Film Cooling Modelling for Gas Turbine Nozzles and Blades: Validation and Application

Abstract

The use of Computational Fluid Dynamics (CFD) for modern turbine blade design requires the accurate representation of the effect of film cooling. However, including complete cooling hole discretization in the computational domain requires a substantial meshing effort and leads to a drastic increase in the computing time. For this reason, many efforts have been made to develop lower order approaches aiming at reducing the number of mesh elements and therefore computational resources. The simplest approach models the set of holes as a uniform coolant injection, but it does not allow an accurate assessment of the interaction between hot gas and coolant. Therefore higher order models have been developed, such as those based on localized mass sources in the region of hole discharge. It is here proposed an innovative injection film cooling model (FCM), embedded in a CFD code, to represent the effect of cooling holes by adding local source terms at the hole exit in a delimited portion of the domain, avoiding the meshing process of perforations. The goal is to provide a reliable and accurate tool to simulate film-cooled turbine blades and nozzles without having to explicitly mesh the holes. The validation campaign of the proposed model is composed of two phases. During the first one, results obtained with the film cooling model are compared to experimental data and to numerical results obtained with the full meshing of the cooling holes on a series of test cases, ranging from single row to multi row flat plate, at varying coolant conditions (in terms of blowing and density ratio). Though details of the flow structure downstream of the holes cannot be perfectly captured, this method allows an accurate prediction of the overall flow and performance modifications induced by the presence of the cooling holes, with a strong agreement to complete hole discretization results. In the second phase, a complete film-cooled vane test case has been studied, in order to consider a real injection system and flow conditions. In this case, film cooling model predictions are compared to an in-house developed correlative approach and full CHT 3D-CFD results. Finally, a comparison between film cooling model predictions and experimental data was performed on an actual nozzle of a GE Oil & Gas heavy-duty gas turbine as well, in order to prove the feasibility of the procedure. The presented film cooling model proved to be a feasible and reliable tool to evaluate adiabatic effectiveness, simplifying the design phase avoiding the meshing process of perforations. Also, refining the mesh near the hole exit, FCM results well approximate the solution coming from a full CHT calculation.

Assessment of modelling strategies for film cooling

“…The model proposed by Andrei et al (2015) is based on the application of the mass source within an injection volume at the discharge of the modelled hole (Figure 2). This can be considered a simplified approach, compared to that one proposed by auf dem Kampe and Völker (2012), which provided distributions of the physical quantities within the injection volume, calculated exploiting in-house developed correlations.…”

“…At the moment in fact, velocity, temperature and turbulence are calculated coherently with Andreini et al (2013a, 2013b) and applied uniformly within the injection volume. Another peculiar characteristic related to this kind of modelling is the significant sensitivity to mesh refinement (auf dem Kampe and Völker, 2012; Andrei et al , 2015). For this reason, this aspect was further investigated in this work.…”

“…At last, an approach similar to that proposed by auf dem Kampe and Völker (2012) was implemented in CFX (Andrei et al , 2015). Notwithstanding some simplifications (e.g.…”

“…However, it is worth underlining that most of the film cooling models presented show a limited validation, usually focused on adiabatic or overall effectiveness only. At this purpose, the main goal of the present work is the further validation of the injection strategies proposed by Andreini et al (2013a, 2013b) and (Andrei et al , 2015) to better understand the capability of the two models in predicting the convective heat loads. In fact, it is worth remembering that the point source model was validated only in terms of overall effectiveness and adiabatic effectiveness for the distributed sources.…”