Ignition Delay Measurements
Fig. 3 is a stainless steel combustion bomb[41] used to measure ignition delay of kerosene and its aluminized versions. The injection port is water cooled to keep the gel (or neat RP-1) at ambient temperature. Fuel temperatures are monitored by a thermocouple at the nozzle close to the tip. The bomb, containing an oxidizing gas of known pressure, is preheated to a defined temperature. Using a piston powered by high-pressure argon, approximately 0.3 cm3 of fuel is injected into the bomb and it auto-ignites at temperatures above about 400°C. An internal transducer measures pressure rise due to combustion. The time is measured from the first movement of the piston forcing the fluid into the chamber until there is a rapid rise of pressure. Generally, the pressure rise due to combustion is 2-4 times the original pressure, depending upon whether the gas is air or oxygen. The inner length of the bomb is 10 cm, which is the maximum length of the spray pattern. The transit time of the spray to the farthest end of the internal space is approximately 25 msec. At lower temperatures where reaction was slow, the spray would deposit on the inner walls, and the ignition delay extended out to hundreds of milliseconds. This appears to be characteristic of the time for auto-ignition of the gel as a film on the wall of the chamber rather than during the spray interval.
Combustion of Metallized RP-1
Fig. 4 shows chemical ignition delay times of neat RP-1, gelled RP-1, and aluminized gelled RP-1 in hot oxygen[40] at a fixed pressure at 0.8 MPa and at four different test temperatures (410°C, 460°C, 520°C, and 580°C). Gelled RP-1 was used as a control rather than neat RP-1, to attain equivalent viscosity and permit comparison of the combustion of control vs. the aluminized fluids under similar spray patterns as predicted by the power law model. The gelled RP-1 (6.5-wt.% silica) had a viscosity at high shear rates equivalent to 30% Alex®/RP-1. The data in Fig. 4 indicate that the 30-wt.% Alexgel ignites faster than gelled RP-1, but about equivalent to that of pure kerosene. However, a surfactant when added to the 30-wt.% Alexgel reduces ignition delay below that of pure RP-1 kerosene (compare curve 1 and curve 2), demonstrating that Alex® acts as combustion accelerant for kerosene. Estimations based on the power law model indicate that apparent viscosity of this gel is comparable to that of RP-1 at injector shear rates (6.3 • 104 sec-Gels produced from 5-mm aluminum did not show any reduction in ignition delay.
Fig. 3 Device for measuring ignition delay times of gels.
Fig. 4 Average ignition delay in hot oxygen vs. temperature.
Similar experiments were repeated except in air[40] rather than oxygen, and at an initial air pressure of 1.1 MPa. Four different fuels were examined: gelled RP-1, 25-wt.% Alexgel, neat RP-1, and 30-wt.% L-Alex®/ 2-wt.% Tween-85/RP-1 fuel. Tests were performed at four different temperatures (430°C, 550°C, 580°C, and 600°C). The data in Fig. 5 indicate that 25-wt.% Alexgel (curve 3) ignites faster by approximately 30% than gelled RP-1 (curve 4). Curve 1 shows that the L-Alex® version ignites faster by factor of 2-3 than neat RP-1 (curve 2). As it was noted earlier, these two fluids have similar fluidity at high shear rates, confirming that L-Alex® is even more superior as a combustion accelerant than Alex® in an air environment.
Fig. 5 Average ignition delay of different fuels in air vs. temperature.
”Combustion” in Nitrogen
Alex® is known to react rapidly with gaseous nitrogen and this is supported by the DTA data of Mench et al.[25] This characteristic would be of benefit in air-breathing engines, permitting incipient combustion to occur and support these engines when they are starved for air. Alex®/RP -1 gels (55-wt.% Al) were sprayed into pure nitrogen at 750°C showing incipient combustion, but substantially slower than in air or oxygen. Nevertheless, the energy generated by this reaction can contribute to heat generation, while minimizing the opportunity for flame-out in air-breathing engines such as RBCC systems.
Small Rocket Engine Tests
The combustion performance of Alexgels was measured in a small rocket engine by Mordosky et al.[42] The objective was to measure flame temperature, specific impulse, and c* combustion efficiency (defined as PcAt/dm/df).[43] In order to compare rocket test results obtained from different test engines, it is beneficial to have exactly the same fuel and oxidizer compositions as well as approximately the same operational conditions. The small rocket engine constructed by Mordosky et al.[42] was 30.5 cm long with an internal diameter of 12.0 cm. Combustion of Alexgels was done in gaseous oxygen. The aluminum loadings (Table 2) were the same, but there were some variations in the surfactant and gellant. Table 3 shows average values of adjusted c* efficiency of the Penn State fuels as compared to the work of Palaszewski and Zakany.[7,9] They show that:
(i) The 5-wt.% Alex®/RP-1 gel burns more efficiently as compared to 5-wt.%Al/RP-1 gel using coarse aluminum.
(ii) The 55-wt.% Alex®/RP-1 gel burns more efficiently as compared to 55-wt.%Al/RP-1 gel.
(iii) The 5-wt.% Alex®/RP-1 gel burns with the same efficiency as neat RP-1, although the viscosity of the neat RP-1 is very much lower than that of gelled RP-1 or Alex®-loaded gels.
These data show that the combustion of the nano aluminum overcomes the deficiency of a larger droplet spray pattern associated with higher viscosity of the Alexgels. They also measured the aluminum oxide that is produced during combustion and found it to be submicron in size.
APPLICATIONS OF ALEXGELS
Hydrocarbon-Based Liquid Rocket Engines
Combustion bomb testing reinforced by small rocket engine tests shows that Alex® nano aluminum powder provides improvement in volumetric energy density over pure kerosene while micron-size aluminum burns inefficiently when dispersed in hydrocarbon. Another benefit is the reduction of metal slagging on the walls of the rocket engine.
Hydrocarbon-Based RBCC Systems
Cryogenic hydrogen is being considered for RBCC engines. However, maintaining this cryogenic fuel over long duration flights, such as its use for re-entry, is problematic. Should hydrocarbon fuel be considered for RBCC engines, then adding Alex® would provide the same potential benefits as its use in hypersonics, i.e., flame stability and increased output in the air augmented mode, while providing increases in volumetric Isp in the rocket mode as well.
Pulse Detonation Engines
Another possible use for aluminizing kerosene is in pulse detonation engines (PDEs). The detonation of kerosene in air has yet to be demonstrated. The potential for reducing ignition delay into the submillisecond regime by adding Alex® is almost certain. Further reductions of particle size, enhanced by finer sprays as well as organic layered aluminum such as the L-Alex®, could result in overcoming the difficulty in detonating kerosene.
Table 2 Compositions of the RP-1/Alex® gel propellants tested at Penn State
|
Metal loading |
Liquid fuel |
Metal powder |
Gellant |
Surfactant |
|
weight percentage |
(RP-1) |
(Alex) |
(SiO2) |
(Tween-85) |
|
0 wt.% |
95% |
0% |
5% |
0% |
|
5 wt.% |
90% |
5% |
5% |
0% |
|
10 wt.% |
86% |
10% |
4% |
0% |
|
30 wt.% |
68.2% |
30% |
1.3% |
0.5% |
|
55 wt.% |
43.3% |
55% |
0.4% |
1.3% |
Table 3 Average values of normalized c*vg efficiency of NASA gels to the Penn State gels
 |
 |
 |
 |
|
Neat RP-1 |
— |
88.9± 1.8 |
— |
|
5 wt.% |
88.3±1.7 |
78.1 ±2.4 |
1.132±0.036 |
|
55 wt.% |
83.0±2.2 |
77.8±2.4 |
1.07 ±0.04 |
Liquid Hydrogen Engines
Adding aluminum to cryogenic hydrogen to form gels has been suggested by Starkovich et al.[14] as a means to increase volumetric energy density and at the same time increase the containment of a liquid hydrogen leak. Such benefits could also be useful for a liquid hydrogen RBCC engine, where Alex® could also serve to increase flame stability during the air cycle.
CONCLUSION
Gels of Alex® in RP-1 are dynamically stable at least over the short term and matched the viscosity of coarse aluminum gelled into RP-1 (Fig. 2). Ignition delays of nano-aluminized gels were always shorter than gelled RP-1 (equivalent viscosity) as well as neat RP-1, which has lower viscosity. Alex® acts as a combustion accelerant for RP-1, and probably other kerosenes as well. An organic coated version of Alex® (L-Alex®) was developed and found to have ignition delays in air lower than RP-1 by a factor of 2-3. These findings are relevant to advanced rocket combined cycle engines, hypersonic (scramjet) engines, and perhaps pulse detonation engines. Coarse aluminum is not a combustion accelerant for RP-1 in oxygen.
Rocket engine testing at Penn State showed that the 5-wt.% and 55-wt.% Alexgels burn substantially more efficiently than coarse aluminum. The 5-wt.% Alex®/ RP-1 gel burns with the same efficiency as neat RP-1, although the viscosity of the neat RP-1 is very much lower than that of gelled RP-1 or micron-size aluminized gels. Apparently, the kinetics of combustion of the nano aluminum overcomes the larger droplets of the spray caused by higher viscosity.
The overall conclusion is that loading Alex® into RP-1 improved its combustion kinetics and efficiency as compared to neat RP-1, and better than published values for micron-size aluminum gels. Moreover, Alex® additions would result in improving the combustion of kerosene in air-breathing engines.
