Heat transfer analysis and experimental verification in flexible MEMS thermal films

Abstract

Besides the convective heat transfer on the surface by a fluid, a considerable portion of the heat in a thermal film is lost to the sensor substrate and measured wall via heat conduction. Heat loss negatively impacts the frequency response and sensitivity of thermal films. To maximise the frequency response and sensitivity, the substrate material must be prioritised according to the frequency response criteria for screening. The effects of wind speed on the frequency response and of substrate thickness on sensitivity were investigated. First, a heat-transfer model was established, mathematical calculations were conducted, and materials with superior comprehensive performance were chosen as flexible substrates. Subsequently, FLUENT was used for three-dimensional simulations of the thermal film heat transfer. The results confirm the appropriate selection of the substrate in terms of heat transfer distribution. The effect of substrate thickness on the thermal film heat transfer was studied. Finally, wind tunnel experiments were conducted to verify the effect of wind speed on the response time and that of substrate thickness on sensitivity. Through mathematical calculations, simulations, and experiments, it was confirmed that wind speed has a positive effect on the frequency response, the substrate with lower thermal-conductivity and diffusivity is the optimal choice, and the substrate thickness has a positive effect on sensitivity. To measure fluid parameters such as the shear stress using the flexible MEMS thermal film, a thicker substrate was chosen as much as possible until it interfered with the fluid.