Flow velocity is an important parameter in turbulent flow studies, fluid mechanics and related applications. Currently, hot-wire anemometers are widely used in measuring flow velocity owing to their relatively high spatial and temporal resolutions. This technique depends on the variation of the wire resistance with temperature. Unlike conventional hot-wire anemometry, fiber-optic hot-wire anemometry offers numerous advantages including resistance to harsh environments, immunity to electromagnetic interference and workability in remote areas. As such, they have been highly considered great potential for next-generation hot-wire anemometry.
Depending on the operation mode, the flow velocity can be deduced as a function of the measured temperature variation or the heating current. Both fiber-optic hot-wire anemometry and conventional hot-wire anemometry have been effectively operated under constant power and constant-current modes, which are almost similar. However, conventional hot-wire anemometry can also be operated under constant temperature mode, which is considered more convenient and efficient. For instance, high measuring speed is achieved at constant temperature due to minimal thermal inertia effects. Unfortunately, the application of the constant temperature mode in the fiber-optic hot-wire anemometer has not been fully investigated.
To this note, Michigan State University researchers: Mr. Nezam Uddin, Dr. Guigen Liu, Mr. Qiwen Sheng, and Dr. Ming Han from the Department of Electrical and Computer Engineering demonstrated constant temperature operation of a fiber-optic hot-wire anemometer. They further investigated and compared the frequency response operation in both the constant temperature and constant power operating modes. The work is published in the journal, Optics Letters.
A fiber-optic anemometer based on a 200 µm silicon thick Fabry-Perot interferometer was used. The anemometer was heated through a diode laser and the Fabry-Perot interferometer temperature was kept constant by adjusting the heating laser power through a feedback control loop. This way, the heating laser power was utilized as the output signal for flow velocity estimation. Additionally, a 1550 nm diode laser was used to measure the temperature. To simulate the effects of the flow changes, the silicon Fabry-Perot interferometer was exposed to radiation from an external intensity-modulated laser beam. This gave a clear picture of the relationship between the anemometer response and radiation power.
The constant temperature operation significantly improved the frequency response as compared to the constant current operation mode that uses the Fabry-Perot interferometer temperature as the output signal. This represented an increase of 2 kHz for constant temperature operation up from 0.5 Hz for constant power operation. On the other hand, a record reduction in the 10%-90% rise time of the step response from 625ms to 1.8ms was recorded for constant power and constant temperature respectively. Furthermore, the anemometer response exhibited relatively good linearity to radiation power.
The constant temperature is a better operation mode for anemometers. It prevents thermal cycles by the wire as observed in constant current and also reduces the effects of temperature on the sensitivity thus ensuring consistent performance. Together with the improved frequency response, these benefits make constant temperature operation of fiber-optic hot-wire anemometry a game-changer in measuring turbulent flows and particularly those with rapidly changing flows.
Uddin, N., Liu, G., Sheng, Q., & Han, M. (2019). Constant temperature operation of fiber-optic hot-wire anemometers. Optics Letters, 44(10), 2578.Go To Optics Letters