CST – Computer Simulation Technology

EM Modeling of Aircraft Fuel Systems

EADS has developed an internal specialised code for modelling the filling of an aircraft fuel tank. The code simulates this scenario and outputs the electric field, charge density and potential at various points in the tank and fuel over the filling period. Results of this code are verified against CST EM STUDIO® (CST EMS) simulations.

Aircraft are subject to restrictions when filling a fuel tank, such as the flow rate (speed) of the fuel entering the tank. The model will be used to assess new tank designs on future aircraft, identify if the designs are safe and if any of the restrictions should be adjusted.

Various tests have been carried out at test houses in both a rectangular tank (Figure 1) and a Faraday pail (Figure 2).

Figure 1: Diagram and photo of rectangular test tank

Input parameters for the model include the charge density of the fuel, the flow rate of the fuel entering the tank and the conductivity of the fuel. The geometry and materials of the tank walls are also defined. Prior to this, internal codes were only capable of modelling perfectly rectangular metallic tanks. The new code allows complex geometries to be defined and different materials to be specified for each wall....

As the code will be used to assist with the development of new fuel system designs in aircraft, complete confidence in the results it produces is required. The code had been specifically developed for EADS’ purposes and limited validation had taken place. Therefore, it was decided to perform further validation against experimental data, other internally developed models and commercially available simulation software.

Figure 2: Diagram and photo of Faraday pail at the large scale electrostatics facility at the Health and Safety Laboratory

The new code provides the e-field, charge density and potential at any specific point within the tank as well as maximum and central surface potential of the fuel as a function of fill fraction. Two geometries, a rectangular tank and the Faraday pail, both shown in Figure 3, were used for the experimental trials.

Figure 3: Rectangular and pail geometries used for the validation

CST EM STUDIO® (CST EMS) has a powerful CAD engine which allows complex geometries to be created which can then be simulated with the internal code. The CST EMS electrostatic solver delivers the potential within the tank at a single fuel depth per simulation run. The internal code is designed to simulate the filling process, the results of which were validated against the static CST EMS results.

Figure 4: Electric potential on a cutplane in the pail model

The structures were defined to be perfect electric conductors and the fuel was defined as a dielectric material with a relative permittivity of 2.1 and a uniform charge density of 150μC/m3 which is a value typically taken as representative of fuel in an aircraft tank (this value was also used in the internal code). The electrostatic solver within CST EMS was run using a hexahedral mesh over several fuel depths.

For each CST EMS simulation, a 2D plot showing the electric potential across the surface of the fuel was generated as shown in Figure 4.

Figure 5: Comparison between codes of fuel surface potential as a function of fill fraction

The maximum voltage across each 2D plot was extracted and plotted over the maximum surface potential results produced by the internal code. It is clear from Figure 5 that there is good agreement between the two codes. This completed an important step in the validation process.

Rhys Phillips, Simon Evans & Jill Ogilvy – EADS Innovation Works, UK

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