Continuous Monitoring of Binary Gas Mixture Concentration With Application to Turbine Blade Cooling Experiments

Turbine blade cooling experiments often use mixtures of air and a heavy gas (CO₂, SF₆) to simulate coolant/mainstream density ratios. If the mixing of the mainstream with the coolant ejected from the blade is of interest, then it may be necessary to determine the spatial distribution of the heavy gas concentration in the flowfield. Commercial analyzers are too slow and have other disadvantages when used for this purpose. To meet this special need, a device has been developed to continuously monitor the heavy gas concentration in a small sample stream by determining the speed of sound in the sample. Together with the temperature of the sample, the information is sufficient to determine the concentration. The device measures the time of propagation for an ultrasonic burst transverse to the stream. The temperature of the gas contained in the device is controlled and measured. Calibration with several gas mixtures (air and CO₂, SF₆, He) has shown an uncertainty (C_meas-C_true) of 2% over the full concentration range of 0–100%, for CO₂ and SF₆. The device is operable in the pressure range from −50 to 100 kPa gauge and in the temperature range from 0 to 40 °C. The instrument is rugged and will survive in noisy, turbulent environments.