Computational fluid dynamics plays a crucial role in the design of cooling systems in gas turbine combustors due to the difficulties and costs related to experimental measurements performed in pressurized reactive environments. Despite the massive advances in computational resources in the last years, reactive unsteady and multi-scale simulations of combustor real operating conditions are still computationally expensive.
Modern combustors often employ cooling schemes based on effusion technique which provides uniform protection of the liner from hot gases, combining the heat removal through the heat sink effect with liner coverage and protection by film cooling. However, a large number of effusion holes results in a relevant increase of computational resources required to perform a CFD simulation capable of correctly predicting the thermal load on the metal walls within the combustor. Moreover, a multi-physics and multi-scale approach is mandatory to properly consider the different characteristic scales of the several heat transfer modes within combustion chambers to achieve a reliable prediction of aero-thermal fields as wall heat fluxes and temperatures. From this point of view, loosely-coupled approaches permit a strong reduction of the calculation time, since each physics is solved through a dedicated solver optimized according to the considered heat transfer mechanism.
The main goal of the combustor pilot within the ACROSS project is to reduce the aero-engine combustor design phase by acting on both physical problem modeling and hardware optimization aspects. In other words, the aim is to improve the quality of numerical results by getting as close as possible to the reference experimental results with reduced calculation times. To achieve this goal, an important step will be to optimize the existing multiphysics and multi-scale loosely-coupled tool called U-THERM3D and developed at the University of Florence within the commercial CFD solver ANSYS Fluent. This academic tool was designed to solve Conjugate Heat Transfer (CHT) problems inside aero-engine combustion chambers in a numerically efficient manner by desynchronizing the time-steps employed to solve all the involved heat transfer phenomena. With this customized numerical procedure, the three main heat transfer mechanisms, convection (intrinsically dependent on flow turbulence and chemical reaction), conduction, and radiation, are solved in dedicated simulations with a parallel coupling strategy.
The optimization phase will be carried out on several aspects of the tool. The main efforts will focus on a new management of data exchange between the different simulations which is currently carried out through I/O operations. The new management of data exchange will lead to a reduction in calculation times. Moreover, a part of the activities will be focused on increasing the manageability of U-THERM3D for use within industrial design frameworks. The computational capabilities of HPC systems allow a whole range of onerous analyses (LES-like simulations) to be carried out in a reasonable timeframe. This is why several academic combustors with features that make them particularly interesting also for an industrial framework will be analyzed during the project. Various combustors will be investigated during the project: in the first year, the RSM combustor developed and tested at the University of Darmstadt [1-2] was studied [3] and simulated on IT4Innovations infrastructures to optimize data management due to the thermal interaction between hot gases and metal structures; the next step will focus on the management and modeling of radiative heat transfer for which the FIRST test case [4-5] will be used. Finally, the new procedure will be tested on a real aero-engine combustor to fully validate the new numerical approach.
References:
[1] Greifenstein, M., et al. “Flame–cooling air interaction in an effusion-cooled model gas turbine combustor at elevated pressure.” Experiments in Fluids 60.1 (2019): 1-13.
[2] Hermann, J., et al. “Experimental investigation of global combustion characteristics in an effusion cooled single sector model gas turbine combustor.” Flow, Turbulence and Combustion 102.4 (2019): 1025-1052.
[3] Amerini A., et al. “Assessment of a conjugate heat transfer method on an effusion cooled combustor operated with a swirl stabilized partially premixed flame”. The work will be published in:Proceedings of the ASME Turbo Expo 2022: Turbomachinery Technical Conference and Exposition
[4] Geigle, K.P., et al. “Soot formation and flame characterization of an aero-engine model combustor burning ethylene at elevated pressure.” Turbo Expo: Power for Land, Sea, and Air. Vol. 55119. American Society of Mechanical Engineers, 2013.
[5] Paccati, S., et al. “Large-Eddy simulation of a model aero-engine sooting flame with a multiphysics approach.” Flow, Turbulence and Combustion 106.4 (2021): 1329-1354.