Session: 02-02 Shape Memory Alloy and Polymer Applications

Paper Number: 164764

164764 - Optimization of a Shape Memory Alloy Engine for Heat Recovery 

A previous body of work at UCF, in partnership with NASA KSC, has considered shape memory alloy (SMA) thermal switches operating in a make or break contact mode. The extension here is to consider aspects of an SMA engine in continuous operation where the SMA is the heat engine and is not necessarily coupled with an external device to transfer heat. The objectives of this work are two-fold. First, it is to establish a framework for designing and implementing an SMA heat engine combining mechanical, thermal and materials aspects. Mechanical aspects of the framework were addressed from force balances in the SMA element and focused on the resulting stress distribution. Thermal aspects were addressed by considering the heat transfer rate between the SMA element and both the heat source and the heat sink. The effect of temperature of the heat source relative to the phase transformation temperatures of the SMA was also evaluated. The total enthalpy of the transformation was considered from chemical, elastic, and frictional contributions. Each of these terms was connected with the material microstructure through alloy selection and thermomechanical processing, making recommendations vis-á-vis the size and distribution of precipitates, variant interfaces through texture, cycling and training, defects, nucleation sites (bulk vs. surface), and multi-step transformations (e.g., a trigonal R-phase transformation). The second objective is to examine conditions for optimized performance based on the targeted application. Previous analyses have merely considered an overall Carnot efficiency without taking into consideration that SMA heat engines are advantageous and can run with heat scavenged from sources that are nearly limitless or are otherwise even wasted.  With heat or energy recovery applications in mind, the framework presented distinguishes between a high speed and a high torque mode of performance in contrast with more traditional power or total amount of heat transferred criteria. This was extended to consider a figure of merit (FOM) for the performance of an SMA heat engine in the various modes of operation.  The importance of separately using both an enthalpy term and a hysteresis term in the proposed FOM is emphasized through competition between the net enthalpy change and the elastic enthalpy change. Emphasis is additionally placed on methods for generating adequate starting torque to automatically activate engine operation, including heterogeneous phase transformation in the SMA and engine design specifics such as an offset axis of rotation or a spring with varying pitch. The implications of this work for recent elastocaloric SMA applications of interest are also presented.

Presenting Author: Maria Chikhareva University of Central Florida

Presenting Author Biography: Maria Chikhareva is currently with The Boeing Company in Seal Beach, CA as a Structural Analysis Engineer. She was a Fulbright Scholar at the University of Central Florida where she received her Masters in Materials Science and Engineering with a thesis on shape memory alloy heat engines. She is a graduate of the Bauman Moscow State Technical University and also worked at The Boeing Design Center and Schlumberger in Russia. She is pursuing her PhD in Materials Science and Engineering at the University of Central Florida working on shape memory alloys.