Measurements of Unsteady Convective Heat Transfer Rates Within a Stirling Regenerator Matrix Subjected to Oscillatory Flow

Abstract

A large-scale porous matrix made of stacked screens and with a porosity of 90% was fabricated to investigate heat transfer within a Stirling engine regenerator. Experiments were run at a Valensi number of 2.1 and a maximum Reynolds number of 800 (based upon the matrix hydraulic diameter and the cycle maximum bulk mean velocity within the matrix). This is considered to be a representative flow situation within the regenerator of a real engine. Thermocouples are employed to measure the instantaneous flow temperature and matrix solid temperature under oscillatory flow conditions within the regenerator matrix where the flow is thermally, fully-developed. Convective heat transfer rates are evaluated and compared to those that would be expected under quasi-steady, unidirectional flow conditions such as the steady flow conditions used to develop many of the correlations in Stirling system design. The comparison suggests that unsteadiness of oscillatory flow has an influence on heat transfer. When oscillatory flow is in the acceleration part of the cycle, there is an enhancement of convective heat transfer rates over those calculated by using the measured temperature difference and a published Nusselt number correlation. Such a correlation can be found in the Stirling modeling software SAGE or in the text Compact Heat Exchangers by Kays and London. When in the peak velocity or deceleration part of the cycle, the measured convective heat transfer rates are nearly the same as those calculated by using the measured temperature difference and the design correlations. Also, near flow reversal, we see a phase difference between the measured heat transfer rates and the measured solid-to-fluid temperature difference. From these data, instantaneous and cycleaveraged Nusselt numbers are evaluated and presented.