Radiofrequency plasma stabilization of a low-Reynolds-number channel flow

Abstract

The effects of plasma heating and thermal non-equilibrium on the statistical properties of a low-Reynolds-number ($Re_{\tau } = 49$) turbulent channel flow were experimentally quantified using particle image velocimetry, two-line planar laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy and emission spectroscopy. Tests were conducted at two radiofrequency plasma settings. The nitrogen, in air, was vibrationally excited to $T_{vib} \sim 1240\ \mathrm{K}$ and 1550 K for 150 W and 300 W plasma settings, respectively, while the vibrational temperature of the oxygen and the rotational/translational temperatures of all species remained near room temperature. The peak axial turbulence intensities in the shear layers were reduced by 15 and 30 % in moving across the plasma for the 150 and 300 W cases, respectively. The plasma did not alter the transverse intensities. The Reynolds shear stresses were reduced by 30 and 50 % for the 150 and 300 W cases. The corresponding Reynolds shear stress correlation coefficient was also reduced, which indicates that the large-scale structures were diminished. Finally, the plasma enhanced the turbulence decay in the zero-shear regions, where the power law decay $t^{-1/n}$ exponential factor $n$ decreased from 1.0 to 0.8.

Spontaneous Raman Scattering Temperature Measurements and Large Eddy Simulations of Vibrational Non-equilibrium in High-Speed Jet Flames

“…Experimental studies have shown turbulence intensity and decay to be affected by plasma-generated V-T non-equilibrium. 2 Similarly, Koo et al 3 showed V-T non-equilibrium to alter jet flame stabilization heights in direct numerical simulations. They found that vibrational temperatures lower than translational hindered reactivity and pushed flame stabilization downstream.…”

Control of vibrational distribution functions in nonequilibrium molecular plasmas and high-speed flows

“…However, the effect of this relatively straightforward coupling mechanism on the high-speed flow appears to be understood only qualitatively, with limited predictive capability. Recent experimental results also point to the existence of other coupling mechanisms between the excitation/ relaxation of internal energy modes and the flow energy spectrum, such as the delay of turbulent transition in hypersonic boundary layer flows caused by vibrational relaxation of CO 2 [7], or turbulence intensity reduction in low Reynolds number flows, when the relaxation time of nitrogen vibrationally excited in an RF discharge matches the turbulence decay time [8]. However, detailed experimental data isolating and quantifying these mechanisms are not available.…”

Thermal non-equilibrium effects in turbulent compressible shear flows

“…The flow is passed through a passive grid, generating a decaying turbulent flow field in vibrational non-equilibrium. Fuller et al 10 and Fuller 11 report a clear coupling between vibrational non-equilibrium and turbulent transport. They observed reductions in the Reynolds stresses of approximately 15% for the lower plasma setting, and 30% for the higher plasma setting.…”

“…They observed reductions in the Reynolds stresses of approximately 15% for the lower plasma setting, and 30% for the higher plasma setting. Further details on the set-up and challenges of experimentally studying such a flow, as well as further analysis is given in Fuller et al 10 The complex flow physics within a boundary layer means understanding the interaction between physical processes is difficult to determine. Isolating these interactions in simpler flows will allow for a better understanding of these interactions in more complicated flows.…”