Unsteady fluid mechanics and heat transfer in low pressure turbines and Stirling engines.

摘要

The present study focuses on the unsteady heat transfer and fluid mechanics in low pressure turbines and the expansion space region in a Stirling cycle engine. The flows in both engines are unsteady and have strong temporal acceleration and spatial acceleration. The interest in these two flows is the separation and laminar-to-turbulent transition in the boundary layer.; First presented are evaluations of performance of transition models when applied against data taken on the suction surface of a low pressure turbine airfoil in the presence of passing wakes. Tested are (1) separated flow transition onset models, such as those of Mayle and Davis et al., (2) attached flow transition onset models, such as those of Mayle, Abu-Ghannam and Shaw, Suzen and Huang and Drela, and (3) transition path models, such as that of Ramesh and Hodson. The Mayle separated and attached flow onset models are shown to be successful for the all three cases investigated when applied in that fashion. However, performance of the transition path model is poor. The effects on transition onset and transition path of unsteady acceleration, as well as turbulence intensity, are described. Deceleration and high turbulence levels promote transition, whereas acceleration and a low level of turbulence can delay transition.; Second part of this study is the experimental investigation of the boundary layer flow on the head wall of a Stirling engine. The expansion space of a Stirling engine, where the thermal energy is accepted into the engine by heat transfer, is the hottest part of the engine. Also the flow in this region is oscillatory, impinging on a two-dimensional concavely-curved surface. The present project is to experimentally investigate the flow and heat transfer in the heater head region. Flow fields and heat transfer coefficients are measured to characterize the oscillatory flow as well as to supply experimental validation for the CFD Stirling engine design codes. Three-Dimensional simulation is also conducted by using ANASYS CFX. Shear Stress Transport model is used in the combination with transition onset models. Vortex and thick boundary layer measured in the experiments are also captured by the simulation.