Abstract
Results obtained with a previously developed technique for measuring local heat transfer to bare and ceramic coated metal wires from a dense, highly thermallyionized plasma are presented and compared with theoretical predictions. A free-burning arc in argon at atmospheric pressure provides a plasma in local thermodynamic equilibrium. Wire probes are swept through this plasma in planes parallel to the anode. The wires do not draw a net current from the plasma. Experimental evidence indicates that the layer between the undisturbed plasma and the probe surface is not in chemical equilibrium. The data show that heat transfer to the wire in this situation is dominated by the energy transport associated with electron and ion partial currents to the probe surface as determined by the potential of the probe with respect to the plasma. The data obtained using this technique provide an experimentally verified model for the heat transfer to wires immersed in a plasma over a wide range of plasma conditions including a 100 percent variation in the degree of ionization.
Zusammenfassung
Es werden Versuchsergebnisse mitgeteilt, die mit einer früher entwickelten Methode zur Messung der örtlichen Wärmeübertragung an blanke und keramikumhüllte Metalldrähte von einem thermisch hochionisierten Plasma gewonnen wurden. Diese Ergebnisse werden mit der Theorie verglichen. Ein frei brennender Bogen in Argon bei Atmosphärendruck ergibt ein Plasma im lokalen thermodynamischen Gleichgewicht. In Ebenen parallel zur Anode werden Drahtsonden durch das Plasma geschwenkt. Die Drähte entziehen dem Plasma keinen Nettostrom. Durch Versuch wird festgestellt, daß die Schicht zwischen ungestörtem Plasma und Sondenoberfläche nicht im chemischen Gleichgewicht ist. Die Werte zeigen, daß hierbei die Wärmeübertragung zum Draht bestimmt wird durch den Energietransport der Teilströme von Elektronen und Ionen zur Sondenoberfläche, wie er sich aus dem Potential der Sonde in Bezug auf das Plasma ergibt. Diese Versuchstechnik liefert ein experimentell bestätigtes Modell für den Wärmeübergang an in Plasma eingetauchte Drähte in einem weiten Bereich der Plasma-Zustandsgrößen, einschließlich einer Variation des Ionisationsgrades von hundert Prozent.
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Abbreviations
- A:
-
cross-sectional area of probe
- Cp :
-
specific heat at constant pressure
- D:
-
wire diameter
- Db :
-
ambipolar diffusion coefficient
- e:
-
unit charge
- i:
-
enthalpy
- j:
-
current density
- k:
-
thermal conductivity
- kB :
-
Boltzmann constant
- L:
-
length of wire immersed in plasma
- n:
-
particle number density
- q:
-
heat flux (heat transfer per unit area and time)
- rp :
-
probe radius
- Re :
-
electrical resistance
- T:
-
temperature
- VF :
-
floating potential of probe with respect to the plasma=Vplasma−Vprobe
- a :
-
thermal diffusivity
- δ:
-
boundary layer thickness
- Δ:
-
difference
- μ :
-
dynamic viscosity
- v :
-
kinematic viscosity
- ξ:
-
degree of ionization=nelectron/(nion+ nneutral) (singly charged ions only)
- ρ :
-
mass density
- ρ e :
-
electrical resistivity
- τ :
-
characteristic time
- Φ :
-
electron work function
- Xi:
-
ionization energy
- NuD :
-
Nusselt number based on wire diameter=qD/kΔT
- Pr:
-
Prandtl number=μcp/k
- ReD :
-
Reynolds number based on wire diameter=UD/v
- Sc:
-
Schmidt number=v/Damb
- ψ :
-
dimensionless probe potential=eVF/kBTplasma
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Petrie, T.W., Pfender, E. The effect of ionization on heat transfer to wires immersed in a highly thermallyionized plasma. Warme- und Stoffubertragung 5, 85–100 (1972). https://doi.org/10.1007/BF01438410
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DOI: https://doi.org/10.1007/BF01438410