This paper documents the construction of a Virtual Engine, with particular reference to its geometry and conceptual description.. The phrase Virtual Engine denotes a system which allows simulations of whole gas-turbine engines to be undertaken at any desired level of fidelity or physical modeling. In order to be of any practical use, the system must allow the computations to be setup as automatically as possible and needs to contain provisions for the exchange of boundary data between adjacent computational domains — e.g. solid-gas interfaces. The paper illustrates the application of the system to the representation and analysis of a modern commercial turbofan engine.
The design process of a gas turbine engine involves interrelated multi-disciplinary and multi-fidelity designs of engine components. Traditional component-based design process is not always able to capture the complicated physical phenomenon caused by component interactions. It is likely that such interactions are not resolved until hardware is built and tests are conducted. Component interactions can be captured by assembling all these components into one computational model. Nowadays, numerical solvers are fairly easy to use and the most time-consuming (in terms of man-hours) step for large scale gas turbine simulations is the preprocessing process. In this paper, a method is proposed to reduce its time-cost and make large scale gas turbine numerical simulations affordable in the design process. The method is based on a novel featured-based in-house geometry database. It allows the meshing modules to not only extract geometrical shapes of a computational model and additional attributes attached to the geometrical shapes as well, such as rotational frames, boundary types, materials, etc. This will considerably reduce the time-cost in setting up the boundary conditions for the models in a correct and consistent manner. Furthermore, since all the geometrical modules access to the same geometrical database, geometrical consistency is satisfied implicitly. This will remove the time-consuming process of checking possible mismatching in geometrical models when many components are present. The capability of the proposed method is demonstrated by meshing the whole gas path of a modern three-shaft engine and the Reynold’s Averaged Navier-Stokes (RANS) simulation of the whole gas path.
The growth in diameter of turbofan engines exacerbates problems related to the interaction of the Outlet Guide Vanes (OGV), pylon and intake because it reduces the ratio between components gaps and disturbance wavelength. The main components of this interaction are the potential fields generated by the intake and by structural components in the bypass, the pylon and the Radial Drive Fairing (RDF). The OGV bladerow and the fan are immersed in these potential fields and suffer performance degradation as well as integrity issues as a result. Simple actuator-disc analysis shows that a uniform OGV cascade amplifies the effect of the pylon potential flow. Therefore, a number of methods have been proposed over the years to compute OGV exit flow angle patterns that result in an approximately circumferentially uniform static pressure field at fan exit. Within actuator disc approximations, the determination of the optimal exit flow angle pattern can be accomplished analytically but little information is obtained on how the geometry of the vanes ought to be modified. Consequently, it is not difficult to generate by this method OGV cascades that stall or choke locally. More recent contributions use CFD computations coupled to optimization methods to determine OGV patterns that reduce the distortion at the fan exit, while minimising some measure of OGV loss. Whilst in principle more rational, these methods encounter practical difficulties due the computational power needed to obtain reliable loss estimates while exploring large design spaces. In this paper the view is taken that the performance of the OGV bladerow can be preserved during the optimization process if the loading distribution of each vane is made to match the loading distribution of the nominal vane (i.e. the aerodynamic design intent with axisymmetric inlet and exit flow). As loading distributions are readily available from inviscid-type analysis, the generation of optimal OGV patterns can be accomplished with very reasonable computational expense using a method based on the model described in part I of this paper.
The fan systems of typical high bypass civil engines encounter strong flow distortions originating in the intake and in the bypass duct. These flow distortions cause the fan stage operation point to vary from its design intent, thus reducing the fan stage performance and increasing low engine-order fan blade forcing. A cyclic pattern design for the fan Outlet Guide Vanes (OGV) can be effectively used to recover the fan stage performance and to control its system-level aeromechanical behaviour. This paper presents the development of a OGV pattern design philosophy using numerical experimentation technique. Multiple fan-intake unsteady CFD computations are conducted by clocking the circumferential pressure profile at fan exit. The study revealed that a mild, low-harmonic fan back pressure profile with a suitable clocking position is able to improve the fan rotor efficiency and reduce the 1EO fan forcing simultaneously. Such a profile can be generated by designing a cyclic OGV pattern that allows the bifurcation potential fields of controlled intensity and phase to pass through the OGV blade row; thus termed as the translucent design philosophy. Further, a sensitivity study is performed to assess the effects of simultaneous distortions upstream and downstream of the fan. The study showed that a correctly clocked intra-stage static pressure profile can consistently improve both the performance and aeromechanical behaviour of fan system having different intake lengths and at different flight conditions. The implementation of the proposed translucent design philosophy in a new OGV pattern design tool is discussed.
The preliminary design of labyrinth seals requires fast and accurate estimate of the leakage flow. While the conventional bulk flow models can quickly predict labyrinth seal discharge characteristics, they lack the accuracy and pragmatism of modern CFD technique and vice-a-versa. This paper presents a new 1D loss model for straight-through gas labyrinth seals that can provide quick seal leakage flow predictions with CFD-equivalent accuracy. The present seal loss model is developed using numerical experimentation technique. Multiple CFD computations are conducted on straight-through labyrinth seal geometries for a range of pressure ratios. A distinct post-processing methodology is developed to extract the through-flow stream tube in seal. Total pressure losses and flow area variations experienced by the flow in seal stream-tube are systematically accounted for based on the well-known knife-to-knife (K2K) methodology. Regression analyses are conducted on the trends of variations of loss and area coefficients to derive the independent pressure loss and flow area correlations. These novel correlations can predict the bulk leakage flow rate, windage flow rate and inter-knife static pressures over a wide range of variation of flow and geometry parameters. Validation study shows that the leakage mass flow rate predicted by this model is accurate within +/-8% of measured test data. This fast and accurate model can be employed for various applications such as, in seal design-analysis workflows, for secondary air system (SAS) performance analysis and for the rotor-dynamic and aeroelastic assessments of seals.
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