Buckling Prevention of a Single Long NiTi Tube Compressive Elastocaloric Regenerator

Abstract

Superelastic NiTi tubes have been used in elastocaloric cooling devices due to the good balance between the mechanical stability and the heat transfer efficiency, and the increase of the tube length is crucial to the reduction of the driving force and the increase of the total cooling power of the devices. The key problem to adopt long superelastic NiTi tubes in compressive elastocaloric cooling devices is how to prevent long tubes from column buckling. In this article, a buckling prevention method of entraining a single long tubular material into a rigid and thick outer shell was proposed to ensure the uniform compression of the long tubular material. The enhancement in the maximum compression load of a single long tubular material with an outer shell compared with the bare single long tubular material was derived theoretically using the energy method. The proposed buckling prevention method was validated by compression experiments using a single long stainless-steel tube and outer shells of different materials and outer diameters. The buckling prevention method was successfully applied to a single long polycrystalline superelastic NiTi tube in a compressive elastocaloric regenerator. A buckling prevention structure consisting of four layers (a flow distributor, a long NiTi tube, a thermal insulation layer, and an outer shell) was developed to restrain the redundant freedoms and uniformly compress the long NiTi tube of outer diameter 5 mm, wall thickness 1 mm, and initial length 305 mm. There were several stress rise-drop-recover processes because of the friction and sliding of the tube right before the tube was applied to the largest stress in the first loading. The dependences of the stress-strain response of the long tube on the applied stress and strain rate were also investigated. An elastocaloric cooling device based on the single-long-tube compressive elastocaloric regenerator was constructed and its cooling performance was measured and discussed. With the buckling prevention, the single-long-tube elastocaloric regenerator reached the highest temperature span, specific cooling power, and COP values of up to 5.7 K, 150 W∙kg−1, and 4.0, respectively. The results of the article are significant for developing large-power elastocaloric cooling devices.

Keywords: Buckling and buckling prevention, Shape memory alloys, Tubes, Elastocaloric refrigeration, Heat transfer