Dietmar Giebert

A Combined Eulerian and Lagrangian Method for Prediction of Evaporating Sprays

Polydisperse sprays in complex three-dimensional flow systems are important in many technical applications. Numerical descriptions of sprays are used to achieve a fast and accurate prediction of complex two-phase flows. The Eulerian and Lagrangian methods are two essentially different approaches for the modeling of disperse two-phase flows. Both methods have been implemented into the same computational fluid dynamics package which is based on a three-dimensional body-fitted finite volume method. Considering sprays represented by a small number of droplet starting conditions, the Eulerian method is clearly superior in terms of computational efficiency. However, with respect to complex polydisperse sprays, the Lagrangian technique gives a higher accuracy. In addition, Lagrangian modeling of secondary effects such as spray-wall interaction enhances the physical description of the two-phase flow. Therefore, in the present approach the Eulerian and the Lagrangian methods have been combined in a hybrid method. The Eulerian method is used to determine a preliminary solution of the two-phase flow field. Subsequently, the Lagrangian method is employed to improve the accuracy of the first solution using detailed sets of initial conditions. Consequently, this combined approach improves the overall convergence behavior of the simulation. In the final section, the advantages of each method are discussed when predicting an evaporating spray in an intake manifold of an internal combustion engine.

A conservation-based discretization approach for conjugate heat transfer calculations in hot-gas ducting turbomachinery components

Abstract A numerical procedure has been developed for simulation of conjugate heat transfer in generalized coordinates and used in some typical turbomachinery applications. Discretized equations for nodes located exactly on the solid–fluid interface were derived using energy conservation principles, yielding the corresponding temperatures directly, without the need for inter- or extrapolation from adjacent nodes. A finite-volume-based computer code was used along with the SIMPLE algorithm and the k – ϵ turbulence model. The turbulent flow and heat transfer in a stepped labyrinth seal and in an effusion-cooled combustor liner have been studied and results were compared with measured data showing good agreement. In the labyrinth-seal case, the comparisons were in terms of surface temperatures and Nusselt numbers, while for the effusion-cooling case in terms of the streamwise velocity and the film-cooling effectiveness for different blowing and density ratios. In the latter case, two different liner materials were used to study the influence of the thermal conductivity on film-cooling characteristics and the agreement was better for the lowest of the two conductivities. The dominant flow structures could be captured with good accuracy.

Film-Cooling From Holes With Expanded Exits: A Comparison of Computational Results With Experiments

A 3D Navier-Stokes code, together with the standard k-ϵ model with wall function approach, was used to investigate the flowfield in the vicinity of three different single scaled-up film-cooling holes. The hole geometries include a cylindrical hole, a hole with laterally expanded exit, and a hole with forward-laterally expanded exit.Comparisons of numerical results with detailed flowfield measurements of mean velocity and turbulent quantities are presented for a blowing ratio and density ratio of unity. Additionally, experimental data for different blowing ratios and a density ratio of about two are taken to perform validation of the code for adiabatic film-cooling effectiveness prediction.Results show that for both the round and the expanded hole geometries the code is able to capture all dominating flow structures of this jet in crossflow problem. However, discrepancies are found when comparing the flowfield inside the hole and at the hole exit. In particular, jet location at the hole exit differs significantly from measurement for the expanded hole geometries. For the adiabatic film-cooling effectiveness, it is shown that for round and expanded hole exits the intensity of the shear regions and the source of turbulence, respectively, have a strong influence on the predictive capability of the numerical code.Copyright

Exploiting the flexibility of unstructured grids for computing complex flow patterns more precisely

Abstract In comparison to structured meshes, the most important advantage of unstructured grids is geometrical flexibility provided by the lack of strict topological rules. The present work proposes techniques to exploit this feature by generating and optimizing unstructured meshes to achieve more accurate results in less computing time. In this article a suited layout of initial grids is shown to improve convergence speed while reducing memory requirements and preserving numerical accuracy. To control the process of solution adaptive grid optimization, normalized gradients of transport variables are proposed. These quantities prove to be effective in several test cases and a demanding turbomachinery application. Although the calculations were carried out assuming two-dimensional flow, the methods introduced here can be applied under three-dimensional conditions without difficulty.