Abstract:
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The presented work paper describes a high accuracy method of gas flow velocity measurements. The method employs a continuous, sine thermal wave rather than a short pulse. Applied probe consists of three thin wires. One of them is a transmitter of thermal wave and operates in CTA mode. Computer controlled CTA system constraints transmitter's overheating ratio to vary sinusoidal with predicted amplitude and frequency regardless of flow velocity. Two remaining wires, placed downstream to the transmitter are wave detectors operating in CC mode. Signals from the wave detectors are sent via an A/C converter to the computer and then are processed. The flow velocity is derived on the basis of measured thermal wave phase shift passing detectors. Presented analysis, based on the heat conduction equation leads to formulas necessary to flow velocity calculation as well as requirements for probe and range of velocity to be measured. There are two techniques applied in velocity measurements: the first consists of the phase shift measurements at established wave frequency and the other where the phase shift was measured at some manifold wave frequencies and then the dependence the phase shift on the frequency was used. For both methods an analysis of uncertainty was performed and some of the results of the testing in a laminar air flow for flow velocity ranging from 20 cm/s to 300 cm/s was compared. Achieved accuracy of the velocity measurements was better than 0.2 cm/s. In contrary to PWA technique presented method doesn't need calibration of probe, and can be used under some restrictions to gases with changeable composition. Described method is destined to calibration of wind tunnels as auxiliary methods and to precise measurements of laminar flows.
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