[CCoE Notice] Peng Lin_CHE Dissertation Defense Announcement

Wed Jul 28 12:25:30 CDT 2021

Name: Peng Lin

DATE: Friday, July 30, 2021

TIME: 1:00 PM

COMMITTEE CHAIRs: Prof. Demetre Economou and Prof. Vincent Donnelly

LOCATION: TEAMS Link:

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________________________________

TITLE:

NUMERICAL SIMULATION OF PLASMA FLOW IN ATMOSPHERIC PRESSURE PLASMA JETS

Atmospheric pressure plasma jet (APPJ) reactors are popular mainly due to their applications in plasma medicine. In APPJ a plasma is generated in a capillary dielectric tube under a working gas flow. The jet entrains ambient gas to produce a complicated array of plasma chemical reactions. To achieve better control of the plasma chemistry a “shielding” gas may flow in an annular tube concentric with the main tube.

A 2-D (r, z) numerical simulation of an APPJ was performed. The helium working gas and the nitrogen shielding gas emerged, in laminar flow, in dry air ambient. With the shielding gas present, more nitrogen penetrated into the helium core, causing a shorter plasma “plume”. The shielding gas hindered the diffusion of oxygen and argon from the ambient air into the helium jet. The flow rates of the working and shielding gas critically affected the gas temperature, and in turn the discharge characteristics. For a He flow of 2 standard liters per minute (slm), switching on the nitrogen shielding gas flow (at 4.5 slm) reduced the on-axis O2 and Ar mole fractions from [cid:image009.png at 01D783AB.4A9E3ED0]  to [cid:image010.png at 01D783AB.4A9E3ED0]  and from [cid:image011.png at 01D783AB.4A9E3ED0]  to [cid:image012.png at 01D783AB.4A9E3ED0]  , respectively, at an axial distance of 3 mm downstream of the nozzle. The radial profiles of the mole fractions of the ambient gases monotonically and strongly decreased towards the system axis, for short axial distances from the nozzle (~1 mm), but became progressively flatter at longer distances downstream (3 and 5 mm). Simulation predictions captured the salient features of experimental data of ambient species mole fractions in the APPJ.

 In cases extensive surface coverage is desirable, a jet array may be used. A 2-D numerical simulation of an APPJ jet array was conducted. In order to maintain cylindrical symmetry, the actual 3-D system was simulated by a central jet surrounded by a coaxial annular flow jet. Argon emerged from the tubes in laminar flow into nitrogen ambient. For the base case, the annular plasma jet merged with the central jet, forming a single jet. Radial propagation of the combined plasma jets helped uniformity of substrate-jets interaction. The jet-array operated in “parallel” jets mode for high values of the in-between nitrogen flow. A new mode of operation, namely “converging-diverging” mode was identified for intermediate values of the in-between nitrogen flow. Increasing the distance between the central and annular jets, extended the argon diffusion length and affected the mixing layer. This, coupled with Coulomb forces between the positive ion clouds of the jets, caused the annular jet to diverge away from the central jet. The simulation predicted experimentally-observed features of interaction among jets in jet-arrays.

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