liquid Engine Jet Atomization Workshop Report", May 1987, NASA, Huntsville, AL 35812, U.S.A. Hinze, J.., "Fundamentals of the Hydrodynamic Mechanism of Splitting in Dispersion Processes", AIChE J. Vol. 1, No. 3, Lane, W. R., "Shatter of Drops in Streams of Air", Ind. Eng. Chem. Vol. 43, No. 6, 1951, pp , cited in (Cheremisinoff, 1986). Lefebvre, A. H., "Atomization and Sprays", Hemisphere Publ. Co. New York, Operation Manual # B-3113A, EG&G ELECTRO-OPTICS, 35 Congress, Salem, MA 197, Przekwas, A. J., Chuech, S. G. and Singhal, A. K., "a Status Report on Numerical Modeling for Primary Atomization of Liquid Jets", ILASS-AMERICAS 3rd Annual Conference, 1989, Irvine, CA. Reitz, R. D. and Bracco, F. V., "Mechanisms of Atomization of a Liquid Jet", The Physics of Fluids, Vol. 25 (1982) p Simmons, H. C., "The Atomization of Liquids; Principles and Methods", Parker Hannifin Report No. 791/2-, Sirignano, W. A. and Rangel, R. H., "A Computational Fluid Dynamics Approach to Jet-Blast Atomization Studies", ILASS- AMERICAS 3rd Annual Conference, 1989, Irvine, CA.

Effect of Viscosity on the Breakup Length of Liquid Sheets Formed by Splash Plate Nozzles. University of Toronto, Toronto, ON M5S 3G8

ILASS Americas, 19 th Annual Conference on Liquid Atomization and Spray Systems, Toronto, Canada, May 2006 Effect of Viscosity on the Breakup Length of Liquid Sheets Formed by Splash Plate Nozzles M. Ahmed