Experimental determination of the thermal regenerator loss in Stirling engines by high-speed gas temperature measurements including numerical reproduction

The regenerator is the most crucial component for achieving high efficiencies in Stirling engines. To improve the performance of regenerators, and thus that of the cycle in which the regenerator is operating, it is important to be able to quantify their losses. Therefore, this contribution presents a method for the direct determination of the thermal regenerator loss in an experimental Stirling engine by evaluating the closed-loop integral of the enthalpy flow. For this purpose, the engine is equipped with fine-wire thermocouples between the cold end of the regenerator and the adjacent heat exchanger to measure the gas temperature with high temporal resolution. The thermal regenerator loss is then derived from these measurements according to the outlined processing procedure. The distinguishing feature of the presented method is that it can be applied in any regenerative machine. Thereby, the maximum measurement error yields a satisfactory value of about 5%. Initial deviations between the experiment and a one-dimensional differential model amounted to an average of 32% in relative difference across all operating points investigated. Following a comprehensive analysis of these deviations, the model was improved, reducing the mean deviation between model and experiment to 7.1%. By applying heat transfer correlations for oscillatory flow in heat exchangers on trial, it was demonstrated that these are indispensable for an accurate reproduction of the gas temperature courses, but not necessarily for the reproduction of the overall regenerator loss.