Investigation of flow characteristics in a twin-surface dielectric barrier discharge reactor by Schlieren imaging

Abstract

Dielectric barrier discharges are an emerging technology for the plasma-catalytic removal of volatile organic compounds and other gas purification challenges such as the removal of O2 traces from H2. Packed-bed reactors are mainly used for these applications, but surface dielectric barrier discharges (SDBDs) typically printed on thin dielectric plates are promising alternatives for the treatment of large volumetric flow rates due to their low flow resistance causing a low pressure drop. Especially for SDBDs the flow conditions are crucial, because the active plasma filled volume covering the mentioned plates with a typical thickness of 0.1 mm is small in comparison to the overall reactor volume with a typical distance of some tens of millimeters to the reactor wall. In this study, the flow conditions of a twin-surface dielectric barrier discharge were investigated by Schlieren imaging applied in converting O2 traces in H2 containing gas mixtures to H2O and compared to fluid dynamics simulations. Schlieren imaging was used to visualize local gradients of the refractive index inside the SDBD reaction chamber, while gas composition, dissipated power, or flow rate were varied. Without a plasma discharge, laminar flow dominates, resulting in a conversion below 10% over a Pt‑coated electrode configuration in the reaction of O2 traces with H2. With the plasma discharge, full conversion was achieved for the same reaction without catalyst, although the plasma is also confined to the surface of the electrode configuration. Schlieren structures covering the complete cross section of the reaction chamber were observed, showing that strong radial mass transport is induced by the plasma. The shape and extent of the Schlieren structures is ascribed to a superimposition of gas flow, thermal expansion from the plasma volume, thermal buoyancy as well as an electrohydrodynamic force between the electrodes and the grounded reactor walls. Fluid dynamics simulations show vortex formation above and below the electrode, created by the electrohydrodynamic force further implying extensive mass transport by the plasma, which is visualized in addition by carbonaceous deposits on the reactor lid.These findings reveal new possibilities to improve SDBD reactors for gas purification applications based on their favorable flow conditions.

The effect of toluene trace admixtures on the plasma‐assisted oxygen trace removal from hydrogen‐rich gas mixtures in a surface dielectric barrier discharge reactor

“…The initial design was implemented by Offerhaus et al, [12] while Peters et al, [14] Schücke et al and Nguyen-Smith et al [15] characterized different aspects of the discharge, largely in the context of volatile organic compound oxidation. Additionally, Ollegott et al [6] investigated radial mass transport induced by the plasma discharge. The electrode configuration consisted of a thin α-Al 2 O 3 plate onto which two nickel-plated metallic grids were imprinted on opposite sides.…”

“…(b) Photograph of the twin SDBD reactor setup with plasma ignition in N 2 . [6] SDBD, surface dielectric barrier discharge.…”

“…[1] A surface dielectric barrier discharge (SDBD) reactor has been recently demonstrated to be an effective means of removing oxygen traces from hydrogen-rich gas mixtures such as coke oven gas (COG). [5,6] However, aromatic hydrocarbons may be present as trace impurities in these mixtures and can also be activated in the plasma. [7] Thus, we investigated the influence of toluene traces on the oxygen trace removal.…”

Surface dielectric barrier discharge (sDBD) for flow control in plasma conversion

“…The scale-up of this sDBD has been studied by Böddecker et al by combining 5 electrodes in parallel to treat up to 500 slm of gas [12]. The flow characteristic inside these sDBD has been investigated by means of the Schlieren imaging technique to analyze the flow pattern to assess the conversion as it depends on the gas flow and the plasma chemistry [13].…”

“…These experiments revealed that the gas conversion in these sDBD can be very large despite the fact that the plasma is located at the very electrode surface and fills the gas volume only to a very small extent. It has been observed [13] that the sDBD exerts a force on the gas, which induces a distinct flow pattern that leads to an efficient mixing of the gas and plasma species. This plasma-induced flow was triggered by the ion wind generated by the electric stray fields.…”