The study presented in this article provides detailed description about a newly developed experimental technique to determine three key convective heat transfer parameters simultaneously in hot gas path of a modern high pressure turbine–recovery temperature (Tr), heat transfer coefficient (HTC), and adiabatic film cooling effectiveness (Eta). The proposed technique, dual linear regression technique (DLRT), has been developed based on the 1D semi-infinite transient conduction theory, is applicable toward film cooled heat transfer experiments especially under realistic engine environment conditions (high Reynolds number along with high Mach numbers). It addresses the fundamental three temperature problem by a two-test strategy. The current popular technique, curve fitting method (CFM) (Ekkad and Han, 2000, “A Transient Liquid Crystal Thermography Technique for Turbine Heat Transfer Measurements,” Meas. Sci. Technol., 11(7), pp. 957–968), which is widely used in the low speed wind tunnel experiments, is not competent in the transonic transient wind tunnel. The CFM (including schemes for both film cooled and nonfilm cooled experiments) does not provide recovery temperature on the film cooled surface. Instead, it assumes the recovery temperature equal to the mainstream total temperature. Its basic physics model simplifies the initial unsteady flow development within the data reduction period by assuming a step jump in mainstream pressure and temperature, which results in significant under prediction of HTC due to the gradual ramping of the flow Mach/Reynolds number and varying temperature in a transient, cascade wind tunnel facility. The proposed technique is advantageous due to the elimination of these added assumptions and including the effects of compressible flow physics at high speed flow. The detailed discussion on theory and development of the DLRT is followed by validation with analytical calculation and comparisons with the traditional technique by reducing the same set of experimental data. Results indicate that the proposed technique stands out with a higher accuracy and reliability.
In this paper, detailed experimental results of total pressure loss and secondary flow field are presented for a high turning (127°) airfoil passage in presence of an upstream purge slot (with and without coolant injection). The experiments were performed at Virginia Tech’s quasi 2D linear turbine cascade operating at transonic conditions. Measurements were made at design exit Mach number 0.88 and design incidence angle. The selected coolant to mainstream mass flow ratio (MFR) was set at 1.0%. In order to match engine representative inlet/exit blade loading, a diverging endwall was utilized where the span increased from the inlet to the exit at a 13 degree angle. A 5-hole probe traverse was used to measure exit total pressure. Pressure loss coefficients were calculated both along pitchwise and spanwise directions at 0.1 axial chord downstream of the blade trailing edge. CFD studies were conducted to compliment the experimental results. The backward facing step present with the upstream slot affects the approaching boundary layer and influences the passage horse-shoe vortex strength. The addition of coolant from the purge slot further increased the aerodynamic losses. However, the backward facing step of the upstream slot seems to be the predominant factor in affecting pressure losses when compared to with or without blowing cases. These results provide further understanding of the passage secondary flow characteristics and aid towards improved design of endwall passages. The heat transfer experiments, designed to find the heat transfer coefficient and the film cooling effectiveness are described in detail in part II of this paper [1].
Comparison of heat transfer performance of a nonaxisymmetric contoured endwall to a planar baseline endwall in the presence of leakage flow through stator–rotor rim seal interface and mateface gap is reported in this paper. Heat transfer experiments were performed on a high turning turbine airfoil passage at Virginia Tech's transonic blow down cascade facility under design conditions for two leakage flow configurations—(1) mateface blowing only, (2) simultaneous coolant injection from the upstream slot and mateface gap. Coolant to mainstream mass flow ratios (MFRs) were 0.35% for mateface blowing only, whereas for combination blowing, a 1.0% MFR was chosen from upstream slot and 0.35% MFR from mateface. A common source of coolant supply to the upstream slot and mateface plenum made sure the coolant temperatures were identical at both upstream slot and mateface gap at the injection location. The contoured endwall geometry was generated to minimize secondary aerodynamic losses. Transient infrared thermography technique was used to measure endwall surface temperature and a linear regression method was developed for simultaneous calculation of heat transfer coefficient (HTC) and adiabatic cooling effectiveness, assuming a one-dimensional (1D) semi-infinite transient conduction. Results indicate reduction in local hot spot regions near suction side as well as area averaged HTC using the contoured endwall compared to baseline endwall for all coolant blowing cases. Contoured geometry also shows better coolant coverage further along the passage. Detailed interpretation of the heat transfer results along with near endwall flow physics has also been discussed.
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