Flash Boiling Sprays produced by a 6-hole GDI...
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17 th International Symposium on Application of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 07 – 10, 2014 3.16.1 Flash Boiling Sprays produced by a 6-hole GDI Injector A. Wood 1* , G. Wigley 1 , J. Helie 2 1: Aeronautical and Automotive Engineering, Loughborough University, Loughborough, UK 3: Continental Automotive SAS, Toulouse, France * Correspondent author: [email protected] Keywords: flash boiling, sprays, GDI, interstitial streams, entrainment During certain stages of GDI engine operation fuel will be injected into the cylinder in a superheated state. Upon injection the fuel will then undergo flash boiling which causes significant changes in spray morphology and a reduction in mean drop size. An investigation was undertaken into the spray behavior of a 6-hole, 60 o cone angle GDI injector under superheated conditions. Fuel temperature, T, and chamber pressure, P, conditions were chosen to range from a non-flash boiling condition to a flare flashing condition with intermediate conditions in which the spray was in a partial flashing state. An imaging study showed that as the superheat degree (SD) was increased from a non-superheated case through to a flare flashing case the spray morphology changed. Under non-flashing conditions there were the original six spray plumes from this injector. As superheat was increased these plumes were shown to widen and eventually interact with one another. With sufficient superheat for flare flashing the spray was shown to collapse towards the core region and form interstitial streams in the gaps between the original spray streams with the original spray stream locations empty of fuel, as shown in Figure 1. These spray shapes show a high level of similarity to other published results (Xu 2013). P=0.9 bar T=23 o C (SD=-75) P=0.1 bar T=71 o C (SD=36) Fig. 1 Planar Mie-images at an axial distance, Z, of 40mm from the injector at time, t, of 1100!s after electronic start of injection Further planar Mie-imaging revealed that the interstitial streams were formed in the range 10-40mm for the flare flashing experiments, as in Figure 2, which, according to a flashing model, is the expected region for flashing to occur (Razzaghi 1989). Phase Doppler data showed the important role droplet entrainment has on the spray morphology under flashing conditions. Flash boiling produces a large number of small droplets which have very low momentum levels. A feature of these droplets is that they are readily entrained, hence, under flashing conditions the droplets tend towards regions of low pressure such as the spray core region which has a lower pressure. This collapsing process appears to be aided by the recirculating tip vortices on the leading edge of a spray. The after effect of these vortices is a continued radial velocity component towards the spray core for the remainder of the spray event. Mean velocity data in both the mains and interstitial streams were compared which revealed that the droplets in the mains streams are more likely to be entrained into the spray core than those in the interstitial streams, as shown in Figure 3. This is a possible explanation as to why under flashing conditions fuel exists in the interstitial streams, but not in the mains streams. Z=5mm @t=700!s Z=10mm @t=700!s Z=20mm @t=800!s Z=40mm @t=800!s Fig. 2 Planar Mie-images on different axial planes Fig. 3 Radial velocity profiles for mains and interstitial streams Z=40mm x=7.5mm for P=0.1bar T=71 o C Drop size measurements showed that under flash boiling conditions the large droplets within the spray were all broken down into smaller droplets by the flash boiling process. This led to a significant reduction in mean drop size; a D32 reduction of 42% for a superheat degree of 36 o C. References • Razzaghi M (1989) Droplet size estimation of two-phase flashing jets. Nuclear Engineering and Design, 114:115- 124. • Xu M, Zhang Y, Zeng W, Zhang G, Zhang M (2013) Flash Boiling: Easy and Better Way to Generate Ideal Sprays than the High Injection Pressure. SAE International Journal of Fuels and Lubricants. 6:1:137- 148
Transcript of Flash Boiling Sprays produced by a 6-hole GDI...
17th International Symposium on Application of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 07 – 10, 2014
3.16.1
Flash Boiling Sprays produced by a 6-hole GDI Injector
A. Wood1*, G. Wigley1, J. Helie2
1: Aeronautical and Automotive Engineering, Loughborough University, Loughborough, UK
3: Continental Automotive SAS, Toulouse, France * Correspondent author: [email protected]
Keywords: flash boiling, sprays, GDI, interstitial streams, entrainment
During certain stages of GDI engine operation fuel will be
injected into the cylinder in a superheated state. Upon injection the fuel will then undergo flash boiling which causes significant changes in spray morphology and a reduction in mean drop size.
An investigation was undertaken into the spray behavior of a 6-hole, 60o cone angle GDI injector under superheated conditions. Fuel temperature, T, and chamber pressure, P, conditions were chosen to range from a non-flash boiling condition to a flare flashing condition with intermediate conditions in which the spray was in a partial flashing state. An imaging study showed that as the superheat degree (SD) was increased from a non-superheated case through to a flare flashing case the spray morphology changed. Under non-flashing conditions there were the original six spray plumes from this injector. As superheat was increased these plumes were shown to widen and eventually interact with one another. With sufficient superheat for flare flashing the spray was shown to collapse towards the core region and form interstitial streams in the gaps between the original spray streams with the original spray stream locations empty of fuel, as shown in Figure 1. These spray shapes show a high level of similarity to other published results (Xu 2013).
P=0.9 bar T=23oC (SD=-75 ) P=0.1 bar T=71oC (SD=36)
Fig. 1 Planar Mie-images at an axial distance, Z, of 40mm from the injector at time, t, of 1100!s after electronic start of injection
Further planar Mie-imaging revealed that the interstitial streams were formed in the range 10-40mm for the flare flashing experiments, as in Figure 2, which, according to a flashing model, is the expected region for flashing to occur (Razzaghi 1989).
Phase Doppler data showed the important role droplet entrainment has on the spray morphology under flashing conditions. Flash boiling produces a large number of small droplets which have very low momentum levels. A feature of these droplets is that they are readily entrained, hence, under flashing conditions the droplets tend towards regions of low pressure such as the spray core region which has a lower pressure. This collapsing process appears to be aided by the recirculating tip vortices on the leading edge of a spray. The after effect of these vortices is a continued radial velocity component towards the spray core for the remainder of the spray event. Mean velocity data in both the mains and interstitial streams were compared which revealed that the droplets in the mains streams are more likely to be entrained into the spray core than those in the interstitial streams, as shown in Figure 3. This is a possible explanation as to why under flashing conditions fuel exists in the interstitial streams, but not in the mains streams.
Z=5mm @t=700!s Z=10mm @t=700!s
Z=20mm @t=800!s Z=40mm @t=800!s
Fig. 2 Planar Mie-images on different axial planes
Fig. 3 Radial velocity profiles for mains and interstitial streams Z=40mm x=7.5mm for P=0.1bar T=71oC
Drop size measurements showed that under flash boiling conditions the large droplets within the spray were all broken down into smaller droplets by the flash boiling process. This led to a significant reduction in mean drop size; a D32 reduction of 42% for a superheat degree of 36oC.
References
• Razzaghi M (1989) Droplet size estimation of two-phase
flashing jets. Nuclear Engineering and Design, 114:115-124.
• Xu M, Zhang Y, Zeng W, Zhang G, Zhang M (2013) Flash Boiling: Easy and Better Way to Generate Ideal Sprays than the High Injection Pressure. SAE International Journal of Fuels and Lubricants. 6:1:137-148
