Digitalization of High Accuracy Thermo-Anemometer for Heat Flux Measurement in Stratified Flow

In this study, the high-accuracy thermo-anemometer was developed to execute simultaneous measurement of temperature and velocity by employing Field Programmable Gate Array (FPGA) devices. A cold-wire temperature sensor has first-order lags due to thermal time constants, that means that a phase compensation is essential to measure temperature fluctuations with high frequency. The digital filter comprised the exact first-order lead characteristics was realized by the bilinear s-z transformation. It was verified that the temperature signals were appropriately compensated with an uncertainty of less than ±3.9% in frequency up to 10 kHz. For velocity measurements, a temperature compensation is indispensable owing to velocity and temperature dependence of response of a hot-wire anemometer in thermally stratified flow fields. The accuracy of the velocity measurements at high temperature was improved with the strict algorithm of the temperature compensation calculation incorporating the temperature dependence of the thermal properties of the air. The high-accuracy velocity measurement was realized within the accuracy of ±1.6% in the temperature range of 1.5 times as wide as that of the previous analog based system. The simultaneous measurement system for temperature and velocity was applicable to measure heat flux in thermally stratified flows.