Development and Validation of a C++ Object Oriented Cfd Code for Heat Transfer Analysis

This paper describes the development and validation steps of computational sub-models for gas turbine heat transfer applications, within an open source CFD code based on the Field Operation and Manipulation C++ class library for continuum mechanics (OpenFOAM, http://www.opencfd.co.uk). Open FOAM is based on a polyhedral finite-volume approach with a co-located variables arrangement. In order to set up OpenFOAM toolbox to analyze heat transfer problems with RANS approach, it was necessary to add and implement some additional submodels. First of all a SIMPLE like algorithm was specifically developed to solve the fully three dimensional, steady state form of compressible Navier Stokes equations. Moreover several eddy viscosity models such as the standard, the Two Layer version and the realizable k − ε model and the k − ω SST model have been implemented. The accuracy of the implementations was validated comparing results with experimental data available both from standard literature test cases and from in house performed experiments. The geometries considered as validation tests cover the typical heat transfer problems in gas turbine design . On the whole, during the tests, OpenFOAM code has shown a good accuracy and robustness. The purpose of this work is to show the ability of an innovative CFD tool as support for gas turbine designers and to verify its role as an effective substitute for standard commercial CFD packages. INTRODUCTION One of the most demanding problem in gas turbine design is the proper evaluation of heat transfer phenomena which involve all hot components of the engine. Furthermore, all topical design criteria make heat transfer problems more and more difficult. An improvement in gas turbine performance, for example, can be produced by increasing turbine inlet temperature, which is usually well above the metal critical temperature. In addition, new design concepts adopted for combustors, based on lean premixed flames, reduce the amount of air available for wall cooling. These are only two typical examples that justify the increasing interest in developing more and more advanced cooling systems. The complexity of geometries usually adopted in such designs and the high costs required for accurate heat transfer measurements justify the increasing use of CFD analysis in each phase of the design process. Nevertheless, CFD simulations for evaluation of thermal loads and effectiveness of the cooling devices in gas turbine engines are demanding both in terms of physical modeling and geometrical mesh handling. Actual cooling geometries are characterized, for example, by intricate shapes, with non aerodynamic turbulators such as pins and ribs that must be properly discretized, or they involve complex flows such as impinging jets that make turbulence modelling a key point. Such issues usually require CFD codes to satisfy some essential features: a quite large set of turbulence models, in order to have accurate predictions with all possible flows and the capability in handling hybrid unstructured meshes. The consequence of such strict requirements is that a very reduced set of CFD codes is available worldwide, and the choice is usually limited to few 1 Copyright c © 2007 by ASME well known commercial codes. Commercial software have dominated, in the last decade, CFD analysis of heat transfer problems for turbomachinery applications both in industrial and academic field. Besides their numerous advantages, such as the simplicity of use via practical graphical interfaces, they present some common drawbacks: for example the waste of system resources with a large part of packages not used in standard simulations, which is one of the source of their poor performances in terms of calculation times. However, according to experts, we think that the main drawback of commercial CFD codes is their nature of “black box solution maker”. Advanced users in heat transfer applications need to understand the physics and sometimes the use of “ad hoc” models or modifications suitable for specific cases. User subroutine features provided by commercial packages become quickly inadequate as the complexity of modifications grows. Furthermore R&D department of big companies usually need to tune built-in models in order to feed calculations tools with their design practice frequently based on detailed and expensive experimental tests. The objective of the work presented in this paper is to show the capabilities of a new open-source software environment where it is possible to implement new models, renew the existing ones and experiment with model combinations. The OpenFOAM package (Field Operation And Manipulation) [1, 2] is an object-oriented numerical simulation toolkit written in C++ language [3, 4, 5]. Besides its advanced basic native CFD features, which will be described in the next parts, its essential characteristic is the opportunity to build new models and solvers with high simplicity and in less time than with standard Fortran based codes. Object-oriented programming of C++ drastically reduces the probability of bugs introduction with a consequent saving in debugging time. This paper describes the attempt to build a CFD package suitable for typical steady state heat transfer analysis which could be able to assist gas turbine design process. As will be described later, to reach such goal it was necessary to introduce specific modules to the standard release in order to overcome the limitations of built-in approaches: first of all a compressible steady state solver capable at handling transonic flows, then a set of turbulence two-equations closures with particular reference to a detailed near wall treatment. Additional features such as temperature dependent thermo-physical properties and generic grid interfacing have also been developed. It’s important t remark that such features are not available in the released version of the toolkit, as built models are mainly focused on unsteady weakly compressible flows. As a consequence it’s not possible to draw out specific comparisons between default and developed models. As confirmation of the work done a set of validation testcases were performed. In particular, in this paper, we will focus our attention on the validation of the code with some complex configurations typical of heat transfer problems such as film cooling and impingement cooling. Both film and impingement cases were analyzed with single and multi-hole configurations. In particular the well known Sinha experiment was considered for single hole film cooling test [6], while for multi-hole case an experiment performed in our Department was chosen [7]. Film cooling geometries here considered belong to full-coverage film cooling case also known as effusion cooling, a promising technique used in combustor wall and turbine end-wall cooling. For single hole impingement tests, we referred to the classical ERCOFTAC C.25 test [8] while for multi-hole case again an experiment performed in our Department was considered [9]. Comparison with experimental data are reported in terms of adiabatic effectiveness for film cooling tests and wall heat transfer coefficient for impingement runs. Furthermore, in order to verify the accurate implementation of selected turbulence models, a simple flat plate tests was considered, while to show the robustness of the steady state solver developed results of classical validation tests are briefly commented.