Thermal analysis in the heat transfer coefficient measurement for NiTi tubular elastocaloric regenerators based on a lumped model

The convective heat transfer coefficient is important for the design and optimization of advanced geometries of NiTi-based shape memory alloys in elastocaloric refrigeration. However, the direct convective heat transfer coefficient measurement methods are currently limited by the small sizes of flow channels and nonlinearity of the temperature along the flow direction in the flow channel. In this work, a lumped model for the thermal analysis during convective heat transfer coefficient determination between NiTi shape memory alloys and the heat transfer fluid using only the temperature of either the NiTi or the fluid side has been proposed and developed for elastocaloric refrigeration applications. The thermal analysis indicates that using the temperature signal of single solid or fluid side avoids the difficulty in acquiring both solid and fluid temperatures in elastocaloric regenerators with advanced geometry, and eliminates the systematic error resulting from the measurement of the internal volumetric temperature of heat transfer fluids. Taking the heat transfer fluid air as an example, the model was experimentally validated by measuring the convective heat transfer coefficient between NiTi shape memory alloy and air (50 ± 5 W m −2 K −1 ) according to the dependence of the temperature of NiTi-based shape memory alloys on the operation time. Moreover, parametric studies have shown the high measurement sensibility for various fluid types, regenerator structures, and fluid flow velocities. The proposed lumped model for the thermal analysis in the convective heat transfer coefficient measurement may be further extended to other caloric materials and heat transfer fluids.