As portable electronics technology advances, systems are becoming smaller and more energy intensive. While batteries are currently the only commercial power source for these applications, work is being done to create liquid fuel based portable power packs. These systems would leverage the higher energy density (W-hr/l) and specific energy (W-hr/kg) of liquid hydrocarbon fuels over available battery chemistries. For micro engines and small fuels cells there are advantages to preheating and vaporizing the fuel in a microchannel.
The work presented in this dissertation focuses on understanding and characterizing the temperature and pressure signatures that result from microscale boiling of fuels in etched silicon channels approximately 100 mm in diameter. Building on previous microscale boiling work which used water as the working fluid for electronics cooling applications, the studies presented in this dissertation use both water and fuels including methanol, ethanol, and octane. Results are presented in the form of pressure and temperature measurements for a range of working fluids, volumetric flow rates, superheat temperatures and channel geometries. From a Fourier transform analysis of the pressure signatures, it was found that the frequency of the pressure fluctuations increases with superheat for ethanol as the working fluid while for methanol the frequency increases with volumetric flow rate. Tests were also conducted with sudden expansion geometries, which reduce the amplitude of the pressure fluctuations and create a localized cooling in the working fluid. Results are compared using fluid properties, including surface tension and heat of vaporization, and non-dimensional numbers including the Weber and the Jakob number. This study presents a significant contribution to the body of knowledge on microscale boiling.
One application of microscale boiling for portable power technologies is also presented. Fuel cracking, breaking apart of long hydrocarbon chain molecules into smaller, quicker combusting, lighter weight compounds is presented and preliminary results are given using thermal energy as the input to the system. While these studies conclude that it is difficult to crack fuels at temperatures similar to those measured on the outside of an engine combustion chamber, novel designs are presented which would incorporate a catalyst and significantly improve the probability of cracking.
May 31, 2006
Haendler, B. (2006). Microscale Phase Change of Fuels for MEMS Power Applications. (n.p.): University of California, Berkeley.