S. E. Parrish * and R. J. Zink General Motors Global R&D Mound Road Warren, MI USA

Transcription

1 ILASS-Americas, 23 rd Annual Conference on Liquid Atomization and Spray Systems, Ventura, CA, May 211 Development and application of an imaging system to evaluate liquid and vapor envelopes of sprays from a multi-hole gasoline fuel injector operating under engine-like conditions S. E. Parrish * and R. J. Zink General Motors Global R&D 3 Mound Road Warren, MI USA Abstract A novel high-speed imaging system capable of acquiring schlieren and Mie scattering images in a nearsimultaneous fashion along the same line-of-sight has been developed. Liquid and vapor envelopes of sprays from a multi-hole fuel injector operating under engine-like conditions were systematically investigated. For the conditions evaluated, the maximum liquid penetration length was found to be inversely proportional to injection pressure. Penetration lengths derived from the Mie scattering envelops and schlieren envelopes were found to be quite similar up until the occurrence of the peak liquid penetration, implying that little vapor exists ahead of the penetrating liquid. * Corresponding author

2 Introduction It is well known that the fuel economy and emissions of direct-injection gasoline engines are significantly influenced by the fuel injection process and in particular the characteristics of the fuel spray [1, 2]. As a result an extensive amount of work has been performed in the area of fuel injector spray characterization [3-5]. While extensive, the large majority of the work has been confined to characterization of the liquid phase. Far less has been reported regarding characterization of the vapor phase although a significant body of work related to laser-induced fluorescence and exciplex exists [6, 7]. While quite valuable, these techniques are often difficult to implement and require specialized equipment. Additionally, they are typically limited to slow-speed acquisition rates. The motivation for this work was to develop an imaging system capable of identifying the liquid and vapor envelopes, or perimeters, of fuel sprays at highspeed. Such a system allows systematic studies to be performed to improve understanding of spray characteristics under various operating conditions. The data generated would be of interest to computational modelers for model validation and to combustion system engineers to assist in combustion system development. Schlieren imaging has been used for decades to image a myriad of fluid flow phenomena. Its sensitivity and relative simplicity have made it a popular optical diagnostic. Although rather straightforward in many ways, schlieren imaging is not without difficulties. The technique is sensitive to gradients of index of refraction, and therefore in the case of a multiphase flow, such as a fuel spray, both the liquid and vapor phases contribute to the schlieren signature. This makes it impossible to definitively distinguish one phase from another without an accompanying measurement. The work presented here describes a high-speed imaging system capable of acquiring schlieren and Mie scattering images in a near-simultaneous fashion along the same line-of-sight. Acquiring the images along the same line-of-sight allows the Mie scattering images, which are sensitive only to the liquid phase, to be overlaid onto the schlieren images enabling the phase boundary to be determined. Previously, we reported on the development and use of a similar imaging system [8]. This work involves the adaptation of the previous system to allow measurements to be acquired at high-speed. The ability to acquire images at high-speed is advantageous for a couple of reasons. First, it allows the temporal evolution of an injection event to be investigated. Our previous investigations have indicated that vapor phase envelopes can vary considerably from injection-toinjection. This makes it difficult to discern temporal evolution from a collection of single shot measurements at discrete points in time. To improve our understanding beyond what occurs on average, the ability to capture continuous temporal evolutions becomes a necessity. Secondly, and perhaps unique to the schlieren imaging technique, high-speed imaging alleviates problems associated with the identification and removal of artifacts in the background of images. Often schlieren systems are setup to be extremely sensitive to capture minute gradients of index of refraction related to the physics under investigation. In this work we were interested in identifying the boundary of lean fuel vapor mixtures. Unfortunately this often leads to background schliere due to minute gradients in density and temperature within the test section. When acquiring images on a single-shot basis, background schliere are different from shot-to-shot and elaborate image processing must be employed to extract them. In contrast, with highspeed imaging the background schliere does not change significantly between successive frames. This greatly simplifies the process of eliminating the background schliere. Details regarding the procedure for doing so are provided later. Imaging System Figure 1 shows the imaging system that was developed. It is basically a folded z-type schlieren system. A single high-speed LED was used as a schlieren light source. Output from the LED was focused onto a diffuser utilizing a five power microscope objective as a condensing lens. The use of a diffuser was necessary to produce a uniform light intensity free of any LED structure. A m slit was placed next to the diffuser to provide geometric definition to the light source. Light emitting from the slit was collimated by a parabolic mirror of 1.2 m focal length to produce a beam approximately 1 mm in diameter. Once collimated a flat mirror was used to turn the beam and direct it through the vessel test section. The mirrors on the receiving side of the imaging system were identical to those used on the sending side. The receiving parabolic mirror focused the beam and produced a well defined image of the slit in its focal plane. A knife edge, mounted parallel to the long side of the slit, was used as a cut-off and was placed at the focus of the beam. The use of a slit and knife edge results in a schlieren system that is sensitive only to gradients of index of refraction that are perpendicular to the slit and knife edge orientation. This does not present a problem, necessarily, because index of refraction gradients are produced at all angles. The orientation of the slit and knife edge with respect to the spray does, however, affect the appearance of the resulting schlieren images. Typically an intensity gradient exists in the direction perpendicular to the slit and knife edge arrangement. Therefore, to make the image intensity appear symmetric about the 2

3 H.S. Camera Knife Edge Folding Mirror Parabolic Mirror Vessel Injector Circular H.S. LED Strobe Array (1 µs duration) Diffuser H.S. LED (5 µs duration) Parabolic Mirror Folding Mirror Source Slit Condensing Lens Figure 1. Folded z-type schlieren imaging system. spray axis the slit and knife edge were oriented perpendicular to the spray axis. This results in an intensity gradient from the leading edge to the trailing edge of the spray. Other than appearance to the eye, this intensity gradient had no discernable impact. Illumination for Mie scattering was provided by a circular array of high-speed LEDs placed directly opposite of the injector. The resulting Mie scattering images were collected along the same optical path utilized for the schlieren imaging. This resulted in the acceptance of only small angle scattering yielding a large depth of field. Although undesired, the knife edge effectively reduced the Mie scattering intensity by one half. Images were captured with a high-speed digital camera equipped with a 15 mm lens that was focused at infinity. Mie scattering and schlieren images were captured in a nearly simultaneous fashion utilizing frame straddling. Frame straddling, most commonly used in high-speed PIV, results in image pairs that are slightly temporally separated. The primary advantage of utilizing a common camera along with frame straddling, compared to utilizing two cameras, is that it eliminates the need to spatially register independent images. One apparent disadvantage is that the images cannot be acquired simultaneously. The degree to which this becomes an issue depends on the speed of the event being captured in comparison to the overall time required to perform the measurements. E O E O Mie Schlieren Mie Schlieren Figure 2. Frame straddling timing diagram. 2 khz camera framing rate translates to image pairs at 1 khz. Mie scattering exposures occurred at the end of the even numbered frames while schlieren exposures occurred at the beginning of the odd numbered frames. Figure 2 illustrates a timing diagram depicting the image acquisition process. To acquire image pairs at a rate of 1 khz, a 2 khz camera framing rate was required. The camera exposure time was set to be nearly equal to the frame interval. The Mie exposure occurred at the end of even numbered frames while schlieren exposure occurred at the beginning of the odd numbered frames. Image exposures were governed by the strobe durations and were set to 1 µs and 5 µs for the Mie and schlieren imaging, respectively. These durations were chosen because they produced adequate signal strength; although ideally shorter durations would have been utilized. The total amount of time from the beginning of the Mie exposure to the end of the schlieren exposure was approximately 17 µs. This translates to 1.7 mm of displacement for objects traveling at m/s. This magnitude of velocity is not uncommon for direct-injection gasoline engine fuel sprays but it is usually associated with the early stages of injection and 3

4 dissipates quickly. Therefore for most conditions of interest the image exposures were adequately small. While it is believed that the imaging system developed in this work is novel, it would be amiss to not acknowledge that others have developed imaging systems of similar nature. Pickett et al. [9] developed a dual camera system that acquired Mie and schlieren images simultaneously but from nearly orthogonal perspectives. In comparison, one apparent advantage of the system presented here is that by utilizing the same lineof-sight the Mie and schlieren images always correlate regardless of the spray shape or symmetry. Image Processing In order to visualize lean mixtures the schlieren system was adjusted for high sensitivity. Unfortunately this often leads to background schliere due to minute gradients in density and temperature within the test section. As mentioned previously, with high-speed imaging background schliere do not change significantly between successive frames. This allows the previous frame to be used for background subtraction and greatly simplifies imaging processing. Pickett et al. [9] used a similar methodology to process the high-speed schlieren images they acquired. The following briefly describes the sequence of image processing operations that were utilized to remove the background from the acquired schlieren images: 1) Subtraction of the current image from the previous image (eliminates nearly all background schliere). 2) Binarize with low level thresholding (removes low level background and enables subsequent logical operations). 3) Small feature filter (eliminates small spurious features that are unattached from the major feature). 4) Dilation utilizing a small circular disc as a structuring element (adds continuity to the major feature but slightly inflates perimeter). 5) Fill operation (fills holes in the major feature). 6) Erosion utilizing the same structuring element used for dilation (slightly reduces perimeter countering the effect of previous dilation). 7) Perimeter detection (identify perimeter). Experiment Details To assess the capabilities of the imaging system a series of experiments were conducted. A high temperature pressure vessel was utilized to allow measurements to be performed at elevated pressures and temperatures. A continuous flow of nitrogen gas passed through the vessel to provide evacuation of the injected mass. The pressure and temperature within the vessel were fixed at kpa and 8 K, respectively. Fuel was injected into the pressure vessel by an 8- hole, VCO (valve covered orifice) style injector with a nominal included spray angle of 6 degrees. Multi-hole injectors are of interest because their sprays tend to maintain their shape when exposed to high ambient pressures. Additionally they offer considerable flexibility in fuel placement. A coolant jacket surrounding the injector mount was used to control the injector temperature. The fuel temperature was assumed to be equal to the coolant temperature and was set to 9º C to mimic fully warmed up conditions. Three injection pressures, 5 MPa, 1 MPa and 2 MPa, were investigated and maintained using a PID controlled hydraulic accumulator. The injection durations required to meter 2.5 mg, 5. mg, and 1 mg at the three injection pressures of interest were predetermined and are shown in Table 1. Indolene was chosen as the test fluid due to its realistic multi-component composition. Injected Mass 5 MPa 1 MPa 2 MPa 2.5 mg 382 s 5 s 27 s 5. mg 662 s 47 s 428 s 1. mg 134 s 9 s 671 s Table 1. Injection durations. Exemplary Results Exemplary raw images along with the resulting envelope identifications are shown in Figure 3. Each row corresponds to a different point in time referenced from the start of injection. Although image pairs were acquired at a rate of 1 khz, only four image pairs are shown at a.5 ms interval. While it appears that only four spray plumes are visible, each plume is actually a pair of plumes, one behind the other. Early in time (.5 ms) the Mie scattering and schlieren images appear very similar because insufficient time has passed to produce much vapor. Although the spray is not perfectly symmetric about its axis, the Mie scattering and schlieren images correlate to one another because both images were acquired along the same line-of-sight. As time proceeds, however, the intensity exhibited in the Mie scattering images diminishes while the schlieren images exhibit continual expansion of the individual spray plumes. Late in time (2. ms), individual plumes are no longer discernable in the schlieren images and no Mie scattering is detectable due to complete vaporization of the injected mass. By comparing each of the schlieren images it can be seen that the background schliere does not change significantly between successive frames. As mentioned previously this simplifies the process of eliminating the background schliere by allowing the previous frame to be used for background subtraction. The results of the background subtraction and envelope identification are shown in the far right column of Figure 3. The inner contours were derived from the Mie scattering images and therefore represent only the liquid phase. The outer 4

5 contours were derived from the schlieren images and therefore represent both liquid and vapor phases. Overlaying the liquid phase contour onto the schlieren image allows for the determination of the line-of-sight phase boundary..5 ms 1. ms 1.5 ms 2. ms Mie Scattering Schlieren Overlay (liquid only) (liquid & vapor) (liquid & vapor) Figure 3. Raw Mie scattering and schlieren images and the corresponding liquid and vapor envelopes. M injected=1mg, P injection =1 MPa, T amb =8K, P amb = kpa Influences of Injected Mass and Injection Pressure Figures 4, 5, and 6 show liquid and vapor envelopes for an injected mass of 2.5 mg, 5. mg, and 1 mg, respectively. Each column represents a different injection pressure. Liquid and vapor penetration curves for each case are shown in Figure 7. In all cases, liquid penetration is shown to increase to a maximum value and then go to zero due to complete vaporization. As expected, early in time (.5 ms) and prior to significant vaporization, liquid penetration lengths are directly proportional to injection pressure and are independent of the injected mass. As time progresses, however, it is apparent that the lower the injection pressure the longer the liquid phase persists. Additionally, in general, the maximum liquid penetration length is inversely proportional to injection pressure. This trend strengthens with injected mass and is due to the longer injection durations and larger droplet sizes associated with lower injection pressure. It is worth mentioning that this trend is dependent on the ambient gas temperature and is therefore not universal. At lower temperatures one would expect liquid penetration to be directly proportional to injection pressure throughout the entire time period shown. Comparing one injected mass to another at the same injection pressure, it is apparent that the larger the injected mass the longer the liquid phase penetration and persistence. This is expected as a larger fuel quantity has more momentum and requires more time to vaporize. By comparing the schlieren envelopes to the Mie scattering envelopes several common trends, independent of fuel pressure and injected mass, can be observed. Early in time (.5 ms) the envelopes are nearly identical to one another. This is reasonable to expect and suggests that little vaporization has occurred. As time progresses the envelopes begin to differ significantly. In the case of the Mie scattering envelopes, individual plumes remain thin as penetration continues. In contrast, the schlieren envelopes exhibit a broadening of individual plumes. This indicates that vaporization is occurring along the perimeter of the plume. Penetration lengths derived from the Mie scattering envelopes and schlieren envelopes are quite similar up until the occurrence of the peak liquid penetration. This implies that little vapor exists ahead of the penetrating liquid. Incidentally, this behavior is much different than what is typical of a diesel spray where liquid penetrates far less than vapor [9]. Shortly after the occurrence of the peak liquid phase penetration, complete vaporization of the injected mass occurs. At this point in time the schlieren (vapor) envelopes continue to penetrate but at a reduced rate. Comparing one injection pressure to another at the same injected mass, it can be seen that in general vapor penetration increases with injection pressure as expected. As was the case for liquid penetration, the larger the injected mass the longer the vapor penetration. Sensitivity and Mixture Strength Estimate The primary objective of this work was to develop a diagnostic capable of identifying the boundary of lean vapor mixtures. For the conditions investigated, in most cases a clear and rather sharp boundary was identifiable suggesting good sensitivity. To further assess the sensitivity of the system, an attempt was made to estimate vapor mixture strengths. Among other things, this involves estimating the volume of the vapor mixture. This was simplistically accomplished by first determining the average radius at each location along the axis of the mixture. This implies that the mixture is geometrically symmetric about its axis. While that is never the case, the intention here is to provide an engineering estimate. Upon determination of the average radii, the total volume of the mixture was calculated through simple summation. Figure 8 5

6 .5 ms.5 ms 1. ms 1. ms 1.5 ms 1.5 ms 2. ms 2. ms 5 MPa 1 MPa 2 MPa Figure mg liquid and vapor envelopes. T amb =8K, P amb = kpa.5 ms 1. ms 1.5 ms 2. ms 5 MPa 1 MPa 2 MPa Figure 5. 5 mg liquid and vapor envelopes. T amb =8K, P amb = kpa 5 MPa 1 MPa 2 MPa Figure 6. 1 mg liquid and vapor envelopes. T amb =8K, P amb = kpa Penetration (mm) Penetration (mm) Penetration (mm) 5 MPa 5 MPa 1 MPa 1 MPa 2 MPa 2 MPa 2.5 mg 5. mg 1. mg Time (ms) Figure 7. Liquid (solid line) and schlieren/vapor (dashed line) penetration. T amb =8K, P amb = kpa 6

7 illustrates the actual line-of-sight vapor boundary in comparison to the average radius boundary. Figure 8. Mixture volume and strength estimation. The thin contour represents the actual line-of-sight vapor boundary while the thick contour represents the average radius at each location along the axis of the mixture. A summation of volume 18 cc and an injected fuel mass of 2.5 mg, yields a concentration of 23 µg/cc and a mass fraction of 9:1. (P fuel =2MPa, T amb =8K, P amb = kpa, T=4 ms). Estimation of the vapor mixture strength also requires the assumption that the mixture is homogeneous. That is most certainly not the case during the injection event and for sometime afterward. As time passes, however, it is reasonable to expect an increased level of homogeneity. Therefore, this approximation is only valid well after the end of injection. Mixture strength can be expressed in terms of concentration by simply dividing the injected mass by the estimated total volume. Alternatively, and perhaps more meaningful, a mass fraction can be calculated by estimating the mass of air (nitrogen) contained within the volume and dividing by the injected fuel mass. Concentration and mass fraction estimates for the 2.5 mg injected mass are shown in Figure 9. The motive for performing these estimations was to demonstrate the capability of the imaging system to identify the boundaries of lean mixtures. Therefore only the 2.5 mg case is shown because it resulted in the leanest mixtures. Note that estimates were performed starting at 2. ms after the start of injection in an attempt to satisfy the homogeneous mixture assumption. As time progresses the vapor cloud continuously expands reducing the concentration. Not surprisingly the higher the injection pressure the lower the concentration. This is due to the mixture occupying a larger volume. Even a small increase in axial and radial penetration can lead to a substantial increase in volume. This is apparent when viewing the 2. ms images associated with the 2.5 mg injected mass as shown in Figure 4. When expressed in the form of mass fraction the trends are completely opposite of the concentration trends, as expected. The minimum concentration shown in Figure 9, corresponding to the 2 MPa fuel pressure at 4. ms, is approximately 23 µg/cc which translates to a mass fraction of around 9:1. The schlieren image used for this estimate is shown in Figure 8. While this mixture is quite lean, additionally measurements have shown that the imaging system has the sensitivity to visualize even leaner mixtures. Concentration Mass Fraction 2.5 mg 5 MPa 1 MPa 2 MPa Time (ms) 5 MPa 1 MPa 2 MPa 2.5 mg Time (ms) Figure mg mixture strength estimates. Concentration is calculated by dividing the injected mass by the estimated total volume and is shown in units of µg/cc. Mass fraction is calculated by estimating the mass of air (nitrogen) contained within the volume and dividing by the injected fuel mass. Summary and Conclusions A novel high-speed imaging system capable of acquiring schlieren and Mie scattering images in a nearsimultaneous fashion along the same line-of-sight has been developed. Acquiring the images along the same line-of-sight allows the Mie scattering images, which are sensitive only to the liquid phase, to be overlaid onto the schlieren images enabling the determination of the line-of-sight phase boundary. Acquiring images at high-speed allows the temporal evolution of an injection event to be investigated. High-speed imaging also simplifies the process of removing background schliere because they do not change significantly between successive frames allowing the previous frame to be used for background subtraction. A series of systematic experiments was performed with an ambient pressure and temperature of kpa and 8 K, respectively. For the conditions investigated, the maximum liquid penetration length was found to be inversely proportional to injection pressure due to the longer injection durations and larger droplet sizes associated with lower injection pressure. As ex- 7

8 pected, liquid phase penetration and persistence was found to increase with injected mass. Penetration lengths derived from the Mie scattering envelopes and schlieren envelopes were found to be quite similar up until the occurrence of the peak liquid penetration, implying that little vapor exists ahead of the penetrating liquid. Mixture strength estimates were performed by estimating the total volume of the mixture and by assuming a homogeneous composition. Estimates of vapor concentration down to approximately 23 µg/cc (or 9:1 mass fraction), indicate that the imaging system is capable of visualizing lean vapor mixtures. References 1. Zhao, F., Harrington, D. L., and Lai, M-C., Automotive Gasoline Direct-Injection Engines, SAE International, ISBN Number: , 22, pp Spiegel, L., and Spicher, U., Mixture Formation and Combustion in a Spark Ignition Engine with Direct Fuel Injection, SAE Park, S.W., Kim, H.J, and Lee, C.S., An Experimental and Numerical Study on Atomization Characteristics of Gasoline Injector for Direct Injection Engines, 15 th Annual Conference on Liquid Atomization and Spray Systems, Madison, WI, May Parrish, S.E., and Farrell, P.V., Transient spray characteristics of a direct-injection spark-ignited fuel injector, SAE Dahlander, P., and Denbratt, I., Experimental investigation of fuel pressure influence on droplet size and velocity for a gasoline multi-hole direct injection injector spray under vaporizing conditions in a constant pressure chamber, 18 th Annual Conference on Liquid Atomization and Spray Systems, Irvine, Ca, May Melton, L. A., Spectrally separated fluorescence for diesel fuel droplets and vapor, Appl. Opt. 22 (1983) Fansler, T.D., Drake, M.C., Gajdeczko, B., Düwel, I., Koban, W., Zimmermann, F.P., and Schulz, C., Quantitative liquid and vapor distribution measurements in evaporating fuel sprays using laserinduced exciplex fluorescence, Meas. Sci. Technol. 2 (29) Pawlowski, A., Kneer, R., and Parrish, S.E., Multihole injector spray characterization utilizing white light volume illumination and schlieren techniques, 2 th Annual Conference on Liquid Atomization and Spray Systems, Chicago, Illinois, May, Pickett, L.M., Kook, S., Williams, T.C., Visualization of Diesel spray penetration, cool-flame, ignition, high-temperature combustion, and soot formation using high-speed imaging, SAE