Advanced Combustion Theory and Modeling April 8, 2011

1. Advanced Combustion Theory and Modeling April 8, 2011 Microgravity Droplet Combustion: Space-Based Experiments and Detailed Numerical Modeling Anthony J. Marchese, Ph.D. Associate Professor Dept. of Mechanical Engineering Colorado State University

2. Overview Microgravity Combustion and Heat Transfer • Why study liquid fuel combustion? • Why study liquid fuel combustion in microgravity? • Spherically symmetric, time-dependent numerical modeling • Ground based microgravity experiments • 2.2 second drop tower • 5 second Zero Gravity Facility • Space based microgravity experiments • FSDC-1, DCE and FSDC-2 • Continuing Research • Microgravity flame spread through layered gas mixtures • Microgravity boiling heat transfer

3. Why are we still studying fossil fuel combustion? • Reliability on fossil fuels continues... • 85% of all energy consumed in the U.S. is derived from the combustion of fossil fuels. • 39% of all energy consumed in the U.S. is derived from the combustion of liquid fossil fuels. • 97% of all energy consumed in the transportation sector is derived from the combustion of liquid fossil fuels. • Meanwhile, emissions standards continue to tighten… • California NOx Standards for Gasoline-powered* • light duty vehicles: • 1971 4.0 g/mile • 1993 0.4 g/mile • 2003 0.2 g/mile • * Note: Diesel-powered light-duty vehicles no longer for sale in California.

4. Why are we still studying fossil fuel combustion? Developing accurate models of the combustion process is the key to designing more efficient, cleaner burning engines... • The physical phenomena occurring in an internal combustion engine includes: • vaporization, • mass transfer, • heat transfer, • turbulent fluid mechanics, and • complex chemical kinetics (~ 100 species). • The problem is three dimensional and time-dependent…and impossible to solve with even the most powerful computers!

5. What fossil fuel combustion problem can we solve? The spherically symmetric combustion of a single liquid droplet in an infinite oxidizing medium can be solved numerically in full detail: r Multi-component liquid fuel droplet Flame n-C7H16 C2H5 CH3 C8H18 H OH O2 • Real vapor/liquid equilibrium, • multi-component gas-phase transport, • liquid-phase heat and mass transport, • radiative heat transfer, • detailed gas-phase chemistry (~100 species), • time-dependent.

6. Spherically Symmetric Droplet Combustion By creating and igniting a single liquid droplet in microgravity it is possible to achieve spherically symmetric combustion ... r 1 g 10-6 g ...of droplets large enough to permit accurate photographic analysis. The experimental results are compared directly with detailed numerical modeling.

7. Time-Dependent, Spherically Symmetric, Bi-component Droplet Combustion Model (Cho, et al., 1992; Marchese and Dryer, 1996) • Gas Phase: • Multicomponent molecular diffusion • Complex chemical kinetics • (e.g. 50 species, 250 reactions) • Non-luminous thermal radiation • UV flame emission Mass Conservation: Species Equations: Energy Conservation: • Droplet Surface: • Surface regression • Evaporation of fuel • Condensation of products • Radiative heat addition Net Radiative Heat Flux Droplet Interior:

8. Gas Phase Chemical Kinetic Mechanism N-Alkane Droplet Combustion • Goals: • Generate test matrix, and analyze results of DCE n-heptane experiments using detailed, transient numerical model. • Existing Chemical Kinetic Mechanisms: • Too large: • Chakir (1992) - 72 species • Lindstedt (1995) - 109 species • or too empirical: • Warnatz (1984) - 32 species,96 reactions • for detailed, time-dependent, one-dimensional diffusion flame modeling. • Result • A new compact semi-empirical n-heptane mechanism* has been developed that includes: • Fuel thermal decomposition • Site-specific H-atom abstraction • 37 species, 241 reactions *Held, Marchese and Dryer (1997).

9. N-Heptane Droplet Combustion Fuel Consumption Path • For typical DCE conditions (He/O2)during quasi-steady combustion: • C7H16 products (~ 50%) • C7H16+ H  products(~ 49%) • Decomposition generally dominates over isomerization for n-alkyl radicals.

10. How do we perform experiments in microgravity? Drop Towers Parabolic Flight Aircraft “The Vomit Comet” Orbiting Spacecraft

11. Earth-Based Microgravity Facilities NASA 5 Second Zero Gravity Facility

12. Earth-Based Microgravity Facilities NASA Lewis 2.2 second drop tower

13. 2.2 Second Drop Tower Experiments Experimental Apparatus User Interface Microprocessor Video Cameras Optical Access Ports High Speed Camera Back Light Test Chamber Power Supplies Oxidizer/Inert Inlet Ports

14. Earth-Based Microgravity Facilities Rowan 1.1 Second Drop Tower • Deceleration System • 100 ft3 welded steel cage • 22-oz nylon coated polyester airbag (100 ft3) • 12-inch polyurethane foam mat • Four 6-inch PVC Check Valves • 1.5 HP, 127 CFM radial blower

15. Earth-Based Microgravity Facilities Rowan 1.1 Second Drop Tower

16. 2.2 Second Data Analysis System • Back-lit, high-speed movie camera • Video “set-up” camera • Xybion ISG-250 CCD video camera • - UV Transmissive Lens • - Narrow band interference filter centered • at 310 nm; full-width, half-max = 10nm • - Data acquired at 30 fps

17. Drop Tower Experiments Methanol Droplet Combustion Visual Video Image Ultraviolet Flame Image Spherically Symmetric

18. Diameter-Squared History Pure Methanol Droplets For 1 mm droplets, the numerical model accurately reproduces the measured burning rate for pure methanol droplets in various O2/N2 oxidizing environments.

19. OH* Chemiluminescence Data Analysis Relationship Between Measured Signal and Actual OH* Emission Intensity P( r): Line of sight integral projection as measured by the Xybion Camera Recover actual OH* intensity field, F( r), using the Inverse Abel Transform (Dasch, 1992):

20. OH* Chemiluminescence Numerical Modeling (Marchese, et al., 1996) Numerical Modeling Technique: • Incorporate OH* submechanism into gas phase • chemical kinetic mechanism. • Calculated OH* Emission [W/cm3]:

21. OH* Chemiluminescence Methanol Flame Results Flame Structure, t = 0.90 sec Methanol/35% O2/65% N2, 1.0 Atm

22. Instantaneous Flame Position Pure Methanol Droplets Predicted location of maximum OH* emission agrees with experiment to within 1 normalized radii

23. Space Shuttle Experiments • Fiber Supported Droplet Combustion • Investigation - 1 (FSDC-1) • Completed experiment aboard Space Shuttle • Columbia flight STS-73, November 1995. • Droplet diameters: 3 to 5 mm • Fuels: • Methanol • Methanol/Water • Heptane • Heptane/Hexadecane • Droplet Combustion Experiment (DCE) • Isolated droplet experiments, • up to 5 mm • First flew aboard Columbia • flight STS-83 and STS- 94 in • April and July 1997. • Heptane in O2/He environments • Fiber Supported Droplet Combustion • Investigation - 2 (FSDC-2) • Also flew aboard STS-83 and STS-94

24. Fiber Supported Droplet Combustion FSDC-1 • First ever space-based droplet combustion experiment: • Single and multicomponent droplets • 2 to 5 mm initial diameter • suspended on silicon carbide fiber. • Conducted aboard Space Shuttle Columbia as part of the Second United States Microgravity Laboratory (USML-2), October 1995.

25. FSDC-1 Results Pure Methanol Droplets (Dietrich, et al., 1996) • Measured burning rate decreases with increasing initial diameter. • Neglecting radiation, numerical modeling does not reproduce this phenomenon.

26. FSDC-1 Results Pure Methanol Droplets (Dietrich, et al., 1996) • Neglecting radiation, the numerical modeling predicts alinear increase in extinction diameter with increasing initial diameter. • Modeling under-predictsextinction diameter measurements. • Measured extinction diameter appears to increase non-linearly with increasing initial diameter.

27. The Effect of Radiative Heat Loss in Microgravity Droplet Combustion At increased initial droplet diameters, gas phase radiative heat loss can no longer be ignored! In droplet combustion, the vaporization rate is limited by the rates of diffusion of heat and mass, resulting in: Thus, the mass burning rate and overall instantaneous heat release rate in the flame is directly proportional to the droplet radius: Meanwhile, the radiative heat loss varies as the radius cubed:

28. Non-Luminous Gas Phase Radiation Model Results Calculated gas phase species and temperature for 1, 3, and 5 mm methanol droplets at t = 0.4

29. FSDC-1 Results Comparison with Radiation Model * Diameter-squared vs. time for 1, 3, and 5 mm droplets in air. * Marchese and Dryer, 1997

30. Comparison with FSDC-1 Experiments The Effect of Initial Water Addition Diameter-squared vs. time for methanol/water mixtures with 0, 10 and 20% initial water content.

31. Comparison with FSDC-1 Experiments The Effect of Initial Water Addition Instantaneous burning rate for methanol/water mixtures with 0, 10 and 20% initial water content.

32. Comparison with FSDC-1 Results Extinction Diameter vs. Initial Diameter • Model quantitatively predicts radiative extinctionpredicted asymptotically by Chao, et al. (1990). • For methanol in air, flames surrounding droplets greater than about 6 mm rapidly self-extinguish. • Results may have potential impact on spacecraft fire safety.

33. Droplet Combustion Experiment DCE • First isolated space-based droplet combustion experiment: • n-heptane in O2/He mixtures • 2 to 5 mm initial diameter • no suspension fiber. • Conducted aboard Space Shuttle Columbia as part of the Microgravity Science Laboratory (MSL-1), April and July, 1997. • Entire range of droplet combustion phenomena have been observed: • Radiative flame extinction • Diffusive flame extinction • Complete burn out.

34. Droplet Combustion Experiment DCE

35. Space Shuttle Experiments Results

36. DCE Space Shuttle Experiments Results Complete burnout of droplet (do = 3.5 mm) Radiative Extinction of flame (do = 5 mm)

37. Model Predictions Quasi-steady Burning Rate • For n-heptane/air: • Model accurately reproduces measured burning rate and variation with initial diameter. • For n-heptane/O2/He: • Model appears to over-predict the burning rate. • Gas-phase transport properties?

38. Ongoing Work Combustion of Mars-Based Metallized Rocket Propellants The combination of spherically symmetric combustion modeling and microgravity experiments can be applied to a host of problems, such as…

39. Ongoing Work Microgravity Boiling Heat Transfer T < 300ºF As computers become faster, they generate more heat. Is it possible to use boiling heat transfer to cool computer chips in space-based applications? Experimental Apparatus

40. Ongoing Work Microgravity Boiling Heat Transfer Rowan Students Conducting Experiment on NASA KC-135

41. Summary and Conclusions • Experimental techniques have been developed to generate spherically symmetric combustion of large droplets. • Data analysis techniques have been developed to accurately determine burning rates and flame position. • Numerical model accurately reproduces measured burning rates and flame position for 1 mm size droplets neglecting radiation. • For larger droplets, gas phase radiation loss can not be neglected. • Radiation model predicts that methanol droplets of > 6 mm will radiatively extinguish. • Result has now been verified in FSDC-2 and DCE. • Potential significance for spacecraft fire safety issues. • Transport, chemistry, vapor/liquid equilibrium and radiation (non-luminous and UV emission)sub-models are applicable to more detailed flow situations. • Ongoing work in microgravity heat transfer and combustion in support of future manned space activities.