Abstr:Gas Turbine nozzle cascade
Application Area 6: Turbomachinery Internal Flows
Application Challenge AC6-06
The experimental cascade rig here suggested was investigated in the Von Karman Institute Isentropic Light Piston Compression Tube facility (CT2). The facility uses air as working fluid and is especially engineered to provide constant freestream conditions (total temperature, total pressure and mass flow), which can be varied respectively between 300 and 600 K and 0.5 and 7 bar. The downstream dump tank allows static pressure adjustment and thus the independent selection of Mach and Reynolds number. The profile is the nozzle guide vane of a highly loaded transonic turbine, with a design isentropic mach number of 0.9 in the outlet section.
The whole cascade is linear and the instrumentation has been placed on the central blade for both pressure and heat transfer measurements.
From the experimental campaign blade velocity distributions, exit flow angles, free-stream turbulence intensity and spectrum and heat transfer coefficients measurements are available. These different measurements have been performed for different combinations of Mach and Reynolds number. The gas to wall temperature ratio compares to the values currently observed in modern application. A wide set of experimental data is available both for the blade aerodynamic performances and convective heat transfer.
The nominal isentropic Mach number in the outlet section is 0.9 while in the experimental grid the values ranged from 0.7 to 1.10. Transonic regions and recompression shocks can be placed on the SS with varying intensity with the working condition.
The turbulence levels can vary from 1.0 to 6% while Re spans from 0.5x105 to 2.0x105. Both turbulence level and Re widely influence the boundary layers characteristics and thus have a strong impact on the heat transfer coefficient. The velocity distribution, exit flow angles and the effect of freestream turbulence intensity at different Reynolds and Mach numbers have been investigated. The data set about velocity distribution and exit flow angles, available after the experimental campaign, can be used profitably to test the numerical code's capability to capture turbulence transition in the transonic region which is always one of the most challenging tasks. The presence of transictional flow field and shock-boundary layer interaction has a large impact on the heat transfer coefficient. The results can be used to assess and tune turbulence and transition models in order to get CFD codes able to forecast the transition onset of laminar boundary layer on different flow conditions.
Several CFD simulations have been performed for the LS89 data set. Most of the computations have been focused to verify the numerical scheme and the turbulence model capability in predicting transition and heat transfer. Applications reported in literature refer to a wide range of turbulence models ranging from simple algebraic closures to the one and two equation approaches with different transition correlation. Examples can be found in the work of Gehrer et. al. comparing the algebraic model used by Arnone and Pacciani, the one equation of Spalart and Allmaras and the two equation (low-Re k-e) of Biswas and Fukuyama. Different contributions have been presented in the works of Levbre and Arts, Migliorini and Michelassi. Recently Steelant and Dick refer to the same test case for the analysis of a quite sophisticated approach developed for laminar/turbulent transition modelling. Generally the comparison with experiments shows a reasonable agreement, but also reveals problems and the importance required by a realistic prediction of transition and turbulence.
One of the most relevant outcomes of the present experimental program consists of heat transfer distribution along the NGV blade for different free-stream turbulence intensities and spectrum. Besides measurements are available for different combinations of Mach in the transonic region and Reynolds number spanning in the range of typical industrial turbine application. Reliable heat transfer data for high Reynolds transonic flow conditions represent a quite important aspect for modern turbine stages especially for development and analysis of cooling systems needed to allow safe operating conditions of the blades and increase the overall performances of the engine. A good design from a thermal point of view might allow a higher inlet temperature, less cooling or a lighter design. Methods to predict the heat load in the design phase of the turbine are for this reason very valuable engineering tools. In this regard a reliable CFD code represent a very attractive approach for the projection phase thanks to the lower costs and relatively shorter response time in comparison to the experimental testing. But the validation of the CFD code requires important features like numerical accuracy, grid flexibility and validated turbulence and transition modelling, aspects which altogether heavily impact on the numerical forecast of heat transfer coefficient. The high Reynolds number, the transonic conditions and the wide range of free-stream turbulence make of this test case a realistic and demanding application for the performances of a numerical solver. In this regard a quite important aspect resulting from the comparison against experiments lays in the turbulence model capability of predicting the correct onset for the laminar-turbulent transition. In fact transition onset can be clearly detected for different free-stream turbulence levels, simply observing the experimental heat transfer coefficient distribution on the blade suction side.
The experimental data available were originally designed for convective heat transfer but are very attractive also for the validation of CFD code capability to capture the aerodynamic blade performance in different Mach and Reynolds flow condition and both viscous and unviscous methods. The parameters relevant in this regard are represented firstly by global parameters such as efficiency, mass flow rate and blade load as a function of Mach number.
Global efficiency can be computed and compared with experimental for different Reynolds number as a function of the isentropic Mach number. The following relationship can be used to compute efficiency, in which P2 and P02 are the mass averaged value of static and total pressure in the outlet section.The accuracy and grid sensitivity of CFD codes to capture aerodynamic effects both for viscid and unviscous flow regimes can be assessed comparing experimental and numerical mass averaged flow angle in the exit plane. These data are available for different mach Numbers and Re.
Besides the isentropic Mach number distribution for different loading are available for pressure and suction side in order to focus attention on the accuracy of the simulation in specific areas of blade profiles, mainly leading edge stagnation point or TE wake.
Blade heat transfer coefficient distribution on blade profile is finally available to test the transition and turbulence model. The following expression can be used to compute heat transfer coefficient h from measured thermal flux, total inlet temperature and measured wall temperature.
Contributors: Elisabetta Belardini, Francesco Martelli - Universita di Firenze