Determination of thermal properties of gases under flow conditions

This paper presents a calorimetric flow sensor that is capable of determining the thermal conductivity of flowing gases. The measuring principle relies on using AC heat generation at 200 Hz in order to confine the oscillatory thermal boundary layer within a very small distance from the surface, where, due to the non-slip condition, the flow velocity profile is always close to zero. For this, the sensor features a very sensitive Ge thermistor closely surrounded by a thin, circular chromium heater. The measurement signals are the amplitude and the phase shift of the temperature oscillations. A simple analytical model is developed to show that the phase shift signal is a function of the thermal conductivity (k) of the flowing gas only, which allows its direct measurement. This relationship fit experimentally results from many gases for flow rates up to 750 sccm. Measurement of the volumetric heat capacity (ρc_p) under flow conditions is also possible.