Precise simulations of gas turbine performance cannot be done without component maps. In the early days of a new project one often has to use scaled maps of similar machines. Alternatively one can calculate the component partload characteristics provided that the many details needed for such an exercise are available. In a later stage often rig tests will be done to get detailed information about the behavior of the compressors respectively turbines. Performance calculation programs usually require the map data in a specific format. To produce this format needs some preprocessing. Measured data cannot be used directly because they show a scatter and they are not evenly distributed over the range of interest. Due to limitations in the test equipment often there is lack of data for very low and very high speed. With the help of a specialized drawing program available on a PC one can easily eliminate the scatter in the data and also inter- and extrapolate additional lines of constant corrected speed. Many graphs showing both the measured data and the lines passing through the data as a function of physically meaningful parameters allow to check whether the result makes sense or not. The extrapolation of compressor maps toward very low speed, as required for the calculation of starting, idle and windmilling performance calculations, is discussed in some detail. Instead of true measured data one can use data read from maps published in open literature. The program is also an excellent tool for checking and extending component maps one has derived from sparse information about a gas turbine to be simulated.

In the preliminary conceptional design phase of any new gas turbine project the design and off-design performance of many alternative configurations and cycles must be studied. The simulation results for the off-design cases depend very much on the component maps employed in the model. Especially important are the compressor maps, and when the pre-design study covers a wide range in pressure ratio, then the use of a consistent set of maps is essential for high quality simulation results. A statistical analysis of many compressor maps — taken from open literature and from the MTU in house data bank — was performed. In each map a reference point was defined which was employed to normalize it. Then, the topology of the normalized map was captured with three characteristic numbers that describe 1) the region where efficiency is highest, 2) the mass flow – speed relationship and 3) the shape of the speed lines. The characteristic numbers show in the statistic evaluation clear trends with reference pressure ratio. From these trends a new map scaling procedure was derived which describes the systematic change of the compressor map topology with design pressure ratio much better than the conventional map scaling method, which applies constant factors on pressure ratio or on specific work.

Personal Computers are nowadays very powerful. They allow us to do complete gasturbine performance calculations both for the engine design point and the partload behavior. A typical helicopter engine serves as an example for a cycle study. The simple variation of compressor pressure ratio and turbine inlet temperature, however, does not yield a realistic result. Only after including the effects of variable amounts of cooling air needed for constant metal temperature and turbine efficiency as a function of aerodynamic loading one does get results that are in line with the cycle of real engines. Off-design performance calculations need no longer use crude simplifications for reasonable calculation times. Real component maps can be used while the matching of the cycle is done by iterative methods. Transient simulations on a 486DX machine are only a matter of a few minutes. Even the effects of inlet distortion (pressure or temperature) can be dealt with by using the parallel compressor model. The key to userfriendliness is, to hide everything from the user that he does not need to know at the very moment. The things he needs to know, however, must be presented in clear and easily understandable expressions. Results must be shown in graphics whenever it makes sense. This is the only way to recognize quickly problem areas or to convince people, that the selected design is optimal.

Any gas turbine performance simulation tool employs simplifications, some more, some less. It depends on the intent of the simulation which simplifications are appropriate. For beginners, many are necessary for teaching how the gas turbine works from principle. For practical applications — because of the accuracy requirements — many simplifications introduced in textbooks are not appropriate. This paper comments on the simplifications that are typically made. Simplified gas property models are quite acceptable for ideal cycle analysis. For the examination of real cycles, however, especially the model of the burner should be better than those described in most textbooks. This is because these models yield the best cycle efficiency at stoichiometric fuel-air-ratio while a realistic burner model leads to the conclusion that the best thermal efficiency happens to be at significantly lower fuel-air-ratios respectively temperatures. For off-design simulations many simplifications have the aim to avoid iterative solutions or restricting the algorithms to one-dimensional iterations. If more than one iteration variable shows up — which is the case with multi-spool engine simulations — then the problem is solved with fitting several one-dimensional iterations into each other. This methodology is described in most textbooks, but it is nearly never used in industry because the logic is more complex than necessary and difficult to adapt to special needs. The seeming simplification is actually a complication when applied to real world problems. Universities should teach as a standard the multidimensional Newton Raphson iteration technique which allows writing gas turbine cycle codes with nearly no restriction to the methods of formulating the laws of physics. The consequence of simplified mathematics is often an off-design simulation which does not employ compressor and turbine maps. Such a methodology yields accurate values for thermal efficiency respectively specific fuel consumption only within a narrow range of operating conditions; the accuracy of the results is not sufficient for real world applications. Of course also in programs for industrial use the reality is modeled with many compromises. Some simplifications which have not so obvious consequences are discussed. For example, there is an influence of the speed-flow characteristics in the booster map on its operating line if an often used type of fan performance representation is employed. Another example is that an oversimplified description of what happens in the compressor interduct can lead to wrong conclusions when the effects of inlet flow distortion on the stability of compressors in series are sought.

The potential for improving the thermodynamic efficiency of aircraft engines is limited because the aerodynamic quality of the turbomachines has already achieved a very high level. While in the past increasing burner exit temperature did contribute to better cycle efficiency, this is no longer the case with today’s temperatures in the range of 1900...2000K. Increasing the cycle pressure ratio above 40 will yield only a small fuel consumption benefit. Therefore the only way to improve the fuel efficiency of aircraft engines significantly is to increase bypass ratio — which yields higher propulsive efficiency. A purely thermodynamic cycle study shows that specific fuel consumption decreases continuously with increasing bypass ratio. However, thermodynamics alone is a too simplistic view of the problem. A conventional direct drive turbofan of bypass ratio 6 looks very different to an engine with bypass ratio 10. Increasing bypass ratio above 10 makes it attractive to design an engine with a gearbox to separate the fan speed from the other low pressure components. Different rules apply for optimizing turbofans of conventional designs and those with a gearbox. This paper describes various criteria to be considered for optimizing the respective engines and their components. For illustrating the main differences between conventional and geared turbofans it is assumed that an existing core of medium pressure ratio with a two stage high pressure turbine is to be used. The design of the engines is done for takeoff rating because this is the mechanically most challenging condition. For each engine the flow annulus is examined and stress calculations for the disks are performed. The result of the integrated aero-thermodynamic and mechanical study allows a comparison of the fundamental differences between conventional and geared turbofans. At the same bypass ratio there will be no significant difference in specific fuel consumption between the alternative designs. The main difference is in the parts count which is much lower for the geared turbofan than for the conventional engine. However, these parts will be mechanically much more challenging than those of a conventional turbofan. If the bypass ratio is increased significantly above 10, then the geared turbofan becomes more and more attractive and the conventional turbofan design is no longer a real option. The maximum practical bypass ratio for ducted fans depends on the nacelle drag and how the installation problems can be solved.