40th Engine Systems Symposium - 26th/27th of March 2014

Aero Engine Performance Test Procedure and Considerations

Leyre Lirola Fuentes

AbstractTesting of aero engine performance is crucial for reliable and safe operation before the aircraft

comes into service. Depending on the type of engine and the particular test objective, different kind of test cells and instrumentation are used to carry out the measurements. On the whole, test cells measurements comprise several performance parameters, including thrust, temperature, pressure, mass flow, speed and vibrations estimations. Subsequently, the engine performance information must be corrected and processed. The uncertainty inherent in the measurement process and the differences between real and test operation conditions ascribe fundamental importance to the accurate prediction of correction factors.

With the advent of digital computers, CFD has become an increasingly used tool not only to characterize engine performance, but also to design test cells ensuring their best performance. Despite the fact that manufacturers are still measuring results against test cells predictions, CFD has allowed a great improvement in development and maintenance testing programmes in terms of time and costs.

NomenclatureTET Turbine Entry TemperatureOPR Overall Pressure Ratio Heat capacity ratio

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March 2014 40th Engine Systems symposium Cranfield University

1. IntroductionEngine performance verification has been a

major concern for the gas turbine community since its introduction. It is highlighted in this activity that, although engine component rigs are more fully instrumented and measured in detail, the interaction between all the components and their matching do not allow to predict the overall engine performance with enough accuracy. Accordingly, complete engine performance test rather than component testing is the more appropriate method to demonstrate that the projected design can meet the predicted performance [4,2]. However, an engine development programme has to be supported by complimentary component tests to be fully effective. Both programmes provide the required data to certificate the design for use in flight. [4]

One area of great concern to engine test facilities are the techniques used to compare engine’s behaviour. Knowing that cell type and instrument quality have a strong influence on the measurement, is a key factor on engine performance testing [5].

2. Engine Test FacilitiesThe assessment of the complete engine

performance can be carry out in different environmental conditions, as sea-level or altitude. The most common facility is the Sea-Level Test Facility (SLTF) due to the complexity and high operating costs of the Altitude Test Facility (ATF). However, the ATF allow the engine to be operated in a wide range of conditions simulating different altitudes and aircraft flight Mach numbers and hence, obtaining the best possible datum.

2.1. Sea-level Test FacilitiesSLTF comprises a wide variety of

objectives including: maintenance testing of service engines; development tests of new or modified

engine designs; overall engine performance assessment

for Type Certification.

The follow table shows the different kind of tests which are carried out in order to fulfil the previous objectives [6].

Type of Test Objectives

Pass-offDemonstrate performance guarantees after overhaul

EnduranceDemonstrate mechanical integrity and reliability

Fault diagnosis

Identify causes of overall shortfall of component

failure

IngestionDemonstrate withstanding to foreign object ingestion

Systems development

Evaluate new technology systems

Engine development

Evaluate new technology engine components

DemonstratorAssess the overall

performance of a new engine

Table 1 Since each of these tests demands different

requirements, they are normally carried out by specific organisations and hence, particular details of test cell design differ from bed to bed depending on the test purpose. [4]

Sea-level cells are subdivided into two groups, outdoor test beds and indoor test beds. Amongst them, outdoor test facilities are less common although they provide the most accurate and real results.

2.1.1. Outdoor test bedsThe outdoor test bed facility consists of an

open air stand which supports the engine and also contains all the measurement equipment and engine services necessary for testing (Figure 1) [3, 1]. This facility provides the closest approximation to a true sea level static test bed since the air path is not restricted or constrained. Thus, if the test is carried out under zero wind conditions, the true engine thrust is obtained by correcting the measured thrust only for instrumented errors. In spite of providing an accurate performance

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measurement, this facility implies expensive and time consuming tests. This is mainly caused by the impact of the weather conditions, which complicates the repeatability and reliability of the test, and by the strict noise pollution regulation, which demands extensive and remote areas to carry out properly the engine test performance. Accordingly, it is common conduct most engine test in indoor facilities with measurements calibrated versus outdoor facilities. [1,2]

Figure 1 Outdoor Thrust Test Bed [1]

Figure 2 Rolls-Royce expansion of Jet Engine Test Facility at Nasa John C. Stennis Space Center

2.1.2. Indoor Sea-Level Test BedRequirements of performance repeatability

and reliability are met with indoor test cells since ambient conditions are fully controlled. In

addition, the detuner provides sound attenuation, allowing more accessible test areas [1]. Both points contribute significantly to the fact that indoor testing is preferred in most countries.

However, the data acquired in these facilities have to be corrected for enclosure effects. Indeed, the interaction between the engine jets and the detuner originates an ejector pump effect which allows an additional amount of air to enter the cell (secondary flows). Thus, thrust correction factors have to be accounted in the measured thrust for acquiring the real gross thrust delivered by the engine [2].

Figure 3 Trent XWB blade-off test indoor [7]

Concerning the physical facility, the indoor test bed consists of three main components, an air inlet system, a test chamber and an exhaust system.

Air Inlet System. The inlet system must provide an adequate airflow to the test chamber in terms of quality and quantity across the whole operational range. Although horizontal intakes provide a more uniform flow and lower pressure losses, vertical intakes are more used in practice due to be less affected by air direction, better avoid dust and debris ingestion and reduce test bed noise [3,8,9]. The air inlet system typically consists in:

several splitters located at the inlet to attenuate the noise;

a turning cascade in case of vertical intakes where it is necessary to turn the flow;

a debris guard to protect the engine from foreign object ingestion.

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Perforated intakeGround

NozzleAirmeter

Load Cell

Engine

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March 2014 40th Engine Systems symposium Cranfield University

Test Chamber. This is the region where the engine is positioned. The air comes into from the inlet system and leaves the test chamber through the exhaust system. In the test chamber, most of the air is drawn into the engine through

a

bellmouth intake. As it has been previously mentioned, secondary flows surround the engine due to the ejector effect caused by the interaction between the high jet velocity and the exhaust system [3,8,9]. Besides providing a more uniform flow, the bellmouth supports the instrumentation related to mass flow measurement. Finally, the engine is supported with certain degree of movement by the cradle. A load cell located in the cradle converts the strain created by the axial movement is a measurement of thrust.

Exhaust System. This is the region which has higher influence on gas turbine engine performance due to the fact that the exhaust system controls the back pressure and the amount of secondary flows which pass through the test chamber. It also determinates the amount of noise produced by the test cell. The elements which compose the exhaust system can be summarized in:

the detuner which collects the exhaust gases;

the diffuser section which recovers part of the static pressure reducing the velocity at the exit;

the boot section which turns the flow direction from horizontal to vertical by means of turning vanes;

the exhaust stack which expel the gases a sufficiently distance to avoid environmental issues.

A key consideration to take into account is the exhaust system cooling. It is necessary a significant reduction of the exhaust temperature since the acoustic linings used in this section

cannot deal with the values of modern jet engines (up to 1800 °C using reheat).

2.2. Altitude Test Bed

2.2.1. Altitude Test Facility (ATF)2.2.2. Flying Test Beds

3. Instrumentation3.1. Test cell measures and

Apparatus 3.1.1 Thrust measurementOne of the most relevant performance

parameters is the entrainment ratio (μ) (Equation1), which indicates the relation between the secondary and primary mass flow. Indeed, most of the flow phenomena which may appear inside the test cell can be related to this parameter [3]. Equation 2 shows the relation between the load cell measurement and the actual engine gross thrust.

{1 } μ=W secondary

W engin

{2 } Fg=Fm+ IMD+DCrandle+DBellmouth+DBase+DFriction+Tailpipe Pressure Correction+ Additive Dra g

3.2. Calibration3.3. Results processing

4. Certification5. CFD

CFD has become in an increasingly tool in the field of engine performance test. Indeed, this is a powerful tool used in the design and

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Figure 4 Indoor Test Bed Layout with Horizontal Intake

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March 2014 40th Engine Systems symposium Cranfield University

development of test facilities. Comprehensive CFD analysis reduces the time and costs incurred in the determination of thrust correction factors.

6. ConclusionsYou must have a conclusions section. This

is the place to put the outcome of the paper,

results of a comparison or comments about the topic.

7. ReferencesHere go the references you used for the paper. The referencing style to use is the Harvard

Sequentially numerated references. If you use RefWorks it is stated as Harvard (numerated refs, [1] sequential). This puts a number between brackets in the main text, i.e [1], and the reference on the bibliography in the Harvard style.

[1] Gas turbine Performance Walsh [2] Ejector Pump Theory Applied to Gas Turbine Engine Performance inside Indoor Sea-

Level Test Cell-Analytical and CFD Study [3] L.P. Cannock, A study of sea level gas turbine test bed features and their influence on

engine performance [4] Operation and Performance Measurement on Engines in Sea-Level Test Facilities [5] Performance derivation of turbojets and turbofans from tests in sea-level test cells. [6] Data acquisition and processing in sea-level test beds. [7] http://bloga350.blogspot.com.es/2013/03/engine-blade-off-test-indoor-for-first.html [8] A. Le Fay. Flow path analysis within a model engine test bed [9] S.Franco-Trigo. Gas turbine Engine Test Cell Simualtion

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