Abstract
Measurements are presented of the streamwise and spanwise mean velocity and turbulence intensity components as well as Reynolds stresses and turbulence kinetic energy by using Particle Image velocimetry (PIV) for the isothermal flow field in three simulative turbine blade serpentine coolant passages. The transparent plexiglass test section consists of two pass parallelogram duct connected by a 180 sharp turn. The channels have the cross-sectional equal length, 45.5 mm, of adjacent sides and two pairs of opposite angles are 45-deg and 135-deg. Three kinds of in-line rib array arrangements, i.e. smooth-wall, 90-deg attached ribs and 90-deg detached ribs, are studied. Among them, the latter two cases are new studies in the open literature. The rib height to channel height ratio and the pitch to rib height ratio are respectively 0.1 and 10. Two rib-detached distance to rib height ratios, C/H=0 and C/H=0.38, are selected to study C/H effect on the flow structure. All the measurements were performed at a fixed Reynolds number, characterized by channel hydraulic diameter of 32.17 mm and cross-sectional bulk mean velocity of 10000. The smooth-wall channel is first chosen as the base line case to study the typical flow characteristics of a two pass parallelogram channel. It is found that the three dimensional flows are characterized by the inlet configuration induced streamwise double peak mean velocity profile and high turbulence kinetic energy (TKE) in the first pass, secondary-flow vortices generated by sidewall slant effect in the first and second pass, turn induced high TKE, curvature generated Dean vortices inside the turn, and turning geometry induced separation bubble immediately downstream of the turn. Results of the 90-deg attached rib case show shorter reattachment lengths compared to the corresponding square ducts. The aforementioned separation bubble is lacking in the symmetrical plane but only found at the tip of the divider wall near the top wall. These results together with the prevalence of highly anisotropic turbulence in the turn region provide useful reference for selecting turbulence model of CFD simulation. Moreover, the distribution of near-wall mean velocity components, turbulence intensity components, and TKE are found to be able to illustrate the reported heat transfer distribution. Similar to the attached rib case, the rib-wake lengths of detached rib case are found to be shorter than those of square ducts. In contrast to the attached rib case, the separation bubble on the Y*=0 plane in turn region of the detached rib case appears again but does not extend to the second pass. Finally, a comparison between the near-wall mean velocity components and TKE in the present parallelogram and previous square channel with 180-deg sharp turn and inline detach rib array has been attempted and the differences are addressed.