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Heat Engine Driven by Shape Memory Alloys: Prototyping and Design

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Heat Engine Driven by Shape Memory Alloys: Prototyping and Design Heat Engine Driven by Shape Memory Alloys: Prototyping and Design Ean H. Schiller Thesis submitted to the Faculty of Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for a degree of Master of Science in Mechanical Engineering Dr. Charles Reinholtz, Chairman Dr. Harry Robertshaw Dr. Donald J. Leo September 19, 2002 Blacksburg, VA Keywords: shape memory alloy, SMA, Nitinol, heat engine Copyright 2002, Ean Schiller Heat Engine Driven by Shape Memory Alloys: Prototyping and Design Ean H. Schiller (ABSTRACT) This work presents a novel approach to arranging shape memory alloy (SMA) wires into a functional heat engine. Significant contributions include the design itself, a preliminary analytical model and the realization of a research prototype; thereby, laying a foundation from which to base refinements and seek practical applications. Shape memory alloys are metallic materials that, if deformed when cold, can forcefully recover their original, "memorized" shapes, when heated. The proposed engine consists of a set of SMA wires stretched between two crankshafts, synchronized to rotate in the same direction. Cranks on the first crankshaft are slightly longer than cranks on the second. During operation, the engine is positioned between two distinct thermal reservoirs such that half of its wires are heated while the other half are cooled. Wires on the hot side attempt to contract, driving the engine in the direction that relieves the heat-induced stress. Wires on the cold side soften and stretch as the engine rotates. Because the force generated during heated recovery exceeds that required for cooled deformation, the engine is capable of generating shaft power. Limited experimental measurements of shaft speed were performed. An analytical model of the engine predicts that the maximum output power for the prototype, under test conditions, should be 0.75 W. Thermal efficiency, though not measured or calculated in this work, is expected to be low. Potential applications may include the conversion of waste heat into shaft power. Acknowledgements I am grateful to the following individuals for their gracious support of this work: Dr. Charles Reinholtz Dr. Donald J. Leo Dr. Harry Robertshaw Jerry Schiller Noah Schiller William Whittier iii Table of Contents Heat Engine Driven by Shape Memory Alloys: Prototyping and Design ____________ i Heat Engine Driven by Shape Memory Alloys: Prototyping and Design ___________ ii Acknowledgements______________________________________________________iii Table of Contents _______________________________________________________ iv List of Figures ________________________________________________________ vii Chapter 1: Introduction and Overview ______________________________________ 1 1.1 Introduction_____________________________________________________________ 1 1.2 Overview _______________________________________________________________ 1 1.3 Motivation ______________________________________________________________ 1 1.4 Convention______________________________________________________________ 2 Chapter 2: Shape Memory Alloys __________________________________________ 3 2.1 Scope __________________________________________________________________ 3 2.2 Definition _______________________________________________________________ 3 2.3 History _________________________________________________________________ 3 2.4 Novel Behavior __________________________________________________________ 3 2.4.1 Shape Memory Effect__________________________________________________________ 3 2.4.2 Superelasticity _______________________________________________________________ 4 2.5 Crystal Structure ________________________________________________________ 4 2.6 Transformation Temperatures and Hysteresis ________________________________ 7 2.7 Stress-Strain Relations ____________________________________________________ 9 2.8 Constitutive Model ______________________________________________________ 10 Chapter 3: SMA Heat Engines - Prior Art __________________________________ 16 3.1 Overview ______________________________________________________________ 16 3.2 Crank Engines__________________________________________________________ 16 3.2.1 General ____________________________________________________________________ 16 3.2.2 Counter-Synchronized Twin-Crank Engine ________________________________________ 16 3.3 Pulley Engines __________________________________________________________ 19 3.3.1 Unsynchronized _____________________________________________________________ 19 3.3.2 Synchronized _______________________________________________________________ 19 3.4 Field Engines ___________________________________________________________ 20 3.5 Swash Plate Engines _____________________________________________________ 20 3.6 Reciprocating Engines ___________________________________________________ 21 3.7 Sequential Engines ______________________________________________________ 21 3.8 Novel Contribution ______________________________________________________ 22 iv Chapter 4: Analysis and Design __________________________________________ 23 4.1 Engine Description ______________________________________________________ 23 4.1.1 Structure ___________________________________________________________________ 23 4.1.2 Operation __________________________________________________________________ 24 4.2 Analytical Overview _____________________________________________________ 25 4.3 Specifications___________________________________________________________ 25 4.4 Geometry ______________________________________________________________ 25 4.4.1 Basic Dimensions ____________________________________________________________ 25 4.4.2 Perpendicular Distances _______________________________________________________ 27 4.5 Statics_________________________________________________________________ 28 4.5.1 Torque ____________________________________________________________________ 29 4.5.2 Forces _____________________________________________________________________ 30 4.6 Kinematics _____________________________________________________________ 32 4.6.1 Derivation__________________________________________________________________ 32 4.6.2 Illustrations_________________________________________________________________ 34 4.7 Heat Transfer __________________________________________________________ 35 4.8 Shape Memory Alloy Behavior ____________________________________________ 37 4.9 Analytical Compilation___________________________________________________ 39 4.9.1 Method ____________________________________________________________________ 39 4.9.2 Output_____________________________________________________________________ 40 4.10 Implications___________________________________________________________ 42 4.10.1 Shaft Speed________________________________________________________________ 42 4.10.2 Power Output ______________________________________________________________ 42 4.10.3 Trends____________________________________________________________________ 42 4.11 Regarding Efficiency ___________________________________________________ 45 Chapter 5: Design Evolution and Construction ______________________________ 47 5.1 Practical Design_________________________________________________________ 47 5.2 Engine Components _____________________________________________________ 47 5.2.1 Wire ______________________________________________________________________ 47 5.2.2 Cranks_____________________________________________________________________ 49 5.2.3 Synchronizer________________________________________________________________ 50 5.2.4 Bearings ___________________________________________________________________ 52 5.2.5 Frame _____________________________________________________________________ 52 5.3 Total Package __________________________________________________________ 52 5.4 Heating________________________________________________________________ 54 Chapter 6: Experimental Results _________________________________________ 56 6.1 Testing ________________________________________________________________ 56 6.1.1 Heat Gun Testing ____________________________________________________________ 56 6.1.2 Water Bath Testing___________________________________________________________ 56 6.2 Observations ___________________________________________________________ 57 6.2.1 Startup ____________________________________________________________________ 57 6.2.2 Bolt Movement______________________________________________________________ 57 6.2.3 Irregular Rotation ____________________________________________________________ 58 6.2.4 Heating Problems ____________________________________________________________ 58 v Chapter 7: Summary, Applications and Conclusion __________________________ 59 7.1 Closing ________________________________________________________________ 59 7.2 Pros and Cons __________________________________________________________ 59 7.3 Potential Applications____________________________________________________ 60 7.3.1 Distributed Energy Generation__________________________________________________ 60 7.3.2 Thermal Recycling in Industry __________________________________________________ 60 7.3.3 Novelties___________________________________________________________________ 60 7.4 Loose Ends and Opportunities for Improvement _____________________________ 61 7.5 Conclusion _____________________________________________________________ 61 Appendix 1 ___________________________________________________________ 63 Notation __________________________________________________________________
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