Low-order modeling of high-altitude relight of jet engine combustors

A physics-based, low-order ignition model is used to assess the ignition performance of a kerosene-fueled gas-turbine combustor under high-altitude relight conditions. The ignition model used in this study is based on the motion of virtual flame particles and their extinction according to a Karlovitz number criterion, and a stochastic procedure is used to account for the effects of spray polydispersity on the flame’s extinction behavior. The effects of large droplets arising from poor fuel atomization at sub-idle conditions are then investigated in the context of the model parameters and the combustor’s ignition behavior. For that, a Reynolds-averaged Navier-Stokes simulation of the cold flow in the combustor was performed and used as an input for the ignition model. Ignition was possible with a Sauter mean diameter (SMD) of 50 μm, and was enhanced by increasing the spark volume. Although doubling the spark volume at larger SMDs (75 and 100 μm) resulted in the suppression of short-mode failure events, ignition was not achieved due to a reduction of the effective flammable volume in the combustor. Overall, a lower ignition probability is obtained when using the stochastic procedure for the spray, which is to be expected due to the additional detrimental effects associated with poor spray atomisation and high polydispersity.

Determining a Lower Bound Mixed Mode Failure Curve for An Interface Crack Between Single Crystal Silicon and Silicone Rubber

An interface crack between single crystal silicon (SC-Si) and silicone rubber is examined. The first term of the asymptotic solution for this interface crack is derived. Mixed mode fracture tests were performed on Brazilian disk specimens at different mode mixities. Finite element analyses (FEAs) of these tests were carried out in abaqus. A cubic (anisotropic) material model is used for SC-Si. Two different material models were used for silicone rubber: a linear elastic model for the asymptotic solution and a Mooney–Rivlin (hyperelastic) model for the FEA. The FEAs showed that large deformations were relegated to a small region surrounding the crack tip. Hence, a K-dominate region exists in which linear elastic fracture mechanics (LEFM) may be used. From the FEAs of the Brazilian disk specimens, energy release rates were determined using the virtual crack closure technique (VCCT) and displacement extrapolation (DE) methods which were corroborated by J-integral values evaluated using the contour integral method. Elsewhere, it was demonstrated that properly implemented, the VCCT method may be used for interface cracks. A mixed mode failure criterion is obtained from the energy release rate data. The SC-Si failed before the interface crack propagated. Hence, the failure curve obtained in this study should be considered as a lower bound of the critical energy release rate for this material pair.