Transient gas viscosity measurement using tunable diode laser absorption spectroscopy

We have designed and implemented a non-intrusive method for measuring gas viscosity coefficients of gases with accessible absorbing transitions based on transient tunable diode laser absorption spectroscopy (TDLAS) in a 20 µm hollow-core photonic crystal fiber (HC-PCF). As the system contains no moving parts and can be used with a fiber of arbitrary length, it is suitable for viscosity measurements at elevated temperature and under low-density conditions. The gas flow through HC-PCF is driven by a known, constant pressure gradient imposed by gas in two gas cells at either end of the fiber. Infrared radiation from a diode laser is coupled to the fiber to be guided through the gas. A photodetector is placed at the fiber exit to measure the change of light intensity due to absorption from the molecular species. The path-averaged number density of the flow undergoing this transient process can be monitored in real time by TDLAS, based on the Beer–Lambert law, relating absorbance to gas concentration. An existing numerical model describing the time-varying local number density distribution is applied for the numerical determination of viscosity coefficient, by relating the viscosity to the time required for the gas flowing through the fiber to reach a steady state. This measurement method is validated by measuring the viscosity of CO₂ as a reference gas at room temperature and inlet pressure ranging from 29 to 3.6 kPa, extending the lower limit of viscosity measurements to pressures below 10 kPa, where laboratory data are lacking. The experimental outcomes are in reasonable agreement with the theoretical value, confirming the effectiveness of this new measurement technique.