Engine Description and Modeling

The engine that has been chosen in order to undertake the experiments in this research is a CFM56 5B2 engine (Figure 4.3), which belongs to the CFM56 family and is jointly manufactured by Snecma and General Electric. This particular model is a high-bypass turbofan two-spool engine that produces a total of 31000 lbs (137.9 KN) of thrust at take-off. The engine is of a modular design, thus enabling maintenance to be performed more easily by maintenance workshops having limited repair capability. Modular maintenance is concerned primarily with replacement of modular assemblies and parts. The major modules are the fan and booster, the compressor module, the turbine module and the accessory drive module. This general assembly concept is followed by all the models of the CFM56 family, but each displays different specific performance characteristics depending mostly on the age of the engine and the application to which each engine refers. The specific characteristics of the CFM56 5B2 are shown in Table 4.1 at the end of this section (Lufthansa Technical Training, 1995).

Figure 4-3: CFM56 5B2 Cutaway Drawing (http://www.cfm56.com/products/cfm56-5b/cfm56-5b-technology)

This type of engine is designed specifically for Airbus aircraft, and is installed on the Airbus A-321. This is a medium-range narrow-body jet airliner, which was first introduced in 1993, making it a relatively modern aircraft. This particular type of aircraft is used by hundreds of operators due to its ability to perform domestic, regional, and also transatlantic flights.

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Hence, it constitutes a representative contemporary example, which is why this particular engine was chosen for examination (Aircraft Commerce Journal, 2006).

The modelling of the engine was undertaken by means of a performance simulation code, Turbomatch, which was developed at Cranfield University. It is important to note that in the present study, the fidelity of the Turbomatch tool was not taken into consideration, and so the results obtained are considered to be the most accurate and the closest possible to real conditions. As a modelling and computational tool, Turbomatch has the ability to provide results concerning the performance parameters of the engine depending on the operating conditions. These performance parameters concern the sum of the performance data, such as thrust, exhaust gas temperature and efficiencies, but also others such as rotational speed, pressure and temperature. The data provided by the simulation code are identical to those that a true application uses for health management processing, such as the ACMS described in Chapter 3.

An encoded model of the engine is essential for the Turbomatch performance simulation program. This initial model will respond to a specific performance operating point of the engine, with particular given atmospheric conditions. In order for the coded model to be accurate and valid, the simulated data must match the real data for the same specific condition that was used for modelling. In other words, the thrust, turbine entry temperature (TET) and so on, given from the simulation program, must match the real values. This will provide the design point of the simulation code that will later act as a reference point for the other operational conditions. For the current study, the design point of the engine was chosen to be at maximum climb thrust, because all the real data available, such as TET, pressure ratios and efficiencies, were for that operational segment, and so it was possible to validate the results from the simulation code in conjunction with the real one. A number of runs were undertaken until the desired matching was achieved, at which point the simulation was ready.

Once the design point was ready, it was then possible to investigate the behaviour of the engine for different performance and atmospheric conditions by changing them according to the desired scenario each time. Those conditions, different from the initial ones, will be formed by interchanging certain parameters from the design point. This change will eventually change the overall performance model of the engine, which will be then running for off-design points. The present study is concerned with the ISA deviations and the de- rated thrust settings, so these are the parameters that were changed from the design points. A change in one of them will bring about a change to the whole performance model of the

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engine. For example, the design point of the specific engine being studied was carried out at maximum climb thrust of 6.420 lbs (28.6 KN), and gave specific values for pressures, temperatures, rotational speed etc. If the thrust were to change, then the other parameters would also change. The same would also happen if the change were to occur on an atmospheric parameter such as ambient temperature or altitude, or to anything else that is pre-established in the code (Pachidis, 2005; Cranfield University, 2000).

However, for this version of Turbomatch the two necessary initial inputs, even for off-design runs, are the thrust and the TET, and the absence of either will compromise the results. This means that there is a specific matching of thrust and TET for a given atmospheric condition which has to be known beforehand and be entered as initial input, otherwise the tool is unable to provide results. For example, when the engine is operating at 30000 lbs of thrust, the corresponding TET is 1500k; for 28000 lbs of thrust the TET is 1450k etc. Because the matching of thrust and TET was made piecemeal, there is a kind of deviation between the desired thrust value and that which was finally matched with the TET. For example, the TET at cruise thrust is considered to be 1279.39 K. However that TET was matched for thrust of 25.94 KN with the nominal cruise thrust being at 25.98 KN. Such small deviations exist in all of the code runs, but are considered negligible. Nevertheless, they mentioned in order to be completely accurate with our assumptions. The input code file can be found in Appendix A along with a small explanation of it.

Table 4-1:CFM56 5B2 Technical Characteristics (Lufthansa Technical Training, 1995)

CFM56 5B2 Technical Characteristics

Max take-off thrust (lbs) 31000

Max climb thrust (lbs) 6430

Max cruise thrust (lbs) 5840

Flat Rated Temperature (oC /oF) 30/86

By Pass Ratio 5,5/1

Mass Flow (lbs/sec) 956

Overall Pressure Ratio 35.5

EGT 950

N1 (RPM) 5200

N2 (RPM) 15183

Length (inch) 102,4

Fan Diameter 68,3

Basic Dry Weight (lbs) 5250

Fan Stage Numbers 1

Low Pressure Compressor Numbers 4

High Pressure Compressor Number 9

High Pressure Turbine Stage Numbers 1

Low Pressure Turbine Stage Numbers 4

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