Numerical Simulations and Characterization of Thermally Driven Flows on the Microscale

Large thermal gradients can cause very nonintuitive effects in the flowfield, as flow motion and even a force (often referred to as a Knudsen thermal force) can be induced even with a freestream velocity of zero. These flows can be exploited on the microscale, where temperature gradients of 10E6K/m are achievable. These flows have been studied experimentally many times, and it has been shown that Knudsen forces have a bimodal relationship with pressure, where the peak is in the transitional flow regime. It has also been shown that these thermal gradients cause thermal diffusion, or species separation in a mixture. A MEMS based device called the Microscale In-Plane Knudsen Radiometric Actuator (MIKRA) was developed to use Knudsen forces to calculate pressure and gas composition. The direct simulation Monte Carlo (DSMC) method was used to analyze the device to calculate the device forces and calculate the flowfield. DSMC proved to be a reliable method of simulating these types of flows, as the force results agreed well with experiments, and the DSMC results matched the results of other numerical methods. N2 and H2O mixtures were also simulated, and it was shown that the force is sensitive to the composition. At the same pressure, the force is larger for mixtures dominated by N2. Heat flux is also larger for N2 dominated flows.