ICLASS-2006 Aug.27-Sept.1, 2006, Kyoto, Japan

Paper ID ICLASS06-112

HIGH-SPEED PHOTOGRAPHY AND PHASE DOPPLER ANEMOMETRY MEASUREMENTS OF FLASH-BOILING MULTI-HOLE INJECTOR SPRAYS FOR

SPRAY-GUIDED GASOLINE DIRECT INJECTION

Dahlander P. 1, Lindgren R. 2 and Denbratt I. 3

1Chalmers University of Technology, dallas@chalmers.se2Chalmers University of Technology, ronny@chalmers.se

3Chalmers University of Technology, denbratt@chalmers.se

1. INTRODUCTION In a spray-guided combustion system for direct-injected gasoline engines, precise control over the spray is of the utmost importance. Normally, the sprays formed from high-pressure multi-hole injectors have very stable behavior. However, flash-boiling can dramatically change the fuel distribution and formation of these sprays. Flashing phenomena can occur when fuel is injected and its temperature is so high that it becomes superheated at the pressure it expands into. Due to thermal inertia, the temperature of the fuel remains constant as it expands through the nozzle, and is higher than its saturation temperature at the new pressure. The liquid is then in a non-equilibrium state. The process of expansion through an injector is too fast for the heat to be conducted through surface vaporization. Instead vaporization starts, rapidly and explosively, inside the liquid through bubble growth, leading to an explosive break-up of the liquid fuel immediately after it leaves the nozzle holes of the injector. In other words, the atomization is improved and it changes the formation of the spray. This is referred to as flash-boiling [1]. Due to the extreme break-up, many small droplets are formed, which take up a greater volume than fewer larger droplets would, thus increasing the spray angle close to the nozzle. If the (ambient) pressure of the chamber in which the fuel expands is decreased, the boiling point of the fuel is also decreased. Thus, it seems reasonable to believe that lowering the ambient pressure should increase the degree of flash-boiling since the difference between the fuel

ABSTRACT Flash-boiling occurs when fuel at high temperature expands through an injector into a below atmospheric ambient pressure. This paper describes experiments investigating the effects of fuel type, fuel temperature and ambient pressure on flash-boiling and associated changes in the formation and droplet sizes of sprays from a multi-hole gasoline injector. The experiments were carried out in a constant pressure spray chamber in which the pressure could be reduced to sub-atmospheric levels, and the fuel temperature could be controlled. To visualize the liquid fuel sprays, a high-speed camera was used. Two different types of fuel were tested: a single component fuel and a multi-component fuel. The results show that the fuel distribution changes dramatically when the spray flash-boils. The critical parameter is the fuel temperature, but lowering the ambient pressure also increases the fuels’ tendency to flash-boil. Some of the components of the multi-component fuel have lower boiling points than the single component of the other fuel. Consequently, it flash-boiled at lower temperatures. The results from Phase Doppler Anemometry measurements show that when the spray flash boils the diameters of the droplets are much smaller than those of non-flash boiling sprays. Keywords: Multi-hole, Gasoline Direct Injection, Flash-boiling, High-Speed Photography, Phase Doppler Anemometry, Fuel temperature.

temperature and the saturation temperature is increased. In an engine, the fuel can reach high temperatures before the injection due to heat transfer from the cylinder head and the injector, especially at idle when the fuel mass flow is low. If the throttle is almost closed, which causes low cylinder pressures, flash-boiling can occur, accompanied with changes in spray formation and (consequently) the air/fuel mixing. A previous investigation of flash-boiling of fuel injected into an engine by pressure swirl injectors found that: (1) the initial spray angle increases and initial droplet size decreases with increasing fuel temperature and decreasing cylinder pressure, and (2) the radial spray penetration downstream decreases and (3) rapid vaporization occurs immediately after the fuel leaves the injector [2]. 2. SPECIFIC OBJECTIVES In the study presented here, flash-boiling phenomena associated with a multi-hole gasoline injector intended for use in a spray-guided combustion system were investigated. The experiments were carried out in a constant spray chamber using a high-speed video camera and a Phase Doppler Anemometry system. Two different fuels were used: a single-component research fuel (iso-octane) and a multi-component research fuel. The objective was to investigate how liquid fuel distribution and drop sizes change with variations in the fuel temperature, ambient pressure and type of fuel (single/multi-component) under conditions where flash-boiling can occur.

3. EXPERIMENTAL SETUP The sprays were photographed with a high-speed camera in a non-heated constant pressure spray chamber. For studies of flash-boiling sprays it is of utmost importance to be able to control the fuel temperature. Therefore, an injector mounting case with high heat conductivity was manufactured using copper. In this mounting case, an electrical heater and a thermo-couple were mounted and connected to a temperature controller. The fuel pipe was heated and thermostatically controlled. The temperature of the injector tip was also monitored manually before photographing each spray. Since the injector was heated for a long time (ca 1 h) at each measured temperature and there were very few injections (so fuel mass was practically zero), it seems reasonable to assume that the fuel temperature was the same as, or very close, to the injector temperature. A vacuum pump was used to reduce the pressures in the spray chamber to sub-atmospheric levels. A high-speed camera (Phantom v7.1) operating at a frame rate of 24,000 images/s was used to capture the images. The resolution at this frame rate was 320 x 240 pixels and the exposure time 20 microseconds. The experimental setup allowed the injector to be mounted either at the side or the top of the spray chamber. When the injector was mounted at the side, the sprays were directly illuminated and when the injector was mounted at the top, back-lightning was used. A 2D TSI/Aerometrics Phase Doppler Particle Anemometry (PDPA)-system with a TSI FSA-4000 processor was used for the PDA measurements to capture the droplet sizes and velocities of the spray in the spray chamber. The light source used was a Coherent Innova 90 Argon-Ion laser. The data were captured with TSI FlowSizer v1.1 software. A programmable traverse system was used to move the laser transmitter and the receiver around the different measuring points. The measurement data were collected using an off-axis angle of 65 degrees in order to minimize the effects of variations in the spray’s refractive index caused by changes in droplet liquid temperature due to vaporization [3, 4, 5]. The fuel used for all of the PDA measurements was iso-octane. For these measurements, the same accuracy in the fuel temperature as that of the high-speed photographs cannot be guaranteed since many injections are needed to capture the samples and this may reduce the heat transfer rate from the injector to the fuel slightly. A fuel pressure of 10 MPa was used for all measurements. The experimental setup is shown in Figure 1.

Figure 1. Experimental setup, showing the spray chamber, dual lightning and high-speed camera. The

same setup was also used for the Phase Doppler measurements.

4. RESULTS AND DISCUSSION The measurement matrix for the high-speed camera measurements is shown in Table 1. Table 1. Measurement matrix for the high-speed camera

measurements Fuel Fuel

pressure [MPa]

Fuel temperature

[K]

Ambient pressure

[kPa] Single-component (Iso-octane)

10 293, 325, 372

30, 55, 80

Multi-component

10 293, 325, 372

30, 55, 80

The fuel temperatures and ambient pressures were chosen to be relevant to conditions in real engines. For example the highest fuel temperature, 372 K, is in the range of the cooling water temperature and under highly throttled conditions, cylinder pressures can be as low as ca 40 kPa. 4.1 High-speed photography The sprays were photographed from two different positions for each case: from the side and from below (looking towards the spray). The two views of the spray could not be photographed simultaneously, so the images from the two positions are of different sprays. However, the cycle-to-cycle variations between different sprays were small. When the sprays are directly illuminated, the camera captures the scattered light from the fuel droplets, and when the spray is back-lighted it captures the liquid fuel’s shadows. Thus, in both cases the high-speed photographs provide images of the liquid fuel distribution of the sprays. Of course it is difficult to judge the degree of flash-boiling, but to illustrate its range, Figures 2 to 4 show sprays with no flash-boiling, medium flash-boiling and strong flash-boiling. The non-flash boiling spray can be seen in Figure 2.

Figure 2. Non flash boiling spray, obtained with cold fuel/high ambient pressure. The spray plumes from each

hole of the multi-hole nozzle can be clearly discerned. (Left image side view and right image view from below.)

Ambient pressure/Fuel temperature 80kPa/293 K. The spray plumes can be clearly discerned and it is easy to see how the holes on the nozzle are orientated in the view from below. When the fuel temperature is increased and the ambient pressure is reduced the spray starts to flash-boil, see figure 3.

Figure 3. Medium flash-boiling spray, obtained with higher fuel temperature/lower ambient pressure. The

spray formation is blurred, but it is still possible to discern the nozzle hole configuration. Ambient

pressure/Fuel temperature 50kPa/325 K. The spray formation is blurred, but it is still possible to see the hole configuration. In figure 4 the fuel temperature has been further increased, the ambient pressure has been further lowered, and a dramatic change in the fuel distribution can be observed.

Figure 4. Strong flash-boiling spray, obtained with high fuel temperature/very low ambient pressure. The spray

formation is now so blurred that it is impossible to discern the nozzle hole configuration. Ambient

pressure/Fuel temperature 30kPa/372 K. Pairs of high-speed photographs, like those in Figures 2 to 4, showing the effects of all the studied parameters (fuel type, fuel temperature and ambient pressure) are presented in Figures A1 and A2 in the Appendix. Three

matrices of images are presented for each fuel type, representing events at three different times during the formation of the sprays: shortly after the start of the injection (0.13 ms), in the middle of the injection (0.33 ms) and close to the end of the injection (1.38 ms). Within each matrix of paired images the fuel temperature increases from left to right and the ambient pressure increases from the bottom up. In Figure A1 in the Appendix, the sprays obtained with iso-octane are shown. Iso-octane is a single-component fuel which has a boiling point of 372 K at atmospheric pressure. It can be seen that the tendency to flash boil is mostly dependent on the fuel temperature. The influence of the ambient pressure is secondary; the tendency to flash boil increases only when the ambient pressure is very low. This can be seen in the images taken from below the spray. At the highest temperatures, 372 K, strong flash boiling can be observed even for the highest ambient pressure, 80 kPa, which is predictable since the boiling point is then lower than 372 K. From the side views at high temperatures a reduction in the spray’s umbrella angle can be observed when the spray flash boils. This is equivalent to a reduction in the radial spray penetration as observed in [6]. The shape of the spray looks almost like a hollow cone, as seen in sprays from swirling injectors. Vortices are being created at the edge of the spray, which likely change the air entrainment and mixing. The vortices can be seen clearly in the middle of the spray at 0.33 ms. All timings, here and throughout the paper, refer to time after the start of injection. Due to strong flash-boiling at the highest fuel temperature later in the spray event, at 1.38 ms, it is not possible to see how the holes of the injector nozzle are orientated. Figure A2 in the Appendix shows the sprays obtained with the multi-component fuel. Similar conclusions can be drawn as those for the single-component fuel. However, flash-boiling of the multi-component fuel starts to occur at lower fuel temperatures since the lighter components of the fuel have lower boiling points than that of iso-octane. The boiling characteristics for the multi-component fuel are shown in Table 2. Table 2. Boiling characteristics of the multi-component

fuel at atmospheric pressure. Initial Boiling Point (IBP)

10 % evaporated

50 % evaporated

90 % evaporated

Final Boiling Point (FBP)

298.5 K 312 K 346.5 K 431 K 468 K As can be seen in Table 2, the boiling points range between 298.5 K for the lowest/lightest component up to 468 K for the highest/heaviest component, at atmospheric pressure. Decreasing the ambient pressure reduces the boiling points of all the components. Thus, due to the low boiling point of the lightest component of the multi-component fuel, flash-boiling sprays can be expected even for non-heated fuel (293 K), if the ambient pressure is sufficiently low. In the cases illustrated here, flash-boiling tendencies with unheated fuel can be observed in the image obtained with the lowest pressure investigated, 30 kPa, see Figure A2 at 0.13 ms. Hence, the multi-component fuel is much more sensitive to

flash-boiling than the single-component fuel, so flash-boiling can be observed at quite modest fuel temperatures. One observation that is valid for both the single-component and multi-component fuels is that when the sprays flash-boil the different plumes seem to merge into new plumes. The spray plumes were numbered as shown in Figure 5.

Figure 5. Numbering of the spray plumes.

Thus, spray plumes 1 and 2, 2 and 3, 3 and 4, 4 and 5 and 5 and 6 merge to form new plumes. Therefore there appear to be five different spray plumes instead of six. Very clear examples of this can be seen in Figure A2, which shows, from below, the plumes obtained with a fuel temperature of 325 K and ambient pressure of 30 kPa, at 0.33 ms. A close-up of one of these images is shown in Figure 6.

Figure 6. An example of how the spray plumes merge into new spray patterns when the spray flash boils. Multi-component fuel, 0.33 ms after the start of the

spray. Thus, due to the merging of the spray plumes, the spray pattern from the 6-hole nozzle looks more like that of a 5-hole nozzle. The physical explanation for the merging of the spray plumes is that the spray plume angles increase with increased flash-boiling [7, 2], and when the spray plume angle increases sufficiently, the spray plumes will collide. When using a horse-shoe shaped nozzle hole configuration, a suitable location for a spark plug in the absence of flash-boiling could be somewhere in the plane between spray plumes 3 and 4, a position that allows favorable air entrainment and is seldom hit by liquid fuel. However, when the spray flash boils, as shown in this paper, the new spray plume formed by the merger of spray plumes 3 and 4 can cause the air/fuel ratio around the spark plug to change, which may lead to engine misfires. The main parameters investigated were the fuel temperature and the ambient pressure. However, experiments were also carried out to investigate the

influence of the fuel pressure on the fuel’s tendency to flash-boil. A fuel temperature/ambient pressure point representing medium flash-boiling was selected for these experiments, and the results are shown in Figure 7.

Figure 7. Influence of the fuel’s pressure on its tendency to flash-boil. Views from below at 0.13 ms, 0.33 ms and

1.38 ms. Fuel pressures from the left: 5, 7.5, 10, 12.5 and 15 MPa. The fuel was iso-octane at a temperature of 348

K, and ambient pressure of 55 kPa. Again, it is difficult to judge the degree of flash-boiling simply by looking at the images. However, the degree of flash-boiling appears to decrease slightly as the fuel pressure increases. The droplets are superheated to the same temperature and it is therefore surprising that the degree of flash-boiling changes. A possible explanation for this is that the mass flow rate may influence the thermodynamic time-scale it takes for the droplets to reach thermodynamic equilibrium since a greater mass of fuel has to flash-boil in the same time frame as the fuel pressure increases. 4.2 Phase Doppler Anemometry Measurements To characterize the sprays at the microscopic level, i.e. to quantify the droplet sizes and droplet velocities, Phase Doppler Anemometry (PDA) measurements were carried out in addition to the high-speed photographs, at four fuel temperature/ambient pressures points from the measurement matrix presented in Table 1. The measurement matrix for the PDA measurements is shown in Table 3. The sprays in the selected high and low fuel temperature cases (372 and 293 K, respectively) can be regarded as flash-boiling and non-flash boiling sprays, respectively. PDA data were captured at 5-7 different radial positions through the spray core.

Table 3. Measurement matrix for the PDA measurements.

Fuel Fuel pressure [MPa]

Fuel temp. [K]

Ambient pressure [kPa]

Number of measuring points

Iso-octane 10 293, 372 30, 80 5-7

When the spray flash boils, the spray core cannot be clearly defined visually. Therefore, the measuring points for the flash-boiling sprays were chosen at the same downstream position, but through a line along which the axial velocity of the droplets was maximal (a reasonable way of defining the spray core). This line does not necessarily include exactly the same measuring points as in non-flash boiling

cases, but they instead represent the spray core of the merged plumes, as described above. The results from the PDA measurements are shown in Figures 8 to 11. Figure 8 shows the probability density functions (PDFs) of the droplet diameter samples in the spray cores.

0 10 20 30 40 500

5

10

15

20

25

30

Drop diameter [μm]

Pro

babi

lity

[%]

30 kPa, 293 K80 kPa, 293 K30 kPa, 372 K80 kPa, 372 K

Figure 8. Diameter probability density functions (PDFs) in the spray core for flash-boiling sprays (372 K) and

non-flash-boiling sprays (293 K). Data measured 40 mm downstream of the nozzle. Bin size = 2 microns.

The diameter PDFs of the two high fuel temperature cases were very similar, as were those of the two low fuel temperature cases. Thus, the fuel temperature almost exclusively governs the diameter PDFs of the droplets. The ambient pressure only has a small influence on the PDFs. In the high fuel temperature cases (372 K) the PDFs are shifted towards much smaller droplets, while the low temperature sprays show a much larger number of large, slowly vaporizing droplets. Another way to show the differences in droplet diameters is to calculate a mean value for each sample. Figure 9 shows the Sauter Mean Diameters, SMDs, of the diameter samples at each of the measured positions.

18 22 26 30 34 380

10

20

30

40

50

Radius [mm]

SM

D [ μ

m]

30 kPa, 293 K80 kPa, 293 K30 kPa, 372 K80 kPa, 372 K

Figure 9. Sauter mean diameters (SMD). Data measured 40 mm downstream of the nozzle.

As can be seen, the previous conclusion is not valid solely for the spray core, but for all radii. SMDs for the low temperature cases were about 23 microns, compared to about 12 microns when the fuel temperature was increased. The reason for the improved atomization is that the flashing phenomenon causes an explosive and very rapid break-up into smaller droplets, and the large reduction in drop size leads to faster vaporization. The droplet axial velocities were also sampled and the mean values obtained are shown in Figure 10.

18 22 26 30 34 380

10

20

30

40

50

60

70

80

90

Radius [mm]

Axia

l vel

ocity

[m/s

]

30 kPa, 293 K80 kPa, 293 K30 kPa, 372 K80 kPa, 372 K

Figure 10. Mean droplet axial velocities measured 40 mm downstream of the nozzle.

The mean axial droplet velocity is highly dependent on the ambient density, explaining why higher velocities can be seen at 30 kPa than at 80 kPa at the same temperature. For the lowest ambient pressure/highest fuel temperature (30 kPa, 372 K), corresponding to the case with the highest degree of flash-boiling, see Figure 10, it can be seen that the radial position of the spray core shifted almost 10 mm towards the nozzle center compared to the other cases. The same conclusion can be drawn from the means of the radial velocities (Figure 11), which decreased when the fuel temperature was increased, implying that the spray’s umbrella angle decreased, see Figure 10.

18 22 26 30 34 380

10

20

30

40

50

60

70

Radius [mm]

Rad

ial v

eloc

ity [m

/s]

30 kPa, 293 K80 kPa, 293 K30 kPa, 372 K80 kPa, 372 K

Figure 11. Mean droplet radial velocities measured 40 mm downstream of the nozzle.

5. CONCLUSIONS AND DISCUSSION When a spray flash-boils, explosive break-up occurs immediately beyond the nozzle holes of the injector. This phenomenon clearly has a dramatic effect on the fuel distribution and the droplet sizes. In fact, the changes in the fuel distribution can be so dramatic that it is impossible to discern the otherwise stable and visually clear spray plumes. The driving force for flash-boiling is the temperature difference between the liquid fuel as it exits the nozzle and the saturation temperature (boiling-point) of the fuel at the pressure of the chamber it expands into. Thus, the degree of flash-boiling is increased if either the fuel temperature is increased or the ambient pressure is lowered. Lowering the pressures in the chamber the fuel expands into reduces the boiling point(s) of the fuel. For the fuel temperature and ambient pressure intervals studied in this paper, the fuel temperature has the largest influence on the tendency to flash-boil, but lowering the ambient pressure also slightly amplifies the flash-boiling tendency. There is no clear temperature threshold at which the sprays start to flash-boil. However, the temperature range where the sprays start to flash-boil is clearly dependent on the boiling point of the fuel. For the single-component fuel iso-octane, flash-boiling starts when the fuel temperature is close to the boiling point. For temperatures below the boiling point, the iso-octane sprays show little tendency to flash-boil, which is consistent with expectations based on fundamental physics. However, the multi-component fuel starts to flash-boil at a much lower temperature, given by the boiling point(s) of its lightest/lighter component(s), which is 298.5 K at atmospheric pressure, and therefore even lower at sub-atmospheric pressures. An unanswered question is whether flash-boiling of fuels from this injector will affect the mass injected, i.e. if the mass injected will differ from the mass injected in the absence of flash-boiling when the injection duration is the same. If so, engine control systems would have to take such differences into account to ensure that constant masses exit the nozzle regardless of whether or not the spray flash-boils. At the microscopic level for iso-octane, flash-boiling improves the atomization, yielding much smaller droplets due to the explosive and very rapid break-up of the spray. SMD values for heated (372 K) and non-heated fuel are around 13 and 23 microns, respectively. The droplets’ diameter distribution is almost completely governed by the fuel temperature. When seeking a suitable nozzle hole configuration, and an ideal position for a spark plug, for a multi-hole nozzle to be used in spray-guided, gasoline, direct-injection combustion systems, the spray pattern when the spray flash boils must be studied since the spray patterns can be changed so dramatically with variations in cylinder pressure and (even more so) fuel temperatures. During homogeneous operation in an engine, it is important to obtain an even fuel distribution in the cylinder. However, as shown in this paper, the fuel distribution, droplet size and droplet velocities change when the sprays flash-boil, so air entrainment and mixing parameters

probably change too. A flash-boiling spray could be more sensitive to the in-cylinder motions required for proper cylinder mixing. It is therefore of interest to study the fuel distribution from a multi-hole injector in a real engine under flash-boiling conditions. Under stratified conditions (late injection during the compression stroke) the spray is unlikely to flash-boil since the ambient pressure (cylinder pressure) is then much higher. ACKNOWLEDGEMENTS The authors would like to acknowledge CERC (Combustion Engine Research Center) for financial support, GM R&D for supplying the injector, Mikael Skogsberg for interesting discussions and the laboratory personnel, Mr. Torbjörn Sima and Dr. Bo Pettersson, for their assistance. REFERENCES

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Figure A 1. Single-component fuel, iso-octane. High-speed photographs of the sprays obtained at three different fuel temperatures and three different ambient pressures. The photographs are from three different timings, 0.13 ms, 0.33 ms, 1.38 ms. In each pair of images, those to the left were taken from the side and those to the right from below the sprays.

Figure A 2. Multi-component fuel. High-speed photographs of the sprays obtained at three different fuel temperatures and three different ambient pressures. The photographs are from three different timings, 0.13 ms, 0.33 ms, 1.38 ms. In

each pair of images, those to the left were taken from the side and those to the right from below the sprays.