Droplet Size and Volume Fraction in the Near-Injector Centerline Region of Diesel Sprays

1. Droplet Size and Volume Fraction in the Near-Injector Centerline Region of Diesel Sprays Jennifer Labs and Terry Parker Engineering Division Colorado School of Mines Golden, CO Presented At: ILASS-Americas 2003 Monterey, California May 21, 2003

2. Continued Reductions in Allowable Emission Levels Motivate Research • Fuel/Air Mixing Dependent Upon Spray Physics in a Diesel Engine • Further Optimization of Emissions Requires Detailed Knowledge of the Spray • Sparse Data Exists for the Spray Region Near the Injector Tip • Experimental Approach • High pressure and temperature vessel (with infrared optical access) developed to simulate diesel conditions • Optical “probes” have been shifted into the infrared (droplet sizing) • Extinction and scattering techniques are used at “typical” engine conditions to determine spray properties

3. Previous Work • Previous Spray Work • Temporally and spatially resolved droplet size and volume fraction for non-combusting spray • Dodecane, diesel fuel, and methyl oleate • One axial position, multiple radial positions • Temporally and spatially resolved droplet size and volume fraction for evaporating spray • Dodecane, diesel fuel, and 7 various biodiesels • One axial position, multiple radial positions • Error analysis • Other Work (with 9 fuels) • Line-of-sight soot temperature measurements • Post-combustion gas concentration measurements

4. Optical Measurements Made in a Unique Combustion Chamber • Capable of operation up to1000 K and 50 atm • Operated at 873 K and 12.5 atm • Orthogonal optical access via BaF2 windows

5. The Facility Is Used to Simulate Diesel Combustion • Simulator is a cold-wall pressure vessel with a heated air core • System includes central air flow and side arm nitrogen flows • 3-D Translation Capabilities

6. The Fuel Injection System Provides Realistic Diesel Injection Events • Single Shot Pressure Amplifier • Peak injection pressures • Capable: 140 MPa • Operated: 80 MPa • Fuel Injector • Custom Lucas CAV nozzle is drilled with a single hole in the center • (0.16 mm, L/D~4) • Data Acquisition and Timing Control • LabVIEW software controls system timing • Acquire data at 500kHz for 40 ms Trigger chamber Drive chamber Injection chamber To Injector

7. Lasers Nd:YAG (1.06mm) and CO2 (9.27mm) lasers are co-aligned and focused to a 150 mm waist Waist size is experimentally verified Scattering measurements 1.06mm at 90° 9.27mm at 10° Extinction measurements at both wavelengths Beam power monitored to compensate for power fluctuations Droplet Size and Volume Fraction Measurements Use a Pair of Infrared Lasers

8. Scattering Provides the Basis for Droplet Measurements System Parameters Geometry Optical Thickness Correction Size Distribution where: S - detector scattering signal tsys - net transmissivity for optical train G - detector gain Rl - detector responsivity Pl - beam power N - number density of scatters d - detector image width (equation assumes detector image height is greater than beam diameter) W - solid angle from probe volume to collection lens q - angle between beam and scattering collection direction Dd - droplet diameter k - attenuation coefficient I, Io- beam power after and before traversing the spray nDd - probability (log normal distribution) - differential scattering cross section

9. Combusting and atmospheric background conditions Sauter mean diameter (SMD) and volume fraction are reported Modeling indicates that reported diameters are Sauter mean Values reported for dodecane: Axially every 5 mm from 10mm to 45 mm Radially every 0.3 mm from centerline to 4.2 mm Scattering Measurements Provide Spatial Resolution Within the Spray Center Line Axial Position Radial Position

10. Injection Start (0.0 ms) Ignition (~2.2 ms) Ignition Occurs During Injection Event: Combusting Spray • Injection End (3.05 ms)

11. Steady State Axial Position Dependence at Centerline (1.55 ms) • Axial Dependence • Smaller droplets produced by combusting spray • Steeper fall off of liquid volume fraction for combusting spray

12. Developing Spray Radial Dependence at 25 mm from Orifice (0.55 ms) • Radial dependencies during spray development • Similar to axial trends, combusting droplets are smaller • Combusting spray is thinner at centerline • Volume fraction fall off is similar for cold and combusting sprays

13. Steady State Spray Radial Dependence at 25 mm from Orifice (1.55 ms) • Radial dependencies at steady state are similar to developing spray trends

14. Radial Dependence After Spray Shut-off at 25 mm from Orifice (3.25 ms) • Radial dependencies after spray shut-off • Droplet sizes more similar than other cases • Again, combusting spray is thinner at centerline • Volume fraction fall off is similar for cold and combusting sprays

15. Penetration Length and Spray Angle for Combusting Case • Liquid Penetration Length • Predicted* ~32 mm • Measured ~35 mm • Spray Half Angle • Predicted** 2.8-4.3° • Measured ~5.0° *Higgins, B.S., C.J. Mueller, and D.L. Siebers, SAE Paper No. 1999-01-0519. **Wu, K.-J, C.-C. Su, R. L. Steinberger, D. A. Santavicca, and F. V. Bracco, Journal of Fluids Engineering 105:406-413 (1983).

16. Liquid Penetration Length Higgins, et al. correlation not applicable to this case Spray Angle (Half) Predicted** 1.35-2.05° Measured ~3.8° Spray Angle for Cold Case **Wu, K.-J, C.-C. Su, R. L. Steinberger, D. A. Santavicca, and F. V. Bracco, Journal of Fluids Engineering 105:406-413 (1983).

17. Time averaged over 0.1 ms per frame  43 frames Look for: Spray development Steady state spray  high volume fraction area visible Injection shut-off  loss of any structure Combusting Volume Fraction Movie (t = 0  4.3 ms)

18. Time averaged over 0.1 ms per frame  43 frames Look for: Spray development  more obvious here Steady state spray  small droplets near centerline Injection shut-off  loss of any structure Combusting Droplet Diameter Movie (t = 0  4.3 ms)

19. Time averaged over 0.1 ms per frame  43 frames Look for: Spray development Steady state spray  very high volume fraction near injector along centerline Injection shut-off  loss of any structure Occasionally, areas of high volume fractions near periphery ??? Cold Volume Fraction Movie (t = 0  4.3 ms)

20. Time averaged over 0.1 ms per frame  43 frames Look for: Spray development  large initial droplets replaced with smaller ones Steady state spray  lack of structure seen with combusting spray Injection shut-off  larger droplets obvious once again Cold Droplet Diameter Movie (t = 0  4.3 ms)

21. Conclusions From Combusting and Cold Spray Experiments • Three distinct time periods are evident for both cases: development, steady state and shut-off • Consistent with previous work • Axial Dependence • Smaller droplets produced by combusting spray • Steeper fall off of liquid volume fraction for combusting spray • Radial Dependence • During spray development and steady state the droplets formed in the combusting spray are smaller compared to those formed by the cold spray • After injection shut-off droplets are similar in size • For all time periods, the combusting spray is thinner near the centerline and volume fraction fall off is similar for cold and combusting sprays • Globally, there is a disappearance of high liquid volume fraction levels after injection shut-off

22. Future Work: Quantify Multiple Scattering Effects • Illuminate probe volume with vertically polarized light and measure horizontal component of scattered light • Horizontal signal is a result of multiple scattering • Very optically thick systems can produce equal signals in each polarization state • Heavily affected by multiple scattering • Optically thin systems have a very small component in the horizontal polarization state • Basically unaffected by multiple scattering • Optical depth at 1.06 mm is greater than at 9.27 mm • Multiple scattering effects worse at 1.06 mm

23. Acknowledgements • Biodiesel directed studies supported by NREL, Shaine Tyson, Contract Monitor • Facility development supported by a National Science Foundation Career Award, Dr. Farley Fisher, Contract Monitor • Ongoing research support by NSF, Dr. Farley Fisher, Contract Monitor • Graduate student support, GANN award, DOE • Custom drilling of injector nozzle, Raycon Corporation • CSM contributors to the project • Dr. Tom Grover • Eric Jepsen • Dr. Heather McCann • Dr. Jon Filley • Dr. Tony Dean

24. Notes for #17 • Cold spray • Larger Droplets at Edge are Consistent with Work Conducted by Others • Two Possible Reasons For Size Growth Have Been Put Forth by Others • Collisional Effects • Vortex Effects • Hot spray • Diameter growth with radius not dramatic • Implies collisional effects produce radial growth in room ambient spray • Similarity in sizes for room ambient and evaporating spray implies that evaporation acts on all droplets within the distribution (i.e. weak temporal dependence) • Mass loss from the liquid implies that the system may be saturated

25. Typical probe volume 100-150 mm diameter 1-5 mm length Result is ~25 drops per probe volume 4 mm drops 1 mm length 100 mm diameter Phase-Doppler relies on a single, perfectly spherical drop in the probe volume Phase-Doppler Droplet Sizing

26. Scattering measurements agree with extinction measurements on the spray periphery Similar probe volumes Small Drops at Centerline Consistent With Other Work • Edge droplet size results agree generically with K.J. Wu, R.D. Reitz, and F.V. Bracco, 1986 • Cold spray droplet growth also seen by: • C. Yeh, H. Kosaka, and T. Kamimoto, 1995 • O.L. Gulder, G.J. Smallwood, and D.R. Snelling, 1992 • F. Beretta, A. Cavaliere, and A. D’Alessio, 1984

27. Droplets nearly spherical Weber number for these drops ~0.15 Weber number for droplet deformation and break up ~8 Multiple scattering a complication for systems with optical depths greater than 2 and albedos that approach 1 The measured diameters (for room ambient spray) are expected to be relatively unaffected by multiple scattering Evaporating system not affected by multiple scattering Droplet Sphericity and Multiple Scattering Effects

28. Droplet Lifetimes