Gas Turbine Engine Internal and Secondary Flow System

（Imperial College London,UK;p.childs@imperial.ac.uk）

Abstract:The challenging conditions at which gas turbine engines operate mean that careful management of component temperatures is necessary,in order to ensure component integrity and reasonable service life.Pressurised flows extracted from the compressor can be used for cooling,sealing and balancing of components throughout the engine.Cooling is typically necessary for some combustor and turbine components, and sealing flows may be needed to exclude high temperature gases.In addition to cooling and sealing requirements,there is also a need to balance thrust loads in an engine,to limit loads on bearings,arising from the pressure differentials across compressor and turbine disc assemblies.The diverse tasks of cooling, sealing and balancing are generally assigned to a system known as the internal air system or secondary flow system.This paper describes the technologies associated with this system for both industrial gas turbine engines and aero-engines,and the current state of the art and challenges in component and subsystem design.

Keywords：Internal,Air,Flow,Secondary,Disc,Rotor,Stator,Ingress,Ingestion,Turbine,System

1 Introduction

Gas turbine engine efficiency improves with increasing operating temperature and pressure ratios.The associated running conditions mandate the use of cooling in order to enable reasonable service lives for many components,sealing flows in order to contain or exclude fluids and particulates,and pressurising flows in order to manage thrust loads on spool assemblies.These functions are generally associated with the internal air system or secondary air or flow system.Typical subsystems associated with the internal air system include:bearing sealing and cooling,disc thrust management,rim seal ingress and egress,turbine disc cooling,provision of cooling air supply for turbine blade cooling,provision of cooling flows for combustor components,pressure balancing and flow management of various disc,drum and shaft seals,management of wheel-space temperatures,and management of electrical device cooling.Some typical subsystems associated with the internal air system are illustrated in Figure 1.Air for these diverse requirements can be obtained by bleeding off compressed air from one or more stages in the compressor.This air may also be used for cabin air ventilation.

Extraction of air from the compressor results in specific fuel consumption performance penalties.Such penalties arise from both the power required to compress this air and those arising from spoiling or increased entropy associated with cooling and purge flows mixing with compressor and turbine mainstream flows.Significant efforts are therefore made in order to limit the quantity of bleed air used.This paper reviews some of the principal technologies associated with internal air and secondary flow systems.The paper considers the overall design of the internal air system in Section 2,bleed flows in Section 3,disc thrust management in Section 4,compressor wheel-spaces,drums and stator wells and drive cones in Section 5,turbine disc rim seals and associated features and pre-swirl systems in Section 6,bearing chambers in Section 7,electrical subsystem temperature management in Section 8,and overall conclusions in Section 9.There has been substantial technical and scientific attention to each subsystem and as a result there is an extensive literature base,referenced here principally by attention to relevant reviews and recent works in the area concerned.

Fig.1 Typical flow subsystems associated with the internal air system

2 Internal Air System Morphology

The configuration of the internal air or secondary flow system is predominantly defined by initial estimates for principal flows for the various subsystems and bounded by the compressor,combustor and turbine principal geometry.Flow estimates for the subsystems can be developed from scaling of previous engine flows,or from models for the subsystems considering the operating temperature of the various components,and using models to estimate the cooling or sealing flow required.A flow network model can be used to provide an indication of the necessary flow path dimensions and associated pressure drops.Such models tend to be one-dimensional flow networks that rely heavily on empirical and proprietary data.Once a flow network has been established[1]more detailed modelling of the various subsystems can be undertaken using 2D,3D and 3D time-dependent models to verify and optimise the flow budget.

3 Bleed Flows

Air can be extracted from one or more compressor stages in order to provide the various flows required for sealing,cooling and thrust management,as well as any air required for cabin air ventilation.A bleed off take can comprise a pipe extract located in the compressor casing at the relevant stage to give the pressure required for the flow.

Significant quantities of air are required for pressurising and ventilating the air craft cabin.Federal Aviation Authority Regulation FAR 25[2]specifies a flow rate of 0.55 lb/minute per passenger.As a result the total flow requirement can be 0.92 kg/s for a 220 passenger aircraft.In order to ensure a supply of air uncontaminated by oil and other harmful substances,careful attention to filtering and venting of the flow at start-up may be necessary[3-4].

4 Disc Thrust Management

The surface area associated with compressor and turbine disc assemblies is subject to pressure loading and needs careful management in order to ensure that any net thrust can be accommodated by the bearings.The arising load on bearings will vary according to the running conditions with both the magnitude and direction of the net thrust load altering significantly through a loading,start-up or flight cycle.In the case of aircraft the thrust load on a bearing system needs to accommodate in-flight events,such as stalling of a compressor stage,arising say from ice ingestion,and surge.The large diameter of the turbofan means that the arising thrust load is substantial,and this can be a particular challenge to accommodate for both normal running conditions and extreme events.Typical design approaches involve estimating the pressure differential across discs and stages,and summing up the net load on a spool,and use of sufficiently specified thrust bearings to accommodate the worst case scenario.Modelling methods accounting for the variation of pressure with radius for a rotating disc are reported by[5].

5 Compressor Drums and Stator Wells and Drive Cones

A compressor,in the case of an axial configuration,typically comprises a rotating drum made up of a series of discs supporting the rotating blades with lands or end walls for the stator blades(see Figure 2).Industrial gas turbine configurations may comprise a solid or a significantly more solid drum assembly providing useful inertia for accommodating load fluctuations in comparison to an aircraft application where weight is a critical factor.

Fig.2 Compressor stack features.(Adapted from[5])

The use of a disc assembly can result in a rotating cavity formed between two co-rotating discs with a shroud at the outer periphery.Such cavities are subject to wind age heating arising from relative motion between the rotating disc and the air contained,and as a result attention to management of the running temperature of the assembly may be necessary in order to provide acceptable service life.A significant range of flow conditions are possible in a rotating cavity depending on the cavity dimensions,whether there is net through flow of fluid,on the relative temperature of the cavity surfaces,whether the cavity closed,open,or bounded with a co-rotating or contra-rotating inner shaft[6-7].

The case of a rotating cavity with radial outflow has been extensively studied and a series of characteristic features identified including a source region,Ekman type layers,sink region and rotating core[6,8-9].For a cavity with radial inflow,characteristic regions include an entrainment layer at the outer periphery,the source region,interior core and Ekman type layers on the disc and a sink region towards the inner radius[10].For a cavity with axial through flow,the flow conditions can be particularly complex with non-axisymmetric time dependent flow features arising,dependent on parameters such as the radial and axial temperature gradient[6,11].Significant insights have been developed arising from numerical studies,with for example[12]and[13]demonstrating the presence of time varying flow features that change in form and rotate relative to the discs.The use of radial inflow as a means to control compressor drum growth and hence tip clearance has been investigated by[14].Measurement of axial and tangential velocity profiles inside the cavity of a compressor drum rig with four rotating cavities and shroud heat transfer were reported by[15-18]and coupled transient thermal analysis by[19].

For shrouded stator vanes,a trench or recess in the compressor drum is necessary,known as a compressor stator well,see Figure 3.The associated geometry typically com-prises rim seals upstream and downstream of the vane,and some form of seal between the drum and the shroud to limit flow through the well.The design of such configurations will include consideration of ingress and egress associated with the rim seal flows and any flow interactions and flow spoiling,management of any windage heating in the compressor stator well and ensuring that axial and radial movements can be accommodated[20-21].

Fig.3 Compressor stator well

A conical or cylindrical form is typically used to connect the high pressure turbine and compressor.The drive cone can provide a pathway for ducting cooling and sealing under the combustor to turbine components.Flow and heat transfer in this region has been investigated among others,by[22]and undertaking a conjugate heat transfer study,by[23].

6 Turbine Rim Seals,Disc Features and Pre-swirl

A running clearance is necessary between the high speed rotating disc supporting blades and the corresponding casing or stationary disc.In the case of coaxial discs this configuration is known as a rotor-stator disc cavity or wheelspace,see Figure 4.Fluctuations in pressure and flow conditions in the mainstream annulus,combined with instabilities in the cavity under certain conditions result in complex flow interactions between flows in the vicinity of the rim seal,in the mainstream bladed annulus and within the cavity.Flow exiting a cavity is typically referred to as egress and that entering a cavity as ingress.Significant attention is given to these flows in order to minimise the increase of entropy in a stage and to manage disc,rim and blade temperatures at acceptable levels.

Fig.4 Schematic section of a turbine stage illustrating a rotor-stator disc wheelspace[5]

A wide range of modelling approaches have been developed for determining ingress and egress rates in disc rim seals,Figure 5,including empirical correlations and orifice models where ingress and egress flows around the circumference of the seal are moderated by discharge coefficients.A recent version of such a model[24]integrates the ingress and egress assuming a saw tooth profile for the pressure at the nozzle guide vane exit.The seal effectiveness and purge flow is described in terms of two empirical constants,one defining the ratio of the ingress and egress discharge coefficients,and the other scales the performance curves to give the purge flow which is sufficient to prevent ingestion.

Fig.5 Selected rim seal configurations(Adapted from[5])

Experimental data from[25]and[26]indicates that ingestion mechanisms are associated with a range of different length scales,including shear layer interaction between the mainstream and rim seal flows over multiple vane pitches,in addition to vane and blade pressure fields.[26]present a model relating rim seal effectiveness in a rotor-stator cavity to purge flow based on turbulent transport mechanisms.The model assumes that the flow in the rim seal is mixed by a recirculation region or vortex extending around the circumference,and that all length scales of ingress lead to an effective eddy diffusivity that drives ingress across the seal concentration gradient.For a given seal,an effective eddy is assumed to act across the difference in seal effectiveness between the cavity and mainstream values.[26]also account for the reduction in turbulent mixing in the rim seal recirculation region with increasing purge flow.In general the better the seal,the lower the mixing length,which can be taken to indicate confinement of the effective eddy.A longer medial length of the rim seal,indicated by the radial dimension of the constriction,reduces the gradient and the reducing volume fraction occupied by the gap recirculation zone,drives a reduction in diffusivity,and as a result gives a higher turbulent mixing switch-off parameter.The model gives high fidelity matches for both the Cambridge data,[25],[26],and the data from the Bath rig[27-30].

Common practice in many axial turbines is to place a shroud on the inner radius of guide vanes in order to reduce over tip leakage effects.This mandates a recess in the rotor drum assembly known as a turbine stator well,see Figure 5(g).The arising configuration,comprising two rotor-stator cavities normally separated by a labyrinth seal,has been subject to extensive studies in order to improve modelling methods as reported by[31]and[32].

In the design of an internal air system it is necessary to account for windage heating by the various rotating components such as discs and bolts.Bolts are typically designed with aerodynamic features,recessed or covered in order to minimise such effects.A wide range of correlations are available for evaluating the moment coefficient of a disc under differing flow conditions,see[5]and[33].Experimental data for the effect of rotating bolts or stationary bolts in a rotorstator cavity has been produced by[34],and modelling reported by[35].

In order to transfer air from the non-rotating frame of reference to the rotating frame of reference of a turbine disc,a pre-swirl system can be used.This established method typically comprises accelerating the air through a series of inclined nozzles on the stationary disc to give a high tangential component of velocity before the flow enters corresponding feed holes on the rotating disc and is then transferred through passages on board the rotor disc towards the base of the turbine blades where it can be used to maintain these at reasonable temperatures by means of a series of bleed holes in the blades.The acceleration of the flow,relative to the rotor disc,results in a reduction of the total temperature with potential advantages for cooling effectiveness.In many practical systems,however this accelerated cooling flow must pass across the rotor-stator disc cavity and mixes with the cavity air,some of which may be associated with hot ingress flow,negating some of the advantage of the pre-swirl temperature drop[36].The effectiveness of such a pre-swirl system can be described in terms of the maximum achievable temperature decrease and the associated discharge coefficients for the pre-swirl nozzles and receiver holes[37].In order to account for the complex interactions significant insight and modelling capability[38-42]and use of high quality empirically derived coefficients can be necessary[43].

7 Bearing Chambers

Bearings typically require very careful attention to their mounting,sealing and thermal management in order to ensure the necessary alignment,exclusion of dirt and retention of lubricants,and removal of heat generated by friction.In the case of a gas turbine engine,additional challenges can be management of operation at elevated temperatures and the significant thrust loads that can arise from extreme events,and ramping up or down to a different running condition.While the specification of a bearing may be assigned to specialist,the detailed design of a bearing housing will inevitably require interaction with the internal air system,arising from requirements for cooling,and sealing flows.Common choices for bearing seals include labyrinths and carbon face seals,see[44]and[45],as illustrated schematically in Figure 6.

Fig.6 Generic bearing chamber arrangement for a multi-spool engine[45]

8 Electrical Subsystem Thermal Management

The increase in the use of electrical sub-systems in gas turbine engines typically requires management of the thermal environment for the electrical machine concerned.Such machines generally comprise similar geometrical forms to many gas turbine and mechanical engineering components,such as rotating discs,rotor-stator disc cavities,rotating and static disc features,plain and grooved cylinders,annuli,and cones.The flow and heat transfer associated with these configurations are reviewed by[46].

9 Conclusions

Gas turbine engine components are typically operated under high levels of thermal and mechanical stress.This mandates the use of cooling in order to ensure reasonable service life for the components concerned.In addition to managing component temperatures it is also necessary to consider containment or exclusion of fluids and dirt from certain subsystems and the management of thrust loads on bearings.The diverse requirements of thermal management,sealing and cooling and management of thrust loads are generally assigned to the internal air system or secondary flow system.Extensive research has been undertaken on this system arising from the need to minimise the quantity of air used as this must be extracted from the compressor and hence has an impact on the specific fuel consumption,and also to minimise any harmful interactions between sealing and cooling flows and the mainstream annulus flows,as this also can reduce the overall thermal efficiency and hence increase the specific fuel consumption.

Common features associated with the internal air system are discs,cylindrical cavities,cylindrical and conical surfaces and arising flow and heat transfer in static applications,movement relative to a corresponding surface,and rotating frames of reference.Subsystems associated with the internal air system include bearing sealing and cooling,disc thrust management,rim seal ingress and egress,turbine disc cooling,provision of cooling air supplies for turbine blade cooling,cooling flows for combustor components,pressure balancing and flow management of various disc,drum and shaft seals,management of wheel-space temperatures,and management of the thermal environment for varied electrical devices.

Research on disc flows in rotor-stator disc cavities has reached a level of maturity such that modelling of gas ingestion is now possible and routine in many companies and research groups.Flow and heat transfer in a rotating cavity continues to be a research topic with the arising instabilities resulting in transient 3 dimensional flow features that rotate relative to the cavity.Conjugate heat transfer methods have advanced such that their use for the majority of internal air system components is common-place enabling modelling and optimisation of many of the subsystems.