Theoretical and Experimental Development and Optimization of Regenerators with Axially Variable Matrix Structure to Enhance the Efficiency of Regenerative Cycles

• Applicant: Privatdozent Dr.-Ing. Hans-Detlev Kühl
• Subject Area: Technical Thermodynamics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 490755353

Regenerative cycles, the most well-known of which, the Stirling cycle, is adaptable to both engines in clockwise and refrigerators in counterclockwise operation, are well suited for an efficient decentralized energy transformation. Against the background of global warming, attention is particularly focused on applications in the field of cogeneration, combined cooling, heat and power production (trigeneration) as well as thermally actuated heat pumps. Even since the grant of the Stirling engine patent in 1816, it is well-known that the thermodynamic efficiency of these cycle is crucially dependent on the namesake regenerators acting as thermal energy stores and allowing the working gas to shuttle periodically between different temperature levels at almost negligible loss. Efficient regenerators particularly require an excellent heat transfer between the working gas and the storage matrix at minimum flow pressure loss an void volume. These demands should be evenly fulfilled along the entire length, although the flow conditions vary considerably due to spatially varying mass and volume flows as well as the temperature difference frequently amounting to several 100 K and considerably affecting the property data. This clearly suggests to adapt the matrix to these varying flow conditions by modifications of its structure in the axial direction. Although optimization of regenerators has been a mayor research topic for decades, the optimization potential of such axial variations of the matrix structure has hardly been investigated yet, particularly for thermally actuated cycles. In this project, such investigations shall therefore be performed, primarily focusing on sintered metal felts due to their comparatively low manufacturing cost, but also considering alternative materials. For this purpose, an existing experimental machine designed by similarity-based scaling shall be utilized, since it is particularly well suited due to enlarged hydraulic diameters, an easily accessible, comparatively long hot regenerator and other constructive advantages. At the low-temperature end of the matrix, a fine wire thermocouple sensor will be installed to record the temperature of the working gas during in- and outflow at a high temporal resolution and to thus evaluate the regenerator loss on an experimental basis by integration of the enthalpy flow. By comparison to regenerators with a uniform structure and to corresponding simulation results, the optimization potential of axial variations of the matrix structure will thus be demonstrated. Experimentally verified modelling approaches and general design rules will be derived and provided, thus contributing to a further enhancement of the efficiency of regenerative cycles.

• DFG Programme: Research Grants