Cooling Injection Effect on a Transonic Squealer Tip: Part 1 — Experimental Heat Transfer Results and CFD Validation

Abstract

Recent studies have demonstrated that the aerothermal characteristics of turbine rotor blade tip under a transonic condition are qualitatively different from those under a low speed subsonic condition. The cooling injection adds further complexity to the Over-Tip-Leakage (OTL) transonic flow behavior and aerothermal performance, particularly for commonly studied shroudless tip configurations such as a squealer tip. However there has been no published experimental study of a cooled transonic squealer. The present study investigates the effect of cooling injection on a transonic squealer through a closely combined experimental and CFD effort. Part 1 of this two-part paper presents the first of the kind tip cooling experimental data obtained in a transonic linear cascade environment (exit Mach number 0.95). Transient thermal measurements are carried out for an uncooled squealer tip, and six cooling configurations with different locations and numbers of discrete holes. High resolution distributions of heat transfer coefficient and cooling effectiveness are obtained. ANSYS Fluent is employed to perform numerical simulations for all the experimental cases. The mesh and turbulence modeling dependence is first evaluated before further computational studies are carried out. Both the experimental and computational results consistently illustrate strong interactions between the OTL flow and cooling injection. When the cooling injection (even with a relatively small amount) is introduced, distinctive series of stripes in surface heat transfer coefficient are observed with an opposite trend in the chordwise variations on the squealer cavity floor and on the suction surafce rim. Both experimental and CFD results have also consistently shown interesting signatures of the strong OTL flow–cooling interactions in terms of the net heat flux reduction distribution in areas seemingly unreachable by the coolant. Further examinations and analyses of the related flow physics and underlining vortical flow structures will be presented in Part 2.

On Scaling Method to Investigate High-Speed Over-Tip-Leakage Flow at Low-Speed Condition

“…With the computational setup mentioned above, satisfactory agreements between experimental and CFD results have been reported by previous tip studies by Ma et al [25,26]. Further validations of the CFD performance on tip heat transfer at the high speed (exit Mach number 0.86) and a low speed condition (exit Mach number 0.45) are presented in Fig.…”

“…The experimental tip HTC contour shown in Fig 3a was produced with the same experimental data by Ma et al [24] in a high speed condition. The experimental methods and uncertainty analysis were detailed by Ma et al [24] and Ma et al [25]. The overall trend and local variations are very different between high speed flow ( Fig.…”

Influence of tip clearance and cavity depth on heat transfer in a cutback squealer tip

“…It has a better performance when predicting HTC on blade tip surface than the Spalart-Allmaras (SA) model, as is validated by Ma et al. 19 The result from the SST model shows better accordance with the experimental result. The computation domain is a single passage domain with periodic boundaries on both sides.…”

“…The time history of tip surface temperature on the middle blade tip in the cascade is recorded by a FLIR A325 IR camera, which has a spatial resolution of 320 � 240 and a sampling frequency of 60 Hz. The tip heat transfer coefficient is obtained by solving the heat conduction equation and Newton's law of cooling, as the same method in Ma et al 19 and Jiang et al 27 The uncertainty of the experimental result is acceptable on the whole tip surface, as is shown in Figure 5.…”

Aerothermal characteristics of a transonic tip flow in a turbine cascade with tip clearance variations

“…It should be noted that the results of Wheeler's numerical work [26] showed that the cooling injection still has the potential to reduce overtip leakage losses. In the meantime, the results of Ma's experimental work [27] indicated that when the cooling injection is introduced, distinctive series of stripes in surface heat transfer coefficient are observed with an opposite trend in the chordwise variations on the squealer cavity floor and on the suction surface rim.…”