Buoyancy effects on flow transition in low-density inertial gas jets

Pasumarthi, Kasyap S.; Agrawal, Ajay K.

doi:10.1007/s00348-005-0927-2

Cited by 6 publications

(5 citation statements)

References 6 publications

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“…Although the transition Re is about a factor of 4 lower than the typical transition value (Re ∼ 1 760 for flow inside a cylindrical pipe, i.e., Poiseuille flow25), our Reynolds number is consistent with other reports (Re ∼ a few hundred) for the flow transition of helium plasma jet in open air 26, 27. This low Reynolds number flow transition was observed and explained in helium gas jets by the buoyancy effect 27–29. According to these reports, the buoyancy effect may be significant for flow transition in the low momentum and momentum dominated helium jet.…”

Section: Resultssupporting

confidence: 90%

A Simple Approach to Surface Modification Using Polytetrafluoroethylene (PTFE) with Laminar and Turbulent Flows of Micro Plasma Jets at Atmospheric Pressure

Jung¹,

Gweon²,

Kim³

et al. 2011

Plasma Processes & Polymers

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Unlike large area plasmas, small size plasma jets could be useful in treating localized regions. For this reason, we developed a micro plasma jet having a polytetrafluoroethylene (PTFE) part exposed to the discharge inside the plasma source. As the helium gas flow rate was raised from 0.1 to 6.0 slpm, the plasma exhibited a change in characteristic from laminar to turbulent and an abrupt change of appearance at a certain flow rate (2.0–3.0 slpm). The measured rotational temperature was above 500 K under laminar flow conditions, which was a sufficiently high temperature for the pyrolysis of PTFE, and the excitation temperature of the laminar flow was higher than that of the turbulent flow. The corresponding optical emission spectra were different (CN, C2, F I, Hα dominated for laminar conditions while N 2+, OH, O I dominated for turbulent flow conditions). These different plasma characteristics were obtained by simply controlling the gas flow rate. A feasibility study of surface modification demonstrated a change of the surface wettability from hydrophobic to hydrophilic (and vice versa) depending only on the gas flow rate.

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Section: Resultssupporting

confidence: 90%

A Simple Approach to Surface Modification Using Polytetrafluoroethylene (PTFE) with Laminar and Turbulent Flows of Micro Plasma Jets at Atmospheric Pressure

Jung¹,

Gweon²,

Kim³

et al. 2011

Plasma Processes & Polymers

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“…It reaches a maximum at a Reynolds number of roughly 1500 and decreases afterwards [5]. Buoyancy has been found to play a key role in the flow transition from laminar to turbulent flow in the case of helium jets [13,14]. This effect has been observed for Richardson numbers between 3.53 × 10 −3 and 19 × 10 −3 [13].…”

Section: Introductionmentioning

confidence: 99%

Gas flow characteristics of a time modulated APPJ: the effect of gas heating on flow dynamics

Zhang

Sobota

Veldhuizen

et al. 2014

J. Phys. D: Appl. Phys.

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This work investigates the flow dynamics of a radio-frequency (RF) non-equilibrium argon atmospheric pressure plasma jet. The RF power is at a frequency of 50 Hz or 20 kHz. Combined flow pattern visualizations (obtained by shadowgraphy) and gas temperature distributions (obtained by Rayleigh scattering) are used to study the formation of transient vortex structures in initial flow field shortly after the plasma is switched on and off in the case of 50 Hz modulation. The transient vortex structures correlate well with observed temperature differences. Experimental results of the fast modulated (20 kHz) plasma jet that does not induce changes of the gas temperature are also presented. The latter result suggests that momentum transfer by ions does not have dominant effect on the flow pattern close to the tube. It is argued that the increased gas temperature and corresponding gas velocity increase at the tube exit due to the plasma heating increases the admixing of surrounding air and reduces the effective potential core length. With increasing plasma power a reduction of the effective potential core length is observed with a minimum length for 5.6 W after which the length extends again. Possible mechanisms related to viscosity effects and ionic momentum transfer are discussed.

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“…As is well known, cold laminar gas jets (e.g., at room temperature) can only maintain their laminar flow state within a short distance from the exit of the jet nozzle (usually, the jet laminar-region length/diameter ratio does not exceed 10) even at comparatively low Reynolds numbers [1,2]. Linear instability analysis [3] and experimental observation [2] showed that the cold laminar gas jet becomes even more unstable when the density of the gas jet is less than that of the ambient gas.…”

Section: Introductionmentioning

confidence: 99%

“…Linear instability analysis [3] and experimental observation [2] showed that the cold laminar gas jet becomes even more unstable when the density of the gas jet is less than that of the ambient gas. Since the density of a plasma jet is always much less than that of the cold ambient air, one might anticipate that it is hard to generate a stable long laminar plasma jet in atmospheric-pressure air surroundings.…”

Section: Introductionmentioning

confidence: 99%

What do we know about long laminar plasma jets?

Chen

Pan

Meng

et al. 2006

Pure and Applied Chemistry

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Silent and stable long laminar plasma jets can be generated in a rather wide range of working parameters. The laminar flow state can be maintained even if considerable parameter fluctuations exist in the laminar plasma jet or if there is an impact of laterally injected particulate matter and its carrier gas. The attractive special features of laminar plasma jets include extremely low noise level, less entrainment of ambient air, much longer and adjustable high-temperature region length, and smaller axial gradient of plasma parameters. Modeling results show that the laminar plasma jet length increases with increasing jet inlet velocity or temperature and the effect of natural convection on laminar plasma jet characteristics can be ignored, consistent with experimental observations. The large difference between laminar and turbulent plasma jet characteristics is revealed to be due to their different laws of surrounding gas entrainment. Besides the promising applications of the laminar plasma jet to remelting and cladding strengthening of the metallic surface and to thermal barrier coating preparation, it is expected that the laminar plasma jet can become a rather ideal object for the basic studies of thermal plasma science owing to the nonexistence of the complexity caused by turbulence.

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Buoyancy effects on flow transition in low-density inertial gas jets

Cited by 6 publications

References 6 publications

A Simple Approach to Surface Modification Using Polytetrafluoroethylene (PTFE) with Laminar and Turbulent Flows of Micro Plasma Jets at Atmospheric Pressure

A Simple Approach to Surface Modification Using Polytetrafluoroethylene (PTFE) with Laminar and Turbulent Flows of Micro Plasma Jets at Atmospheric Pressure

Gas flow characteristics of a time modulated APPJ: the effect of gas heating on flow dynamics

What do we know about long laminar plasma jets?

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