Volumetric Flow Measurement on a Displacement Piston Model for a β-Type Stirling Engine

Although many thermodynamic analyses and CFD simulations have been conducted on Stirling engines, the internal flow physics of Stirling engines is still unclear from experimental observations. Particularly, there is a significant lack of knowledge about the unsteady/oscillating three-dimensional internal flow field, which is critical to the understanding of the fundamental heat transfer process and performance of Stirling engines. Therefore, in this study, a Particle-Tracking-Velocimetry system: the Shake-The-Box system is implemented on a vertically oscillating disk inside a confined cylindrical container, filled with Sodium Iodide solution, to resolve the three-dimensional flow field around an experimentally simulated displacement piston inside a β type Stirling engine. Using the refractive index matching method, the image distortion due to the curved cylindrical wall was greatly suppressed and clear particle images were obtained. For the first time, the volumetric flow field below the displacement piston model is resolved and investigated experimentally under three different oscillating frequencies when it moves to the Top Dead Center. The flow measurement result suggests that a circular high velocity jet is created through the gap between the displacement piston model and the cylindrical wall of the container. As this high velocity jet is responsible for the heat transfer of convection at the bottom surface of the container, therefore can directly impact the performance of the Stirling engine. With the velocity of the jet estimated from the volumetric flow measurement data, the average heat convective coefficient \(\overline{h }\) value is calculated and estimated at different oscillating frequencies.