TURBOFAN ENGINE PDF
The document will focus on the turbofan engine (a specific type of jet engine courses through the real world example of a turbofan engine. Airplane Turbofan Engine Operation and Malfunctions. Basic Familiarization for Flight Crews. Chapter 1. General Principles. Introduction. Today's modern. Block fuel benefits from reducing specific thrust for a year entry into service conventional turbofan engine for long range applications. Artistic impression of the intercooled core turbofan engine . Variation of low pressure turbine stage count with fan inlet mass flow.
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The turbofan engine is a propulsive mechanism to combine the high thrust of a turbofan engine, the turbine drives not only the compressor, but also a large fan. The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft "The Chevron Nozzle: A Novel Approach to Reducing Jet Noise" (PDF ). NASA Innovation in Aeronautics NASA/TM "The Engine Yearbook". Evolution of turbojet engines to the technology level of today. • new concepts or technological . Solution: principle of the by-pass engine (called turbofan).
Off-design performance and stability is, however, affected by engine configuration. As the design overall pressure ratio of an engine cycle increases, it becomes more difficult to operate at low rpm, without encountering an instability known as compressor surge. This occurs when some of the compressor aerofoils stall like the wings of an aircraft causing a violent change in the direction of the airflow.
As the HP compressor has a modest pressure ratio its speed can be reduced surge-free, without employing variable geometry. However, because a shallow IP compressor working line is inevitable, the IPC has one stage of variable geometry on all variants except the , which has none.
The Snecma M53 , which powers Dassault Mirage fighter aircraft, is an example of a single-shaft turbofan. Despite the simplicity of the turbomachinery configuration, the M53 requires a variable area mixer to facilitate part-throttle operation. Hot gas from the turbojet turbine exhaust expanded through the LP turbine, the fan blades being a radial extension of the turbine blades.
One of the problems with the aft fan configuration is hot gas leakage from the LP turbine to the fan. The Low Pressure spool runs at a lower angular velocity. The High Pressure spool turns more quickly and its compressor further compresses part of the air for combustion. At the smaller thrust sizes, instead of all-axial blading, the HP compressor configuration may be axial-centrifugal e. Boosted two-spool[ edit ] Higher overall pressure ratios can be achieved by either raising the HP compressor pressure ratio or adding an intermediate-pressure IP compressor between the fan and HP compressor, to supercharge or boost the latter unit helping to raise the overall pressure ratio of the engine cycle to the very high levels employed today i.
All of the large American turbofans e. The high bypass ratios i. Three-spool[ edit ] Rolls-Royce chose a three-spool configuration for their large civil turbofans i. The first three-spool engine was the earlier Rolls-Royce RB. Main article: Geared turbofan Geared turbofan As bypass ratio increases, the mean radius ratio of the fan and low-pressure turbine LPT increases. Consequently, if the fan is to rotate at its optimum blade speed the LPT blading will spin slowly, so additional LPT stages will be required, to extract sufficient energy to drive the fan.
Introducing a planetary reduction gearbox , with a suitable gear ratio, between the LP shaft and the fan enables both the fan and LP turbine to operate at their optimum speeds. Most of the configurations discussed above are used in civilian turbofans, while modern military turbofans e.
High-pressure turbine[ edit ] Most civil turbofans use a high-efficiency, 2-stage HP turbine to drive the HP compressor. While this approach is probably less efficient, there are savings on cooling air, weight and cost.
Because the HP compressor pressure ratio is modest, modern military turbofans tend to use a single-stage HP turbine. Low-pressure turbine[ edit ] Modern civil turbofans have multi-stage LP turbines e.
The number of stages required depends on the engine cycle bypass ratio and how much supercharging i. A geared fan may reduce the number of required LPT stages in some applications. Cycle improvements[ edit ] Consider a mixed turbofan with a fixed bypass ratio and airflow. Increasing the overall pressure ratio of the compression system raises the combustor entry temperature.
Therefore, at a fixed fuel flow there is an increase in HP turbine rotor inlet temperature. Although the higher temperature rise across the compression system implies a larger temperature drop over the turbine system, the mixed nozzle temperature is unaffected, because the same amount of heat is being added to the system.
There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio, causing an increase in the hot mixer entry pressure. A similar trend occurs with unmixed turbofans.
So turbofans can be made more fuel efficient by raising overall pressure ratio and turbine rotor inlet temperature in unison. Increasing the latter may require better compressor materials. If the latter is held constant, the increase in HP compressor delivery temperature from raising overall pressure ratio implies an increase in HP mechanical speed. However, stressing considerations might limit this parameter, implying, despite an increase in overall pressure ratio, a reduction in HP compressor pressure ratio.
However, this assumes that cycle improvements are obtained, while retaining the datum HP compressor exit flow function non-dimensional flow. In practice, changes to the non-dimensional speed of the HP compressor and cooling bleed extraction would probably make this assumption invalid, making some adjustment to HP turbine throat area unavoidable. This means the HP turbine nozzle guide vanes would have to be different from the original.
In all probability, the downstream LP turbine nozzle guide vanes would have to be changed anyway. Log In Sign Up. On Intercooled Turbofan Engines. Kyprianidis, Konstantinos Andrew M. Rolt G. Rolt and Vishal Sethi Additional information is available at the end of the chapter Additional information is available at the end of the chapter Introduction Public awareness and political concern over the environmental impact of the growth in civil aviation over the past 30 years have intensified industry efforts to address CO2 emissions .
CO2 emissions are directly proportional to aircraft fuel burn and one way to minimise the latter is by having engines with reduced Specific Fuel Consumption SFC and installations that minimise nacelle drag and weight. Significant factors affecting SFC are propulsive efficiency and thermal efficiency. Propulsive efficiency has been improved by designing turbofan engines with bigger fans to give lower specific thrust net thrust divided by fan inlet mass flow until increased engine weight and nacelle drag have started to outweigh the benefits.
Mission fuel burn benefits from reducing specific thrust are illustrated in Fig. The figure shows that only a modest reduction in block fuel is obtained by increasing the already large fan diameter. The thermal efficiency benefits from intercooling have been well documented in the literature - see for example [2, 3, 7, 9, 11—13, 15]. Very little information is available however, with respect to design space exploration and optimisation for minimum block fuel at aircraft system level.
This is an open access chapter distributed under the terms of the Creative Commons Attribution License http: Creative Commons Attribution License http: Block fuel benefits from reducing specific thrust for a year entry into service conventional turbofan engine for long range applications.
Previously, a comparative study was presented focusing on a conventional core and an intercooled core turbofan engine for long range applications [5, 7]. Both configurations had the same fan diameter and were designed to meet the same thrust requirements.
The intercooled core configuration illustrated in Fig. The installation standard included a flow splitter and an auxiliary variable geometry nozzle. The two concepts were evaluated based on their potential to reduce CO2 emissions and hence block fuel through both thermal and propulsive efficiency improvements, for engine designs to enter service between and Although fuel optimal designs were proposed, only limited attention was given to the effect of design constraints, material technology and customer requirements on optimal concept selection.
A study is presented here that focuses on the re-optimization of those same powerplants by allowing the specific thrust and hence the propulsive efficiency to vary. Rather than setting fixed thrust requirements, a rubberised-wing aircraft model was fully utilised instead.
It was envisaged that different conclusions would be drawn when comparing the two powerplants at their optimal specific thrust and absolute thrust levels. Differences in the optimal specific thrust levels between the two configurations are discussed.
The design space around the proposed fuel-optimal designs was explored in detail and significant conclusions are drawn. Artistic impression of the intercooled core turbofan engine . Figure 3. Conceptual design tool algorithm . Design space constraints.
Methodology To effectively explore the design space a tool is required that can consider the main disciplines typically encountered in conceptual design. The prediction of engine performance, aircraft design and performance, direct operating costs and emissions for the concepts analysed in this study was made using the code described in .
Another code described in , was also used for carrying out the mechanical and aerodynamic design in order to derive engine component weights and dimensions. The two tools have been integrated together within an optimizer environment as illustrated in Fig. The integration allows for multi-objective optimization, design studies, parametric studies, and sensitivity analysis. In order to speed up the execution of individual engine designs, the conceptual design tool minimizes internal iterations in the calculation sequence through the use of an explicit algorithm, as described in detail by Kyprianidis .
For every engine design there are numerous practical limitations that need to be considered.
A comprehensive discussion on design constraints for low specific thrust turbofans featuring conventional and heat exchanged cores can be found in . The design space constraints set for this study are given in Table 1 and are considered applicable to a year entry into service turbofan engine. The effect on optimal concept selection of design constraints, material technology and customer requirements is discussed in the following sections.
Optimising a turbofan engine 3. Fuel-optimal designs Optimizing a turbofan engine design for minimum block fuel essentially has to consider the trade-off between better thermal and propulsive efficiency and reduced engine weight and nacelle drag.
The cycle optimization results for the two powerplants are given in Table 2. Comparison of the fuel optimal intercooled and conventional core turbofan engine designs.
Significant block fuel benefits are projected for the intercooled core engine, but they are smaller than those predicted in previous efforts . This is mainly attributed to a minimum blade height requirement setting a practical lower limit on the intercooled core size for a given OPR.
Increasing the fan diameter at a fixed tip speed inevitably reduces rotational speed, increases torque and hence increases the Low Pressure LP shaft diameter; this further aggravates the problem since the High Pressure Compressor HPC hub to tip ratio needs to increase. As a result, the optimal specific thrust for the intercooled core is higher compared to the conventional core turbofan engine.
Although the high OPR intercooled core benefits from a higher core and transmission efficiency, and hence a better thermal efficiency, the conventional core benefits from a higher propulsive efficiency.
The design space around the proposed fuel optimal designs was explored and in the next sections important observations are presented. Approximating the design space In order to graphically illustrate the design space, a large number of simulations had to be carried out; these simulations were focused around the fuel-optimal designs presented in Section 3.
Variation of low pressure turbine stage count with fan inlet mass flow and fan tip pressure ratio for a fixed size conventional core. Typical design space discontinuities encountered as a result of turbomachinery stage count changes are inevitably distorted in polynomial approximations.
Fan and core sizing Propulsive efficiency benefits from reducing specific thrust by increasing fan diameter can very well be negated by the resulting combination of: This section discusses various aspects of fan and core sizing for the conventional core and intercooled core configurations. When sizing the engine fan, assuming a fixed size core i.
As discussed earlier, the use of smooth surrogate models for approximating discontinuous spaces inevitably results in approximation errors, and it is worth noting that the addition of an extra LPT stage results in approximately kg of additional weight.
Nevertheless, with the fan and nacelle weight including the thrust reverser each being roughly double the LPT weight and directly proportional to the fan diameter, the weight trends illustrated in Fig. The improvement in mid-cruise uninstalled SFC from reducing specific thrust is illustrated in Fig. If installation effects are ignored, then selecting a higher fan diameter and hence a higher bypass ratio at a fixed size core will result in better SFC.
Nevertheless, the increased nacelle drag and engine weight move the optimal level of specific thrust for minimum block fuel to smaller fan diameters, as illustrated in Fig. Variation of engine weight with fan inlet mass flow and fan tip pressure ratio for a fixed size conventional core. Figure 6. Variation of engine specific fuel consumption with fan inlet mass flow and fan tip pressure ratio for a fixed size conventional core.
Looking at the trends illustrated in Fig. However, as one moves towards the upper left corner of Fig.
Wear Prognostic on Turbofan Engines
In order to satisfy - at constant specific thrust - the time to height and FAR Federal Aviation Regulations take-off distance constraints set in this study it is necessary to scale-up the engine i. Variation of aircraft block fuel with fan inlet mass flow and fan tip pressure ratio for a fixed size conventional core. Most of the conclusions drawn in this section are applicable to both the conventional core and the intercooled core configurations.
Nevertheless, the intercooled core is constrained by a practical minimum blade height requirement for the last HPC stage assuming an all-axial bladed HPC. At a fixed core OPR and intercooler effectiveness, this constraint sets a minimum limit for the core mass flow and as a consequence a minimum limit is also set on specific thrust at a fixed engine thrust.
This makes the intercooled core more favourable for very high thrust engines, as they will not be subject to this constraint.
Figure 8. Variation of HPC last stage blade height with fan inlet mass flow and fan tip pressure ratio for a fixed size intercooled core. Variation of HPC last stage blade height with fan inlet mass flow and fan tip pressure ratio for a fixed size conventional core.
Bigger direct drive fans rotating at low speeds result in high torque requirements which increase the LP shaft outer diameter. The HPC inner diameter has to be pushed out and therefore slowed down, so for a given flow area and blade speed, the resulting blade height tends to reduce, as illustrated in Fig.
For a given blade height requirement the core mass flow needs to be increased and it can therefore be concluded that an intercooled core would favour a geared fan arrangement, over a direct drive one, since it could alleviate some of the restrictions set on the cycle. An aft fan arrangement as the one presented in  could further relieve this issue by not passing the LP shaft through the core, though aft fan arrangements set other design challenges.
The optimal OPR level for the conventional core is constrained by the maximum allowable HPC delivery temperature set, as illustrated in Fig.
For the intercooled cycle, this limitation is alleviated but only to be replaced by a practical minimum blade height requirement which consequently sets a minimum allowable core size constraint. The optimal OPR level for the intercooled core at a fixed specific thrust is therefore a trade-off between a better core efficiency and a smaller core size. If one assumed constant component polytropic efficiencies then SFC benefits would arise for the conventional core from shifting pressure ratio to the more efficient High Pressure HP spool.
However, as the HPC pressure ratio rises beyond an upper limit set, the core configuration would inevitably need to be changed to a two-stage High Pressure Turbine HPT.
Figure For that reason, a two-stage HPT has been assumed for the intercooled core while a minimum IPC design pressure ratio was set to avoid potential icing problems during decent. Engine ratings Sizing and rating an engine is a highly complex process that has to consider aircraft performance requirements, fuel consumption, and engine lifing.
Variation of engine weight with combustor outlet temperature at take-off and top of climb conditions for a fixed size conventional core.
The maximum T4 level may also be constrained by combustor design considerations. For example increasing combustor liner cooling requirements essentially reduces the amount of air available for mixing in the combustion zone and hence the flame temperatures and NOx emissions tend to increase. Detail design studies are required for establishing the optimal trade-off between cycle efficiency and acceptable NOx levels.
For these reasons an upper limit was set for T4 that was considered to be a reasonable trade-off for year entry into service turbofan engines.It was of axial-flow design with 15 compressor and 7 turbine stages, annular combustion chamber and many other modern features.
However, the benefits are highly dependent on achieving technology targets such as low weight and pressure losses for the intercooler. The world's first turboprop engine that went into mass production[ specify ] was designed by a German engineer, Max Adolf Mueller , in Although at first glance it seems to be implied through this figure that a low intercooler effectiveness is beneficial for block fuel, it should be noted that a minimum level of intercooler effectiveness has to be maintained at take-off and hence at cruise due to the aforementioned nozzle area variation limitation.
For GE Aviation , the energy density of jet fuel still maximises the Breguet range equation and higher pressure ratio cores, lower pressure ratio fans, low-loss inlets and lighter structures can further improve thermal, transfer and propulsive efficiency.
Intercooler effectiveness In this study the aerodynamic design for most engine components has been carried out at top of climb conditions.
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