| Summary: | 博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 100 === A pulse detonation engine must complete the process of feeding, mixing, and purging within milliseconds. With liquid fuel, such an engine is extremely sensitive to the Sauter mean diameter (SMD - must be less than 10 μm) and particle size distribution of the fuel, requirements which are difficult if impossible for most fuel injectors to achieve. This study uses the aviation fuel JP-8 and an injector of direct injection engine. This injector can be operated in a wide operation pressure range and duration time. The injection timing and equivalence ratio could also be accurately controlled with good response time. The results indicate that a fuel pressure greater than 8 MPa can achieve the SMD of less than 10 μm. This study further incorporated the concept of flash boiling to obtain a smaller SMD. However, this might result in carbon deposition due to cracking or thermal reaction. To circumvent this phenomenon, a deoxygenation device was designed and employed to mitigate oxidization and to investigate the effects of heating temperature on the generation of deposition. The results of spray distribution indicated when the fuel was heated to 373 K, only a fuel pressure of 6 MPa was necessary for achieving fuel droplet characteristics favorable for detonation.
Liquid fuel with sufficient vapor proportion at micron scale is essentially required in order to increase specific energy density and reduce volume requirements for application of a pulse detonation engine. For detonation initiation of liquid fuel with oxygen, the effects of initial temperature were investigated. It is known that the fully vaporized temperature range of JP-8 from 380 to 410 K. At an initial temperature of 373 K, the fuel vapor with oxygen was not enough to induce the reaction, which led to the detonation initiation failure. Condensed fuel was also observed on the bottom of the detonation tube. A temperature of 393 K, the detonation wave was successfully generated even though a portion of fuel was still in a liquid state. At initial temperatures of 413 K, 433 K, and 453 K, the deflagration-to-detonation run-up distance and pressure trace at fully vaporized conditions were similar to those of gaseous mixtures, such as a propane-oxygen mixture. In addition, the tests with different equivalence ratios were conducted to evaluate the DDT run-up distance. A rapid increase in DDT run-up distance was observed at equivalence ratios close to lean and rich limits. It is also noted that there was a reduction in the rich limit with increasing initial temperature. The minimum DDT run-up distance was approximately 200 mm, which was similar to propane and oxygen mixture results. In order to reduce the volume of consumed oxygen, the oxygen concentration was diluted with nitrogen. The effect of the nitrogen/oxygen ratio (β) on the DDT run-up distance was investigated. As β increased, lean and rich limits of the equivalence ratio decreased. When β was great than 0.4, the detonation wave could not be successfully initiated, which is similar to propane mixtures test cases.
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