Determination of Casing Convective Heat Transfer Coefficient and Reference Free-stream Temperature in the Tip Clearance Region of an Axial Flow Turbine

The present study explains a steady-state method of measuring convective heat transfer coefficient on the casing of an axial flow turbine. The goal is to develop an accurate steadystate heat transfer method for the comparison of various casing surface and tip designs used for turbine performance improvements. The free-stream reference temperature, especially in the tip gap region of the casing varies monotonically from the rotor inlet to rotor exit due to work extraction in the stage. In a heat transfer problem of this nature, the definition of the free-stream temperature is not as straight forward as constant free-stream temperature type problems. The accurate determination of the convective heat transfer coefficient strongly depends on the magnitude of the local freestream reference temperature varying in axial direction, from the rotor inlet to exit. The current study explains a strategy for the simultaneous determination of the steady-state heat transfer coefficient and free-stream reference temperature on the smooth casing of a single stage rotating turbine facility. The heat transfer approach is also applicable to patterned casing surfaces. A detailed uncertainty analysis follows a detailed description of the casing heat transfer measurements. The overall uncertainty of the method developed is between 5 % and 8 % of convective heat transfer coefficient. ________________________________________________________ 1 Graduate Res. Assistant 2 Professor of Aerospace Engineering, corresponding author [email protected] INTRODUCTION The convective heat transfer to the static casing of a shroudless HP turbine rotor is a complex aero-thermal problem. The unsteady flow with a relatively high Reynolds number in the tip gap region has strong dependency on the tip clearance gap, blade tip profile, blade tip loading conditions, blade tip geometry and casing surface character. Thermal transport by flow near the casing inner surface is influenced by the unsteadiness, the surface roughness character and the turbulent flow characteristics of the fluid entering into the region between the tip and casing. Since the turbine inlet temperatures are continuously elevated to higher levels, casing and tip related heat transfer issues are becoming more critical in design studies. In gas turbines, the gas stream leaving the combustor is not at a uniform temperature in radial and circumferential directions. According to Butler et al [1] the combustor exit maximum temperature can easily be twice as high as the minimum temperature. The maximum temperature in general is around the mid-span and the lowest gas temperatures are near the walls. The mechanisms related to the distortion of the radial temperature profile as the combustor exit fluid passes through a turbine rotor are complex, as explained by Sharma and Stetson [2] and Harvey [3]. The hottest part of the fluid leaving the upstream nozzle guide vane tends to migrate to the rotor tip corner near the mid pressure surface of the blade. Unfortunately, mostly the hottest fluid originating from the mid span region of the combustor or NGV finds its way to the pressure side corner of the blade tip in the rotating frame. Details of hot streak migration in gas turbines can be found in Roback and Dring [4,5], Takanashi&Ni [6], Dorney et al. [7] and Dorney and Schwab [8].