Directional Sensitivity of Hot Film Sensors in Liquid Metals

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Abstract
In order to determine whether the thick thermal boundary layers in liquid metals lead to reduced directional sensitivity of hot film sensors, steady state calibrations for speed and directional sensitivity of cylindrical hot film sensors having aspect ratios of about 19 were made in a liquid metal by towing them through a long pool of mercury. The directional sensitivity is characterized by an empirical parameter, k, which varies from zero to one as the sensitivity varies from its maximum to zero. The results show that for measurements of flow direction, the sensor aspect ratio must exceed 20 for sensor Peclet numbers less than about 0.35. The k are only weakly dependent on angle of inclination in the range 30–60°, a result which can be used to reduce the labor of calibration.