Seminars

Shape memory alloys (SMAs) exhibit remarkable thermomechanical phenomena, including the shape memory effect and superelasticity, that can enable novel applications as solid-state thermal actuators or resilient structural elements. SMA straight wire can recover a few percent elongation under tension, yet the same wire in helical spring form provides enhanced kinematics that amplify spring axis elongations to large stretch ratios. While this comes at the tradeoff of relatively low axial force capacity, the high compliance is suitable for low-force actuators, soft active composites, and heat engines.

The operation and performance of a benchtop SMA tension-spring heat engine is explored through experiments and modeling. The heat engine has a continuous loop of SMA spring mounted to geared pulleys which automatically revolve the spring between regions of hot and cold air to produce mechanical work against a torque brake. Several parameters, including the output moment, hot reservoir temperature, gear ratio, and spring prestretch, are varied to access their influence on the steady state engine performance. The engine is instrumented with thermocouples and pitot probes to characterize the heat transfer environment, and the distributions of spring stretch ratio and spring temperature are monitored by optical and infrared imaging. This heat engine’s small footprint, solid state, low cost, and simple operation make it suitable for harvesting waste heat from a variety of industrial or commercial sources.

A system-level mathematical model is developed that captures the steady-state behavior of the SMA heat engine. The thermodynamic model uses an accurate yet computationally efficient reduced-order constitutive model for the SMA tension spring. Considerations of periodicity, compatibility, mechanical equilibrium, and energy conservation leads to a few nonlinear integral equations that can be solved numerically. This provides the distributions of temperature, stretch ratio, and tensile force in the SMA spring that are validated against detailed experimental results. It provides, for the first time, a rigorous analysis and design tool for quick parametric studies.