Laser spark mixing and ignition enhancement of hydrocarbon fuels in a scramjet combustor

This work presents the numerical results of a parametric study into the mixing and ignition enhancement capability of laser sparks in a hydrocarbon fuelled scramjet combustor. A laser spark model for hydrocarbon fuels, capable of reproducing key properties of laser sparks, has been derived from the adaption of established spark models. This spark is then applied to a steady scramjet combustor flow field within a single time-step and a transient simulation is conducted. Besides the display of various aspects of mixing enhancement, this study also reveals the local extent of the influence of the laser spark. For the parametric study, different spark properties have been selected and investigated. For the mixing enhancement study, a single spark simulation is used to infer the influence of the four properties spark energy, location, size, and of the fuel type. The results show a strong influence of the variation of the spark energy and its location. The spark energy mainly changes the magnitude of the disruption while the spark location also changes the effects to some degree. The increasing impact on the injector flow the further the spark is placed inside the injector has overall a negative impact on the mixed mass increase within the injector. For the two investigated fuel types, the impact of the spark is similar. Only the enhancement magnitude differs. Compared to the other effects, the spark size has an insignificant influence on the mixing process. An improvement in mixing can be achieved when the laser sparks are repeated. However, an improvement beyond the single spark capabilities is only possible when the repetition rate is related to the dominant frequency of the flow field oscillations. Otherwise, an improvement in absolute terms can be achieved, but the mixing enhancement efficiency drops below the single spark value. Identically to the mixing enhancement study, the ignition enhancement study investigates single and repeated laser sparks. The presented results show that laser sparks can enforce localised ignition from where the flame can spread. However, the main ignition enhancement path way is through the interaction of the blast wave with the bow shock. The spark itself causes only a minor contribution to the heat release. This is mostly caused by a slow mixing with air and its highly reactive period is left unexploited. The repetition rate is less effective when it comes to ignition enhancement. Although repeated sparks can repeatedly force-induce ignition, flow field oscillations caused by a single spark can identically lead to multiple ignition locations. Repeated sparks only enhance the observed effect. The impact of the spark is, therefore, reduced and, as a result, the efficiency for repeated sparks drops when the repetition rate is causes sparks within the region/time of influence of the previous spark.