Methanol Droplet Extinction in Oxygen/Carbon-dioxide/Nitrogen Mixtures in Microgravity: Results from the Internationa

1. Methanol Droplet Extinction in Oxygen/Carbon-dioxide/Nitrogen Mixtures in Microgravity: Results from the International Space Station Experiments Vedha Nayagam1, Daniel L. Dietrich2, Paul Ferkul1, Michael C. Hicks2 and Forman A. Williams3 1National Center for Space Exploration Research 2NASA Glenn Research Center 3University of California, San Diego Eastern States Section of the Combustion Institute, Fall Technical Meeting October 9-12, 2011, University of Connecticut, Storrs, CT

2. Flame Extinguishment Experiments (FLEX) BACKGROUND • FLEX experiments are microgravity droplet combustion experiments, currently underway in the International Space Station (ISS). • Objectives of these experiments are to study fire safety aspects of microgravity . Alcohol (methanol) and alkane (n-heptane) fuel droplets are burned in diluent substituted environments. • Carbon dioxide and helium are used to replace nitrogen. • The Limiting Oxygen Index (LOI) is determined experimentally for different diluents at selected pressures. • Preliminary results for methanol droplets burning in carbon dioxide substituted environments are presented along with simplified theories to explain the observed behavior.

3. FLAME EXTINGUISHMENT EXPERIMENT (FLEX) Investigator Team PI: Prof. Forman A. Williams, UCSD Co-Is: Prof. Mun Y. Choi, UConn Prof. Frederick L. Dryer, Princeton Mr. Michael C. Hicks, NASA GRC Dr. Vedha Nayagam, NCSER/CWRU Prof. Benjamin D. Shaw, UC Davis PS: Dr. Daniel L. Dietrich, NASA GRC Dr. Paul V. Ferkul, NCSER/GRC Ms. Victoria M. Bryg, NCSER/GRC PM: Mr. J. Mark Hickman, NASA GRC

4. FLEX Experiment Operations International Space Station TDRS Satellite POIC, NASA/MSFC Combustion Integrated Rack (Destiny Module of the ISS) Multi-User Droplet Combustion Apparatus (MDCA) TSC, NASA/GRC Cleveland

5. Experiment Videos FLEX-TEST-161: Methanol droplet burning in 16%O2 - 59%N2 - 25%CO2 at 1 atm pressure: d0=2.8 mm Back-lit droplet imaging camera view Color camera view Primary Diagnostics: Droplet-view, color-camera flame view, UV-camera flame view Setup: Partial-pressure mixing on-board from compressed gas bottles (95 liters, free volume) Operations: Remote operations with crew assistance for setup an trouble shooting. Data stored digitally and down liked

6. Data Reduction FLEX-TEST-161: Methanol droplet burning in 16%O2 - 59%N2 - 25%CO2 at 1 atm pressure: d0=2.8 mm igniter retraction UV camera K=d(d2)/dt color camera df flame extinction • Backlit view camera images are used to obtain burning rate constant K, and the color and UV-camera (310 nm) images are used to obtain flame position history. (Cameras run at 30 frames per second)

7. Burning Rate Constants • Burning rate constant K increases (almost linearly) with ambient oxygen concentration, and slightly decreases with increasing carbon dioxide concentration

8. Burning Rate Constants Deviations from quasi-steady values At low oxygen concentrations burning rate deviates substantially from quasi-steady predictions

9. Flame Stand-off Ratio d df *V. Nayagam, Combustion and Flame, 2010

10. Droplet Extinction Diameter Droplet extinction diameter as a function of initial droplet diameter

11. Droplet Extinction Diameter • Zhang and Williams Theory of Methanol Droplet Extinction Induced by Water Evaporation: • Quasi-steady, spherically symmetric combustion with water absorption • Flame-sheet approximation and unity gas-phase Lewis number • Initially, water produced in the flame diffuses back to the droplet surface and gets absorbed • Water concentration gradually builds up and eventually the water begins to vaporize • Diluted mixture leads to a drop in flame temperature • When the adiabatic flame temperature falls below a critical value flame extinguishes This theory leads to a pair of non-linear ODEs for liquid temperature and water concentration as a function of droplet diameter. These ODEs are integrated numerically for the specific atmospheres corresponding to the experimental conditions. *B.L. Zhang, J.M. Card and F.A. Williams, Combustion and Flame, 1996

12. Zhang and Williams’ Theory of Droplet Extinction Text=1800K de(mm)=1.4mm Tl (K) dl(m) d0(mm)=3mm Tf(K) evaporation water flux fraction water absorbed dl(m) dl(m) *B.L. Zhang, J.M. Card and F.A. Williams, Combustion and Flame, 1996

13. Droplet Extinction Diameters

14. Concluding Remarks • Experimental results for methanol droplets burning in carbon dioxide diluted • environments in microgravity are presented • Simplified theoretical models explain many aspects of the observations reasonably well • Methanol droplet extinction is controlled by water absorption for high oxygen concentrations ( > 18%) below a critical initial droplet diameter. • Radiative extinction occurs at low ambient oxygen levels above a critical initial droplet diameter • Transient, ignition driven combustion may occur at low oxygen concentrations (~13-14%) • The limiting oxygen index (LOI) for methanol at atmospheric pressure lies close • to ~13% oxygen

15. QUESTIONS?

16. Concluding Remarks Test FLEX161: t= 8s UV-image Color camera image