Wall-Modelled Large Eddy Simulations of Axial Turbine Rim Sealing

Estimation of hot gas ingestion through turbine rim seals is important for gas turbine design and optimization, but has proved challenging for computational fluid dynamics (CFD). Several studies have found that unsteady Reynolds-averaged Navier-Stokes (URANS) models underpredict ingestion compared to measurements. Large-eddy simulations (LES) using second-order discretization methodologies have shown improvements on URANS predictions but have still underestimated measurements [1]. Recently [2], an explicit LES study employing a linear reconstruction of the gradient at the cell interface was applied to a disc cavity at a modest Reynolds number, showing promising agreement with data for the prediction of the main engineering parameters for a rotor-system. The present paper presents further extension of this methodology aiming to allow prediction of sealing effectiveness at representative engine conditions. This was achieved in two steps.

First, the passive scalar and wall model equations were implemented and validated against data available in the literature. A better agreement with the analytical and experimental data was seen compared to CFD simulations using a standard second order discretization. Moreover, insensitivity to grid-refinement at the wall has been noted suggesting that the full resolution of all the turbulent scales at the wall is not necessary to predict the main engineering parameters of interest.

In the second step, a wall modeled LES approach (WMLES) has been adopted. This is considerably less computationally demanding than fully resolved LES, and has the potential for extension to engine conditions. WMLES results for a simple rotor-stator disc cavity are presented to demonstrate that the model captures the basic cavity flow. WMLES predictions of ingestion are then reported for a chute seal rotor-stator configuration with nozzle guide vanes (NGVs) as considered in previous research. The effects of rotation on unsteadiness and ingestion quantification within the turbine cavity are investigated. Increased ingestion is associated with shifts in the seal unsteady flow structures peak harmonic amplitudes toward higher non-dimensional frequencies and to an increased level of unsteadiness. The results are consistent with the characterisation of the secondary flow structures as inertial waves, and in broad agreement agreement with previous CFD studies. The WMLES predicts considerably more ingestion than a URANS model. In the outer part of the disc cavity the level of ingestion is sensitive to rotational speed. Ingestion to the inner region of the cavity is less pronounced and shows differing trends with rotational speed. Comparison with measurements shows some agreement, but also areas for further investigation.

REFERENCES

1. O’Mahoney, T. S. D., Hills, N. J., Chew, J. W., and Scanlon, T., 2011. “Large-Eddy simulation of rim seal ingestion”. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, 225(12), Dec., pp. 2881–2891.

2. Amirante, D. and Hills, N.J, (2015). “Large-eddy simulations of wall bounded turbulent flows using unstructured linear reconstruction techniques. Journal of Turbomachinery, Vol. 137 / 051006-1”