Thermal expander metadevices can yield a large uniform temperature field powered by a linear heat source. Previous design of thermal expander metadevices can be regarded as a combination of achieved thermal cloaks and their background materials. However, these thermal-cloak-inspired expander metadevices have an inherent flaw, i.e., their thermal functionality will be lost when the background material is changed, thus limiting their practical applications. To solve this problem, the multiscale topology optimization (MTO) method is employed to design thermal expander metadevices that can maintain their expander functionality under different background materials. In MTO, transformation thermotic technology is used to determine the anisotropic thermal conductivities inside a thermal expander metadevice and topology optimization is performed to generate the topological configuration of each microstructure with the target effective thermal conductivity. Subsequently, the thermal functionalities of thermal double and triple expander metadevices with different background materials are numerically verified via simulations. Finally, the thermal double expander metadevice is fabricated via additive manufacturing and experimentally tested for its thermal functionality. The findings of this study address the challenge of designing thermal expander metadevices with background material-independent functionality.