Study of an integrated thermal sensor in different operational modes, under laminar, transitional and turbulent flow regimes

The behaviour of a thermal flow sensor attached to the wall of a square tube is examined, exposed to gas flow for Reynolds numbers in the range 0–4.1 × 104. Consisting of a polysilicon heater and a pair of thermopiles, the sensor’s three output signals (heater power, its electric resistance and the thermopile signal) are analyzed as a function of Re. Employing three different modes of operation (constant power (CP), constant voltage (CV) and constant temperature (CT)), it was found that the sensor sensitivity maximizes in CT mode. Heater power and its resistance vary with Rea where a changes significantly for Re > 2000. The thermopile signal, which corresponds to the streamwise temperature gradient across the heater, in contrast to the other two signals, is more sensitive in detecting flow disturbances. Its fluctuations increase with a high rate close to Re = 1800, a value which is smaller than the corresponding increase of the other two signals. The probability density function (pdf) of its fluctuations is negatively skewed consistently for the range 1500 < Re < 11,000 while for Re > 11,000 it turns to a symmetric one. The pdf of the other two signals do not show any systematic trends with Re. The most striking difference of the thermopile signal behaviour compared to the other two is its multivalued relationship with Re in the range 2000 < Re < 11,000.