MODELLING OF A THERMAL DISPERSION MASS FLOW METER

This paper deals with the modelling of a thermal dispersion mass flow meter. A steady-state, two-dimensional numerical model of a thermal flow sensor is built, and the heat transfer conditions in the thermal flow sensor and the effects of influential parameters on its measurement characteristic are analysed. The modelled sensor is considered to have an axisymmetric internal structure with defined dimensional and material properties. It is inserted into the flow pipe with a gas flow that has a defined pressure, temperature, mass flow rate and a fully developed velocity profile. The free parameters of the model are set to obtain the best fit between the sensor's modelled and measured characteristics. A heat transfer analysis shows that the heat convection is, as expected, the most dominant heat transfer mechanism, but both the heat radiation and the heat conduction along the sensor's stem are significant as well. The thermal conductivity of the sensor's filler material significantly affects the measurement characteristic, the radial temperature gradient in the sensor and its surface temperature. The length of the part of the sensor outside the flow pipe influences the heat loss along the sensor's stem, while the length of the part of the sensor in the flow pipe affects the convective and radiative heat transfer. The effects of these dimensional parameters are relatively larger at lower mass flow rates.