Influence of shock wave propagation on dielectric barrier discharge plasma actuator performance

Interest in plasma actuators as active flow control devices is growing rapidly due to their lack of mechanical parts, light weight and high response frequency. Although the flow induced by these actuators has received much attention, the effect that the external flow has on the performance of the actuator itself must also be considered, especially the influence of unsteady high-speed flows which are fast becoming a norm in the operating flight envelopes. The primary objective of this study is to examine the characteristics of a dielectric barrier discharge (DBD) plasma actuator when exposed to an unsteady flow generated by a shock tube. This type of flow, which is often used in different studies, contains a range of flow regimes from sudden pressure and density changes to relatively uniform high-speed flow regions. A small circular shock tube is employed along with the schlieren photography technique to visualize the flow. The voltage and current traces of the plasma actuator are monitored throughout, and using the well-established shock tube theory the change in the actuator characteristics are related to the physical processes which occur inside the shock tube. The results show that not only is the shear layer outside of the shock tube affected by the plasma but the passage of the shock front and high-speed flow behind it also greatly influences the properties of the plasma

An experimental marker of thermo-diffusive instability in hydrogen-enriched flames

The structure of hydrogen-enriched methane-air flames in a Bunsen burner at low turbulence is investigated using OH planar laser-induced fluorescence (PLIF). Three flames are investigated at identical unstretched laminar flame speeds and turbulence conditions, while hydrogen enrichment is varied up to 70% by volume. An increase in global flame consumption speed is recorded with hydrogen addition, and is attributed to both an increase in flame surface area and fluctuations in stoichiometry along the flame surface as a result of differential diffusion. These fluctuations are found to be well-captured by the gradient of OH-PLIF intensity along the flame front and it is hence identified as a promising experimentally-accessible marker of thermo-diffusive instability. Its correlation with curvature is hereby examined for the first time experimentally. No correlations are found in absence of hydrogen, while increasingly positive correlations are recorded with hydrogen enrichment, consistent with the behavior of local fuel consumption in direct numerical simulations (DNS) of lean hydrogen-air flames. This highlights the potential of OH intensity gradient magnitudes as a marker of thermo-diffusive instability, and a potential surrogate for local fuel consumption speed which is inaccessible experimentally

Effects of hydrogen enrichment on thermoacoustic and helical instabilities in swirl stabilised partially premixed flames

The effects of hydrogen enrichment on flame and flow dynamics of a swirl-stabilised partially premixed methane-air flame are studied using large eddy simulation. The sub-grid reaction rate is modelled using unstrained premixed flamelets and a presumed joint probability density function approach. Two cases undergoing thermoacoustic oscillations at ambient conditions are studied. The addition of hydrogen modifies both thermoacoustic and fluid dynamical characteristics. The amplitude of the fundamental thermoacoustic mode increases with the addition of 20\% hydrogen by volume. A second pressure mode associated with the chamber mode is also excited with the hydrogen addition. Intermittent single, double and triple helical instabilities are observed in the pure methane case, but are suppressed substantially with hydrogen addition. The results are analysed in detail to shed light on these observations. The feedback loop responsible for the thermoacoustic instability is driven by mixture fraction perturbations resulting from the unequal impedances of the fuel and air channels. It is shown that hydrogen addition increases the flame's sensitivity to these perturbations, resulting in an increase in amplitude. This higher amplitude thermoacoustic oscillation, along with a higher local heat release rate in the presence of hydrogen, is shown to considerably modify the flow structures, leading to a suppression of the helical instabilities.Engineering and Physical Sciences Research Council (EP/R029369/1). Mitsubishi Heavy Industries, Ltd., Takasago, Japan. European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement No.682383)