SPRAY DROPLET SIZE MEASUREMENT In this study, the PDA was used to characterize diesel and different blends of palm biofuel spray. The PDA is state of the art apparatus that needs no calibration. It is capable to measure drops size from as small as 0.5µm to 2500µm with estimated uncertainty to be ±2%. The set of PDA supplied by Dantec Dynamic is listed below and shown in Figures 4.10 to 4.12. i. Laser source ii. Laser beam splitter/bragg Cell iii. Transmitter and receiving optics iv. Traverse system v. PDA processor and software package (BSA Flow Sofware) Laser Beam Splitter Laser Source Power Controller Figure 4.10: Laser Source, Laser Beam Splitter and Power Controller
Transmitter Receiver Traverse System Drop Collection Basin Figure 4.11: Transmitter, Receiver and Traverse System Computer Set Emergency switch off button PDA Processor Figure 4.12: PDA Processor and Computer
PDA Principles The main principle of PDA is the Doppler Effect. Doppler Effect described as a change in frequency of emitted waves produced by motion of an emitting source relative to an observer [47]. On the other hand, any change in frequency due to the Doppler Effect is called a Doppler Shift. As in the PDA application, it is based on the Doppler shift from the light reflected and/or refracted from a moving seeding particle. There are several optical parameters that needed to be identified first before explaining the PDA s principles. As shown in Figure 4.13, the optical parameters are i. Beam intersection angle, θ ii. Scattering angle, φ iii. Elevation angle, ψ iv. Polarization (parallel or perpendicular to scattering plane) v. Shape and size of detector aperture Figure 4.13 PDA s Optical Parameter [48] The measurement of PDA system was started with a single beam was shifted into 2 pairs of beams with different wavelength. As in this study, the Bragg Cell was used to shift the laser beam at 40MHz frequency. The reason why a single beam was used is to ensure both
pairs of the beam were coherent in term of frequency with each other. All four of the beams are made to intersect at their waists (the focal point of a laser beam), where they interfere and generate a set of straight fringes as shown in Figure 4.14. Figure 4.14 Fringes form where Two Coherent Laser Beams Cross [26]
A particle scatters light from two incident laser beams as shown in Figure 4.15, generating an optical interference pattern. The sensor or also called the receiver is then aligned to the flow such that the fringes are perpendicular to the flow direction. Both scattered waves interfere in space and create a beat signal with a frequency. Two detectors receive this signal with different phases. The frequency generated from the scattered waves is proportional to the velocity of the particle and the phase shift between these two signals is proportional to the diameter of the particle. Figure 4.15: The Interference Patterns Two Photo-Detector Surfaces [26] The particle velocity U is calculated from the from the beat frequency or also called Doppler frequency, f D of the signal from any one of the detectors [49]; / (4.1) where U is the particle velocity, θ the angle between the incoming laser laser beam wavelength. beams and λ is the The particle size, d p is derived from the phase difference between the signals from two detectors. Since in this study the light is scattered dominated by refraction, the phase shift for refraction [49];
Φ! (4.2) where Φ is phase shift, d p is particles size and n rel is relative refractive index and could be expressed as " #$% &'()* +,(-+. Laser Alignment The laser alignment is the most important thing when dealing with PDA. The alignment is a process to get the intersection of the beams at the beam waists [26]. If the laser beam do not intersect in the beam waists but elsewhere in the beams, the wave fronts will be curved rather than plane and as a result the fringe spacing will not be constant but depend on the position within the intersection volume. If the beams are badly misaligned, it will probably not be able to get results at all, but if just slightly misaligned, the result of the testing will experience relatively small error and the user might not be aware of that. The locations in the PDA system that need to be aligned before starting the measurement are: i. Aligning laser beam from laser source to the Bragg Cell as shown in Figure 4.16. ii. Aligning set of fiber optics at the laser beam splitter which also known as Bragg Cell to obtain highest intensity of light in Figure 4.17. iii. Aligning four laser beams at the transmitter probe to get intersection of the beams at the desired focus point using 100µm as shown in Figure 4.18. iv. Alignment of the eye piece so that receiver probe can be accurately aligned with the beam intersection volume as shown Figure 4.19.
v. Lastly, the phase plot is another tool that can be used to check whether the PDA is in the optimized alignment. The optimized optical alignment and correct parameter settings will have a cloud of dots build up at the centered around the continuous line and mostly within the tolerance band as clearly shown in Figure 4.20. Figure 4.16: Aligning Laser Beams at the Bragg Cell Fiber Optics Cable Figure4.17: Fiber Optics Cable at the Laser Beam Splitter
100µm Pin Hole Figure 4.18: Aligning 4 Laser Beams That Coming Out From Transmitter Probes Using 100µm Pin Hole Figure 4.19: Focusing of the Image onto the Slit-Shaped Spatial Filters [26]
Figure 4.20: Phase Plot for Fiber PDA [26]
Optical Description of PDA System The optical description of the PDA that was used in this study is listed in Table 4.3 Table 4.3: Optical Description of PDA System Laser System Manufacturer Spectra Physics Laser Type Argon Ion Wavelength 476.5nm (violet) Bragg cell frequency shift 40 MHz Power 4 Watt PDA Transmitter Wavelength 514.5nm (green) and 488nm (blue) Focal Length 500mm Beam Diameter 2.2mm Expander Ratio 1 Beam Spacing 74mm PDA receiver Receiver type 112 mm fiber PDA Scattering angle 60deg Receiver focal length 500.000 mm Receiver expander ratio 1.000 Scattering mode Refraction Aperture mask Mask B Spherical validation band 10.00% Special filter 0.100 mm Maximum particle diameter 305.9 μm Medium Properties Maximum sample 2000 Maximum acquisition time 10s
Location of Drop Size Measurement In order to characterize the spray at different location, traverse system was used to move the transmitter probe and receiver probe. To ensure the measurement locations are around the spray cone, the spray cone photo of diesel at 12 bar was captured and the size of the cone was measured. Using the cone size obtained from the photo, the measurement mesh was generated at the BSA Flow Software. To facilitate the process in generating mesh, Microsoft Excel was used and later exported to the BSA Flow software. Figure 4.21 shows the measurement location of the spray and the cone angle. The measurement only takes half of the cone because it is assumed the droplet size is symmetrical. Atomizer θ 2θ 12 Points 20 Points 28 Points 36 Points Figured 4.21: Location of Measurement Point and Cone Angle
Repeatability In order to obtain reliable measurement, the test was conducted three times and the standard deviation of the result was calculated. In order to be satisfied in the establishment of repeatability, several conditions need to be noted [50]: i. Should follow the same measurement procedures. ii. Observed by the same person. iii. Same measuring instrument under the same conditions. iv. In the same location. v. It is being repeated in a short period of time. The standard deviation was calculated using Equation (4.3). / 1 / 4 3 (4.3) Where σ = is the standard deviation N= the number of sample x i = mean value from all three measurement µ = value from measurement
Test Rig Description Figure 4.22 shows the test rig in this study. The test fuel was drawn from the tank using Hydra Cell multi piston pump at designated pressure. The multi piston pump was chosen because it can pressurize the fuel at low pressure pulsation fluctuation. The regulating valve was used to regulate the pressure. As the fuel leave the pump, the flow rate of the test fuel was measured using flow meter. Before it reaches atomizer, it will fill the surge tank. The surge tank was used to stabilize the pressure hence reducing atomizer s vibration. The pressure swirl hollow cone atomizer with orifice diameter 1.6mm was selected since this type of atomizer is widely used in gas turbine engine. The break down components and outer dimension of this atomizer is given in Appendix A. As the liquid leave the atomizer, a droplet collection basin was used to collect excess sample fuel and return it back to the tank using submersible electrical pump. Excessive fume was rejected using blower. Figure 4.22: Experimental Set up
Safety Precaution The safety during the experiment is always taken care off. The laser beam is ensured not directed at any person and terminated at the end of its use path using black board. In addition, the power of the laser is kept at 4 watts or less to avoid burning the fiber optics cable. The respiratory mask and goggles were used to prevent inhaling the mist. On top of that, fire extinguisher always on the reach in case of fire broke out during the experiment. The emergency shut off button was keep near the PDA s operator in case of emergency. Measurement Procedure 1. The test rig as shown in Figure 4.22 was prepared. 2. Starting with petroleum diesel (B0), the fuel was poured into the fuel tank. 3. Then, pump and blower were started. The blower was used to remove fume from the droplet collection basin. 4. The valve was opened at its maximum to allow the pressure to settle. After the spray becomes stable, the valve was regulated at desired pressure, 8 bar. To Determine Spray Cone Angle 5. The black board was positioned at the back of the spray. The DSLR camera was mounted on a tripod to allow more stable image and consistency of the photograph taken. 6. The camera s shutter was released and the image was exported to SolidWork to obtain the cone angle. 7. The cone angle was measured. 8. Then the pressure was increased to 10 bar and 12 bar. Steps 5 and 6 were repeated.
To Determine Droplet Size 9. The water s valve was fully opened to cool the laser system. 10. The laser, PDA Processor and Traverse System were turned on. 11. At the BSA Flow Software, the measurement point was inserted. 12. The laser intersection beam was pin pointed exactly at the atomizer tip. 13. Then the power was set at 4 watts and the measurement was initiated. 14. After the PDA has finish measuring the 96 points, the result was saved in the computer and list of data was exported to Microsoft Excel for further analysis. The histogram was exported as a picture formatted (.jpeg). 15. To ensure consistency and reliable of this results, the measurement was repeated 3 times and the average was taken. The standard deviation for each measurement was calculated. 16. After that, Step 3 onwards was repeated for injection pressure of 10 bar and 12 bar. 17. After finish the measurement, the tank and fuel supply line was cleaned and flushed using the next testing fuel. This is to ensure overall system does not contain any leftover from previous test. 18. For the next testing fuel (B5, B10, B15, B20 and B25), Step 2 onward was repeated.