Theoretical and Experimental Investigation of Thermal Losses in the Appendix Gap of Regenerative Gas Cycles

• Applicant: Privatdozent Dr.-Ing. Hans-Detlev Kühl
• Subject Area: Technical Thermodynamics
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 255045250

A laboratory-scale machine that may be operated as a Stirling engine, a Vuilleumier heat pump or in a so-called hybrid mode featuring both a power generation and a heat pump effect was first realized within a previous, DFG-funded project. Having observed some systematic deviations between experimental and simulated results obtained in this project, the simplified analytical approximations for the so-called appendix gap losses and their underlying simplifying assumptions were scrutinized and found to be inaccurate. This gave rise to the proposal of the current project aiming at the improvement of the numerical modelling as well as the available analytical approximations for those losses on an experimental basis for the first time, i.e., by direct measurements of the both temporally and spatially variable gas temperature profiles in the gap. For this purpose, an available one-dimensional differential simulation program was firstly extended by a numerical model of the gap and tested successfully. Secondly, the experimental machine developed in the aforementioned project, which features enlarged gap dimensions due to similarity-based scaling and is therefore exceptionally well-suited for such investigations, was equipped with fine-wire thermocouple probes. Having completed the required changes and extensions of the plant and its periphery, first gas temperature measurements at high temporal resolution have been performed successfully. Since only a part of the proposed funding period was initially granted, this renewal proposal is submitted for the purpose of conducting the proposed experimental investigations under variation of the relevant parameters and developing enhanced tools for the description of the radial and axial energy transport mechanisms in the appendix gap on the basis of the experimental results, which shall be validated using the accordingly extended program. In particular, the option of toggling between Stirling-, Vuilleumier- and possibly hybrid mode provided by this machine can be most profitably used here in order to investigate the effects of both amplitude and phasing of the cycle pressure. For the processing of the experimental data, the oscillating flow around the thermocouple wire shall additionally be modelled by CFD, since in some cases, the probes apparently do not allow tracing of the gas temperature without a significant delay - in contrast to earlier experiences with comparable measurements, which were performed at different locations in the cycle though. The final aim of the project is to provide an experimentally founded calculation basis for the one-dimensional simulation, which may also be applied by other users, and - on this basis - universal design equations for the optimum appendix gap geometry.

• DFG Programme: Research Grants