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

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

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 NO x Standards for Gasoline-powered* light duty vehicles: g/mile g/mile g/mile * Note: Diesel-powered light-duty vehicles no longer for sale in California.

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! Why are we still studying fossil fuel combustion?

The spherically symmetric combustion of a single liquid droplet in an infinite oxidizing medium can be solved numerically in full detail: 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. r Multi-component liquid fuel droplet Flame What fossil fuel combustion problem can we solve? n - C 7 H 16 O2 O2 C 8 H 18 C 2 H 5 CH 3 H OH

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

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

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) 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).

N-Heptane Droplet Combustion Fuel Consumption Path For typical DCE conditions (He/O 2 )during quasi- steady combustion: C 7 H 16  products (~ 50%) C 7 H 16 + H  products (~ 49%) Decomposition generally dominates over isomerization for n-alkyl radicals.

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

Earth-Based Microgravity Facilities NASA 5 Second Zero Gravity Facility

Earth-Based Microgravity Facilities NASA Lewis 2.2 second drop tower

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

Earth-Based Microgravity Facilities Rowan 1.1 Second Drop Tower Deceleration System 100 ft 3 welded steel cage 22-oz nylon coated polyester airbag (100 ft 3 ) 12-inch polyurethane foam mat Four 6-inch PVC Check Valves 1.5 HP, 127 CFM radial blower

Earth-Based Microgravity Facilities Rowan 1.1 Second Drop Tower

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

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

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 O 2 /N 2 oxidizing environments.

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

OH* Chemiluminescence Numerical Modeling (Marchese, et al., 1996) Numerical Modeling Technique: Calculated OH* Emission [W/cm 3 ]: Incorporate OH* submechanism into gas phase chemical kinetic mechanism.

OH* Chemiluminescence Methanol Flame Results Flame Structure, t = 0.90 sec Methanol/35% O 2 /65% N 2, 1.0 Atm

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

Space Shuttle Experiments Fiber Supported Droplet Combustion Investigation - 1 (FSDC-1) Completed experiment aboard Space Shuttle Columbia flight STS-73, November 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 Heptane in O 2 /He environments Fiber Supported Droplet Combustion Investigation - 2 (FSDC-2) Also flew aboard STS-83 and STS-94

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 Fiber Supported Droplet Combustion FSDC-1

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

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

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

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

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

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.

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

Comparison with FSDC-1 Results Extinction Diameter vs. Initial Diameter Model quantitatively predicts radiative extinction predicted 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.

First isolated space-based droplet combustion experiment: n-heptane in O 2 /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, Entire range of droplet combustion phenomena have been observed: Radiative flame extinction Diffusive flame extinction Complete burn out. Droplet Combustion Experiment DCE

Droplet Combustion Experiment DCE

Space Shuttle Experiments Results

DCE Space Shuttle Experiments Results Complete burnout of droplet (d o = 3.5 mm) Radiative Extinction of flame (d o = 5 mm)

Model Predictions Quasi-steady Burning Rate For n-heptane/air: Model accurately reproduces measured burning rate and variation with initial diameter. For n-heptane/O 2 /He: Model appears to over-predict the burning rate. Gas-phase transport properties?

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…

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

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

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.