Semester

Fall

Date of Graduation

2018

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

Songgang Qiu

Committee Co-Chair

V'yacheslav Akkerman

Committee Member

V'yacheslav Akkerman

Committee Member

Hailin Li

Committee Member

Fernando Lima

Committee Member

Kenneth Means

Abstract

Stirling engines are often overlooked as a means of energy conversion, though its versatility offers many advantages. Since it is an external combustion engine, it can be supplied with heat through the combustion of a variety of fuels. Its maintenance- and degradation-free aspects, as well as its excellent turn down ratio, make it a suitable candidate for applications such as remote power generation, combined heat and power (CHP), cryogenics, solar power, and even space exploration. The continual advancement of additive manufacturing (AM) offers opportunity to potentially further improve the already efficient Stirling cycle. This dissertation presents the design and optimization of a 1 kW Stirling engine with the heater head and regenerator developed through AM.

This work was divided into four specific tasks: heater head geometry selection for reduced conduction losses, analysis of a foil regenerator made through AM, the impact of flexure bearing geometry and clamping method on stress and fatigue life, and how the configuration and quantity of radiation shields in the displacer affect radiation heat transfer losses. The results of these tasks were discussed, and design recommendations for these four components were provided.

A head heater head was designed and developed with AM that allowed for a previously unattainable geometry with a fully integrated pressure vessel and heat exchanger that could significantly reduce detrimental dead volumes. In addition, tapering of the heater head wall was shown to significantly reduce conduction losses; considered taper options had losses, which were 23.8 to 37.7% less than that of a constant thickness heater head. A regenerator was designed and developed with both foil thickness and gaps between foils set as 300 µm. A foil regenerator developed through AM has the advantages of having less friction losses than traditional random fiber regenerators, and the manufacturing process is less expensive than micro-machining. If 100 µm foils could be printed with AM (current technology achieves a minimum thickness of approximately 200 µm), the Sage results show a 3.6 percent increase in cycle efficiency is possible compared to a random fiber regenerator of the same dimensions. FEA showed the clamping method of flexure bearings had a significant effect on stress and fatigue life; the radially clamped condition resulted in maximum stress values 26.3%-28.2% less than those obtained with the outer ring clamped. A spiral flexure was designed and manufactured with Sandvik 7C27Mo2, which can provide theoretically infinite life. The flexure design was validated with laboratory testing. FEA showed that for a given number of radiation shields, there is an optimal placement along the length of the displacer that yields the lowest radiation losses. The results also show that uniformly spacing the radiation shields out along the length of the displacer has the best performance. Based on the dimensions and boundary conditions considered, five radiation shields provide optimal resistance to heat transfer. The insertion of radiation shields can reduce the temperature and total heat flux in the displacer body by 40 % and 55 %, respectively.

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