Post discharge evolution of a spark igniter kernel

In many practical combustion devices, short duration, high-energy spark kernels are used to ignite combustible gases in turbulent flows. Here we examine the development of a high energy (∼0.25 J) spark kernel created by a short duration (<1 μs) breakdown discharge across two opposed electrodes situated in a uniform air flow. Measurements of electrical energy supplied to the electrodes compare well to thermal energy deposited in the flow with deposition efficiencies exceeding 90%. These spark energies are used as inputs to a numerical model that simplifies the computations by replacing the complex, finite duration, energy deposition process with an instantaneously created, uniform kernel. The evolution of the kernel shape and size predicted by the computational model agrees well with experimental data obtained from high-speed schlieren images, including development of an asymmetry of the kernel between its upstream and downstream regions at later times. The predicted kernel evolution is shown to be essentially independent of the initial size and the composition of the kernel for a fixed deposition energy. The numerical results also reveal the importance of rapid entrainment of ambient air into the central region of the kernel, which quickly reduces the maximum temperatures in the kernel. In addition, the predicted O atom concentrations are well above equilibrium values, especially in the lower temperature regions of the kernel. The higher temperatures and O mole fractions found in the leading portion of the kernel are expected to be an important contributor to ignition in non-premixed combustion flows.