Hotwireanemometria

Hotwireanemometria is a flow measurement technique that uses a heated wire or film to infer the velocity of a fluid. The method is widely used in experimental fluid dynamics and turbulence research because of its high temporal resolution and, with appropriate probes, good spatial information in the near-wall region.

Principle and theory: A thin wire is electrically heated. The balance between electrical heating and convective cooling by the surrounding fluid sets the wire’s temperature and its electrical resistance. In practice, a feedback circuit often maintains the wire at a nearly constant temperature (constant-temperature anemometry), and the required power or voltage to do so is related to the flow velocity. The relationship is commonly expressed by King’s law: E^2 = A + B v^n, where E is the bridge voltage or wire power, v is the flow velocity, and A, B, and n are empirical constants that depend on wire geometry, wire material, and gas properties.

Probes and operation: Wires are typically platinum or tungsten with diameters of a few micrometers. For multi-component measurements, X-wires (two wires crossing at about 90 degrees) are used to capture velocity components in a plane, while single-wire probes provide one component. The electrical signal is a rapidly varying voltage corresponding to velocity fluctuations, with useful bandwidth ranging from hundreds of hertz up to tens of kilohertz, depending on wire and circuit design.

Calibration and limitations: Calibration is carried out in a known reference flow and may require corrections for temperature, pressure, and gas properties. Wires are fragile and subject to aging, oxidation, and coating by contaminants, which can drift the response. The technique primarily measures a single velocity component and is sensitive to thermal lag and environmental conditions; X-wire configurations mitigate this but increase complexity.

Applications: Hot-wire anemometry is employed in wind tunnels, combustion chambers, HVAC studies, and microfluidics to characterize mean flow, turbulence intensity, and velocity spectra. It offers high temporal resolution and small sensing volume but demands careful calibration and maintenance.