Cooling Injection Effect on a Transonic Squealer Tip—Part I: 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 I 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. ansysFluent 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 surface 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 II.

Improving Purge Air Cooling Effectiveness by Engineered End-Wall Surface Structures—Part II: Turbine Cascade

“…This method uses discrete deconvolution and a pair of nonsingular analytical solutions to the convective heat transfer equation to derive a digital filter impulse response, then the heat flux can be determined from temperature traces based on the convolution integral. The measured thermal product √ of the end-wall material is 564 √ /m 2 K (previously reported by Ma et al[41]).…”

“…local heat flux can be calculated directly based on the transient wall temperature history and material thermal properties. In this study, a well-established Impulse Response Method, developed by Oldfield[40] and further implemented by a series of research by O'Dowd et al[36], Zhang et al[37][38][39], and Ma et al[41], was employed. This method uses discrete deconvolution and a pair of nonsingular analytical solutions to the convective heat transfer equation to derive a digital filter impulse response, then the heat flux can be determined from temperature traces based on the convolution integral.…”

Recent investigations of shock wave effects and interactions

“…6 Transonic tip gap heat transfer and flow characteristics, from Zhang et al [55], for a flat tip with a 1.5% tip gap. Within the figure, color variations denote tip surface heat flux variations, and greyscale variations denote flow density gradient distributions which were employed in these studies utilized squealer configurations [57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72], partial squealer configurations [61,64,69], and winglet configurations [73,74]. Smooth blade tips were employed by Thorpe et al [75], Green et al [59], Key and Arts [62], O'Dowd et al [76,77], Wheeler et al [78], Zhang et al [55,56], Shyam et al [79], Atkins et al [80], Wheeler and Saleh [70], Anto et al [81], Virdi et al [68], Wheeler and Sandberg [82], Li et al [65], Zhang et al [83], Zhang and He [84], Wang et al [69], Zhou [71], Jung et al [61], Gao et al [85], and Kim et al…”

Aerothermal characteristics of transonic over-tip leakage flow for different tip geometries with cooling injection

“…Some researchers such as Couch et al [21] and Wheeler et al [22,23] investigated the effects of tip coolant injection on transonic OTL loss. Similarly, Ma et al [24,25] numerically and experimentally studied the transonic squealer tip with the cooling injection. It was observed that dominant driving flow structure was a counter-rotating vortex pair (CRVP) caused by the bifurcation of coolant impinging on the casing.…”

“…The strong interactions between OTL flow and cooling injection has been discussed. A part of the computational results was compared with the experimental results of Ma et al [24,25].…”