Calibration of the Measurement System for Methane Pyrolysis in Rocket Nozzle Cooling Channels

Abstract: Methane-based rocket propellant is gaining traction as a green technology with advantages in sustainability, cost-effectiveness, and performance. However, under high temperatures found in rocket nozzle cooling channels, methane can undergo thermal decomposition known as methane pyrolysis, resulting in the generation of hydrogen and solid carbon. This poses challenges to rocket engine performance and can eventually cause engine failure. Understanding and predicting the composition of evolved gases in rocket engine processes is therefore crucial. This thesis focuses on quantifying the production of hydrogen in the exhaust stream. To achieve this objective, a correlational measurement method utilizing sensors was developed and experimentally investigated. This approach involved the detailed mapping of sensor responses to variations in gas composition, temperature, and pressure, which were compared and validated against theoretical data derived from REFPROP; a widely used software tool for calculating gas properties. The sensors employed in this study enabled direct measurements of the speed of sound (SOS) and thermal conductivity (TCD) of the gas. The SOS measurements exhibited strong agreement with theoretical predictions in response to changes in hydrogen content. In contrast, the TCD measurements showed lower sensitivity to hydrogen. It was observed that temperature exhibited a substantial influence on both SOS and TCD compared to pressure. However, the implementation of experimental and theoretical correction coefficients effectively compensated for these effects. The resulting calibration curves demonstrated an absolute deviation of 0.2-0.3%vol in hydrogen concentration, which demonstrates the effectiveness of the developed method of quantifying hydrogen in gas mixtures. Lastly, the occurrence of methane pyrolysis was tested and confirmed.