Through the decades, the aircraft industry always improved their onboard systems to get the best performances, security and comfort. In order to build a lasting travel type, security of the aircraft is one of the main goals for engineers. Due to rough exterior conditions while flying, especially at high altitude, with relative humidity and very low temperatures, the freezing temperature can cause the plane to ice. Ice can have major impacts on the aircraft’s weight and aerodynamical phenomena, – especially the lift – (the lift can decrease to 40% due to ice). Modeling and installing a specific system to prevent ice is a necessity. Therefore, aircraft designers developed an anti-icing system inside the wing to prevent ice.
There are several anti-icing systems on aircraft, mostly depending of the engine’s type. Most of aircrafts use the bleed air system, which consists of using a hot bleed air to warm up the wing leading edge. Another system named de-icing boots system is mostly used on turboprop aircrafts and consists of black rubbers at locations prone to icing which inflate and literally break the ice. Another system is simply an electrical leading edge warm up directly installed in the wing leading edge. Those examples are just an introduction to some anti-icing systems that aircraft industry has develop and are using. Each have pros and cons.
Here, we will focus on the anti-icing system using hot bleed air. This approach is used by the Boeing 737-300/400/500 anti-icing system with hot bleed air warming the leading edges.
Typically, this type of anti-icing system consists of a hot bleed air flow provided by the engine compressor’s stages to warm up the plane’s wing leading edge. The wing anti-icing system is made of two independent pneumatic systems among others, providing hot bleed air from each of the two turbofans separately. The hot bleed air is ducted via the engine bleed valve from the fifth compressor stage. If the pressure isn’t enough, bleed from the ninth compressor stage can additionally be used. Note that the fifth stage bleed air temperature is approximately 340°C and the ninth stage one is approximately 540°C which are too hot to be used in aircraft’s pneumatic systems such as hydraulic pressurization or potable water system pressurization for example. The hot air then runs through a pre-cooler to reduce the temperature to 200°C and this cooled air is distributed via the bleed ducts to consumers like the air conditioning packs for example and the wing anti-icing system. In order to know the moment to use the anti-icing system, the aircraft’s pilots use the visual ice indicator which is situated in the middle beam of the window. Once the probe is icing, the pilots enable the anti-icing system. Hence, hot bleed air is provided to the slates number three, four and five as shown in Figure 1.
Due to the larger diameter and the aerodynamics phenomena, slates number one and two do not need any anti-icing devices. Once the anti-icing system is enabled, the hot bleed air is guided along telescopic pipes then is distributed via piccolo tubes as shown in Figure 2. From there, it exits the piccolo tubes through little holes, warms the wing leading edge and flows out of the wing through exit holes situating on the wing’s lower surface.
AxSTREAM NET™ is a tool which provides the flexibility and convenience to perform various hydraulic, pneumatic and thermal tasks including solving models like the hot bleed air anti-ice system modelling. With AxSTREAM NET™, we are able to study a wide variety of steady or unsteady cases such as:
- Determine the temperature, pressure and velocity profiles of the pneumatic system
- Estimate in steady state the wing leading edge temperature while using the anti-icing system
- Chart in unsteady state the profile temperature in the wing leading edge to estimate the needed time to achieve the right temperature to melt the ice on the wing leading edge
The hot bleed air is provided (bottom-right corner of the model in Figure 3) by the precooler with a pressure of 50 PSI and a temperature of 227°C. As mentioned earlier, the telescopic duct guides the fluid to the piccolo duct, represented by a succession of pipes, chambers and exit holes. To model it properly, the following procedure has been chosen: a chamber with a relative volume has been added to link exit holes with the pipes to similar the pipe holes. Regarding the piccolo duct geometrical parameters, three pipes with chambers and a certain number of holes has been introduced to simulate the real component. The flow exiting through the holes comes into chambers, modeling the cavity between the piccolo duct and the wing leading edge. Then the hot bleed air flows along the wing leading edge before exiting the aircraft with a particular path.
To accurately account for heat transfer, non-swirling convection has been added between the piccolo duct and the wing leading edge. Similarly, heat transfer between the wing leading edge and the environment has also been modeled. The wing leading edge is an alternation of steel slates and insulator slates. Hence, conduction warms the slates in the wing leading edge while convection with the environment cools down the wing leading edge’s last slate.
The temperature profile in the pneumatic system can be seen through the coloring of the fluid ports (squares at the end of each element) in Figure 4. Here, no thermal losses were accounted for in the telescoping pipes and piccolo duct so maximum thermal energy could be transferred to the wing leading edge (best case scenario, although alternatives can also be simulated).
Looking at Figure 5, we observe that the flow inside the system is quite fast. The air flows rapidly in the pneumatic system, leading to a large mass flow rate (0.96 kg/s) for powerful de-icing.
Due to the lack of geometrical/thermal information for some parts of the system, reasonable values were chosen. Despite the assumptions made and the macroscopic approach taken for the system modeling, the results obtained are reasonable including the wing leading edge surface temperature necessary to melt the ice properly.
With AxSTREAM NET™, it is easily possible to create complex systems to perform various fluid and/or thermal modeling and get proper results. The results presented here are just a part of what we can get from this system. As an example, we can link this system to the general pneumatic system of an aircraft as the Boeing 737 – 300/400/500 pneumatic system designed in AxSTREAM NET™ as shown in figure 7.