Thermal property determination of laminar-flowing fluids utilizing the frequency response of a calorimetric flow sensor

We demonstrate that the thermal conductivity and diffusivity of fluids can be measured independently of their motion state utilizing a micromachined calorimetric sensor. The sensor membrane bears a heating resistor and two symmetrically arranged thermistors. By immersing the sensor into the laminar-flowing sample fluid and applying an AC heating current, the frequency response of the thermistor temperatures can be exploited to evaluate the thermal properties of the fluid. We developed a novel analytical model to describe the conductive transfer in the micromachined sensor as well as the fully conjugated heat transfer in the fluid. The validity of this model was confirmed experimentally by a practical example using nitrogen as fluid flowing through a rectangular flow channel. Based on these results, a thermal parameter extraction procedure can be deduced. Moreover, with known thermal parameters, the arrangement can also be used for flow measurements.