Experimental study on the 3-omega sensor depending on surface condition of forced convective heat transfer

Accurate determination of gas-mixture composition is essential for stable and efficient operation of energy systems. Thermal-conductivity sensors are attractive for this task because, compared with catalytic or electrochemical sensors, they offer simpler structures and a wider measurable concentration range. Here, the three-omega (3ω) method was employed to quantify the influence of surface airflow conditions on the measured thermal conductivity of a gaseous sample. Two sensor architectures - a microheater on a polyimide-substrate and a suspended-bridge microheater - were fabricated and tested under forced convection conditions. The devices were operated in a flow chamber, where the inlet flow rate was varied up to 200 L/min, and the 3ω temperature-amplitude signal was recorded. For the polyimide sensor, comparison with theoretical predictions indicates that the surface airflow is insensitive to retrieved substrate thermal conductivity over the tested range. By contrast, the suspended sensor exhibited little change at moderate flow but a pronounced decrease in signal at the highest flow, consistent with enhanced convective heat removal and a modified thermal boundary condition. These results delineate the operating envelope in which 3ω measurements remain robust in flowing gases, providing guidance for gas-composition metrology under flow conditions.