ICLASS-26 Aug.27-Sept.1, 26, Kyoto, Japan Paper ID ICLASS6-142 EXPERIMENTAL INVESTIGATION OF SPRAY IMPINGEMENT ON A RAPIDLY ROTATING CYLINDER WALL Osman Kurt 1 and Günther Schulte 2 1 Ph.D. Student, University of Bremen, Chemical Engineering Department, kurt@iwt.uni-bremen.de 2 Professor, University of Bremen, Chemical Engineering Department, gs@iwt.uni-bremen.de ABSTRACT The present work focuses on an experimental study of spray impingement onto a rotating cylinder wall at operating conditions relevant for industrial application. The spray process is used here as non-contact coating technique of rapidly moved surfaces. The aim of the investigations is to study the fluid dynamic interaction between the polydisperse spray and a moved surface, i.e. the wall of a rapidly rotating cylinder. Water has been sprayed onto the wall of a non-rotating / rapidly-rotating cylinder (d C =4mm) up to 12.4m/s by means of a flat spray atomizer. Plexiglass has been used as cylinder wall material. The operation conditions (rotational speed of cylinder, spray impact angle, mass flow rate, etc.) have been varied systematically. For visualization of the spray-cylinder interaction, a laser light sheet has been used in combination with the CCD-camera. In addition to the visualization technique Phase Doppler Anemometry (PDA) has been applied to perform an analysis of drop size, drop velocity and drop number flux at a key position in the spray cone near the moved surface. The results helped to understand the fluid dynamic interaction mechanisms of spray impingement onto a rotating cylinder and to properly adjust operating parameters on the pilot plant. Keywords: Spray impingement onto a rotating cylinder wall, spray-wall interaction, coating moved surfaces, laser light sheet spray analysis 1. INTRODUCTION The fluid dynamic interaction mechanisms of spray impinging onto a solid substrate is of importance in many engineering applications, such as, spray coating, spray painting, spray drying, spray cooling in electronics, fire fighting, fuel-air mixing in combustion systems, agricultural and industrial processes. Therefore, many numerical and experimental studies have been performed in the past on the behavior and atomization characteristics of the impinging spray on surfaces mostly in the state of resting [1,2,3,4], by the aim of a detailed understanding of the spray impingement characteristics. However, there is still a lack of experiments on impingement of sprays on moved surfaces at various experimental conditions. In the present work the spray / rotational cylinder wall interaction has been studied at operating conditions relevant for industrial application. The operation conditions (rotational speed of cylinder, spray impact angle, mass flow rate, etc.) have been varied systematically. 2. EXPERIMENTAL WORK An image of the laboratory spray rig is shown in Fig.1. The central device of the set-up was a rotation cylinder and a flat spray atomizer, which were mounted in a box. The box was fixed on a three- dimensional traversing unit, allowing the positioning of cylinder wall and atomizer relative to the control volume of the fixed PDA system. The rotation cylinder was driven by an electrical motor whose speed was carefully controlled and adjusted by a frequency speed controller. The cylinder wall material was an acrylic glass and the wall surface had a technically smooth mean roughness height of R Z =.14µm. The cylinder had a diameter of d C =4mm and a length of 2mm. It was rotated resulting in different cylinder wall velocities (v CS ) from m/s to 12.4m/s. Additionally an air deflector was mounted to provide the deflection of the air-layer adherent to the cylinder wall so hindering the air-layer from reaching the control volume. A flat spray pressure atomizer (TP611 from Spraying Systems) was used with water as model fluid. The atomizer was operated at pressures of p=3bar and 5bar, which resulted in a liquid flow of.5l/min and.7l/min, respectively in a spray cone angle of 6 and 66. The atomizer was located at the 12-o clock position above the cylinder (see Fig.1). The vertical distance between the nozzle exit and the cylinder wall was fixed at z w =1mm. The impingement angle was varied from α=9 to 3. The liquid pressure at the atomizer was measured using a manometer. The liquid emerges at the outlet of atomizer as a thin lamella, then breaks up into droplets and impinges onto a cylinder wall. A rubber lip has been used to remove the film of fluid that deposited on the cylinder wall. The operation conditions were varied systematically (Table 1). Table 1: Operating Conditions Flat Spray Atomizer Atomizer Pressure Liquid Flow rate Spray Cone Angle Cylinder Wall Velocity (v CS ) Cylinder Wall Roughness (R Z ) Atomizer-Wall distance (z w ) Impingement Angle of Spray (α) Air Deflector (AD) TP611 3 and 5[bar].5 and.7[l/min] 6[ ] and 66[ ] ; 6.2 and 12.4[m/s].14[µm] 1[mm] 9[ ]; 6[ ] and 3[ ] with / without The instrumentation set-up is shown schematically in Figure 2. For visualization of the deflection of the spray cone, when it impinges onto the cylinder wall under
different process conditions (resting / moving), a laser light sheet modul (LLS, Nd:YAG Laser, cylindrical lenses, laser line generator lense and a mirror) was used in combination with a CCD-camera. In addition to the LLS-technique a standard Phase Doppler Anemometry (PDA) system was applied to perform a more detailed analysis of drop size, drop velocity and drop number flux at a key position in spray cone near the surface. The measurement volume was positioned by adjusting the cylinder and nozzle relative to the PDA by means of the three-dimensional traversing unit. 3. EXPERIMENTAL RESULTS 3.1 Laser Light Sheet Analysis Visualization of the spray impingement process onto the moved cylinder wall (= to 12.4m/s) was carried out by means of laser light sheet in combination with a CCD-camera. The laser light sheet crossed the spray along its central axis plane y-z (side profile of flat spray). In the following figures indicates the surface speed of the cylinder wall, α the adjusted impingement angle of the spray and zw the distance of the cylinder wall from the atomizer tip, respectively. The laser light is scattered at the droplets in the illuminated plane therefore a light region indicates a high concentration and / or smaller droplets. The lower the intensity of scattered light the lower the concentration and / or size of droplets. Figure 3a-d show the images of the spray impingement process onto resting cylinder wall (=m/s) in the y-z plane recorded at different impact angles and pressures. (a) α=9 ; p=3bar (c) α=9 ; p=5bar (b) α=3 ; p=3bar (d) α=3 ; p=5bar Fig. 1: Spray rig for analysis of Spray Impingement on a wall of a rotating cylinder Fig. 3: Influence of conditions for α and p (=m/s; zw=1mm) FLUID MANOMETER P PDA MEASUREMENT VOLUME AIR DEFLECTOR y RECEIVER LASER MOTOR WITH SPEED CONTROL x x z ϕ LLS r =2[mm] CCD CCD-CAMERA MIRROR ω RUBBER LIP LASER y-axis x-axis z-axis Fig. 2: Spray rig with adapted Laser Light Sheet modul and Phase Doppler Anemometry The results show that for the impingement angle of α=9 and pressure of 3bar (Fig.3a), the spray impinges onto the surface and spreads out symmetrically along the wall in both directions (: in rotation direction and : against rotation direction). In the case of impingement angles smaller than α=9 (Fig.3b), the enlargement of the spray in rotation direction () is larger and against rotation direction () is smaller compared with perpendicular spraying. The pictures show, that the spreading in direction () -plane clearly increases with the decrease of the impingement angle, whereas the spreading in the other direction ()-plane heavily decreases [5]. Furthermore the influence of the atomizing pressure can be seen in the Fig.3c-d. It is shown, that by increasing the pressure, the spray impingement behavior, as described before, is more pronounced. This could be explained by the increased spray cone angle, due to larger expansion of the liquid lamella and the higher volume flow rate. Figures 4 7 show the results of the spray impingement onto the moved cylinder wall (=6.2 and 12.4m/s) recorded at atomizing pressures of p=3bar and 5bar,
different impact angles α (9-3 ) and without / with air deflector. We consider a continuous cylinder rotation in a quiescent ambient air. The cylinder surface moves with a constant velocity of and accelerates the ambient air in the running direction ( ) to a velocity equal to the cylinder speed. If the radial distance increases beginning from the cylinder surface, the air velocity decreases. This behavior was established by using Hot Wire Anemometry. The same tendency on the behavior of air layer on moving continuous surfaces is also known from literature. (a) with Air Deflector ; α=9 (b) with Air Deflector ; α=3 (a) without Air Deflector (b) with Air Deflector Fig. 5: Influence of spray impact angle α (=6.2m/s; p=3bar) The influence of cylinder surface velocity on spray impingement (α=6 ) for two velocities (=6.2m/s and =12.4m/s) is shown in Fig.6a-b. (a) with Air Deflector ; =6.2m/s Fig. 4: Influence of Air Deflector (AD) (α α=9 ; =6.2 m/s; p=3bar) The influence of installing an air deflector is clearly seen in Fig.4a,b (=6.2m/s, p=3bar and α=9 ). In the case without an air deflector the spray in Fig. 4a spreads out. This occurred due to the aerodynamic force induced by the air-layer moving with the wall surface. As result, the spreading against rotation direction ( ) here does not occur, the smaller droplets with their low kinetic energy follow the stream of the air layer in rotating direction, thus probably never reaching the surface. By using an air deflector (Fig.4b), the spray impact is similar to the spray impact case onto a resting cylinder wall (Fig.3a) but the thickness of the film formed at the wall by impingement of spray is smaller. The following Figures show results of the case with air deflector because this is of importance for practical application. Fig.5a-b show the influence of spray impact angle at α=9 and 3 on spray impingement. It can be observed, as in case of impact on a resting surface, that by decreasing the impact angle from α=9 to 3 the spray spreading is more pronounced and it seems that the fine droplets follow the path of cylinder surface. Furthermore at a small impact angle of α=3, the spray spreading (Fig.5b) does not exist. It can be founded by reduced effect of wall impingement. (b) with Air Deflector ; =12.4m/s Fig. 6: Influence of cylinder surface velocity (α α=6 ; p=5bar) The above photograph (Fig.6a) shows a formation of a cloud of spray followed the path of cylinder surface against rotation direction (), whereas this cloud in the photograph (Fig.6b) is spiralled and deflected in the rotation direction for the case with higher cylinder velocity.
From the presented photographs appears that the cylinder velocity affects the spray impingement process. So far it is not clear what could be the reason for this phenomenon. For this purpose more differentiated investigations will be done. Figure 7a-b show the effect of atomization pressure on the spray impact process at cylinder velocity v CS =12.4m/s. By an atomization pressure of p=5bar the mass flow rate increases, the flat lamella becomes wider and the lamella break-up into droplets takes place nearer to the nozzle exit. Based on this, smaller droplets with larger velocities, larger drop concentrations and additionally higher kinetic energy reaches the wall. The recorded picture (Fig.7b) shows that the tendency of impinging characteristics on the cylinder surface are similar compare to the experiments with a lower pressure of p=3bar (Fig.7a). However at higher pressure, the radial enlargement and a height of spray on the cylinder surface in the rotating direction increases due to the fact of higher mass flow rate of the liquid and spray impingement velocity. At lower pressure the effect of a formation of a cloud of spray, as described before, is less pronounced. (a) with Air Deflector ; p=3bar surface. This position was found in the pre-examination experiments as notable sensitive to the process. A A: Measuring Volume (M V ) B B: α = 3, 6 and 9 α x +x y ω z z ω -x B B A z w = 1mm Measuring Volume (M V ) +y -y A v CS (b) with Air Deflector ; p=5bar v CS Fig. 7: Influence of operating pressure p (α=6 ; v CS =12.4m/s) 3.2 Phase Doppler Anemometry Measurement Drop size-, velocity and number flux analysis of the impinging spray was done by means of standard Phase Doppler Anemometry (PDA). The rotating cylinder and nozzle were adjusted relative to the PDA instrument by means of a three dimensional traversing unit. Fig.8 indicates the measurement location. Comparing measurements have been made in the free spray and in the impinging spray for the case of a non-rotating cylinder (cylinder surface velocity v CS =m/s: CW_) and rotating cylinder (cylinder surface velocity v CS =6.2m/s: CW_6.2 and v CS =12.4m/s: CW_12.4). Additionally the influence of the air deflector (AD) was investigated. Data were taken for the central axis position near the wall (see Fig. 8). The measuring volume (M V ) was positioned 6mm above the Fig. 8: Measurement location for the PDA measurements The results of particle flux values for a pressure of 5bar, the case without air deflector and all impingement angles in question are shown in the diagram Fig.9a. The particle number flux values for the two cases (CW_ and CW_6.2) have a maximum at 9 and decrease by decreasing the impact angle. For the CW_6.2 case the measured values of particle flux for the impingement angles in question are lower than for the case of CW_. For the case of CW_12.4 the lowest particle flux values vary only slightly with impingement angle. The measured values of particle flux for the two interaction cases (CW_6.2 and CW_12.4) can be explained by the air-layer dragged on the surface of moving cylinder wall. This tendency also has been shown by Arcoumanis and Cutter [6], which investigated the interaction mechanisms of spray impingement onto the non-moving wall by existing cross-flow. Figure 9b shows for the case with air deflector and the same interaction cases a diagram with a particle number flux versus impingement angle. The particle number flux values for the three interaction cases have a maximum at 9 and decrease by decreasing the impact angle. Comparing the results for the two interaction cases (CW_6.2 and CW_12.4) with CW_, it is shown that the rotating cylinder at the velocities under observation has no effect on the particle number flux values. The number flux of droplets is similar to CW_. This tendency verifies the presented results of spray impact images before. The effect of a lower operation pressure (p=3bar) on the results of the drop number flux values at different interaction cases with and without air deflector was the same. But with the difference that these measured values were smaller. This can be explained by getting larger
droplets at lower operation pressures. Particle Number Flux [1/sec mm²] Particle Number Flux [1/sec mm²] 14 12 1 8 6 4 2 14 12 1 8 6 4 2 Fig. 9: (a) without Air Deflector ; p=5bar ZW_ ZW_6.2 ZW_12.4 3 6 9 12 (b) with Air Deflector ; p=5bar ZW_ ZW_6.2 ZW_12.4 3 6 9 12 Particle Number Flux versus impingement angle for different interaction cases at M V =6mm Figure 1a shows for a pressure of 5bar and the case without air deflector a diagram with the modal diameter versus impingement angle. For the case of CW_ nearly the same low values of modal diameter (~25µm) were measured for all impingement angles under observation. For the case CW_12.4 there was observed a larger value of modal diameter (~45µm) for the three observation angles. The measured value of modal diameter for the case CW_6.2 starts with a same high value as in CW_12.4 case for the 3 spray angle and reaches the same low value as in case CW_ for the 9. One can assume that in the CW_ case all drops, also the smallest ones, reach the measurement volume i.e. the surface of wall. Increasing the wall surface velocity cause an effect of separating smaller drop, so that they do not reach the measurement volume because they tend to follow the air-layer on the moved surface. This is even more pronounced for smaller impingement angles. Figure 1b shows for the case with air deflector the accordant diagram. It can be seen clearly, that the modal diameter is independent in this case as well from the impingement angle and as well as from the velocity of the cylinder surface. The effect of the air-layer seems to be excluded. The separating effects cannot be observed here. At a lower operation pressure a similar trend of drop size values for the interaction cases in question with and without air deflector was observed. But with the difference that the measured drop size values were larger. Modal Diameter d [µm] Modal Diameter d [µm] As it was concluded from the above results the trajectory and velocity of the droplets are influenced by the air-layer [6]. In the experiments the PDA (one component) was aligned normal to the plane of impingement, so that only the vertical velocity component (v z ) of the impinging spray droplets could be measured. So we considered the vertical drop velocity of CW_ as reference. Modal Velocity vz [m/sec] 12 1 8 6 4 2 12 1 8 6 4 2 12 1 8 6 4 2 (a) without Air Deflector ; p=5bar ZW_ ZW_6.2 ZW_12.4 3 6 9 12 (b) with Air Deflector ; p=5bar ZW_ ZW_6.2 ZW_12.4 3 6 9 12 Fig. 1: Modal Diameter versus impingement angle for different interaction cases at M V =6mm (a) without Air Deflector ; p=3bar ZW_ ZW_6.2 ZW_12.4 3 6 9 12 Fig. 11: Modal Velocity versus impingement angle for different interaction cases at M V =6mm
Figure 11 shows the velocity component v z versus impingement angle for different operation pressures and the case without air deflector. In case of α=9 and of resting cylinder (CW_) the velocity component v z coincide with a velocity v eff in the position of measurement volume, having a maximum value. Decreasing of spray impingement angle leads to smaller values of v z according v z = sin a x v eff, as seen in the diagram reaching the lowest value for α=3. This is even more pronounced in the case of rotating cylinder because this case results in an α eff < α. For CW_12.4 we got the basic velocity values of the 3 position also for the other impingement angles. It should be noted here that we run this experiment also at the higher operation pressure of 5bar, getting equivalent results. All these experiments were carried out three times for each operation condition and agreeing PDA results were consistently obtained. 3. Özdemir, I. B. and Whitelaw, J. H.: Impingement of an unsteady two-phase jet on unheated and heated flat plates, J. Fluid Mech., vol. 252, pp. 499-523, 1993. 4. Brenn, G.., Durst, F. amd Zambotti, St.: Experimental investigation on multiple spray interacting with solid walls, Entropie, vol. 2, pp. 29-36, 1996. 5. Naber, J.D., Reitz, R.D.: Modeling Engine SprayWall Impingement, SAE-Paper, 88179, 1988 6. Arcoumanis, C. and Cutter, P.: Flow and Heat Transfer Characteristics of Impinging Diesel Sprays Under Cross-Flow Conditions, SAE Technical Paper, 95448, 1995. 4. SUMMARY In the present investigation an experimental study of spray impingement onto a resting and moving cylinder wall at different operating conditions important for industrial applications has been carried out. For visualization of the spray-wall interaction, a laser light sheet has been used in combination with the CCD-camera. In addition to visualization technique Phase Doppler Anemometry (PDA) has been applied to perform a drop size, drop velocity and drop number flux analysis at a key position in spray cone near the moved surface (M V =6mm). The recorded images of the spray-wall interaction have shown that the spray impingement onto the moved cylinder in the case without air deflector is influenced by the air-layer on the cylinder surface dragged by the rotating cylinder. This caused a strong deviation of the spray where the smallest drops even do not reach the surface of the rotating wall. Installing an air deflector could help to avoid this effect. The thickness of the film formed at the wall by impingement of spray is smaller as in case of impact onto resting surface. The PDA measurements have shown that the movement of cylinder surface and the spray impingement angle influence the drop size, drop velocity and drop number flux measured at the key position of interest. The influence of the cylinder surface velocity on these leading spray parameters is more significant than that of spray impingement angle. Further experiments for a more detailed analysis are planned in different spray cone positions and planes. In this context the trajectories of the deflected droplets will be included in the investigations. 5. REFERENCES 1. Mundo, C., Tropea, C., Sommerfeld, M.: Numerical and Experimental Investigation of Spray Characteristics in the Vicinity of a Rigid Wall, Experimental Thermal and Fluid Science, vol. 15, pp. 228-237, 1997. 2. Mundo, C., Sommerfeld, M., Tropea, C.: On the modelling of liquid sprays impinging on surfaces, Atomization and Sprays, vol. 8, pp. 625-652, 1998.