Design of a laser absorption spectroscopy based compact gas analysing sensor for high speed flow diagnosis

Abstract

To diagnose the operation state of a high-speed aerospace engine, a non-intrusive sensor that can measure the flow properties could be required. The tunable diode laser absorption spectroscopy (TDLAS), which can characterize the flow through spectroscopic measurement and analysis by using a tunable laser source, is a suitable technique for such high-speed flow diagnosis. In this study, a laser absorption spectroscopy-based compact gas analysing sensor for high-speed flow diagnosis is designed and applied to stationary gas conditions and supersonic flow conditions which have various pressures and temperatures.

A gas analysing sensor for an air-breathing engine intake is designed based on laser absorption spectroscopy. The number of optical and electrical components is reduced and the characteristics are optimized to detect the weak absorption lines of oxygen molecules while making the gas analysing sensor smaller. A triangular spiral-shaped laser beam path, which could be applied to a rectangular flow passage, is proposed for the doppler shift detection and absorption strength amplification. The design concepts of adjustable hardware components are proposed to miniaturize the gas analysing sensor. The procedure of converting the temporal raw signal measured by the gas analysing sensor into a spectral absorbance is introduced.

A spectroscopy method for the designed gas analysing sensor is proposed to measure multiple gas properties from a single absorption line. Although the usage of multiple absorption lines can improve the measurement accuracy and the number of measurable gas properties, measurement of multiple absorption lines could be difficult in the designed gas analysing sensor because it utilizes a single laser source. Therefore, a spectroscopy method that can measure the multiple gas properties is required. In the singleline absorption spectroscopy method proposed in this study, absorbance loss that may occur in the direct absorption spectroscopy is predicted and the measured absorption line is recovered by using the predicted absorbance. Multiple gas properties such as pressure and temperature are then determined by matching the recovered absorption line with the calculated line. To verify that the proposed single-line absorption spectroscopy method could be used for gas property measurement, air conditions with various pressures and temperatures are made in a gas chamber. The gas properties of the test conditions measured using the gas analysing sensor are compared with the gas properties measured using the traditional sensors.

To verify that the designed compact gas analysing sensor is applicable for the high-speed flow diagnosis, supersonic wind tunnel tests are performed. A shock tunnel and a supersonic wind tunnel with an electric heater are used as the ground facilities. The pressure, temperature, and speed of the generated supersonic flow are measured using the designed compact gas analysing sensor with the proposed single-line absorption spectroscopy method. The measured flow properties are compared with the values measured and calculated using traditional techniques such as pressure sensors, shock angle measurement via flow visualization, and isentropic relation calculation. It is confirmed that the compact gas analysing sensor designed in this study could be applicable for the high-speed flow diagnosis.