Hot-wire measurement technology

 
Hot-wire probe Copyright: © RWTH Aachen | IST

The operating principle of this measuring technique is the transfer of heat from a hot surface into a flow. The heat transfer depends on the surface temperature and the flow variables temperature, density and velocity.

Measurement Procedure

At IST the CTA (constant temperature anemometry) method is used, in which a wire (usually made of tungsten) is kept at a constant temperature. If the temperature difference to the flow medium is known, there is a clear correlation between heating power and the heat transfer coefficient. The heat transfer coefficient is proportional to the so-called mass flow density (product of density and flow velocity) perpendicular to the measuring wire.

The relationship is non-linear and directional. The measuring arrangement supplies a voltage as an output signal which is dependent on the change in heat dissipation. The calibration of the probe should be carried out at the flow temperature which also prevails in the measured flow. If this is not possible, there are correction procedures which, however, have a negative effect on the measuring uncertainty. From the voltages of a 3-wire probe, the angle of incidence of the probe as well as the mass flow density can be determined with the use of calibration polynomials. To determine the flow velocity, the (mean) temperature and pressure of the flow must also be known.

Limits of the field of application

Hot-wire probes work with sensor strengths in the micrometer scale and are therefore not suitable for use in harsh environments. Due to the mutual influence of the wires and the angles under which they are arranged, the measuring range is reduced compared to 5-hole probes. However, the sensors can detect unsteady flow processes very well.

Measured variables

  • mass flow density
  • Flow angle (pitch and yaw angle)
  • Turbulence intensity
  • Reynolds Stresses