Development and Experimental Analyses of Meso and Micro Scale Combustion Systems
Open Access
- Author:
- Wu, Ming-hsun
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 27, 2007
- Committee Members:
- Richard A Yetter, Committee Chair/Co-Chair
Vigor Yang, Committee Member
Srinivas A Tadigadapa, Committee Member
Kendra Sharp, Committee Member - Keywords:
- microcombustion
micropropulsion
microfluidic
flame propagation - Abstract:
- The present research was concerned with the development and analysis of meso and micro scale combustion systems for propulsion applications. In a more general sense, the research focused on understanding combustion, heat transfer, and fluid mechanics of chemically reacting flows in small volumes and channels as well as the fabrication and diagnostic aspects of the systems. A series of small scale combustion systems were investigated that operated with gaseous, liquid, and solid reactants. Both steady-state and transient modes of combustion were studied in confinements where the characteristic length scale of the combustion volume approached that of the reaction zone. Scaling issues related to combustion kinetics as well as fluid and thermal transport were reviewed. The insufficient flow residence time compared to the characteristic chemical time was identified as the critical issue for combustion at the microscale. This challenge was addressed by increasing the flow residence time using vortex flows and decreasing chemical reaction times with oxygen enrichment. Liquid nitromethane combustion, which has a higher volumetric power density than gaseous fuels, was also studied in mesoscale vortex chambers. Pressurization and oxygen enrichment were utilized to achieve high efficiency liquid propellant combustion in miniaturized chambers. However, combustion with high oxygen concentration in small confinements raises safety concerns. In the present study, deflagration-to-detonation (DDT) was studied to address these concerns and DDT was indeed observed for gaseous flames ignited in capillary tubes with diameters less than 1 mm. Three propagation modes were identified as the size of the tube diameter approached the quenching limit. Propagation of nanoscale Al/MoO3 thermite reactions in channels with dimensions on the order of a hundred microns was also demonstrated. The shorter reaction time of these transient processes can potentially overcome many of the thermal management problems of microscale combustion, since the time scales of the important chemical reactions become much smaller than the thermal diffusion time scales. Micro counterflow diffusion flame burners were fabricated using low temperature co-fired ceramic (LTCC) tape technology, and were successfully operated with homogeneous gas-phase combustion. Unique to these burners was the incorporation of optical quality windows into the build process. A microscopic imaging spectrometer was developed to analyze the flame structures of the micro diffusion flames stabilized in the burners.. One-dimensional spatial distributions of CH* and C2* species across the sub-millimeter scale flames were obtained using this technique, and compared with three-dimensional numerical model predictions based on detailed kinetics. The present research revealed the achievability of many different types of combustion processes at millimeter and sub-millimeter scales. In addition to micro propulsion, the knowledge obtained and techniques developed can be applied to many other micro reacting flow applications (e.g., fuel reforming, actuation, fluid pumping, power generation, toxic incineration, etc).