John W. Chew

Heat Transfer Measurements to a Gas Turbine Cooling Passage With Inclined Ribs

The local heat transfer coefficient distribution over all four walls of a large-scale model of a gas turbine cooling passage have been measured in great detail. A new method of determining the heat transfer coefficient to the rib surface has been developed and the contribution of the rib, at 5 percent blockage, to the overall roughened heat transfer coefficient was found to be considerable. The vortex-dominated flow field was interpreted from the detailed form of the measured local heat transfer contours. Computational Fluid Dynamics calculations support this model of the flow and yield friction factors that agree with measured values. Advances in the heat transfer measuring technique and data analysis procedure that confirm the accuracy of the transient method are described in full.

The computation of convective heat transfer in rotating cavities

Abstract In this paper, numerical solutions of the Reynolds-averaged Navier-Stokes equations are presented for convective heat transfer inside axisymmetric rotating disc cavities. The study involves the examination of three different disc-cavity configurations and the use of four different mathematical models of turbulence. The three configurations are a rotating cavity with flow entering axially at the center and leaving radially through the outer shroud, a rotating cavity with central axial throughflow, and a rotor-stator system with axial flow injection through the stator center and outflow through the annulus formed between the rotor disc and the outer shroud. Of the four turbulence models, three are based on the zonal modeling approach with the k - e model in the main flow region and alternative low-Reynolds number treatments across the near-wall regions. These near-wall alternatives consist of two versions of the mixing-length hypothesis and a one-equation k -transport model. The fourth turbulence model is the mixing-length hypothesis applied over the entire cavity. Comparisons with available heat transfer measurements show that none of the models is successful in all cases examined. Considering overall performance, the k - e model with the one-equation near-wall treatment is preferred.

Performance of Radial Clearance Rim Seals in Upstream and Downstream Rotor–Stator Wheelspaces

A new experimental facility for the investigation of rim sealing is described and measurements are presented for two representative radial clearance seals with a nominally axisymmetric external flow. One radial seal has an upward rotor lip and is upstream of the rotor while the other has an upward stator lip and is downstream of the rotor. Measurements include surface pressures, tangential velocities in the core region of the disc cavity flow, and traverses of gas concentration in the cavity showing the distribution of mainstream ingestion. Tests were conducted at rotational Reynolds numbers up to 3 × 106 with nominal seal clearance to radius ratios in the range 0.002 to 0.01. For the radial seals a differential pressure criterion is found to overestimate the minimum sealing flow. Tangential velocity measurements in the wheelspace are in excellent agreement with other workers measurements and with theoretical predictions.

Interaction of Rim Seal and Annulus Flows in an Axial Flow Turbine

A combined computational fluid dynamics (CFD) and experimental study of interaction of main gas path and rim sealing flow is reported. The experiments were conducted on a two stage axial turbine and included pressure measurements for the cavity formed between the stage 2 rotor disc and the upstream diaphragm for two values of the diaphragm-to-rotor axial clearance. The pressure measurements indicate that ingestion of the highly swirling annulus flow leads to increased vortex strength within the cavity. This effect is particularly strong for the larger axial clearance. Results from a number of steady and unsteady CFD models have been compared to the measured results. Good agreement between measurement and calculation for time-averaged pressures was obtained using unsteady CFD models, which predicted previously unknown unsteady flow features. This led to fest response pressure transducer measurements being made on the rig, and these confirmed the CED prediction.

Rim sealing of rotor-stator wheelspaces in the absence of external flow

Sealing of the cavity formed between a rotating disc and a stator with an asymmetric external flow is considered. In these circumstances circumferential pressure variations in the external flow and the pumping action of the disc may draw fluid into the cavity. Gas concentration measurements, showing this effect, have been obtained from a model experiment with a simple axial clearance seal. In the experiment, guide vanes, fitted upstream of the rim seal, generate an asymmetric external flow. The measurements are shown to be in reasonable agreement with three-dimensional computational fluid dynamics (CFD) calculations and are also compared with more elementary models. The CFD results give further insight into the effects of ingestion within the cavity.

Computational and Mathematical Modeling of Turbine Rim Seal Ingestion

Understanding and modelling of main annulus gas ingestion through turbine rim seals is considered and advanced in this paper. Unsteady 3-dimensional computational fluid dynamics (CFD) calculations and results from a more elementary model are presented and compared with experimental data previously published by Hills et al (1997). The most complete CFD model presented includes both stator and rotor in the main annulus and the inter-disc cavity. The k-e model of turbulence with standard wall function approximations is assumed in the model which was constructed in a commercial CFD code employing a pressure correction solution algorithm. It is shown that considerable care is needed to ensure convergence of the CFD model to a periodic solution. Compared to previous models, results from the CFD model show encouraging agreement with pressure and gas concentration measurements. The annulus gas ingestion is shown to result from a combination of the stationary and rotating circumferential pressure asymmetries in the annulus. Inertial effects associated with the circumferential velocity component of the flow have an important effect on the degree of ingestion. The elementary model used is an extension of earlier models based on orifice theory applied locally around the rim seal circumference. The new model includes a term accounting for inertial effects. Some good qualitative and fair quantitative agreement with data is shown. Copyright

Efficient Finite Element Analysis/Computational Fluid Dynamics Thermal Coupling for Engineering Applications

An efficient finite element analysis/computational fluid dynamics (FEA/CFD) thermal coupling technique has been developed and demonstrated. The thermal coupling is achieved by an iterative procedure between FEA and CFD calculations. Communication between FEA and CFD calculations ensures continuity of temperature and heat flux. In the procedure, the FEA simulation is treated as unsteady for a given transient cycle. To speed up the thermal coupling, steady CFD calculations are employed, considering that fluid flow time scales are much shorter than those for the solid heat conduction and therefore the influence of unsteadiness in fluid regions is negligible. To facilitate the thermal coupling, the procedure is designed to allow a set of CFD models to be defined at key time points/intervals in the transient cycle and to be invoked during the coupling process at specified time points. To further enhance computational efficiency, a “frozen flow” or “energy equation only” coupling option was also developed, where only the energy equation is solved, while the flow is frozen in CFD simulation during the thermal coupling process for specified time intervals. This option has proven very useful in practice, as the flow is found to be unaffected by the thermal boundary conditions over certain time intervals. The FEA solver employed is an in-house code, and the coupling has been implemented for two different CFD solvers: a commercial code and an in-house code. Test cases include an industrial low pressure (LP) turbine and a high pressure (HP) compressor, with CFD modeling of the LP turbine disk cavity and the HP compressor drive cone cavity flows, respectively. Good agreement of wall temperatures with the industrial rig test data was observed. It is shown that the coupled solutions can be obtained in sufficiently short turn-around times (typically within a week) for use in design.

Computational fluid dynamics for turbomachinery internal air systems

Considerable progress in development and application of computational fluid dynamics (CFD) for aeroengine internal flow systems has been made in recent years. CFD is regularly used in industry for assessment of air systems, and the performance of CFD for basic axisymmetric rotor/rotor and stator/rotor disc cavities with radial throughflow is largely understood and documented. Incorporation of three-dimensional geometrical features and calculation of unsteady flows are becoming commonplace. Automation of CFD, coupling with thermal models of the solid components, and extension of CFD models to include both air system and main gas path flows are current areas of development. CFD is also being used as a research tool to investigate a number of flow phenomena that are not yet fully understood. These include buoyancy-affected flows in rotating cavities, rim seal flows and mixed air/oil flows. Large eddy simulation has shown considerable promise for the buoyancy-driven flows and its use for air system flows is expected to expand in the future.

Application of Computational Fluid Dynamics to Turbine Disk Cavities

A CFD code for the prediction of flow and heat transfer in rotating turbine disk cavities is described and its capabilities demonstrated through comparison with available experimental data. Application of the method to configurations typically found in aeroengine gas turbines is illustrated and discussed. The code employs boundary-fitted coordinates and uses the k-e turbulence model with alternative nearwall treatments. The wall function approach and a one-equation near-wall model are compared and it is shown that there are particular limitations in the use of wall functions at low rotational Reynolds number. Validation of the code includes comparison with earlier CFD calculations and measurements of heat transfer, disk moment, and fluid velocities. It is concluded that, for this application CFD is a valuable design tool capable of predicting the flow at engine operating conditions, thereby offering the potential for reduced engine testing through enhanced understancing of the physical processes

Measurement and Analysis of Ingestion Through a Turbine Rim Seal

Experimental measurements from a new single stage turbine are presented. The turbine hits 26 vanes and 59 rotating blades with a design point stage expansion ratio of 2.5 and vane exit Mach number of 0.96. A variable sealing flow is supplied to the disk cavity upstream of the rotor and then enters the annulus through a simple axial clearance seal situated on the hub between the stator and rotor. Measurements at the annulus hub wall just downstream of the vanes show the degree of circumferential pressure variation. Further pressure measurements in the disk cavity indicate the strength of the swirling flow in the cavity, and show the effects of mainstream gas ingestion at low sealing flows. Ingestion is further quantified through seeding of the sealing air with nitrous oxide or carbon dioxide and measurement of gas concentrations in the cavity. Interpretation of the measurements is aided by steady and unsteady computational fluid dynamics solutions, and comparison with an elementary model of ingestion.