An experimental investigation of secondary flow characteristics in a linear turbine cascade with upstream converging slot-holes using TR-PIV Jian Pu a , Jun Yu a , Jian-hua Wang a,⇑ , Wen-shuo Yang a , Zhi-qiang Zhang b , Lei Wang b a Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Jinzhai Road No. 96, Hefei 230027, Anhui, PR China b Aero-engine Institute of Aviation Industry Corporation of China, Wanlian Road No. 1, Shenyang 110015, Liaoning, PR China article info Article history: Received 7 May 2014 Received in revised form 20 July 2014 Accepted 23 July 2014 Available online 1 August 2014 Keywords: Time-Resolved PIV Secondary flow End-wall film cooling Converging slot-hole Free-stream turbulence level abstract To study the time-mean and temporal characteristics of secondary flow within a linear GE-E 3 high pres- sure turbine cascade, a planar Time-Resolved Particle Image Velocimetry (TR-PIV) system is used. In the double-passage cascade, a row of six converging slot-holes is placed upstream of center blade to generate film cooling effect, and different turbulence grids are replaced to create various free-stream turbulence (Tu in ) levels. In this experiment, the time-mean characteristics of secondary flow, the fast switch process of unsteady leading edge horseshoe vortex (LEHV), and the temporal characteristics of corner vortices (CVs) are completely exhibited by the TR-PIV technique. The influences of the upstream coolant injection and Tu in level on the flow characteristics of LEHV and passage vortex (PV) are discussed. The discussion reveals that: (1) in the case of no coolant injection, a high Tu in level slightly moves the LEHV toward the blade, changes the shape of the PV, increases the fluctuations of the LEHV and PV, and reduces the fre- quency of the LEHV switch process; (2) at various Tu in levels, the coolant injections suppress the forma- tion of the LEHV, and the LEHV disappears at a high coolant-to-mainstream blowing ratio (BR) of 1.5; (3) a high BR of 1.5 can greatly weaken the PV at various Tu in levels, and relative to the case of low Tu in , the high Tu in level induces a larger reduction; (4) for a low BR, at various Tu in levels, a slight change in the LEHV results in a distinct difference of the PV characteristics; (5) for a high BR, since the LEHV disappears, the Tu in effect on the secondary flow characteristics is slight. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Secondary flow, as a common phenomenon in turbine cascades, usually is defined as any flow, which is not in primary or stream- wise flow direction. Secondary flow usually causes aerodynamic loss and high convective heat transfer coefficient at end-wall [1–3]. As Langston [1] reviewed, understanding, analysis, predic- tion and control of the secondary flow are conducive to improve the performances of turbine cascades. Currently, a detailed mea- surement of the secondary flow features in a real turbine blade row is still a challenge due to several complicated factors, such as blade geometry, rotation, high Mach number, and high free-stream turbulence level, just to name a few [1–2]. To provide turbine designers with a relatively comprehensive understanding of the secondary flow structures in blade rows, linear turbine cascades have been extensively used in previous investigations [4–20], due to simple geometries and better performances of opti- cal measurement under limited experimental capacities. Generally, to ensure flow periodicity, the cascades were consisted of more than three passages [4–12], but to reduce the required flow-rate and improve optical access, the cascades were also made of only one or two passages in previous investigations [13–20]. Up to date, several typical secondary flow patterns [1] in linear turbine cascades have been developed through flow visualizations. Wang et al. [4] presented a transient multi-vortex structure in a linear cascade through multiple smoke wires and laser light sheet. Ma et al. [5] studied the unsteady characteristics of the secondary vortices near the end-wall of a linear cascade with different inci- dence angles using hydrogen bubble technique, and their results indicated that a fluctuation HV system appears near the junction of the LE and end-wall, and a PV forms due to the interaction between the HVs on the both sides. Besides qualitative flow visual- izations, some quantitative measurement techniques, such as five- hole pressure probe, hot-wire probe, Laser-Doppler Velocimetry (LDV) and PIV, have been widely used to understand the secondary http://dx.doi.org/10.1016/j.expthermflusci.2014.07.014 0894-1777/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Tel.: +86 551 63600945; fax: +86 551 63606459. E-mail address: jhwang@ustc.edu.cn (J.-h. Wang). Experimental Thermal and Fluid Science 59 (2014) 56–71 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs