According to the certification specifications for large civil airplanes (FAR-25 or JAR/CS-25), aircraft environmental control systems (ECSs) must ensure adequate cabin conditions for passenger safety. For instance, crew and passengers must receive enough fresh air, free of hazardous concentration of gases, and pressurized cabin compartments must provide a pressure altitude of no more than 8000 ft. On top of that, aircraft manufacturers equip the ECS to provide an optimal temperature for passengers comfort during flight.
Simcenter Amesim encapsulates all the experience in ECS modeling that we matured over the years contributing to international research projects and working closely with our customers. We developed a methodology that, starting from the aforementioned requirements, allows engineers to derive the required power to be delivered by the ACMs to cool down or heat up the cabin, to model the equipment and to integrate it with the ECS model. This methodology, summarized in the following sections, is explained in more detail in the standard demonstrators included in Simcenter Amesim and applied to industrial use cases as described in papers  and .
The first step of the methodology consists in discretizing the aircraft structure by applying a nodal approach with goal of capturing its thermal behavior. This can be achieved using the Simcenter Amesim Thermal library. The equivalent thermal circuit of a section of a double deck fuselage aircraft is illustrated in the picture below on the left. On the right, the corresponding Simcenter Amesim model is depicted.
This nodal approach can be applied to the entire fuselage discretizing it along the longitudinal axis as shown in the figure below.
Using this approach, the desired cabin temperature is imposed, while the atmospheric boundary conditions, computed with the dedicated components of the Aeronautics and Space library, vary according to the specified mission profile. As a result, it is possible to compute the heat flow rates in the cabin during the flight mission. These are plotted below for different thicknesses of the insulation layer. The heat flow rates represent the required power that the ECS must deliver to maintain the desired cabin temperature throughout the flight.
Once the preliminary power requirements for the ACM are available, these can be modeled using the Gas Mixture and Moist Air library. This is the second step of the methodology. An example of a 4-wheel ACM is depicted in the figure below. For more information about this model, watch this video.
With this kind of model is possible to study the gas temperature and pressure evolution in the ACM components and size them effectively. Furthermore, the start-up procedure was simulated as follows. The aircraft is at rest on the ground. At t = 2s, the flow control valve connecting the left engine bleed system to the ACM is opened, letting hot pressurized air flow through it. The right flow control valve is opened at t = 3s. The mixing point target temperature is set at 293 K, and then lowered at 288 K and 283 K at 12 and at 15 s respectively.
The results are plotted below. In the first subplot one can notice that as the flow control valve is opened, the pressure builds up accelerating the turbine. In the second sub-plot, the target and computed temperature of the mixing point are plotted, together with the trim air valve opening fraction.
From a performance modeling perspective, the rest of the ECS mainly consists of pipes and other equipment dedicated to the distribution of the pressurized air from the engine bleed ports to the cabin. This portion of the system can be easily modeled with the Gas Mixture and Moist Air library.
Finally, the models of the aircraft thermal structure and the ACMs and distribution network can be integrated together. This allows you, among other things, to verify that the cooling units are correctly sized and to simulate the overall system performance during failure conditions.
The initial objective of this article was to simulate the “pull-down” or cabin cooling case scenario. This is possible with the integrated model and the results achieved are plotted below. The first subplot shows the aircraft mission profile, i.e. the Mach number and the altitude from which the static and total pressure and temperatures are derived. The second subplot shows instead the atmospheric static temperature and the temperature inside the cabin. You can note that the initial cabin temperature is set to 45°C (113°F) and the atmospheric static temperature on ground is 40°C (104°F). The plot cursor is set at t=0s, i.e. 30 minutes after the ACM start-up. It can be seen how the cabin temperature falls below the requested threshold, validating the system performance for the pull-down or cabin cooling case.
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About the author:
Federico Cappuzzo is product manager for Simcenter Amesim with the Aerospace and Defense team. His main activities are focused on the development of solutions dedicated to the modeling of aircraft systems and their virtual integration. Before joining Siemens at the end of 2014, he worked as a system engineer at Airbus in Germany.