Development of simultaneous TDLAS and BOS techniques for enhanced accuracy in combustion gas temperature measurements

There remain many uncertainties regarding the ignition promotion effects of the micro-rocket torch used for forced ignition in a scramjet engine. Therefore, a detailed investigation of the physical properties of the ejected gas is necessary as a preliminary step toward understanding the ignition promotion mechanisms. While tunable diode laser absorption spectroscopy (TDLAS) has been developed for high-precision optical measurements of combustion gases, it has become evident that accurately measuring the combustion gas temperature is challenging due to the presence of multiple variable fitting parameters. Therefore, in this study, TDLAS was employed to measure the gas temperature, and the accuracy was enhanced by integrating the background-oriented schlieren (BOS) technique to precisely determine the gas jet diameter. We successfully visualized the gas ejected from the torch nozzle using the BOS method. This visualization allowed us to fix the optical path length, which was previously treated as a variable fitting parameter. As a result, the TDLAS+BOS method improved the temperature measurement accuracy compared to TDLAS-only. Specifically, the simultaneous TDLAS + BOS measurements provided more accurate gas temperatures than TDLAS-only, with errors reduced to 7.5% for an equivalence ratio of 0.51 and 10.7% for an equivalence ratio of 1.38. These findings demonstrate that the integration of BOS with TDLAS significantly enhances the precision of temperature measurements in micro-rocket torch applications, contributing to a better understanding of the ignition mechanisms in supersonic flows.