| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 88 === This study analyze the thermal and flow characteristics of the sensor tube in a thermal mass flow meter (TMFM) via the thermal-difference experiment and numerical simulations. Experiments were performed to study the thermal responses of two types of sensing circuit (constant-temperature and constant-current modes), and effects on the heating section with three different flow passages of sensor tube. Numerical simulations obtained from software packages (FLUENT) were also performed to validate experimental results. Simulations emphasized on the following issues: 1) analysis of the heat loss phenomena from the sensor tube wall, 2) predictions of temperature variation inside the tube, 3) effects of flow passages on the thermal flux of the heating section of the tube.
Measurements from the thermal-difference experiments were made via the constant- temperature and constant-current sensing circuits; which were used to analyze the relations among the thermal-difference (temperature and thermal flux) from the two heating sections and the flowrate passing through the sensor tube. Experimental results show that sensor tube with the constant-temperature mode has faster response time, its output voltage and flowrate relation is nonlinear, and it was prone to influence of environmental temperature. The characteristics of sensor tube with the constant-current sensing circuit was: wider range of working temperature, slower response time, and roughly a linear relation between output voltage and flowrate.
Disagreement of the thermal flux of the two heating sections among the measurement data with the constant-temperature mode and the numerical simulations do exist. Such discrepancy should caused by the unequal length of two handmade heating coils wrapped around the sensor tube. The discrepancy was eliminated by comparison in terms of the thermal flux difference from two heating sections, and good agreement were obtained between numerical and experimental approaches.
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