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EDWARD A. FLETCHER,' HAMPTON H. FOSTER, and DAVID M. STRAIGHT Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio
Aluminum Borohydride and Mixtures with Hydrocarbons in Jet Engine Combustor Ignition Chemical ignition may be an excellent technique for use in thermal-jet engines. However, aluminum borohydride reacts violently with air, water, and other reducible substances to produce solids that plug orifices and fuel lines. Boron alkyls (not very reactive with water) or aluminum alkyls might b e used, but ignition characteristics would b e impaired. Protracted ignitor injection might have another application. If a jet engine were to encounter flame-out near its operational ceiling, the hydrocarbon fuel might not reignite because the temperature and pressure were outside the burning limits of hydrocarbon fuels. Because aluminum borohydride always ignited, an ignitor fluid might be used to start the engine and get it back to operating conditions. In a turbojet the ignition would b e used to bring the compressor-turbine up t o a speed where the pressure and temperature will sustain a hydrocarbon flame. In a ramjet, combustion o f the ignitor would create a momentary area blockage, slow down, compress, and heat the air, with the same result.
A-r approaching steadystate burning limits jet engine combustors CONDITIOM
can be ignited with electrical ignition systems only when ignition capability has been specifically designed into the combustor. Where this is undesirable, chemical ignitors may be useful, particularly at high altitude engine-windmilling conditions where ignition is most difficult. Several compounds can be considered-e.g., alkylboranes, alkylaluminum compounds, and boron hydrides. In this study, aluminum borohydride was used because it is one of the most flammable substances known. Combustor. A single tubular 533 combustor was installed in a directconnect duct test facility ( 3 ) which supplied the desired altitude inlet and exhaust conditions. A 3 x 11.5 inch transparent plastic window in the outer shell permitted visual observation of the flame. A 10.5-gallon-per-hour, fixed-area fuel nozzle having an 80' Present address, Department of Mechanical Engineering, University of Minnesota, Minneapolis 14, Minn.
spray-cone angle was used. As the fuel passed through a copper cooling coil 50 feet long and inch in outside diameter immediately before entering the nozzle, the fuel temperature was very near that of the inlet air. The low-volatility jet-type fuel was obtained by removing volatile components from MIL-F-5624A stock to adjust the Reid vapor pressure to a nominal 1 p.s.i. Ignition Systems. CHEMICAL IGNITION. Aluminum borohydride was introduced into the combustor from an injector fitting into the sparkplug hole. A glass capsule containing 2 ml. of the mixture was mounted in the injector. A manually operated trigger arm broke the sealed tip from the capillary tube (inside diameter 0.025 inch), allowing the contents to be injected into the combustion chamber. All tubes were notched 3/4 inch from the body of the capsule to provide a reproducible breaking point. The capsule was charged with hydrocarbon diluent, stoppered with a selfsealing biological-vial rubber stopper, cooled in a dry ice slush bath, and evacuated through a hypodermic needle. Aluminum borohydride was added from a buret equipped with a similar needle. The: capsule was finally pressurized with nitrogen to slightly above 1 atm. through a third hypodermic needle and stored in dry ice. Before insertion into the injector it was warmed by hand and thoroughly mixed. ELECTRICAL IGNITION ( 7 ) . Energylevels of 0.3 and 10 joules of stored condenser energy per spark with a spark rate of 8 sparks per second were used. The estimated efficiency was 20 to 25y0 of stored energy available at the spark gap. Test Procedure. BURNING LIMITS. The: minimum combustor-inlet air pressurcs at which burning could be maintained were determined for air flows of 1.87, 3.75, and 6.0 pounds per second per square foot based on a combustor maximum cross-sectional area of 0.267 square foot at a constant air and fuel temperature of 10' F. (minimums observed as fuel-air ratio was varied a t constant air velocity). The burning limit was also determined for a reduced inlet air and fuel temperature (-46' F.) a t 3.75 pounds per second per square foot. IGNITION LIMITS WITH ELECTRIC SPARKS. The minimum combustorinlet air pressure at which ignition could be obtained with spark energies of 0.3 and 10 joules (at 8 sparks per
RUBBER STOPPER WIRED I N PLACE
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Injector used pressurized glass capsules with capillary tubes for single charges of chemical ignitor
second) was obtained for the combustor inlet conditions ( 2 ) . IGNITION LIMITS WITH ALUMINUhf BOROHYDRIDE. After the desired conditions were established, fuel was admitted at a rate previously determined to be optimum for ignition (fuel-air ratio, approximately 0.01 5). Aluminum borohydride mixture was then injected. Ignition was indicated by temperature rise and observation of flame through the window. Except with a few injections of 7.5 and 15% blends, flame was always visible, whether or not the combustor was ignited. The criterion for satisfactory ignition was that the flame fill the combustor and continue burning after the source of ignition had been dissipated. Most of the data were obtained with JP-4 as the diluent; aluminum borohydride concentrations were 7.5, 15, 32, and 100%. Some data were obtained with a solution of 32y0 aluminum borohydride in mineral oil.
Results and Discussion Static chamber tests ( 4 ) showed that 207, aluminum borohydride in npentane ignited spontaneously when injected into relatively dry air a t 1 inch of mercury absolute. Preliminary
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Figure 1. Chemical ignition was achieved at more severe conditions than those that limited spark ignition Diluent, JP-4 fuel; combustor-inlet air and fuel temperatures, 10' F.; capillary tube diameter, 0.025 inch Chemical ignition, a t AI(BH4)a concentrotion indicated - Spark ignition, spark energy, E/sec., at ioules indicated / / / / / Steady-state burning limit, below which there was no burning or ignition 0 - 4 6 ' F., burning limit 0 -46' F., chemical ignition limit A 46' F., 10-ioule spark limit
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Figure 2. A spark energy from 50 to several thousand joules is required for ignition at aluminum borohydride ignition limit Combustor-inlet air and fuel temperature, 10' F.
in inches of mercury absolute (Figure 2). ] combustor ignition tests with isopentane containing 32Yo aluminum borohydride Extrapolation of the curve to values of were disappointing. The combustor V/&P corresponding to the aluminum could be ignited only at conditions borohydride ignition limit shows that a simulating those near sea level operation. spark energy from 50 to several thousand This was attributed to excessively rich joules would be required for ignition at vapor phase fuel-air mixtures in the these severe conditions, if the extrapolaignition zone; possibly the high spontion is assumed valid. taneous ignition temperature of isoEFFECT O F INLET AIR AND FUEL pentane was a factor. Isopentane was TEMPERATURE. The steady-state burntherefore dixarded as a diluent. Subing limit and ignition limits with 1007, sequent tests with diluents having aluminum borohydride and a IO-joule lower volatility, primarily JP-4, and spark system were also determined a t lower spontaneous ignition temperatures an inlet air temperature of -46" F. were much more satisfactory. a t 3.75 lb./sec.-sq. ft. (Figure 1). EFFECTO F ALUMINUM BOROHYDRIDE Each limit was raised appreciably by COUCEUTRATION AND AIR FLOWRATE. the 56" temperature decrease. Mixtures of aluminum borohydride in MINERIAL OIL A S DILCENT.Mineral JP-4 (7.5Y0) did not ignite the comoil was used in place of JP-4 for runs at bustor at the highest air flow rate 3.75 Ib./sec./sq. ft. At 10' F., ignition (6 pounds per second per square foot), was consiqtently achieved with the mixand only occasionally was ignition tures at pressures as low as 10.4 inches of obtained even at lower rates. Fifteen mercury, the lowest pressure a t which this per cent mixtures brought about igniexperiment was run and about 1.5 inches tion a t all the air flow rates studied, above the burning limit. With a JP-4 but ignition was erratic and undependdiluent, occasionally ignition was not able. I'l'ith the blends containing achieved at these conditions. At -40" F., 32 or lOO7, aluminum borohydride one ignition was obtained in five tries there was a marked improvement in a t 13 to 14 inches. ignition characteristics. The minimum EFFECTO F INJECTION DURATION ON IGpressures for ignition were reduced and NITION LIMITS. The heat of combustion ignition was much more consistent. of the aluminum borohydride, 32,000 Combustor ignition limits are shown in joules per ml., is orders of magnitude Figure 1, As the air flow rate increased, higher than the spark energies with the pressure which limited ignition inwhich they are compared. Much smaller slugs should therefore bring creased. With 10070 aluminum boroabout ignition. Alternatively, if 2-ml. hydride ignition could be achieved a t slugs are injected more slowly, it might slightly lower pressures than with 3270 be possible to nurse a cold combu.tor to borohydride in .JP-4. a steady-state burning condition. AtChemical ignition was achieved a t tempts to increase injection time by the conditions considerably more severe use of smaller capillaries resulted in than those which limited spark ignition. plugging, but larger capillaries-Le., The 32Y0 aluminum borohydride blends shorter injection times-gave the adverse were appreciably better than the 10effect shown in Figure 3. As the joule spark, and the neat aluminum injection time was decreased from 0.9 borohydride lowered the limit further. to 0.3 to 0.01 second, ignition became The efficacy of chemical ignition can erratic and the pressure limit for ignibe compared with that of sparks by tion became higher. If injection times use of an empirical relationship (2) can be increased by smaller capillaries, between the minimum spark energy lower injection pressures, or ignitor fluids for ignition and V / d P [where V less prone to form plugs, it should be is the reference velocity in feet per possible to approach the steady-state second based on inlet air density and burning limit with chemical ignitors maximum combustor cross-sectional more closely than in this study. area, and P is the inlet-air total pressure literature
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COMBUSTOR IGNITED COMBUSTOR F A I L E D TO IGNITE
Aluminum borohydride con32%; diluent, centration, JP-4 fuel; inlet-air and -fuel 10' F.; temperatures, air-flow, 3.75 Ib./sec./sq. ft. capillary length, 0.75 inch
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INJECTION DURATION, SEC.
Figure 3.
1 390
Increase in injection duration improves combustor ignition limits
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cited
(1) Foster, H. H., Natl. .4dvisory Comm. Aeronaut., NACA RM E51A24 (1951). (2) Foster, H. H., Straight, D. M., Ibid., NACA R M E52J21 (1953). ( 3 ) Rayle, W. D., Douglass, H. \V., Ibzd., NACA RM E50H16a (1950). ( 4 ) Straight, D. M., Fletcher, E. .k, Foster, H. H., Ibtd., NACA RM E53G1.5 (1953).
RECEIVED for review November 25, 1958 ACCEPTED June 2, 1959 Division of Petroleum Chemistry, Symposium on High Energy Fuels, 135th Meeting, ACS, Boston, Mass., April 1959.
