Optical Measurements of Water Droplet Characteristics in Turbulent Gasoline Pipe Flow

Transcription

1 SAE TECHNICAL PAPER SERIES Optical Measurements of Water Droplet Characteristics in Turbulent Gasoline Pipe Flow Choongsik Bae and Jeong Heon Kim Korea Advanced Institute of Science and Technology Dong-Hyon Sheen SK Corporation Reprinted From: Diesel and Gasoline Performance and Additives 2001 (SP 1628) International Spring Fuels & Lubricants Meeting & Exhibition Orlando, Florida May 7-9, Commonwealth Drive, Warrendale, PA U.S.A. Tel: (724) Fax: (724)

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3 Optical Measurements of Water Droplet Characteristics in Turbulent Gasoline Pipe Flow Choongsik Bae and Jeong Heon Kim Korea Advanced Institute of Science and Technology Dong-Hyon Sheen SK Corp. Copyright 2001 Society of Automotive Engineers, Inc. ABSTRACT The liquid fossil fuel contaminated by water can make some troubles in combustion processes and endurance of a combustion system. The optical sensor monitoring the water concentration instantaneously in a fuel pipeline is an effective means for controlling the fuel quality. In two component liquid flows of oil and water, the flow pattern and characteristics of water droplets are changed with various flow conditions. Then, the light scattering of the optical sensor measuring the water concentration is also dependent on the flow patterns and droplet characteristics. Therefore, it is important to investigate the detailed behavior of water droplets in the pipeline of the fuel transportation system. In this study, the flow patterns and characteristics of water droplets in the turbulent pipe flow of two component liquids of gasoline and water were investigated using optical measurements. The dispersion of water droplets in the gasoline flow was visualized, and the size and velocity distributions of water droplets were simultaneously measured by phase Doppler technique. The water droplets were spherical and dispersed homogeneously in all ranges of this experiment. The velocity of water droplets was not dependent on the droplet size and the mean velocity of droplets was equal to that of the gasoline flow. The mean diameter of water droplets decreased and the number density increased with Reynolds number of the gasoline flow. INTRODUCTION Water is the commonest contaminant in a liquid fossil fuel, and the liquid fuel contaminated by water can make some troubles in combustion processes and induce the corrosion of a combustion system. The water content is also important in the combustion system to use the water-in-oil emulsion because the combustion characteristics were varied largely with the water concentration. Then, the water concentration in a liquid fuel is another factor to decide the fuel quality. For controlling the fuel quality, it is needed to monitor the water concentration instantaneously in a fuel pipeline. On-line optical sensor is considered to be an effective means for monitoring it instantaneously. In the pipe flow of two component liquids of gasoline and water, the flow pattern and characteristics of water droplets are changed with flow conditions. The flow pattern and droplet characteristics also influence the light scattering of the optical sensor [1, 2]. Therefore, for the development of the optical sensor, it is very important to investigate the characteristics of water droplets in the pipe flow of the fuel transportation. Many studies on combustion characteristics of the liquid fuel containing water have been reported over the years. Most of these researches were studied on reductions of NO x emissions and the levels of smoke and soot particulate in combustion systems [3-8]. Several researches were carried out for new technology of the water introduction into the liquid fuel oil [6, 8]. From those studies, it has been perceived that the characteristics of combustion and spray were varied largely with the water concentration. However, investigations on the microscopic behavior of water droplets in the gasoline pipe flow have scarcely reported. Chantrapornchai et al. [1] studied the influence of the droplet characteristics on the optical properties of waterin-oil emulsion, and reported that spectral reflectance increased with increasing the droplet concentration and decreasing the droplet size. In their study, the droplet concentration was varied in the range of 0 wt.% to 20 wt.% and the mean droplet diameter was in the range 0.26 µm to 26 µm.

4 P P P N 2 Gasolin Water Tank Water F Flow of Gasoline and Water Receiving Optics o 30 Stroboscope P F Pum Pressure Flowmete Transparent window Control valve of flow rate F Test Section 1 Mixing Device Ar-ion Transmitting Optics Test Section 2 CCD Camera Phase Doppler Anemometer Fig. 1 Experimental set-up for two-component liquid pipe flow of gasoline and water. This study was carried out to investigate the influence of the gasoline velocity on characteristics of water droplets in the turbulent pipe flow of gasoline containing water, as the representatives of the pipeline for the gasoline transportation. The flow patterns and droplet behavior were investigated by the visualization, and the distributions of size and velocity of water droplets were measured simultaneously by the phase Doppler measurement technique, for the gasoline flows of Reynolds number from 4 x 10 4 to 1 x 10 5, EXPERIMENTAL SYSTEM To investigate the characteristics of water droplets, the experimental system for the turbulent pipe flow of two component fluids of gasoline and water was set up like Fig. 1. It was composed of the gasoline supply system, water supply system, mixing system of water and gasoline, pipelines and the test sections with transparent windows. A rotary pump was used to supply gasoline, and the flow rate of gasoline was controlled with the gate valve and the by-pass pipe. Water was supplied through a nozzle using the static pressure of nitrogen, and the flow rate of water was varied with controlling the static pressure. The water nozzle was installed at about 2 m downstream from the gasoline pump, and the diameter of the nozzle orifice was 0.2 mm. Mixing system was composed of two meshes and installed downstream of the nozzle. The cell size of the mesh was 74 µm. In this experiment, the water nozzle and mixing system were used so that water could disintegrate into small droplets and mix with the gasoline flow. The size of water droplets could be controlled with the number of meshes. The rectangular pipe (28 mm x 28 mm) was used for the facility of the visualization through the transparent windows. The transparent window was installed at two locations, which were described as test sections 1 and 2 in Fig. 1. At the test section 1, the behavior of water droplets injected from nozzle was visualized. At the test section 2, the droplet characteristics in the steady state gasoline flow was measured by the visualization and phase Doppler technique. The interval between the test section 2 and the mixing system was about 70 times of the pipe hydraulic diameter. In this experiment, the Reynolds number of the gasoline pipe flow was varied in the range of 4 x 10 4 to 1 x 10 5, representing the pipe flow for the gasoline transportation. The water concentration was varied in the range of 50 ppm to 300 ppm under consideration that the permission of the water content in gasoline was limited to about 100 ppm generally. VISUALIZATION AND PHASE DOPPLER MEASUREMENTS The photographic images of the water droplet behavior in the gasoline flow were obtained using the CCD visualization technique. The pixel number of CCD image was 640 X 480. The optical fiber stroboscope of electric pulses was used for the light source, and the diffusion paper was equipped on the front of optical fiber for a homogeneous illumination. The pulse duration of the light was 10 µs. The influences of the gasoline flow on the size and the velocity of water droplets were also investigated by detailed measurements using a phase Doppler measurement technique. Simultaneous measurement of droplets size and velocity is very important for the investigation of the droplet flow because the correlation characteristics of velocity and size of water droplets can

5 reveal some important information about the behavior of droplets [9]. Nozzle Nozzle The phase Doppler measurement technique is one of the most versatile and accurate techniques currently available in simultaneous measurements of droplet s size and velocity [10, 11]. It is an extension of Laser Doppler Velocimetry. Coherent light from a laser is separated into two beams and focused by a converging lens. Interference fringes appear in the intersection volume of the two beams. Light scattered by a particle crossing this measurement volume features a periodic signal with a frequency that is proportional to the particle s velocity and to the inverse of the fringe interval. This interval is constant and depends on the geometry of optical set-up and the wavelength of a laser. If the scattered light is observed from two different angles of two photo-multipliers, two Doppler signals with different phase can be detected. The difference is proportional to a droplet s diameter. Thus, the diameter and velocity of droplets can be measured simultaneously. A one-dimensional phase Doppler anemometer system (TSI) was used for simultaneous measurements of velocity and diameter of water droplets. The optical configuration of phase Doppler measurements was optimized for this experiment. The light source was an Ar-ion laser. A wavelength of nm (green) was used for velocity and diameter. Each light power of the two laser beams was 70 mw, and the frequency shift was 40 MHz. The scattering angle was 30 degree, which was determined considering the polarizing angle and the refractive index. The focal lengths of the transmitting and receiving lenses were 250 mm and 310 mm, respectively, in order to optimize the measurement volume and the measurement range [12]. The resolution of the droplet diameter was µm/degree. The detailed values of optical parameters were shown in Table 1. Table 1. Optical configurations for phase Doppler measurements. Transmitting optics Ar-ion wavelength : µm Focal length : 250 mm Spot diameter : 233 µm Fringe spacing : 3.21 µm Fringe number : 72 Receiving optics Focal length : Receiver aperture height : Where Frequency shift : Polarization : Off-axis angle : Phase to diameter 310 mm 2.0 mm 40 MHz Orthogonal 30 degree (a) Q w = 7 g/m Fig. 2 Behavior of water droplets in gasoline flow at R g= 0. Nozzle Gasoline Flow (a) Q w = 7 g/m 1 mm Conversion factor : RESULTS AND DISCUSSIONS (b) Q w = 10 g/m Nozzle (b) Q w = 10 g/m 1 mm 1 mm Gasoline 1 mm Flow Fig. 3 Behavior of water droplets in gasoline flow at R g = µm / degree Behavior of Water Droplets at Nozzle Exit The behavior of water droplets injected in the gasoline pipe flow was investigated by the photographic images using the CCD visualization as indicated in Figs. 2 and 3. A magnifying zoom lens was used to obtain the enlarged images. The visualization was performed at the test section 1 under the exit of the water nozzle, where water was injected and disintegrated into small droplets. The behavior of the water droplets at the Reynolds number of the gasoline flow, Re g = 0 are shown in Fig. 2, of which the water flow rate were Q w = 7 g/m and Q w = 10 g/m. At the low flow rate of water, the water droplets behaved like a dripping flow as shown in Fig 2(a). The generated droplets were relatively large and showed irregular shapes, which was changed into spherical droplets downstream with time. The mean droplet diameter was about 400 µm, two times larger than the diameter of the nozzle orifice. At the high flow rate of water, the water droplets behaved like a spray flow and consequently a lot of relatively small droplets were produced as indicated in Fig. 2(b). It is because the droplet break-up was promoted with the velocity of the water jet. This difference between two cases of Figs.

6 2(a) and (b) indicates that the mechanism of liquid break-up is dependent on the water flow rate. Figure 3 shows the behavior of the water droplets at Re g = 41.4 X 10 3, where the water flow rate of Fig. 3 was equal to that of Fig. 2. Compared to Fig. 2, the mixing between water and gasoline was developed remarkably. However, there was no large difference in the droplet size with increasing the Reynolds number of the gasoline flow. For improving the disintegration and mixing of water droplets, two meshes were installed at the mixing system behind the water nozzle. Flow Patterns of Water Droplets in Steady State Gasoline Turbulent Pipe Flow To investigate the flow pattern of water droplets, the droplet dispersion was visualized in the steady state turbulent gasoline pipe flow. This measurement was carried out at the test section 2. Figures 4 and 5 show the droplet dispersion at the water concentration of C w = 100 ppm and C w = 200 ppm, respectively. The water droplets were spherical and distributed homogeneously. With increasing the water concentration, the number density of droplets increased largely, and the droplet size decreased just a little. With increasing the Reynolds number of the gasoline flow, the number density of droplets increased and the droplet size decreased largely. It is believed that the droplets at the mixing system were disturbed largely and the droplets disintegrated better with increasing the gasoline velocity. It implies that the influence of the obstacle like a mesh on the droplet atomization was large. 1 mm 1 mm (a) Re g = (b) Re g = Fig. 4 Visualization of water-droplet dispersion 1 mm 1 mm (a) Re g = (b) Re g = Fig. 5 Visualization of water-droplet dispersion (a) Stratified flow (b) Transient flow (c) Dispersed flow Fig. 6 Flow patterns of two-component liquid flow of gasoline and water. The flow pattern of water droplets in two-component liquid flow of gasoline and water is similar to that of twophase flow and classified into three types generally as shown in Fig. 6 [13, 14]. The first is the flow pattern of water droplets in the laminar flow at the high concentration of water and the low Reynolds number of gasoline. The flow is separated due to the density difference between two liquids as shown in Fig. 6(a). This flow pattern is called a stratified flow. The second is the flow pattern of water droplets in the turbulent flow at the low concentration of water and the high Reynolds number of gasoline. The small water droplets are distributed homogeneously and flow steadily in the gasoline flow as shown in Fig. 6(c). This flow pattern is called a dispersed flow. The third is the flow pattern of the intermediate range between the laminar flow and the turbulent flow. The large droplets with an irregular shape flow intermittently in the gasoline flow as shown in Fig. 6(b). This pattern is called a transient flow. In this experiment, the water concentration was very low under C w = 300 ppm and the gasoline flow was turbulent with high Reynolds number over Re g = 4 x Thus, the flow pattern must be the dispersed flow at the entire range as shown in Figs. 4 and 5. Size and Velocity of Water Droplets To investigate the droplet characteristics quantitatively in the steady state turbulent gasoline pipe flow, the simultaneous measurements of the size and velocity of the water droplets were performed using the phase Doppler technique. The measurements were also performed at the center of the test section 2, which was the same location as the visualization of Fig. 4 and 5. This single measurement point was considered sufficiently valid for investigating the droplet characteristics because droplets were the complete spheres and the flow pattern was a dispersed flow. Ten thousand droplets were sampled and analyzed for various conditions. The droplet size distributions at the water concentration of C w = 100 ppm are indicated in Fig. 7. At the low Reynolds number of Re g = 41.4 x 10 3, the size distribution was wide and had three peeks as shown in Fig. 7(a). At the high Reynolds number of Re g = 82.8 x 10 3, the size distribution narrowed into a normal

7 20 15 Mean Diameter D 10 = 69 mm Mean Velocity V m = 0.74 m/s PDF [%] Droplet Diameter [mm] (a) Re g = (a) Re g = Mean Diameter D 10 = 55 mm Mean Velocity V m = 1.48 m/s PDF [%] Droplet Diameter [mm] (b) Re g = Fig. 7 Size distributions of water droplets with Reynolds number at C w = 100 ppm. (b) Re g = Fig. 8 Correlations of velocity and size of water droplets. distribution with one peak as shown in Fig. 7(a). The size distribution was shifted into low diameter because large droplets disintegrated more easily than small droplets and the water break-up was promoted with the Reynolds number. Figures 8(a) and (b) indicate the correlation of the velocity and size of water droplets at the same conditions as the Figs. 7(a) and (b), and then the mean velocities of the gasoline flow were V g = 0.74 m/s and V g = 1.48 m/s, respectively. The droplet velocity was constant irrespective of the droplet size and the mean velocity of droplets was equal to that of the gasoline flow. The difference of the density between water and gasoline is small and the momentum of the gasoline flow was very large compared to that of the water droplets. Thus, it is believed that water droplets could exchange the momentum with the gasoline flow and had the same velocity as the gasoline flow. Effects of Gasoline Flow on Water Droplet Size The effects of the gasoline flow on the water droplet size were investigated quantitatively as shown in Fig. 9. The gasoline flow was expressed by Reynolds number of a pipe flow. The droplet size decreased monotonously with increasing the Reynolds number of the gasoline Mean Diameter [mm] Re g = Re g = Re g = Reg = Water Concentration [ppm] Fig. 9 Effects of flow conditions on mean diameter of water droplets. flow. When the Reynolds number increased from Re g = 41.4 x 10 3 to Re g = 82.8 x 10 3, the droplet diameter decreased by about 25 percent. It proves that the influence of the gasoline flow on the droplet disintegration was large. In addition, the droplet diameter decreased with the water concentration. The

8 droplet size at C w = 200 ppm was reduced by about 15 % compared to that at C w = 50 ppm. SUMMARY AND CONCLUSION In this study, the influences of the gasoline flow on the characteristics of the water droplets were investigated experimentally in the steady state turbulent pipe flow. The flow pattern and the behavior of water droplets were visualized, and the size and velocity of water droplets were measured simultaneously using phase Doppler measurements. The findings from the results can be summarized as follows: 1. Water droplets of initially irregular shapes were changed into spherical droplets at the downstream distant sufficiently from the mixing system, and then the flow pattern became a dispersed flow. 2. The water droplet velocity was independent on the droplet diameter and the mean velocity of droplets was equal to the mean velocity of the gasoline flow. 3. The disintegration of the water droplets was promoted and the droplet size decreased largely with increasing the Reynolds number of the gasoline flow. 4. The flow rate of water influenced the initial break-up and the droplet size decreased with the water concentration. ACKNOWLEDGMENTS The programs of National Research Laboratory and BK21 of Korea and SK Corporation have provided financial support for this project. Series B, Fluid thermal Engineering, Vol. 41, No. 4, pp , G. Greeves, I.M. Khan and G. Onion, Effects of Water Introduction on Diesel Engine Combustion and Emissions, Sixteenth Symposium (Int.) on Combustion, pp , A. Kufferath, K. Ehrardt, C. Heyse and W. Leuckel, Continuous Generation and Air-Assisted Atomization of Fuel Oil-Water-Emulsions, Combustion Science and Technology, Vol. 148, No. 1-6, pp , J.H. Kim, Y. Ikeda and T. Nakajima, Characteristic Measurements of Electrostatic Fuel Spray, Proc. Ninth International Symposium of Laser Techniques to Fluid Mechanics, Lisbon, pp , F. Durst and M. Zare, Laser Doppler Measurements in Two Phase Flows, Proceedings of the LDA Symposium, Copenhagen, pp , A.A. Naqwi and F. Durst, Light Scattering Applied to LDA and PDA Measurements, Part. Part. Syst. Charact. 8: , Y. Ikeda, T. Hirohata and T. Nakajima, Measurement Uncertainties of Phase Doppler Technique due to Effects of Slit Location and Control Volume Size, Proc. Eighth International Symposium of Laser Techniques to Fluid Mechanics, Lisbon, pp , G.F. Hewitt, Measurement of Two Phase Flow Parameters, Academic Press, London, J.G. Collier and J.R. Thome, Convective Boiling and Condensation, Oxford University Press, Oxford, pp. 8-33, REFERENCES 1. W. Chantrapornchai, F. Clydesdale, D M. Julian, Influence of Droplet Characteristics on the Optical Properties of Colored Oil-in-Water Emulsions, Colloids and Surfaces, A: Physicochemical and Engineering Aspects, Vol. 155, No. 2, pp , L. Levi, Applied Optics, John Wiley & Sons, Inc., New York, pp , M.A.A. Nazha, H. Rajakaruna and R.J. Crookes, Soot and Gaseous Species Formation in a Waterin-Liquid Fuel Emulsion Spray, Energy conversion and management, Vol. 39 No. 16, pp , J. M. Ballester, N. Fueyo and C. Dopazo, Combustion Characteristics of Heavy Oil-Water Emulsions, Fuel, Vol. 75, No. 6, pp , F.L. Dryer, Water Addition to Practical Combustion System Concepts and Applications, Sixteenth Symposium (Int.) on Combustion, pp , K. Takasaki, T. Fukuyoshi, M. Otsubo and S. Abe, Improvement of diesel combustion using a fuelwater-fuel injection system, JSME international,