Miniature fiber-optic probes in wind tunnels: how to decode the secrets of high-speed flow fields in a small body?

The development of high pressure compressors and turbines has been playing a key role in modern aero-engine manufacturing. As one of the core components of an aero-engine, the turbine faces great challenges in its design and testing process. Harsh operating environments and complex flow field conditions make the performance evaluation of turbines extremely complex. The interaction of rotating and non-rotating components in the turbine, secondary flow or blade wake shedding usually results in highly fluctuating flow fields. Therefore, the sources of non-constancy must be understood by some technical means before designing the turbine.

With the development and optimization of Computational Fluid Dynamics (CFD), some flow states can be learned numerically, but experimental studies are still of interest to the scientific community and industry. Experiments on turbines occupy an important place in obtaining compressor blade stage performance as well as propeller blade flow. A wide variety of measurement techniques and instruments are used for turbine applications, including particle image velocimetry, laser Doppler velocimetry, laser bifocal methods, hot-wire velocimetry, and porous pressure probes.

In turbine measurements, the lack of sufficient optical access prevents the use of PIV, LDV and L2F, which are mainly driven by free-space optics.In addition, HWA (Hot-Wire Anemometry) suffers from mechanical stability and gas contamination, and is therefore less efficient in some cases. Therefore, in order to solve these problems, some studies have been conducted to install the sensors close to the MHP ports in order to reduce the pneumatic transmission distance and improve the accuracy and reliability of the measurements.

In recent years, fiber optic sensing technology has emerged as a new measurement option. Fiber optic sensors have the advantages of small size, geometric flexibility, easy distribution and networking, as well as immunity to most electromagnetic interference. Recent research has focused on the application of fiber optic sensors for measuring velocity magnitude in airflow fields, including fiber optic anemometers based on dual-channel Fabry-Perot interferometers, as well as single-channel differential pressure FP sensors and diaphragm sensors with built-in fiber optic Bragg gratings.

Although a variety of MHP devices have been designed for discretization of flow velocity vectors and associated flow characteristic measurements, the conventional THP remains the best known and widely used.The THP typically consists of a streamlined axisymmetric body pointing towards the airflow field for two-dimensional flow field measurements. It has three pressure-sensing ports that can be used to measure the direction and magnitude of the flow velocity vector. Depending on the yaw angle of the flow velocity vector, the pressure distribution of the pressure ports will be different.

In order to reduce the aerodynamic transmission distance, an aerodynamic cylindrical probe with a piezoresistive Wheatstone bridge-type pressure sensor chip on the top was developed to confirm the rapid decay of the radial compressor impeller outlet. Also, a number of smart probes were designed to measure the flow angle with multiple pressure sensors mounted on these probes to sense the pressure at the tip port.