Experimental and numerical determination of thermohydraulic properties of regenerators subjected to oscillating flow




Schopfer, Sandro

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Regenerators are key components in many thermal devices such as Stirling cryocoolers, magnetic refrigeration devices etc. They act as temporal thermal energy storage and therewith separate two thermal reservoirs. Regenerators are typically made up of porous structures referred to as the packing material that can lead to complex flow pathways of the heat transfer fluid through the regenerator. The nonisothermal and periodically reversing flow type allows for thermal energy exchange with the packing material of the regenerator. The performance of such devices depends greatly on the geometry of the porous structure, itsmaterial properties, length scales involved as well as operating conditions. This thesis is a study of thermohydraulic properties of thermal regenerators under oscillating flow conditions. In the first part of this thesis, thermodynamic models are developed for the extraction of the friction factor and Nusselt number from an experiment based on a harmonic approximation technique. These models are verified using a two dimensional pore scale model that allows to calculate friction factor and Nusselt number on a theoretical basis independent from an experiment. The second part of this thesis is devoted to the application of the models presented in part one to an experiment. A test apparatus that allows to measure temperature and pressure drop for various types of regenerators is presented. The measurements for a microchannel and packed bed of spheres regenerator are characterized using spectral analysis. Friction factor and Nusselt numbers are evaluated and parametrized using the models derived in the first part of this thesis. Themethodology presented in this thesis reveals insights in the dynamic effects of oscillating flow type heat transfer. The theoretical findings are applied to experimentally obtained data for a correct interpretation of friction factor and Nusselt number.



thermal, heat transfer, energy, flow