Experimental Evidence of Temperature Ratio Effect on Turbine Blade Tip Heat Transfer

Abstract

Determination of a scalable Nusselt number (based on “adiabatic heat transfer coefficient”) has been the primary objective of the most existing heat transfer experimental studies. Based on the assumption that the wall thermal boundary conditions do not affect the flow field, the thermal measurements were mostly carried out at near adiabatic condition without matching the engine realistic wall-to-gas temperature ratio (TR). Recent numerical studies raised a question on the validity of this conventional practice in some applications, especially for turbine blade. Due to the relatively low thermal inertia of the over-tip-leakage (OTL) flow within the thin clearance, the fluids' transport properties vary greatly with different wall thermal boundary conditions and the two-way coupling between OTL aerodynamics and heat transfer cannot be neglected. The issue could become more severe when the gas turbine manufacturers are making effort to achieve much tighter clearance. However, there has been no experimental evidence to back up these numerical findings. In this study, transient thermal measurements were conducted in a high-temperature linear cascade rig for a range of tip clearance ratio (G/S) (0.3%, 0.4%, 0.6%, and 1%). Surface temperature history was captured by infrared thermography at a range of wall-to-gas TRs. Heat transfer coefficient (HTC) distributions were obtained based on a conventional data processing technique. The profound influence of tip surface thermal boundary condition on heat transfer and OTL flow was revealed by the first-of-its-kind experimental data obtained in the present experimental study.

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

“…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.…”

“…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.…”

Conjugate heat transfer: Some fundamentals and recent progress

“…It thus has no differentiating ability needed for accounting for the history effect as observed, which has to be corrected locally, as highlighted by Maffulli and He [14]. Strong impact of the wall temperature condition on HTC has also been revealed for complex flows around a turbine blade tip, both computationally (Zhang and He [16]) and experimentally (Jiang et al [17]).…”

“…noted that Eq (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). only includes the nonlinearity in the time-averaging on the fluid side.…”