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

Abstract

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 laminarflowing 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.

Determination of the thermal conductivity of gases under flow conditions

“…As most of the state-of-the-art works [6][7][8] suggest, it is necessary to know the flow velocity in order to determine the thermal properties of the fluid. The natural solution is thus to stop the fluid motion or to bring the heat transfer process into a region where the fluid is basically motionless.…”

“…Other researchers have also successfully used oscillatory (AC) thermal excitation to simultaneously determine k and α of stagnant or moving liquids [6,7]. The main drawback is that the flow speed must be known beforehand.…”

Gas concentration and flow speed measurements using a polymer-based membrane sensor

“…In this experiment it could be shown that only the heater and the downstream temperature sensor are necessary for determining gas concentration and flow speed. The fabrication process is presented in detail in a previous work [7]. …”

“…Most research groups working on this problem use oscillating heat generation. In these investigations either the flow rate has to be known in advance or the sensors operate under no flow conditions [7].…”

Simulation of the frequency response of a thermal flow sensor in gaseous media

“…AC driven calorimetric flow sensors are also used to characterize the physical properties of the fluid only when the flow conditions were already established [3,[8][9][10], i.e., the flow speed is known. Frequency analysis of thermal flow sensor does not represent any novelty itself [3,8,9,11,12].…”