Semi-Empirical Approach to Gas Flow Velocity Measurement by Means of the Thermal Time-of-Flight Method—Further Investigation

Abstract

This paper describes a study to expand the knowledge as to whether a thermal wave anemometer can be used to measure the velocity of flowing gases or gas mixtures in situ. For this purpose, several series of measurements were performed in laboratory conditions using both the previously used probe and other probes of similar design. The probes were not modified mechanically or electrically in any way. The obtained results were compared with each other, and on this basis, the optimal, though purely empirical, form of the calibration function was determined (4). The analysis of the relative differences between the measured and set velocity values showed that they do not exceed 1% in the velocity range from 0.05 to 2.5 m/s. Lowering the sensitivity of the method for velocities below approx. 0.05 m/s results in a rapid increase in the observed deviations, reaching 15% for 0.015 m/s. The conducted research also revealed an increased resistance of the proposed measurement method to small flow disturbances, both longitudinal and transverse, and a reduced sensitivity to non-optimal positioning of the probe in relation to the flow direction, relative to the methods using both detectors.

Micromachined Thermal Time-of-Flight Flow Sensors and Their Applications

“…With the known hotwire driving frequency and comparing the sensing wire signal to the same, the spatial wavelength of the heat convection pattern was measured, and the air mean convection velocity was calculated. In recent years, quite a few excellent research works based on this concept have been published [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ]. In these studies, the approach was also named pulsed anemometers.…”

“…According to the numerical and analytical study on the thermal time-of-flight sensor [ 34 ], the time shift (time-of-flight) related to the flow velocity depends on both advection and convection shifts. The advection time shift is nearly proportional to the reciprocal of the flow velocity with some nonlinearity added by thermal diffusion.…”