Enhancement of hydrogen microcombustion via field-emission dielectric barrier discharge

Abstract

A particle-in-cell (PIC) model is constructed for a ∼100 μm field-emission dielectric barrier discharge (FE-DBD) actuator in atmospheric pressure mixtures of oxygen, hydrogen, and water vapor. The discharge is characterized, including the effects of gas temperature and gas composition. Quantities relevant to plasma-coupled combustion, such as Joule heating, Lorenz body force, and radical generation are estimated for gas temperatures and compositions relevant for combustion energy conversion. Due to its low ionization energy, O 2 increases the magnitude of Joule heating and body forces in the discharge. However, water vapor weakens these effects because of its high ionization energy and many excitational degrees of freedom that remove energy from electrons through inelastic collisions. At higher gas temperatures the discharge becomes more diffuse on account of lower gas densities. At room temperature, and operated at an easily achieved 300 V at 10 MHz, the FE-DBD produces both intense local body forces (10 7 N/m 3 ) and Joule heating (10 9 W/m 3 ) at the corner of the exposed electrode. A boundary layer model provides an estimate that the FE-DBD can induce flow velocities of around 10-20 m/s and temperature increases of around 5-20 K in room temperature H 2 /O 2 /H 2 O mixtures. Sources based on the PIC model are applied in continuum simulations of non-premixed hydrogen combustion in a microchannel. Without the FE-DBD, the inflow of fuel and oxygen is hindered by viscous losses, thermal conduction to wall boundaries, and thermal expansion of gasses, leading to flame extinction. The FE-DBD actuators pre-heat and dissociate the fuel and produce body forces that counter viscous resistance, sustaining combustion. In another example, FE-DBD plasma coupling reduces the autoignition time of a stoichiometric, 950 K H 2 -O 2 mixture from 117 μs to 49 μs.

Field-emission plasma enhancement of H2–O2 micro-combustion

“…Current density is enhanced further by the Schottky effect, which lowers the surface energy barrier for the emission of electrons, as well as geometric features like sharp corners that amplify the local electric field. FE-DBD actuators are attractive for sub-millimeter combustion because of their small scale, low voltage, and strong effect on flow and gas chemistry [13,19,20].…”

Plasma-flow interactions in field-emission discharges with applications in microcombustion

“…The channel is made of quartz and sits atop a silicon bottom plate. The design is based on the microburner concept presented by Mackay et al [29]. The location of the actuators is close to the fuel and oxidizer inlets in order to enhance mixing and increase residence time in the channel.…”

“…More recently, Babaeva et al [26] performed fluid and hybrid modeling to quantify the effect of polarity and secondary electron emission on streamer propagation in nanosecond surface discharges. Fluid model for plasma was combined with Navier-Stokes solution for fluid flow in [27][28][29], and Euler solution for fluid flow in [30]. Shan et al [31] used a combination of empirical and cir cuit models with unsteady reynolds averaged Navier Stokes (RANS) and large eddy simulations (LES) solutions to simu late DBD plasma actuation.…”

“…Benard et al [32] modeled the time evolution of the DBD induced volumetric force based on particle image velocimetry (PIV) flowfield measurements. In the present work, we adopt a method similar to [20,29], and perform kinetic modeling for plasma combined with CFD modeling for fluid flow.…”

Dynamical level-crossing model for the time-dependent electron emission from dielectric surfaces in symmetric dielectric barrier discharges

“…These models solve the Boltzmann transport equation, and thus take the volumetric effect of electron-impact ionizations and diffusive loss mechanisms into consideration, but require an initial electron density and coefficient of electron emission from surfaces for closure [17]. The role of surfaces becomes more important as devices are miniaturized; already, micro and nano scale plasma generators have found a wide variety of applications [18], and micro-combustion applications of asymmetric dielectric barrier discharge plasmas, with one metallic and one dielectric surface, have been recently investigated [19]. In the case of direct-current field-emission plasmas, the connection between metallic electrodes and gaseous regions is well-established -electron emission from surfaces has been investigated in the context of field and thermionic emission, and understood in terms of the Fowler-Nordheim formula [20,21], while the coefficient of secondary electron emission, γ se , quantifies the effect of impinging ions on electron emission.…”