Towards Thermal Flowsensing With pL/s Resolution

摘要

This paper focuses on the development of highly sensitive calorimetric flow sensors. Both the hydrodynamics of the flow channel, as well as the heat transfer are analyzed in detail. With the expressions for the hydraulic resistance of the flow chanenls and the hydraulic system requirements for the flow sensor, it is possible to optimize the design of the flow channel. It is theoretically shown why for small v the calorimetric flow sensor output is linear in v. Also it follows from gheory that for a symmetrical configuration, in this linear regime the heater temperature is independent of v for constant heating power. This suggests that for low Reynolds Number, King's Law has to be modified. Different configurations and methods are analyzed: absolute, differential, two beam, three beam, CPA, CTA and TBA. The latter is a real thermal balance measurement and allows the use of non-linear sensing elements. From our experiment with acoustical measurements it is possible to estimate practical attainable sensitivity. In combination with proper flow channel design, and a fabrication technology for narrow channels, it is shown that pL/s sensitivity is in reach.