Enhancement of solid-fuel ramjet combustor performance using energetic fuel additives

Abstract

In this paper, experimental investigation for the solid fuel ramjet combustor was performed using a compact gas generator. This thesis discussed three technical issues for the investigation of solid fuel ramjet at the university level. : Test facility, Ignition delay, Combustion characteristics. First of all, A gas generator using the catalytic decomposition of ethanol-blended $H_{2}O_{2}$ was investigated for application in experimental research on high-speed air-breathing engines. The gas generator should supply product gas with an oxygen concentration of 21 mol.% at a given pressure of 3-12 bar and a temperature near 500 °C, which are the flow conditions in a ramjet combustor flying at Mach 3. To avoid detonation of the liquid monopropellant, an appropriate mixture ratio of $H_{2}O_{2}$, water and ethanol was selected. In the batch reactor test, the tested catalysts were manganese oxide and platinum with gamma-alumina pellets. The temperature measurements indicated that the platinum with alumina pellets was more appropriate for decomposing propellant than manganese oxide with alumina pellets. In addition, product gas species with an oxygen concentration of 21 mol.% were confirmed through the test. In the demonstration test, the system was operated with a propellant mass flow rate near 100 g/s. A catalytic reaction was sustained for the whole operation time with an adiabatic decomposition temperature efficiency of over 0.98. The measured flow temperature was near 500 °C, and the chamber pressure was near 5.5 bar, which satisfied the flow conditions in a ramjet combustor flying at Mach 3. And then, the ignition delay requirement and ignition characteristics of a solid fuel in the ramjet combustor were investigated. The shock in a simple diffuser geometry was calculated in order to obtain the upper limit of ignition delay, and it was determined to be shorter than 1 s. In order to achieve this short ignition delay, a high-density polyethylene (HDPE) fuel grain coated with an ignition support material and a fuel-rich propellant (FRP) were prepared. The ignition support materials were nitrocellulose with boron potassium nitrate (NC/$BKNO_{3}$) and an ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB)-based composite propellant. For testing purposes, a compact gas generator was built to simulate the conditions in the ramjet combustor. The simulation results revealed that the NC/$BKNO_{3}$ ignition support material showed the shortest ignition delay of 1.27 s but the flame in this case was not sufficient to ignite the HDPE. In a demonstration test, an ignition support material composed of NC/$BKNO_{3}$ and the AP/HTPB-based composite propellant was applied to the FRP fuel grain, in which case a stably sustained flame was obtained. The test results demonstrated that ignition of the FRP and that of the ignition support material occurred simultaneously with an ignition delay of 1.74 s. After that, additional combustion test was conducted through controlling the content of selected ignition support material, and it was experimentally confirmed that ignition delay can be shortened by 0.5 s. Finally, in order to improve the combustion characteristics of the fuel grains, effect of metal additives (Al and B particles) with high heating value on FRP mixed with 15 wt.% AP particles were investigated. Depending on the composition of metal particles, three types of fuel grain with 15 wt.% AP particles were prepared and tested at supplying air simulant speed with Mach 0.21. Through the test, the noticeable effect of B particles on the combustion was confirmed by a high regression rate over 0.5 mm/s and combustion efficiency over 80%. Meanwhile, Al particles were not effective additives individually

however, Al particles contributed to the ignition of B. On the other hand, as the flow rate increases, the regression rate decreased sharply and the combustion performance decreased. As a method to enhance the combustion performance of the fuel grain, Mg particles with superior vaporization characteristics was applied. The effect of Mg on both ignition and combustion characteristics of fuel grain was noticeable and rapid sustained operation of the fuel grain was achieved. And it also burned well in the high speed oxidizer flow. After that, thermal analysis was performed for the fuel grains to observe their combustion in the real air condition. The superior heat release in the thermal analysis revealed that the fuel grain could also burn well in the real air condition and the feasibility of the solid fuel ramjet application with superior performance of the fuel grain was confirmed.