THE SIMULATIONS OF FLOW AND HEAT OVER MICROSCALE SENSORS IN SUPERSONIC RAREFIED GAS FLOWS USING DSMC

As the use of MEMS-based devices and systems are continuously increasing, the understanding of their correct characteristics becomes so serious for the related researches. In this study, the supersonic rarefied gas flow over microscale hotwires is investigated using the Direct Simulation Monte Carlo (DSMC) method. Indeed, the DSMC has been accepted as a powerful method to study the rarefied gas flow especially in transitional regime. Therefore, it can be considered as a reliable method to investigate the rarefied supersonic flow over microscale objects including the microscale hotwires. In this work, we study the effective parameters, which affect the performance of these sensors at constant sensor surface temperature conditions. We use our developed DSMC code to perform our investigation. This code uses the DSMC algorithm to solve the rarefied gas flow on unstructured grid distributions. To validate our developed DSMC code, we solve the supersonic rarefied gas flow and heat transfer in microchannel considering different Knudsen number magnitudes. Comparing the achieved flow and heat transfer solutions with other available results and data reported on microchannel studies, we verify the accuracy of achieved results. Next we focus on hotwire sensor, which often consists of the combinations of different long narrow circular cylinders. We study the effects of grid resolution, time step size, and the number of simulated particles on the obtained results. We further study the effects of sensor temperature and sensor diameter on the sensor thermal performance. The achieved results indicate that the surface heat flux performs very similarly in different studied cases. For example, the achieved local Nusselt number distributions around the circular sensor show that the surface heat flux would gradually increase from the sensor stagnation point to its rear end as the temperature gradient increases. It reaches to a maximum magnitude and it then starts decreasing resulting in effective heat flux reduction. Finally, there is a low pressure zone at the rear side of cylinder, which is not considerably affected by the flow properties. The results also show that if the wire surface temperature increases, the Nusselt number would reduce. However, the amount of Nusselt Number reduction rate would decrease as the temperature increases. Furthermore, the results show that the Reynolds number decreases and the Knudsen number increases as the sensor diameter decreases, which is due to the transitional regime behavior. As is known, the flow at boundaries change the condition from the slip to transitional regime when the Knudsen number increases sufficiently; and the flow become rarefied. There is a reduction in the total heat flux rate as the sensor diameter is reduced.