Thermomechanical Characterization of Hot Surface Ignition Device Using Phenomenological Heat Flux Model

Abstract

Modern aircraft compression ignition engines that seek to achieve fuel-flexible operation require ignition-assisting devices to perform reliably. Many military propulsion systems, and unmanned aircraft in particular, can use hot surface energy addition to improve engine performance. An analysis of the thermomechanical stress behavior of hot surface ignition devices is therefore critical to ensure that they can satisfy new operational requirements. In this paper, a phenomenological heat-flux model is proposed to simulate heat generation inside a heating element. Two finite element analysis models are established using the proposed heat-flux model in order to characterize the transient thermomechanical behavior of an ignition assistant: a two-dimensional (2-D) axisymmetric model with an equivalent cylindrical heating element and a three-dimensional (3-D) solid model with realistic heating element geometry. The transient behavior of the ignition assistant is investigated at standard conditions by characterizing the temperature and thermal stress inside the device for various input voltages. The effect of the nonaxisymmetric heating element geometry on the temperature and thermal stress distribution is assessed. Furthermore, the 3-D ignition assistant model is used to analyze the thermomechanical behavior of an ignition device subjected to a combustion event inside a rapid compression machine. Multiple circumferential orientations of the heating element plane relative to the spray axis are investigated. Results demonstrate that the observed maximum principal stress depends significantly on heating element orientation.