Surface Mounted Hot-Film Anemometry

 
Copyright: © RWTH Aachen | IST

The surface hot film anemometry is based on the same principle as the hot wire measurement technique. An electrical resistance in the form of a thin structured metal film ("surface hot film sensor") is cooled by a passing flow. In constant temperature anemometers, for example, the electrical power that must be applied to maintain a fixed temperature of the hot-film sensor is used as the measured variable. The heat transfer on a wall, and thus also on a hot-film sensor, is particularly dependent on the wall shear stress. By using several hot-film transducers in line, the qualitative profile of the wall shear stress on turbomachinery blades can be determined. From this, the point of laminar-turbulent transition can be determined.

Application

Hot films are applied to compressor and turbine blades to determine the location of transition. The individual sensors consist of a 10 μm thick gold or nickel layer 0.1 mm wide and 0.9 mm long. They are vapour-deposited at a distance of 2 - 3 mm on a 50 μm thick base film. In the example shown above, the connection of the sensors to the measuring system was realized with flexible and individually manufactured conductor tracks that connect the sensors to the measuring bridge.

By determining the temperature-dependent resistance of the hot-film sensor, the profile of the wall shear stress can be qualitatively determined from the measurement data (the so-called quasi-wall shear stress).

Measured variables

  • Quasi-Wall Shear Stresses
  • Location of Transition