Multi-phase measurements on a GDI spray using optically efficient fluorescent tracers

Transcription

1 Multi-phase measurements on a GDI spray using optically efficient fluorescent tracers Diego Angarita-Jaimes 1, Mourad Chennaoui 2, Catherine Towers 1, Anita Jones 2, David Towers 1 1: School of Mechanical Engineering, University of Leeds, Leeds, UK, d.p.towers@leeds.ac.uk 2School of Chemistry, University of Edinburgh, Edinburgh, UK, a.c.jones@ed.ac.uk Abstract Multi-phase air/fuel measurements have been performed on a high pressure multi-hole GDI injector. Previously developed UV-excitable fluorescent tracers have been used to seed the gas phase with the naturally occurring droplets in the fuel used as tracers for the other phase. The co-registration offered by a 3CCD colour camera enables images recorded in the three colour planes (red, green and blue) to give flow constituent as well as pulse order information using standard solid state lasers and a 532nm pumped pulsed dye laser. A high-pressure multi-hole GDI injector was mounted in a rig with a glass barrel to simulate the engine cylinder and provide optical access; images were obtained under controlled conditions of fuel pressure and injection time. Suitable corrective processes have been adapted and implemented to account for crosstalk and chromatic aberrations so that the uncertainty of the velocity vectors produced is comparable to that of conventional PIV using 532nm illumination. The fluorescent dyes used in this work can also be used in other multi-constituent flows and flare removal experiments to allow for velocity measurements in complicated engineering rigs. The current technique shows potential to offer a more detailed understanding of the interaction between the air and fuel in GDI sprays. Due to the simplicity of the imaging system, future stereo PIV measurements will be possible and a range of experimental conditions consistent with the different operating regimes of a GDI engine are planned. 1. Introduction More stringent regulations on green house emissions as well as the shortage of fuel resources have led to a new set of technologies aimed at producing engines with higher efficiencies and lower emissions levels; new alternatives have been introduced such as High Speed Direct Injection Diesel (HDI) engines, and Gasoline Direct Injection (GDI) engines. GDI engines are showing the potential to achieve considerable improvement (25%) in fuel economy, power output and a reduction of cold-start hydrocarbon emissions; this is achieved mainly by reducing throttling and heat losses during stratified combustion and a better control over fuel injection. GDI injection systems use a considerably higher pressure, compared to conventional port injection engines, which increases the degree of atomisation and vaporisation making it possible to achieve stable combustion on the first cycles without requiring extra fuel injection (Zhao et al. 1999). On a GDI engine, the spray parameters have a greater significance compared to port injection engines; factors such as cone angle, penetration, mean drop size and delivery rate are critical and an optimum matching of these factors is essential for the development of the GDI concept (Zhao et al. 1999). Unlike port injection engines, there is less time for mixture preparation on GDI engines hence this process needs to be aided by the spray characteristics. Whereas most of the port injected engines can operate using a spray with 200 µm SMD, a GDI engine will require at least 20µm in order to achieve acceptable levels on emissions (Zhao et al. 1999). For a GDI injector, it is important that fuel atomisation has taken place before the spark occurs since the GDI concept relies on obtaining - 1 -

2 rapid vaporisation of fuel droplets (Anderson et al. 1996) In order to understand the interaction mechanisms on GDI engines simultaneous 3-component velocity in both phases (air/fuel) and droplet size measurements in the spray are required; these parameters need to be temporally and spatially resolved throughout mixture preparation and characterised over a range of temperatures and pressures consistent with the injection strategies (early and late fuel injection) to be used in GDI engines. PIV has become a powerful tool for measuring fluid velocity owing to its non-intrusive nature (Adrian 1991). A laser beam is formed into a light sheet which illuminates light scattering tracer particles added the flow. Light from the particles or tracers is then elastically scattered and imaged onto a CCD camera or photographic film. One of the challenging application areas for PIV is in the quantification of multi-phase and multi-constituent flows (Hassan 1998). When conventional PIV is applied to two-phase flows, images are obtained from the tracer particles in both constituents and velocity vectors are obtained on the image field; however, this approach does not allow separation of the flow field for each component and important information concerning the interaction of the phases such as relative velocity and mixing cannot be obtained. Different groups have addressed multi-phase flows in Gasoline Direct Injection (GDI) sprays using a 532nm laser light sheet with conventional tracers in one phase and fluorescent tracers in the other. A par of monochrome cameras were used so that one camera would produce fluorescence images and the other Mie scattered images. Data for the 2 phases was obtained but the discrimination of the conventional tracers from the fluorescent tracers was not completely successful (Boedec and Simoens 2001 and Rottenkolber et al. 2002). The desirable sub-pixel accuracy, for reliable PIV data, is difficult to achieve due to the distortions introduced by the optical filters on each camera. An alternative strategy has been developed for multi-constituent gas phase flows using fluorescent tracers, a 3CCD colour camera and solid state lasers for which separate velocity vectors have been obtained (Ormsby et al. 2007). UV-excitable fluorescent tracers with high quantum yield have been developed with their emission spectra contained in the blue (Bis-MSB with o-xylene as solvent) and red (DCM with DMSO as solvent) channels of a 3CCD colour camera. The use of UV excitation, using a frequency tripled solid state laser, allows the spectral separation of the excitation and emission wavelengths. Using a 3CCD colour camera provides simultaneous images on three colour planes (red, green and blue) enabling both flow constituent and pulse order to be determined. The use of a single colour camera for 2D PIV can be easily extended to 3D PIV in a stereo arrangement consisting of a pair of 3CCD colour cameras. 2. Imaging system and experimental setup It is required that sufficient spectral separation exists between excitation and emission wavelengths for the fluorescent tracers with the spectra ideally contained within the primary colour bands of the camera. Flow constituent identification in the fuel/air system will be achieved by adding micron sized UV-excitable Bis-MSB tracers (using o-xylene as the solvent) emitting in the blue channel to one constituent (gas phase) and the naturally occurring fuel droplets will act as tracers for the other phase (fuel). The first laser pulse will consist of both 532nm and 355nm wavelengths spatially overlapping; nonfluorescent (fuel) droplets will produce images on the green layer and fluorescent particles (gas - 2 -

3 phase) will produce an image on the blue channel and the green channel. The red image channel remains unused and hence an Nd:YAG-pumped dye laser can generate a second pulse, giving Miescattered images from both types of tracer in the nm region also identifying pulse order. Imaging red Mie scattering provides enough spectral separation with respect to the blue and green images and therefore crosstalk levels will be reduced. Figure 1 below shows a schematic diagram of the laser light sheet and the three layers of the implemented colour imaging system. Any Mie scattered light from the UV laser sheet is blocked by a 400 nm long pass wavelength filter. Figure 1. Imaging system for a 3CCD colour camera Experiments were performed using a combination of a twin-cavity solid state laser and a dye laser. An Nd:YAG laser (Continuum Surelite II-10) was fitted with a tripling crystal giving a first pulse of both 532nm and 355nm light; a 532-pumped Quanta Ray Pulsed Dye Laser, using DCM dye with ethanol as the solvent, gave a pulse with an emission peak centred at 650nm. A dispersive fused silica 15 mm equilateral prism was used to separate the harmonics of the first pulse by refraction with the 532nm and 355nm components subsequently realigned at the laser sheet focus. In order to align the foci of the 3 laser sheets it was required to expand each beam using separate plano-concave lenses at different distances relative to the common cylindrical lens. The three lenses (f = -200mm) were positioned in each beam and mounted on micro-blocks, giving 3- axes of linear adjustment, to aid the alignment. The experimental setup is shown in Figure 2 below. Fused silica prism Iso-octane Tripled Nd:YAG 532nm pumped dye laser Air with fluorescent tracers Figure 2. Experimental setup Hitachi HV- F22F - 3 -

4 14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Each beam was steered using suitable mirrors for each wavelength: 355nm and 532nm dichroics for green and UV and broadband dielectric mirrors for the red beam. The red and green beams met at a 532nm dichroic mirror and then the 3 beams met at a 355nm dichroic mirror. Both the 355nm and 532nm dichroic mirrors were clear-backed allowing the other wavelengths to pass through unhindered. The exact alignment of the three light sheets was important; a JAI CV-A50 1/2 inch camera, protected by different neutral density filters (positioned separately on each beam to account for the differences in the spectral response of the camera), was placed directly in the sheet focus. This allowed sheet positioning (co-alignment of the three different wavelengths) and also ensured that the three beams focused at the same position. The fuel system used consists of a high pressure multi-hole GDI injector which was mounted in a rig with a glass barrel giving optical access. Fuel was provided with a high pressure pump (pressurised using a nitrogen cylinder) with a pressure transducer placed on the high pressure side of the system to have an accurate reading of the fuel pressure just before injection. Injection time and delay (with respect to the lasers) were controlled by changing the width and start of the pulse sent to the injector control unit. A 555 monostable timer circuit was implemented to obtain single injections (to avoid quick accumulation of fuel on the glass barrel which could block the view of the camera) synchronized with the repetition rate of the lasers and the camera. Unleaded nonfluorescent gasoline was used to avoid any emission of the additives normally present in commercial gasoline. The spray rig with the glass barrel is shown in Figure 3 below. Figure 3. Fuel injector and spray rig system A pressure-driven medical nebuliser (Med2000 Andy Flow) was used to generate Bis-MSB droplets at a concentration of 10-2 M (using o-xylene as the solvent) to seed the gas phase; tracers were fed into the glass chamber at a pressure of ~0.5 bar. Images were recorded using a Hitachi HV-F22F 3CCD colour camera focused with a Nikon 105mm lens with the aperture set at F#11 to obtain a sharp focus on the three colour planes. The camera was triggered externally (using the laser repetition rate) to ensure that lasers, camera and injectors would be synchronised. Brightness and contrast on the camera were adjusted to control noise levels with the red gain increased to compensate for the reduced sensitivity of the camera on the red channel. Distortions in the images introduced by the glass barrel were corrected using a meniscus cylindrical lens (f=4030 mm) placed in front of the camera; its position and rotation were adjusted to obtain -4-

5 sharp images of the particles on the three channels. Any possible Mie scattering at 355nm due to the large fuel droplets was blocked using a long-pass filter with a cut-off wavelength of 400nm. An image of the multi-hole injector is shown in Figure 4 below with 532 illumination; the white area indicates the region where multi-phase PIV data was later obtained. Figure 4. Flow from the multi-hole injector using 532nm illumination. 3. Image acquisition and vector calculation The tip of the injector was placed on the same vertical plane as the light sheet; the focus of the sheets was placed 2cm from the tip of the injector to ensure that one of the main jets from the injector was imaged. Images were taken from the section indicated in Figure 4 with one representative image shown in Figure 5 below with a pulse separation of 4µs and under conditions corresponding to 2000r.p.m. with injection on the induction stroke

6 Figure 5. RGB image of the air/fuel system with a pulse separation of 4µs. In order to avoid colour blind areas the spectra in colour CCD cameras are designed to overlap; however this can cause crosstalk between the colour channels with images in one channel producing a small signal in the other two channels. Due to the bigger droplets in the fuel phase crosstalk between colour channels can be present in the system; crosstalk was corrected using an approach initially applied in 3D surface contouring (Huang et al. 1999) and then adapted to obtain a quantitative estimation of crosstalk between colour channels using a 3CCD colour camera (Zhang et al. 2006). Using this approach crosstalk effects were quantified by estimating the percentage of the intensity in a particular channel that is detected in the other two channels; a coupling matrix, as proposed by Zhang et al. 2006, was established with the recording settings used and then inverted to account for crosstalk. Chromatic and lens aberrations were also accounted for. In a colour 3CCD camera the three channels are not exactly co-aligned; given that both types of tracers produce a signal in the red channel, this was taken as the reference. There is a small offset between the red and the green and between the red and the blue channels which produces false displacements on the calculated vectors. A correction procedure was used in which images from a target were taken; the target covered the whole field of view ensuring that vectors would be obtained in every interrogation window. Images were split into their RGB channels with pairs of images (blue to red and green to red) then processed using DaVis 7.2 with the same settings later used to calculate the vectors fields. The displacements obtained in each interrogation window can be then subtracted from the vectors for each phase to account for false displacements due to the offset between the channels. Following crosstalk compensation, figure 5 was split into its RGB channels; fluorescent and nonfluorescent tracers from the first pulse were discerned; a particle with signal on both green and blue channels was identified as fluorescent and a particle with signal just on the green channel was identified as non-fluorescent and corresponds to the fuel phase. Mie scattering from both types of tracers was obtained in the red channel corresponding to the second pulse from the dye laser

7 Pairs of images (air tracers and red, fuel and red) were processed using DaVis 7.2 with the adaptive multi-grid cross-correlation algorithm; vector maps corresponding to the CCD offset were subtracted. Separate sets of vector maps were obtained for air phase and fuel phase both of which are plotted in Figure 6 below. Figure 6. Multi-phase vectors corresponding to tracers in Figure 5 for a pulse separation of 4µs. The red vectors represent the fuel phase and the blue vectors the gas phase velocities. Blue vectors represent the air (gas phase) and red vectors represent the fuel. A different scale (by a factor of 5) has been used for the vectors due to the difference in velocity for the 2 phases with the gas phase velocity being considerably less than that in the fuel phase. 4. Conclusions Multi-phase air/fuel measurements have been presented from a high pressure GDI injector system; flow phase and pulse order have been determined with a set of velocity vectors obtained for the fuel phase and the air phase. Suitable corrective processes have been adapted and implemented to account for crosstalk and chromatic aberrations so that the uncertainty of the velocity vectors produced is comparable to that of conventional PIV using 532nm illumination. The technique has the potential for detailed measurements of the interaction between the air and the fuel phases on a GDI injector. Future work will include stereo PIV measurements (by adding a second 3CCD colour camera) under experimental conditions corresponding to different operating regimes in a GDI engine

8 Acknowledgments The authors would like to thank the Engineering and Physical Sciences Research Council (EPSRC) for funding under grant reference GR/S69108; D Angarita is partially funded by a Roberto Rocca Fellowship. Injectors and control system were kindly provided by Jaguar. References Adrian RJ (1991) Particle imaging techniques for experimental fluid-mechanics. of Fluid Mechanics 23: Annual Review Anderson R, Brehob D, Yang J, Vallance J, Whitetaker R (1996). A new direct injection spark ignition combustion system for low emissions. FISITA 96. Technical paper N P0201 Boedec T and Simoens S (2001) Instantaneous and simultaneous planar velocity field measurements of two phases for turbulent mixing of high pressure sprays. Experiments in Fluids 31: Hassan YA (1998) Handbook of Fluid Dynamics. Chapter 36: Multiphase Flow Measurements Using Particle Image Velocimetry. C RC Press, editor RW Johnson. ISBN-10: Huang PS, Hu QY, Jin F, Chiang FP (1999) Color-encoded digital fringe projection technique for high-speed three-dimensional surface contouring. Optical Engineering 38, Ormsby M, Chennaoui M, Angarita-Jaimes D, Angarita-Jaimes N, Towers C, Jones A, Towers D. Optically efficient fluorescent tracers for multi-constituent PIV and flare removal. 7th International Symposium on Particle Image Velocimetry University La Sapienza Via Eudossiana 18, Roma ITALY September 2007 Rottenkolber G, Gindele J, Raposo J, Dullenkopf K, Hentschel W, Wittig S, Spicher U, Merzkirch W (2002) Spray analysis of a gasoline direct injector by means of two-phase PIV. Experiments in Fluids 32: Zhang Z, Towers CE, Towers DP (2006) Time efficient colour fringe projection system for 3D shape and color using optimum 3-frequency selection. Optics Express: Vol. 14 N Zhao F, Lai M. Harrington D (1999) A review of mixture preparation and combustion control strategies for spark-ignited Direct-injection gasoline engines. SAE