Two-color frequency-multiplexed IMS technique for gas thermometry at elevated pressures

The development and demonstration of a high-bandwidth two-color temperature sensor for high-pressure environments using intensity-modulation spectroscopy (IMS) is presented. The sensor utilized rapid intensity modulation, beam coalignment, and frequency multiplexing to deal with common challenges for laser absorption spectroscopy systems at high pressures and achieved a sensor bandwidth of $100$ kHz. The P(16) and R(6) transitions of the $H_{2}O$ fundamental antisymmetric stretch rovibrational band near $2.5\mu$m were chosen for initial development of this temperature diagnostic concept. Temperature validation experiments were conducted with shock tubes for both reactive and non-reactive environments. Shock tube experiments were first conducted with $H_{2}O$ and $N_2$ mixtures at pressures of around $8.2$ atm, yielding temperature measurements with a standard deviation of $2.9\text{K}$ within the steady-state test time. The performance of this system was then validated at $36.9$ atm, yielding temperature measurements with a standard deviation of $8.4\text{K}$. By comparing the measured temperatures with calculated temperatures based on ideal shock jump relations, the sensor achieved an average accuracy within $4.3\text{K}$ of the known temperatures across multiple experiments spanning a range of 1030–1450 K, 8–38 atm. These results demonstrate that the IMS-based sensor enables high-precision measurements of temperature at high pressures.