Multiphysics Modeling

Thermal-Energy Assisted Combustion Device for UAV Engines

Compression ignition (CI) technique can be implemented in unmanned aerial vehicle (UAV) propulsion systems since this technique can secure reliable engine operability with decent mechanical durability.

Unlike the ground-based engines, maintaining the stable engine operation may become challenging at high-altitude environment due to the low oxygen level and subsequent ignition failure. Therefore, active ignition control strategy is preferred to remedy such undersirable issues. To this end, CI engines for UAV systems need to be integrated with the ignition assistant (IA) device such as glow plug, which is like a diesel-engine cold-start assist device.

Mechanical Breakdown of the IA Device (UIUC experiment)

Following repeated experiments with the ignition-assist (IA) device using direct fuel injection, the UIUC research team led by Prof. Tonghun Lee observed significant mechanical failure of the device. The exact cause of this breakdown remains unclear and may stem from thermomechanical stress loading or a more complex multiphysics interaction involving high-temperature, high-pressure ignition and spray-device coupling, which could induce intense localized heat transfer or pressure loading.

This motivates a deeper discussion of the system’s behavior from a multiphysics standpoint.

Simulation on RCM combustion chamber with the IA device

The current test adopted a BERU glow plug which comprises a co-annular type, electrically conductive heating element core embraced by an externally exposed ceramic sheath. Efforts by the team at University of Illinois Urbana-Champaign (UIUC) is acknowledged in measuring detailed composition of the ceramic material sheath, and the measurement was made by using a Jeol 7000F scanning electron microscope (SEM) with Thermo Electron energy dispersive spectroscopy (EDS) accessories.

Image shown below is the heat flux indicaterd illumination by the UIUC IR camera measurement and its cross-section view of the IA device configuration. The last image set on the bottom row shows the simulated IA device surface temperature and consequently established thermal boundary layer formed around the IA tip surface.

Thermal-Energy Assisted F24 Fuel Ignition and Spray-Wall Impingement

The directly injected F-24 spray strikes the top surface of the IA device, where it mixes with the surrounding high-temperature air to facilitate ignition. This assisted ignition event is characterized by monitoring the in-chamber pressure and heat-release-rate history.

The resulting intense spray impingement and ignition process subject the IA device’s structural components to transient and severe thermomechanical loading.

Finite Element Analysis (FEA) coupled with the CFD simulation

Upon provided thermodynamic properties from the CFD simulations, the FEA model can simulate dynamic thermo-mechanical stress behavior over the CFD time scale. The detailed geometry of the IA device was obtained from direct measurement using a SEM.

To solve the transient thermo-mechanical stress of the IA device, ABAQUS was used as a FEA model solver. 3-D linear hexagonal elements with 8 nodes (C3D8T) were utilized, which can provide the transient solutions of the temperature field and the stress field simultaneously. The heat flux was assigned to the heating element using the same approach to the CFD analysis.

Series of images shown below is captured by the FEA simulation present the surface temperature and maximum principal stress distributions during the spray impingement and ignition process.

Images shown below show the history of the temperature and maximum principal stress during the fuel injection and combustion events for the three selected locations; i.e., SURF (the round tip surface where the fuel impinges), INT1 (the maximum stress region after the voltage activation, closest to the SURF), and INT2 (the maximum stress region after the voltage activation, farthest to the SURF).

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