Abstract
This work presents a thermal model for thin film anemometers and shows the importance of the characteristic length of the heater in convection phenomena. The techniques of silicon micro-machining were used to manufacture an anemometer with low electric consumption and great sensitivity. A thermal model of the sensor is described and then confronted with experimental results. The calculation of thermal conductance due to convection, radiation and conduction is presented. The coefficient of convection was deducted and it was found very high in comparison with the one of a macroscopic structure: 200 W m-2 K-1 typically for free convection. The response of the sensor in the airflow is also presented as a function of the coefficient of convection or a function of the flow velocity and it shows that the coefficient of convection does not follow the macroscopic laws. Then, a new sensor with a larger characteristic length was manufactured and its response and coefficient of convection are compared to the smaller sensor ones.