Characterization and modeling of a MEMS-based resonant micro heat engine.

摘要

The focus of this dissertation is investigating the dynamic and the thermal behavior of a MEMS-based resonant micro heat engine. The thermodynamic cycle of the engine is also investigated. To completely characterize the operation of the engine a new experimental setup is built and used. A lumped parameter model of the engine is developed.;The model is validated against measured data. The velocity transfer function shows a broad plateau with a maximum value of 2.5mm/s/W. No resonant peak is observed over the frequency range 0.1--1000Hz. To reduce the resonant frequency of the engine a small mass is placed on top of the engine. The resonant frequency of the engine is reduced to 100Hz. The amplitude of the velocity transfer function reaches a maximum value of 10mm/s/W at resonant frequency. Thus, at resonance the amplitude of velocity transfer function is increased by a factor of 4. Experiments show that the amplitude of velocity transfer function increases by a factor of 1.5--8.5 when added mass is thermally isolated from the engine. Pulse durations less than 10% of the total engine cycle period are desirable.;Both resonant and off-resonant operations of the engine are investigated. The results show that resonant operation is valuable. For a fixed amount of energy delivered to the engine, as the resonant operation is approached the cycle opens up and more mechanical work is observed. However, for an off-resonant operation pressure and volume become coupled and less mechanical work is observed.;The thermodynamic cycle of the engine has been acquired experimentally. Vapor pressure, vapor volume, vapor temperature, and vapor entropy changes inside the cavity of the engine are determined. The maximum measured pressure and volume changes are 45Pa and 0.55mm3, respectively. The data show that the engine operates at vapor temperature gradient less than 1°C. Albeit the temperature gradient is low, the measured second law efficiency of the micro heat engine is about 16%. Major sources of irreversibility in the engine are heat transfer over finite temperature differences during heat addition and rejection, heat transfer into and out of engine thermal mass and viscous losses due to liquid working fluid motion.