Jennifer Labs and Terry Parker Engineering Division Colorado School of Mines Presented at:

1. Multiple Scattering Effects on Infrared Scattering Measurements used to Characterize Droplet Size and Volume Fraction Distributions in Diesel Sprays Jennifer Labs and Terry Parker Engineering Division Colorado School of Mines Presented at: ILASS Americas 17th Annual Conference on Liquid Atomization and Spray Systems Arlington, VA, May 2004

2. Classic measurement methods do not address optically thick sprays • Diesel sprays are transient and optically thick • Diffraction based instruments • Ensemble measurement based on line-of-sight diffraction, collects scattering in the forward direction • Refractive index insensitive, maximum optical depth approximately 0.7 • Ineffective for the dense region of diesel sprays • Single particle measurements • PDPA, LDA, monitor single droplet/particle in probe volume • Diameter range 0.5 to 2.0 mm (extremes, dynamic range typically smaller), can provide velocity and number density (time averaged) • Ineffective for the dense region of diesel sprays

3. Diesel sprays are transient and optically thick The critical issue is how close to the orifice the measurements can successfully be applied Discussion today Diesel simulator Overview of the measurement system Overview of Results The multiple scattering problem Multiple scattering results Conclusions Infrared scattering measurements have beenused to monitor the dense region of diesel sprays Labs, J. and T. E. Parker (2003). "Diesel Fuel Spray Droplet Sizes and Volume Fractions from the Region 25 mm Below the Orifice." Atomization and Sprays 13(1): 45-62. Parker, T., E. Jepsen, et al. (1998). Measurements and Error Analysis of Droplet Size in Optically Thick Diesel Sprays. Twenty-Seventh International Symposium on Combustion, The Combustion Institute. Parker, T. E., L. R. Rainaldi, et al. (1998). "A Comparative Study Of Room Temperature And Combusting Fuel Sprays Near The Injector Tip Using Infrared Laser Diagnostics." Atomization and Sprays 8: 565-600.

4. Optical Measurements Are Made in a Unique Diesel Simulator • Capable of operation up to 1000 K and 50 atm • Simulator is a cold-wall pressure vessel with a heated air core • Operated at 873 K and 12.5 atm • Orthogonal optical access via BaF2 windows • System includes central air flow and side arm nitrogen flows • 3-D translation capabilities • Data rates are 500 kHz in order to capture spray transients

5. The Fuel Injection System Provides Realistic Diesel Injection Events • Single Shot Pressure Amplifier • Peak injection pressure ~150 MPa (22,000 psi) • Standard Fuel Injector • Custom Lucas CAV Nozzle • (D ~ 0.16 mm, L/D ~ 4) Trigger chamber Drive chamber Injection chamber To Injector

6. Laser scattering measurements Nd:Yag (1.06 mm) at 90° Tunable CO2 (9.27 mm) at 11° Scattering detectors calibrated for absolute measurements Beam power monitored to compensate for power fluctuations Lasers are focused to an experimentally verified diameter of 150 mm Measurements utilize focused and co-aligned infrared lasers

7. Infrared lasers replace the more classic visible light sources • To decrease optical thickness effects, the optical “probe” wavelengths have been shifted into the infrared • Lower Attenuation Levels • System must avoid hydrocarbon absorption features near 10.6 mm • Extinction and angular scattering techniques are used to determine droplet sizes and volume fraction as a function of position and time in the near field spray region

8. The governing scattering equation produces as a function of measured signal A ratio of two signals produces the droplet size Number density cancels out for common probe volume Number density is calculated using the diameter from corrected scattering signals (which gives the differential scattering cross section) and the 9.27 mm signal This signal is used because thickness correction is smaller and well known Result is the average volume fraction over the probe volume Scattering signals are proportional to the product of number density and optical cross section

9. Scattering Measurements Provide Spatial Resolution and an Increased Sizing Range • Ratio of Scattering Measurements at Different Wavelengths and Angles Can Be Used to Produce Spatially Resolved Droplet Size Measurements • Modeling Indicates Reported Diameters are Sauter Mean • Insensitive to Distribution Width • Diameter values greater than 16 mm cannot be uniquely sized due to multiple roots

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11. Extinction and scattering probe volumes similar at the spray edge Agreement for results confirm validity of measurements Data plotted are 100 point averages with time zero at the spray onset Results from Extinction and Scatteringfor Droplet Sizes at Spray Edge Agree

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13. Results allow examination of radial and axial trends in sauter mean diameter volume fraction • Axial Dependence • Smaller droplets produced by combusting spray • Steeper fall off of liquid volume fraction for combusting spray

14. Dodecane 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 Sequential spray events allow construction of “movies” of spray properties

15. Multiple scattering will ultimately limit the utility of this measurement technique • Mirrored volume fraction profiles of cold and evaporating sprays Cold spray Possible Multiple Scattering zone Evaporating spray

16. Multiple Scattering Introduces Error Into the Measurement • Multiple scattering is a complication for systems with optical depths greater than 2 and albedos that approach 1 • Multiple scattering produces error by introducing optical signal in a way that is difficult to quantify • Polarized light scattered from a spherical droplet only once will retain the polarization of the incident light • Optically thin systems are basically unaffected by multiple scattering • Multiply scattered light from very optically thick systems will produce randomly polarized light • Equal signals in each polarization state • Multiple scattering “error” can be estimated by monitoring the cross polarization signal • Attenuation and signal loss was already quantified using the optical thickness correction

17. Cross polarization signal for the 1.06 mm laser is used to estimate multiple scattering • Optical depth at 1.06 mm is greater than at 9.27 mm for droplets smaller than 3 mm • Majority of drops on interior of spray where multiple scattering effects will be greatest are on the order of 3 mm • Multiple scattering effects are greatest if it is assumed that the 1.06 mm signal is the only one affected (assume the 9.27 mm measurement is unaffected by multiple scattering) • Droplet sizing based on a ratio of signals

18. Perpendicular and parallel scattering signals used to determine multiple scattering effects • Illuminate probe volume with vertically polarized light and use a cube polarizer • Entire horizontally polarized signal is a result of multiple scattering • Simultaneous measurement of vertical and horizontal components of light scattered from probe volume • Horizontally polarized light  parallel signal S// • Vertically polarized light  perpendicular signal S • Multiple scattering  Sm • Single scattering  Ss

19. By measuring the scattering for the two polarization states, the single scattering signal can be estimated • All of the light scattered with horizontal polarization is multiply scattered • Singly scattered light retains vertical polarization • Because the multiply scattered light is randomly polarized, half of the total signal due to multiple scattering (Sm) is represented by the parallel signal • The total signal scattered in the vertical plane contains all of the signal due to single scattering (Ss) and half of the total signal due to multiple scattering (Sm) • Then light scattered only once can be represented by the equation • As R gets smaller, Ss S and multiple scattering effects can be neglected

20. Calculated Error Due to Multiple Scattering in the 1.06 mm Data • Error due to multiple scattering results in reported droplets that are small • Relative change in volume fraction can be either positive or negative • Competing mechanisms

21. Data from Three Experimental Cases with Calculated Error Bars • Combusting case is expected to be most optically thin due to evaporation rates • Data confirms this hypothesis • Room ambient and combusting cases are least susceptible to errors due to multiple scattering • High pressure case most adversely affected by multiple scattering • Multiple scattering at 9.27 mm will offset some of the computed error 25 mm below orifice

22. Measured Ratio is a Function of Position Within the Spray • High pressure data shows ratio values of approximately 0.5 for both axial locations near centerline • Ratio quickly drops off at an axial distance of 10 mm • Ratio remains relatively high throughout the spray at 25 mm, falling with distance from centerline • Room ambient data exhibits highest ratio near centerline, with both axial positions falling quickly to below 0.2

23. Multiple Scattering Effects Are Not Only a Function of Optical Depth • Multiple scattering effects depend upon the system geometry as well as the optical depth of the system • If optical depth were the only consideration, multiple scattering errors would be greatest in regions of spray where optical depth is greatest Combusting Dodecane

24. Conclusions • Experimental results indicate that for the combusting case, infrared scattering can be applied near to the injector outlet • The combusting case demonstrated negligible susceptibility to multiple scattering for all regions of the spray • The room ambient case was susceptible to multiple scattering effects only near the centerline • High pressure and room ambient cases will require measurement of multiple scattering in both signals • The high-pressure case was most adversely affected by multiple scattering throughout the spray • Results indicate that multiple scattering effects are not only a function of optical depth, but system geometry as well

25. Acknowledgements • Biodiesel directed studies supported by NREL, Dr. Shaine Tyson, Contract Monitor • Facility development and ongoing research supported by National Science Foundation (#CTS-9502481 and (#CTS-0072967), Dr. Farley Fisher, Contract Monitor • Graduate student support, GAANN award, Department of Education • Custom drilling of injector nozzle, Raycon Corporation, Ann Arbor, Michigan • New injection system, Sturman Industries, Woodland Park, Colorado • CSM contributors to the project • Dr. Tom Grover • Dr. Heather McCann • Eric Jepsen