Numerical Prediction of Heat Transfer Coefficient and Adiabatic Effectiveness on a Nozzle Guide Vane With Representative Combustor Outflow

Abstract

The employment of lean-premix combustors in modern gas turbines allows to reduce NOx emissions by controlling the flame temperature at the expense of highly unsteady and strongly non-uniform flow fields which are necessary to stabilize the flame. This highly complex swirled flow field characterized by evident temperature distortions alters the aerodynamics and heat transfer in the first high pressure turbine stator with potential detrimental consequences on engine life and efficiency. From a numerical point of view, the mutual combustor-turbine interaction has been studied by using standard turbulence modeling approaches, as commonly employed during the design phase, even if more advanced scale-resolving methods have been proven more reliable and benchmarked against various experimental findings. From the experimental perspective, film-cooling adiabatic effectiveness and heat transfer coefficient (HTC) measurements on the external surface of the nozzle guide vanes, in the presence of representative combustor outflow characteristics, are not common since the relevant temperature distortions that are present make such kind of measurements really challenging to perform. For this reason, very limited assessment of such approaches regarding this aspect is available in literature. In this study, an experimental test case with a combustor simulator and a nozzle cascade, where both adiabatic effectiveness and HTC measurements have been carried out, is investigated by carrying out a systematic computational study, through RANS calculations of the combustor-cascade integrated domain. The film cooling system performance has been predicted by meshing the whole vane internal cooling system, while the heat transfer coefficient is calculated using the conventional two-point method, normally adopted for heat transfer calculations in gas turbines. The comparison between numerical predictions and experimental results was exploited to assess the capability of traditional modeling approaches in the characterization of both adiabatic effectiveness and heat transfer coefficient. This evaluation represents an effective means to assess if conventional/industrial approaches can be reliably used, when representative and highly unsteady combustor outflows are considered, or advanced and more time-consuming methods shall be adopted.

Study of Combustor–Turbine Interactions by Performing Coupled and Decoupled Hybrid RANS-LES Simulations under Representative Engine-like Conditions

“…As a matter of fact, several studies have shown that the vortical structures that characterize the combustor flow field are able to maintain a non-negligible part of its characteristics even at a relevant distance from the regions of the combustor where they are generated, in correspondence to the swirler. In particular, at the exit of the combustor, a highly non-uniform velocity distribution can be observed, characterized by high swirl and pitch components, along with temperature non-uniformities ("hot streaks") [1] and high turbulence levels [2][3][4] that can be conserved up to the inlet of the following component, the first-stage nozzle of the high-pressure turbine (HPT), as proven both experimentally and numerically [5][6][7][8]. Such severe conditions are particularly exacerbated due to the employment of modern gas turbine (GT) combustors, such as lean-burning systems [9].…”

“…However, since RANS is able to limit the computational cost, it has historically played a central role in studying combustor-turbine interaction phenomena. For example, recently, to meet the goals of the STech (smart technologies) program, an annular sector rig, including a trisector non-reactive simulator and nozzle cascade, has been investigated by performing RANS simulations and then benchmarking the numerical findings to the available experimental results [4] in order to assess if traditional RANS modeling, usually employed for industrial best practice, is able to successfully predict the heat loads and the film cooling behavior on the S1N. It has been proven that instead more sophisticated CFD modeling should be considered.…”