This study reports on the implementation of a non-intrusive measurement technique for the measurement of temperature in chemically non-reacting internal flows. The proposed technique is based on laser absorption spectroscopy. A low pressure, laboratory scale hybrid rocket motor simulator testbed was used to obtain a high temperature gas flow inside an electrically heated stainless steel pipe. The test section where the temperature measurements were made is located downstream of the heater and just before a convergent-divergent nozzle. This test section incorporates two viewing ports made of 1mm thick quartz plate to accommodate the laser beam of the Tunable Diode Laser Absorption Spectrometer (TDLAS) system. The TDLAS was originally designed to make measurement of temperature and velocity of hot gases inside the exhaust plume of a hybrid rocket motor. Consequently, the lasers were tuned to the absorption frequency of water vapor inside the combustion products.

For the present study, air was used as the working fluid. It was supplied from a compressed air cylinder which had low moisture concentrations. Therefore, the gas (air) flow had to be seeded with water vapor to obtain the necessary vapor concentrations for TDLAS measurements. This was accomplished by implementing a water vapor injector upstream of the heated section of the test apparatus. The superheated water vapor was supplied to the injector from an electrically heated, high pressure steam generator.

Temperature of the hot gas flow was also measured using flow-through thermocouples. Measurements were made and recorded during unsteady (startup and shutdown of the flow system) and steady state operation of the test apparatus. Experiments were repeated for different heat input rates and flow rates. Comparisons of the measurements indicate that TDLAS underestimate gas temperature by a constant value in the temperature ranges considered. A calibration (zeroing) of the TDLAS measurement improves the TDLAS readings. Based on this study, it is concluded that the TDLAS can be used to make real time temperature measurements of heated internal gas flows with reasonable accuracy.

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