Chemistry Reference
In-Depth Information
chemistry seems to remain rather underdeveloped in this field. Until now, these
principles were only considered in special cases. Some examples are mentioned:
•
For high-power arc discharges (thermal plasmatrons and torches) detailed
investigations on scaling and similarity were performed, but for the most
part without satisfactory physical interpretation, see [10].
•
Within a macroscopic description (compare Section 4.4) Warburg and his
school [7], Becker [8] and Eremin [9] connected the efficiency of chemical
reactions in gas discharges with important discharge parameters. Some of
these pure empirical considerations include one fundamental point of view:
the chemical changes of reactants during the flow through the discharge
zone are determined by the power
P
deposited in the plasma volume
V
A
and
multiplied by the residence time τ of the gas in the reaction zone. These
discharge operation parameters are combined to the
specific energy
τ
0
P
V
A
as the decisive quantity, and the mixture composition is then controlled by
kinetic curves n
i
=
/
.
•
In the field of glow discharge polymerization (film deposition) often the so-
called Yasuda parameter Y is applied which combines the specific energy
with the molecular weight of deposited monomers by
Y
n
i
(
τ
0
P
/
V
A
)
, where
W
is the invested electric energy [Ws],
F
the flow rate [sccm], and
M
the
molecular weight [g/mol] [11,12].
•
For various low-pressure nonthermal plasmas and ion beams, the etching
rates have been summarized successfully using a similarity presenta-
tion [13].
=
W
/(
F
·
M
)
On the other hand, the similarity principles of nonthermal plasmas, for example, of
the low-pressure positive column, are mostly restricted to pure electronic similarity
[14,15].
4.5.2 S
IMILARITY
P
RINCIPLES
A very general starting point of the similarity analysis is the formulation and prepara-
tion of all the equations, relevant to the problem by transformation to dimensionless
forms. The central equation for reactors in process engineering is the generalized
equation of transport (heat, particles, momentum) given by the following generalized
transport equation [16]
∂
∂
t
+
div
(
w
)
−
div
(
δ grad
)
+
f
−
G
=
0,
(4.14)
where
is the generalized transport quantity (mass, particle densities, energy, heat, etc.)
div
(
w
)
the transport by convection and diffusion
div
(
δ grad
)
the transport by conduction (e.g., of heat)
f
the transport by phase transitions ( is the energy transfer coefficient and
f
the transfer area/volume)
G
the sources (e.g., effective source terms of particles)
