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All the University partners in ICAS-GT run undergraduate courses on turbomachinery and propulsion. ICAS-GT has provided them with a wealth of teaching material to enhance the air system, fluid flow and heat transfer elements of such courses. Experimental techniques have been developed and applied for the first time as part of this project. These include heat flux gauges suitable for application to rotating components, and Particle Image Velocimitry (PIV) via an endoscope for velocity measurements in enclosed cavities. These and other advanced measurement techniques are available for sharing between the Universities, with benefits to a wide range of future research projects in aero-thermal disciplines.
The gold standard modeling methodology describes the meshing and convergence strategies for producing a fully three dimensional, time-dependent prediction of hot gas ingestion at a turbine disc rim. The solution domain includes the main annulus flow, and the front and rear wheel space sealing flows. Solution of the main annulus flow is necessary to establish the correct boundary conditions at the rim seal gap. The methodology has been demonstrated by application to the turbine rim sealing rig geometry, and is believed to capture the important physical flow mechanisms, although further work is required to completely validate the modeling approach. The result is relevant to the design of turbine rim seals in new gas turbine engines, and the analysis of existing rim seal performance. It has the potential to reduce the amount of testing and instrumentation required during an engine development programme, and to reduce engine specific fuel consumption through a reduction in the required rim sealing flow. This result includes attempts to simplify the CFD methodology to a level more consistent with engine design cycles.
Prior to ICAS-GT, the database of air system experiments was limited in terms of non-dimensional parameter ranges. In many cases the experiments failed to capture flow and heat transfer mechanisms that are relevant in gas turbine engine internal air systems. ICAS-GT has addressed this by conducting experiments designed to reproduce engine representative flow and heat transfer phenomena in five important areas of the air system. All the experimental programmes have successfully generated data. Some of this has been used to develop physical understanding, some for validating CFD and FE modeling methods, and some for generating correlations which are applicable to gas turbine design and analysis. The database is a lasting attribute which will be exploited variously by all the consortium partners in developing and validating design and predictive tools.
A fully three dimensional, unsteady CFD method for modeling the complete pre-swirl system has been developed. This has been applied to the Karlsruhe University test rig geometry and found to give very good agreement in terms of pressure change through the complete system, and through the component parts of the system. The CFD calculations have also provided valuable insight in to the pre-swirl chamber flow distribution. Despite the use of discrete pre-swirl nozzles rather than a continuous cascade ring, the peak to peak pressure fluctuation in the chamber has been shown to be very low, and has little influence on the rotating hole discharge characteristics. This modeling methodology is now available for optimising the pre-swirl systems of new engine designs with benefits in terms of blade cooling (and therefore blade life), or specific fuel consumption (through the more efficient use of less air).
Experimental data from the Sussex University rotating cavity rig tests has been analysed to give Nusselt numbers for the disc surfaces, and for the inner surface of the conical drive arm. Two correlations have been derived for heat transfer from the cone inner surface. For low values of Rosby number, where the flow is governed by rotationally dominated forces, Nusselt number has been correlated against axial Reynolds number, position in the cavity and Grashof number, where Grashof number is a measure of the degree of buoyancy. For higher values of Rosby number where the flow is dominated by the action of through-flow beneath the disc bores, Nusselt number has been correlated against axial Reynolds number and cavity position only. These correlations are of direct use in predicting drive come metal temperatures in new engine designs, and have already been embedded in the design methods of at least one of the partner companies.
Experimental data from the turbine rim sealing experiments conducted at Aachen University have been collapsed into correlations of rim sealing effectiveness as a function of rim sealing flow and rim seal geometry. Axial type seals up and downstream of the rotor have been investigated, for a range of seal gaps and seal positions relative to the blade and vane rows. The result is relevant to the design of turbine rim seals in new gas turbine engines, and the analysis of existing rim seal performance. It has the potential to reduce the amount of testing and instrumentation required during an engine development programme, and to reduce engine specific fuel consumption through a reduction in the required rim sealing flow. The correlations are currently being refined for axial rim seals, and will be available for incorporation in industrial partner design systems by mid 2001.
Pre-swirl systems are used in gas turbine engines to swirl HP turbine blade cooling air up to disc speed in order to minimise the total temperature of the air fed to the blade. Optimisation of these systems requires a detailed knowledge of the flow discharge characteristics of the (static) pre-swirl nozzles, and of the rotating holes in the disc. Discharge coefficients for rotating holes and for static nozzles have been determined experimentally for a wide range of operating conditions. The dependence of discharge coefficient on swirl ratio, system pressure ratio and disc rotational speed for a range of rotor-stator axial gaps, rotor hole inlet treatments and length to diameter ratios has been determined. In addition to pre-swirl systems, these results are relevant to other parts of the internal air system where flow is passed axially through rotating holes. These results have already been used by one of the partner companies to assist in understanding the operation of an existing pre-swirl system. They have the potential to be correlated and embedded in existing design systems.
A combination of experimental and numerical modeling approaches has given an insight in to the mechanisms that can cause a high degree of heating in compressor stator well cavities formed between compressor stator shroud rings and the compressor drum. These are as follows:
- High shearing of the flow at the gap between the rotor and the downstream stator row as the flow enters the stator well cavity, causing significantly more heat generation than predicted by conventional windage heating correlations for a free disc.
- Windage heating of the flow in the downstream cavity which is broadly in line with conventional windage heating correlations for a free disc.
- Windage heating of the flow in the upstream cavity which is higher than predicted with conventional windage heating correlations for a free disc due to the impingement of the jet flow from the labyrinth seal.
- Windage heating of the flow as it passes through the labyrinth seal beneath the stator well shroud, which is broadly in line with conventional windage heating correlations for seals.
- The re-ingestion in to the stator well of air which has already passed through the stator well and been ejected back in to the main annulus.
This understanding provides a good basis for further work to derive a simple model for predicting compressor stator well heating. The result is relevant to all gas turbine engine manufacturers using the shrouded compressor stator design philosophy.
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