Flow And Heat Transfer In Micro Fluid-based Inertial Devices And Their Engineering Applications

Inertial sensors have long been widely used in many fields including the automobile industry, the military industry, industrial control, aeronautics and astronautics. Miniaturization, low cost and high performance of inertial sensors have been the topic of extensive research. The traditional designs of inertial sensors involve a solid proof mass or rotation part, which have some disadvantages such as low life, low reliability, difficult manufacturing and high cost. Fluid-based inertial sensors, developed in recent years, may be able to overcome these shortcomings. In the present work, two kinds of fluid-based inertial sensors were investigated theoretically and experimentally: (1) a micromachined convective accelerometer, and (2) a two-directional synthetic jet gyroscope. Compared with the common inertial devices, they offer significant practical advantages.Thermal optimization of the micromachined convective accelerometer was theoretically conducted and experimentally tested. The effects of sensor position, cavity size, operating power and gas media on the sensitivity, linearity and response frequency of the convective accelerometer were numerically analyzed. The results show that the convective accelerometer can achieve good linearity when the Grashof number ranges from 10-2 to 103. Temperature sensor position has obvious influence on the device's sensitivity; good linearity and high sensitivity can both be achieved when the non-dimensional sensor position, defined by the ratio of the sensor distance from the heater to the half cavity length, is around 0.3. Increasing heating power can increase sensitivity. The greater the density of the gas media, the higher is the sensitivity, however, the response frequency decreases. Based on the above theoretical calculations, the structure (i.e., cavity size, heater width, sensor position, gas media) of the existing convective accelerometer can be improved, the experimental tests on the optimized device confirm these expectations. To achieve even higher sensitivity and better linearity, a design in which four temperature sensors are distributed in four bridge arms is proposed, this design remains to be tested experimentally.To further improve the accelerometer, size effect on free convection was numerically studied. The results demonstrate that, compared to the inertial force, the effect of viscous force on free convection increases with decreased size; this results in heat transfer correlation for small Ra different from the standard one for macroscale free convection. For the heat transfer of free convection in a two-dimensional square cavity, three regions are specified and the corresponding correlations are fitted. Study on the relative importance of free convection and surface radiation shows that the relative importance of surface radiation either in large space or in square cavity, decreases as the characteristic size declines; free convection will gradually dominate the heat transfer. In the second part of this work, the design criterion and the flow characteristics of synthetic jet were studied. For the piezoelectric actuator, the resonance frequency was analytically studied based on the energy method, knowing the resonance frequency will facilitate the development of engineering applications for the actuator. The square wave signal can make the piezoelectric actuator gain a higher amplitude than with a sine wave, triangular wave, or toothed wave signal. The synthetic jet exhibits self-similarity and is like the conventional jet only at distances greater than 10 times the orifice diameter away from the orifice. Based on the investigations of the synthetic jet, the concept of a two-directional synthetic jet gyroscope is proposed for the first time. The preliminary design of such a gyroscope is presented, and some optimal parameters for the synthetic jet and the sensor position are given.