Sep

5

Environmental control systems requirements and design challenges

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.

Another requirement expressed by airlines concerns the so-called “pull-down”, or cabin cooling case. Here, aircraft manufacturers are requested to demonstrate that the aircraft on the ground on a hot day is able to reach a temperature lower than 27 degrees Celsius (80 degrees Fahrenheit) in less than 30 minutes. This shall be achieved using the auxiliary power unit (APU) as a power source, with an initial cabin temperature after heat soak of 45°C (113°F) and no passengers aboard. This requirement has a direct impact on the aircraft turnaround time, hence the aircraft operating cost, as the cooling down time delays the aircraft full operability.

From an engineering perspective, this requirement represents one of the main dynamic sizing cases for the aircraft ECS. In particular, it drives the sizing of the air cycle machines (ACMs) or air conditioning packs. These refrigeration units are based on the air cycle cooling process and represent the core of the ECS.

A typical ECS architecture is shown in the schematics below. Compressed hot air is bled from the engine compressors or the APU (not represented here), cooled by a pre-cooler heat exchanger installed downstream the engine bleed ports and conveyed to the ACMs. Each ACM consists of a compressor, a turbine and a fan mounted on the same shaft.
Hot air bled from the engine compressors passes through in the primary heat exchanger where it is cooled before entering in the compressor. Compressed air passes then through the main heat exchanger, a re-heater and a condenser before entering in the turbine. At the exit, air humidity is removed by another passage through the condenser. At this point, the cooled air is conveyed to the mixing unit.
This architecture is known as 3-wheel machine, and a common variant is the 4-wheel machine where a second turbine is added on the same shaft downstream the condenser.
From the mixing unit, the fresh air is then distributed to the cabin, here divided into three main zones, namely the flight deck, the forward and the aft cabin.
It is clear that in order to speed up the design of the ACMs and hence reduce the development cost and the time-to-market, it is necessary to assess their performance integrating them into the ECS with the appropriate environmental conditions. Simcenter Amesim software provides unrivalled capabilities which help you model and integrate multi-physic systems, evaluate their performance and design for different sizing scenarios.
  • How Simcenter Amesim supports ECS design
  • Aircraft thermal model: assess power requirements
  • Air cycle machines modeling: optimize performance
  • Integration and results
  • Watch the video Air Cycle Machine start-up
  • Related content

How Simcenter Amesim supports ECS design

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 [1] and [2].

Aircraft thermal model: assess power requirements

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.

Air cycle machines modeling: optimize performance 

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.

Integration and results

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.

 

Watch the video

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This article is repubblished from the Simcenter Blog, https://community.sw.siemens.com/s/article/how-to-assess-the-aircraft-cabin-cooling-sizing-with-simcenter-amesim , by tim.mila

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