Study on the transient characteristics of the sensor tube of a thermal mass flow meter

Abstract

Thermal mass flow meters (TMFMs) are most widely used for measuring mass flow rates in the semiconductor industry. A TMFM should have a short response time in order to measure the flow rate rapidly and to measure the time-varying flow rate with accuracy. Therefore it is important to study transient heat transfer phenomena in the senor tube of a TMFM. Many models have been presented by previous investigators. But all of them focused on steady heat transfer phenomena, so it is impossible to analyze transient heat transfer phenomena in the sensor tube using previous models. Furthermore, it is impossible to predict the response time using results from previous research works. In the present thesis, a simple numerical model for transient heat transfer phenomena of the sensor tube of a TMFM is presented. The proposed model treats the fluid region and the tube region separately. Numerical solutions for the tube temperature and the fluid temperature at a transient state are obtained using the proposed model and compared with experimental results to validate the proposed model. Based on numerical solutions, heat transfer mechanism at a transient state in the sensor tube is explained. In the present thesis, a correlation for predicting the response time of a sensor tube is also presented. The functional form of the correlation is obtained using scale analysis and coefficients appearing in the correlation are obtained by the proposed numerical model. The correlation is verified by experimental results. Using the proposed correlation, physical meaning and characteristics of the response time of the sensor tube are presented.