Fuel Tank Protection Research at NASA GRC Clarence T. Chang NASA Glenn Research Center Cleveland, Ohio 44135 USA INTERNATIONAL AIRCRAFT SYSTEMS FIRE PROTECTION WORKING GROUP MEETING November 1-2, 2005 Atlantic City, New Jersey USA

Adaptive OBIGGS Inert gas generation Ullage ignition model O2 sensor flammability sensors O2 sensor Corrective responses Ignitable fuel tank ullage mixture Detonation transition Ignition sources Fuel tank constraints The Problem CTC 9-15-2005 Outside air No ignition detonation Tank over- pressure Limited Mostly FATAL May be survivable Survivable Damage No Problem Always The Outcomes The Inputs Fuel What does it take to ignite a air mixture? What does it take to sustain an ignition into a significant over-pressure event? What does it take to cause a detonation? An aircraft’s fuel tanks are open to the outside air. Fresh air enters and leaves the tank as the outside air pressure changes. The fuel vapor mixes with the air inside the ullage and can become combustible under certain conditions When an ignition source is introduced, this premixed ullage gas can reaction in a range of manners from no ignition to full over pressure to detonation (shown on the right side). Currently, a proposed FAA regulation to mandate flammability reduction is pending decision. One of the solutions to this requirement is to use inert gas to dilute the tank ullage oxygen down to 12%. Experiments are done in the lab, scaled up to the real tank, and verified that the inert condition applies for limited flight profiles. This is a good first step, but there are still much more room to improve. How would we make a flexible fuel tank protection device to accommodate the changing fuel tank flammability condition? NASA Glenn’s Adaptive OBIGGS (On-Board Inert Gas Generation System) concept seeks to go another step further by addressing actual need. We measure the ullage composition, assess its ignition and shock transition potentials, and then activate an advanced inert gas generator to increase the dilution, slow down the chemical kinetics and the combustibility. This work is complemented by assessment in fuel composition to look at how a fuel can be inherently safer. Our technical development currently focuses on six widely ranging subjects. The three basic science areas deal with ignition science, reacting flow dynamics, and fuel science. Sensors are developed to monitor fuel tank flammability and OBIGGS system health in real time. And finally an highly effective inert gas generator can make such an adaptive OBIGGS more economical. The fundamental concept is very simple: Find out the condition in the tank, assess the danger, do something to reduce the danger.

Fuel Modification Reduces Ignition Overpressure n-Octane iso-Octane MTBE Vapor Pressure Increases With Branching Ignition Energy Increases with Branching Energy Density Increases with Branching Vapor Phase iso-Octane Concentration is 3 Times That of n-Octane at 40oC, MIE is a Factor of 6 Higher Substitution of Branched for Linear Alkanes Reduces Pressure Impulse Thus Reducing Structural Failure Branched Alkanes Increase Ignition Delay That Decreases Reaction Rates and Leads to Flame Extinction Changing the fuel composition can affect storage safety by limiting the ignition overpressure. This chart compares the various isomers of octane and their properties. Isomers are molecules with the same chemical form but different physical structure. On the right are the n-octane and branched-chain iso-octane. The noted comparison on the lower left is that the combustion overpressure of iso-octane is much lower than that of n-octane. This is partly because iso-octane’s MIE is 6 times higher than that of n-octane, making the former harder to ignite. But more importantly, the iso-octane pyrolysis produces a reaction quenching species which inhibits follow on reaction. If more branched hydrocarbons can be introduced into the jet fuel mix, it is possible to make the fuel harder to burn in storage. Linear Species (n-Octane) Form Reactive Ethylene While Branched Species (iso-Octane, MTBE) Form Reaction Quenching Isobutylene

n-Octane MIE vs. Spark Duration Ignition Sensitization: Minimum Ignition Energy Dependent on Spark Duration! n-Octane MIE vs. Spark Duration 10-1 100 101 102 103 104 Spark Duration, μs Spark Energy ,mJ n-Octane, 35.9 ºC, Φ =2 n-Octane, 18.3 ºC, Φ =1 n-Octane, 17 ºC, Φ =0.95 n-Octane, 19.8 ºC, Φ =1.06 n-Octane, 19.8 ºC (no spark) What is the ignition limit of a fuel? What must we do to ignite it? What must we do to keep it from igniting? Part of our work looks at what we can do from the fuel side to make ignition harder. The first thing we had to do was to establish a reliable ignition test. We reviewed data from past minimum ignition energy tests and noticed that the data scatter is in the order of 300%. That’s an unacceptable high amount. As a result, we developed the three electrode system which we can produce data with 10% scatter. Once this is done, however, we discovered that some major components of jet fuel ullage vapor exhibit sensitizing behavior. Contrary to common perception, the minimum ignition energy for medium weight hydrocarbon seems to be rich, around an equivalence ratio of 2. What makes the situation really interesting is that the same fuel near soichiometric exhibit discontinuities in the MIE with respect to the spark duration. Around the 1 ms time frame, MIE can vary as much as a factor of 300 in some situation. Thus if an ignition test is done with variability in duration just around this discontinuity, the MIE can vary significantly. A sudden change in the MIE of a compound is a sensitized behavior. A change in the reaction kinetics can make this kind of behavior. So are there families of chemical in the jet fuel blend that is acting as sensitizer? If we remove these components, can we make the fuel more difficult to ignite in storage?

Fuel tank Protection - Deflagration-to-Detonation Transition B Ullage gas Liquid fuel at bottom Unspecified Ignition Source Deflagration Detonation The real danger in fuel tank ignition is ullage gas detonation. An unspecified ignition source starts a constant volume combustion inside compartment A. As the pressure increases, a high-speed jet surges into the adjacent compartment B. This jet has energy level several orders of magnitude higher than the initial spark. At such high energy level, the deflagration wave transitions into detonation. This transition does not have to happen in a multi-compartment fuel tank. Any large enough tank with protrusion can provoke a deflagration wave to transition into detonation. One such well known device is the Shelkin coil used in pulsed-detonation engine to deliberately trigger the detonation transition. A simple ignition overpressure may be survivable. A detonation is not. By diluting the reactants with more inert gas, the kinetics will slow down. If ignition cannot be avoided, how much inert gas must we add to keep a detonation transition from happening? Ignition in compartment A results in constant-volume combustion Constant volume combustion results in over-pressure in A maximum 8x Pressurized gas jets into compartment B at sonic speed. Jet poses 4-10 order of magnitude stronger ignition energy than the MIE Jet ignites ullage gas in B, transition from deflagration to detonation wave. Some deflagration over-pressure may be survivable. Detonation is not survivable.

Improved Inert-Gas Recovery from Combustion Derived Inerting 1/3 More N2 Recovery or ¼ less Bleed Air Needed Φ = 0.70 Dry Combustion Products and Air at 150 ºF and 180 ºF at 40 psig 80 82 84 86 88 90 92 94 96 98 100 10 20 30 40 50 60 70 Inert-Gas Recovery Fraction, % Inert-Gas Purity, %   CDI, Φ = 0.70, N2+CO2 CDI, Φ = 0.70, N2 only ASM, N2 only 180 ºF 150 ºF Ignition and detonation transition will not happen if the reaction chemical kinetics are slowed down. We can do this chemically, such as using Halon, or we can do it physically by diluting the reactants with inert gas. Dilution spreads the reaction heat to a larger pool of inert gas, thus lowering the temperature rise. This in turn slows down the reaction rate, which in turn slows down the heat release and slows down the reaction rate even more. This unstable cascading effect is what puts out an ignition event. The currently favored means of inert gas generation is by air separation by hollow-fiber membrane air separation modules (ASM). Bleed air goes in one end, oxygen-richer air comes out the side and oxygen-deficient air (inert gas) comes out the other side. The operation of an ASM is a trade-off between inert gas purity and inert-gas recovery fraction. If you want a purer gas, then the recovery fraction is lower. Improving the selectivity of the membrane can increase the recovery fraction, effectively moving a curve to the right. On this graph, the blue curves show that lowering the air temperature from 180 ºF to 150 ºF improves the membrane selectivity. However, lowering the temperature also reduces the permeability which requires an increase in the amount of membrane area to compensate, thus increasing the membrane mass. One way to improve the membrane selectivity is to change the feed gas. The limitation to the selectivity is that O2 and N2 are very similar. If we were able to tag the O2 so that it looks very different than the N2, then the selectivity could be increased significantly. We can do this. By burning the air, the O2 is converted to H2O and CO2. These gases are much faster than O2 and N2, thus these oxygen-bearing species can be removed much easier than O2 from an ASM. The red curves shows the amount of N2 recovered from a partially burned air (75% burned) is about 1/3 better than the comparable air separation using ASM. However, if the definition of inert gas also includes CO2, and the fuel tank ullage application can tolorate 2% CO2, that amount could be some 50-60% more than the baseline ASM configuration. This means a great reduction in bleed air consumption.

In-Tank Real-Time Multi-Species Fiber Optic Flammability Sensor Real-Time: 5 second update interval for feedback control, and critical time-dependent process monitoring Multi-Species Analysis: N2, O2, CO2, H2O, CO, CH4, other HC’s, H2, H2S, NO, SO2,… Precise: currently has 1% precision in 5 seconds for N2; future versions will require < 1 second for same precision Rugged and Reliable: system has no moving parts to go out of alignment or consumables to wear out Intrinsically-Safe: No electrical penetration into measurement volume Cost-Effective: Monitor multiple locations simultaneously with multiple fiber sensors and one base unit Real-Time Fiber Optic Gas Analyzer Gas Inlet Gas Outlet Respiration Gas Monitoring Example Low-Volume Gas Sampling Chamber (or Fuel Tank Ullage) Advantages Fiber Optic Sensor Probe How do we know if the situation in a tank is dangerous? How do we know how dangerous it is? We need a sensor to tell us the composition of the ullage gas mixture. We now have a flammability sensor suitable for the fuel tank. It is an optic based unit that is impervious to contamination and does not introduce electrical power into the fuel tank. The graph at lower left shows such a technique being used to monitor its inventor’s breath through a sampling chamber, alternating from fresh air to exhaled breath. This instrument can detect hydrocarbon species simultaneously as well. With such an instrument, we are now able to establish the equivalence ratio and the dilution ratio in real time. This technique is currently under going paten processing.

Contact Information at NASA Glenn Research Center CDI-OBIGGS & Ignition Mitigation Clarence Chang, Ph.D. Clarence.T.Chang@nasa.gov Deflagration-to-Detonation Transition Science Nan-Suey Liu, Dr.-Ing. Nan-Suey.Liu@nasa.gov In-Tank Real-Time Flammability Sensor Quang-Viet Nguyen, Ph.D. Quang-Viet.Nguyen@nasa.gov Fuel & Ignition Science Marty Rabinowitz, Ph.D. Martin.J.Rabinowitz@nasa.gov