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High Heat Flux Surface Coke Deposition and Removal Assessment
Abstract
: The internal surfaces of liquid hydrocarbon-fueled rocket engine thrust chambers, throats, and nozzles are exposed to high pressure combustion products at temperatures beyond 6000 F. Regenerative cooling is widely used in these engines to prevent overheating of the copper alloy liners. As a result, high heat flux fuel-wetted surfaces reach temperatures where fuel carbon deposits (coke) form. Coke has a much lower thermal conductivity than copper - thicknesses of only a few millionths of an inch can cause liner temperatures to increase to dangerous levels. Moreover, reusing launch vehicles and main engines increases the likelihood that unsafe levels of coke will be deposited over the course of multiple missions. Therefore, there is a need for a method to survey coke layer thicknesses and locations in the cooling channels so that engine operating margins, service intervals, and lifetimes can be determined. Unfortunately, the cooling channel geometry combined with thin coke layers makes this a difficult and challenging problem. Reaction Systems, Inc. has developed a low temperature oxidation method that can rapidly remove the coke layers in the cooling channels and at the same time map their location. We demonstrated this technique in a recent SBIR Phase II effort, which included depositing coke on copper surfaces at heat fluxes in excess of 20 Btu/in2-s under pressure, temperature, and flow conditions that match those experienced in liquid hydrocarbon-fueled rocket engines. Surface analysis was used to characterize the carbon concentration on the surface of the copper substrate after the coking cycle and also measure the thickness of the carbon deposit. These analyses were used to also demonstrate that the carbon was completely removed from the substrate using our low temperature oxidation process.
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Electromigration is increasingly relevant to the physical design of electronic circuits. It is caused by excessive current density stress in the interconnect. The ongoing reduction of circuit feature sizes has aggravated the problem over the last couple of years. It is therefore an important reliability issue to consider electromigration-related design parameters during physical design. In this talk, we give an introduction to the electromigration problem and its relationship to current density. We then present various physical design constraints that affect electromigration. Finally, we introduce components of an electromigration-aware physical design flow.
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A series of electrically heated tube experiments was conducted to investigate the potential of JP-7 as a coolant under conditions relevant to a Mach 8 propulsion system. The heat transfer capabilities, carbon deposition, and material compatibility of JP-7 at surface temperatures up to 1700 F (927 C) were tested in 0.125 in. diameter tubes of 304 SS, Inconel 617, Haynes 188, Haynes 230, and 50150 Moly-Rhenium. The heat transfer to the coolant was modeled well by a Dittus-Boelter correlation at lower heat fluxes. At higher heat fluxes, audible instabilities were observed and corresponded to a significant enhancement in the coolant heat transfer. The carbon deposition rates in these tests were comparable to those in previous experiments at lower heat fluxes and much longer residence times. This result suggests that alternative paths of the deposition mechanism may be enhanced under high heat flux test conditions. Microscopic investigation of the post test tubes indicated that there was a significant layer of ordered carbon deposits that had not been seen in the tests at lower heat flux.
The hydrocarbon fuels RP-1, commercial-grade propane, JP-7 and chemically pure propane were subjected to tests in a high pressure fuel coking apparatus in order to evaluate their thermal decomposition limits and carbon deposition rates in heated copper tubes. A fuel thermal stability parametric evaluation was conducted at 136-340 atmospheres, bulk fuel velocities of 6-30 m/sec, and tube wall temperatures of 422-811 K, and the effect of inside wall material on deposit formation was evaluated in tests using nickel-plated tubes. Results show RP-1 deposit formation at wall temperatures between 600 and 800 K, with peak deposit formation near 700 K. Substitution of deoxygenated JP-7 for RP-1 showed no improvement, and the carbon deposition rates for propane fuels were found to be higher than those of either of the kerosene fuels. Nickel plating of the tube walls significantly reduced RP-1 carbon deposition rates.
This paper presents the results of material compatability experiments using Mil-Spec RP-1, n-dodecane, propane, and methane fuels in contact with OFHC, NASA-Z, and ZrCu coppers. The objectives of the current research are 1) to define the corrosive interaction process that occurs between hydrocarbon fuels and copper combustion chamber liner materials, and 2) to develop and demonstrate protective measures against this corrosive process. Two distinct test methods were employed. Static tests, in which copper coupons were exposed to fuel for long durations at constant temperature and pressure, were used to provide compatability data in precisely controlled environments. Dynamic tests, using the Aerojet Carbothermal Test Facility, were used to expose copper specimens to fuel under realistic booster engine service conditions. Tests were conducted using 1) very pure grades of each fuel and 2) fuels to which a contaminant was added to define the role played by fuel impurities. The findings from metallurgical and chemical analyses of the copper specimens and chemical analysis of the hydrocarbon fuels are provided. Conclusions are reached as to corrosion mechanisms and effects, methods for the elimination of these mechanisms, selection of copper alloy combustion chamber liner and materials, and hydrocarbon fuel specifications as related to advanced booster engine systems.