As the complexity of modern aircraft systems increases, model-based design represents a mean for reducing development cost and optimizing performance. The jet engine is the main power source for such systems, and therefore its performance assessment in an appropriate modelling environement is required.
The Simcenter Amesim gas turbine library offers a set of dynamics gas components allowing to model various systems containing up to 100 species. Mixture composition can vary over time and along the system, accounting for chemical reactions occuring in the combustion chamber. The gas turbine library allows you to assess the jet engine performance for a broad range of operating points, studying the impact of ambient conditions on thrust, pollutant emissions and fuel consumptions.
Let us consider a typical high by-pass ratio turbofan, which are commonly used by medium-range and long-range aircrafts. A twin-shaft configuration is considered, where the low pressure turbine (LPT) is driving the fan and where the high pressure turbine (HPT) is driving the high pressure compressor (HPC). The primary air flow goes through the fan core, HPC, combustion chamber, HPT, LPT and nozzle. The secondary airflow, which is the main contributor for the aircraft thrust, goes through the fan by-pass. The combustion chamber is composed of a flame tube and annulus. This geometry allows controlling the chemical reaction process and the air temperature before it enters the turbine.
The species editor helps you define the mixture composition. Six species are considered: oxygen, nitrogen, fuel, water vapour, carbon dioxyde and argon. Species thermodynamics and transport properties are available in two formats: linear and NASA. The first one allows tackling real-time applications by considering simplified properties varying linearly between two temperature points. The NASA format provides accurate representation of species properties by using polynomial approach.
The combustion definition tool allows you to define the chemical reaction occuring in the engine combustion chamber. Fuel is reacting with oxygen to produce carbon dioxyde and water vapour. This equation is used by the combustion chamber components to compute the pollutants mass flow rates and enthalpy flow rates, along with heat realease.
In this example, the aicraft executes a descent manoeuver from 35000 ft to 29000 ft at constant speed. Ambient conditions are function of aircraft altitude and Mach number and directly impact the engine behaviour. Static and total conditions are used as boundary conditions for the system through use of the flight mission definition component and sender/receiver components. Injected fuel mass flow rate is controlled by thrust requirement. As air density increases with decreasing altitude, thrust tends to increase as well. In this scenario, the injected fuel mass flow rate must decrease to maintain the required thrust.
This manoeuver also has an impact on the combustion chamber behaviour. Air temperature in the primary zone tends to decrease due to moving operating point of the turbomachine. However, the impact on the TIT (turbine inlet temperature) is damped by maintaining the same air mass flow rate fractions in the flame tube and in the annulus.
This example provides an overview of the gas turbine library capability for the assessment of jet engine performance. Further investigation may be performed by considering the impact of bleed-off take and sub-systems and/or equipments integration.
Video: Assessing the jet engine performance with the gas turbine library