Elastocaloric Effect Characterization of a NiTi Tube at Elevated Temperatures for the Application in A Compressive Cooler

Abstract

Comprehensive characterizations of the superelastic and elastocaloric effects of the superelastic and elastocaloric effects of NiTi and NiTi-based shape memory alloys (SMAs) in the operation temperature region are highly desirable for the adoption of them in elastocaloric coolers and heat pumps with a large operation temperature lift. In this article, we report superelastic and elastocaloric effect data of a commercially available superelastic polycrystalline NiTi SMA tube with an outer diameter of 5 mm, a wall thickness of 1 mm at temperatures from 293 to 323 K. The NiTi tube sample was subjected to a superelastic training of 250 cycles to stabilize its superelastic and elastocaloric effects. We observed that there were temperature dependences for both superelastic and elastocaloric effects, and the stress-strain curves differed much between isothermal and adiabatic conditions. The largest temperature rise and temperature drop values during loading and unloading were measured as 21 and 11 K, respectively at 293 K under an applied strain of 3.66% and applied strain rate of 0.1 s−1. The loading conditions (loading function and holding time) also had an impact to the superelastic effect of the NiTi tube. We identified two major reasons of the adiabatic temperature change irreversibility – the hysteresis heat dissipation and the temporary residual strain after unloading and they affected the cooling performance of the elastocaloric cooler/heat pump differently. We investigated the dependences of the superelastic and elastocaloric effects on the maximum applied strain and the temperature distribution on the NiTi tube during loading and unloading. The results obtained are beneficial to the modelling and optimization of elastocaloric coolers and heat pumps with large temperature lifts.

Keywords: Shape memory alloys, Elastocaloric effect, Elastocaloric refrigeration, Heat transfer