Heat And Mass Transfer in Reactor with Confined Plasma Jet in
Nanopowder Production Processes
A.V. Samokhin, A.G. Astashov, S.N. Polyakov, Yu.V. Tsvetkov, E.V. Troitskaya, N.V. Alekseev
A.A. Baikov Institute of Metallurgy and Materials Science RAS, Moscow, Russia
Abstract: A computational 3D CFD model of a plasma reactor based on the software package ANSYS /
CFX is developed. The calculations show that sudden expansion of plasma flow into the reactor with
water cooled wall provides necessary quenching of high temperature flow for the formation of
nanoparticles by condensation from the gas phase. Flow in the reactor with confined plasma jet is
characterized by the presence of stationary vortices in different temperature zones. The distribution of
heat flux on the reactor wall was experimentally investigated using the partitioned reactor. The
formation of a layer of copper nanoparticles was studied experimentally during recondensation of copper
powder in a nitrogen plasma and a layer of tungsten nanoparticles during reduction of WO3 powder in a
hydrogen-nitrogen plasma in the reactor of 20 kW in power. The distribution of local mass flux density
of nanoparticles along the reactor has a maximum of joining region of high-temperature flow to the
reactor wall reactor as well as the distribution of local energy flux density.
Keywords: plasma jet, nanopowder, heat flux, mass flux, layer.
1. INTRODUCTION
This research covers heat and mass transfer which
takes place when nanopowders are synthesized in
the thermal plasma jet in the reactor with a
confined plasma jet. The plasma jet generated in
the DC arc plasma torch flows into the cylindrical
reactor with water-cooled walls, and the ratio of
plasma nozzle and reactor diameters above 10 [1].
The chemical syntheses leading to generation of
condensed products in the described reactor make
possible to generate the target product in the form
of nanopowders with the size of particles over
100 nm. Nanoparticles are generated in plasma
reactors as the final products are condensed from
the gas phase. As the plasma jet flows, after the
channel suddenly expands, into the reactor
volume, the high temperature jet quickly cools
down. As the jet cools down lower than the
condensation temperature, nanosized particles are
formed as the result of condensation from the gas
phase and continue to grow. As particles evolve
from the generation point to exit from the reactor
or transfer on its surface, their dispersion, phase,
and chemical structures are changed after
particles collide and interact with the gas phase.
These processes are controlled by changing
synthesis parameters - plasma jet composition,
velocity and enthalpy, agent concentration, and
reactor size.
As nanoparticles are generated in the reactor, they
inevitably deposit on the surface limiting the
high-temperature gas-dispersion flux. Direct
depositing of nanoparticles on the water-cooled
surface prevent their sintering, but unlimited
growth of the nanoparticles layer thickness
depositing on the reactor surface brings about an
increase in the temperature of this layer and
subsequently nanoparticles degradation as the
result of sintering, and possible changes in their
phase and chemical composition.
In order to obtain the final product of plasma
synthesis in the form of nanopowder in which
nanoparticles preserve the properties determined
by conditions of their generation in the gas flux,
any physical and chemical conversions in the
deposited particle layer should be excluded or
minimized. This is possible if the layer thickness
does not exceed a specific value at which the
temperature at the layer border does not exceed
the specific threshold values of indicative
physical and chemical processes– chemical and
phase-related conversions of nanoparticles, and
sintering of nanoparticles in this layer.
2. CFD MODELING
A 3D computational model was designed to
simulate transfer processes in the plasmachemical reactor to determine rate and
temperature profiles in the reactor volume based
on computational equation system solution
including the Reynolds-averaged Navier-Stokes
equations and thermal energy transfer equation. In
order to close the equation system, additional
equations for kinetic turbulence energy and
dissipation rate (two-parameter k-ε turbulence
model) were used. The properties of hightemperature gas were set in the local
thermodynamic equilibrium approximation. The
solution for the related hydrodynamic and heat
problem was obtained with ANSYS/CFX 12.1
software system.
mm in diameter and 600 mm in length for plasma
fluxes of nitrogen and hydrogen and nitrogen
mixture (22 vol.% Н2), for different values of
thermal plasma enthalpy and flow rate, and
plasma nozzle diameter. The experiments were
carried out for the following range of parameter
changes: plasma-forming gas flow rate: 0.5 – 2.0
m3STP/h, plasma jet enthalpy at the reactor
entrance: 6.5 – 8 kWh/m3STP, plasma torch
effective power: 6.6 – 12.3 kW. It was found that
the heat flux density distribution along the reactor
is not monotonous, and its maximum lies in the
area where the high-temperature flow connects
with the reactor wall (Figure 2). The heat flux
density value is determined by the plasma torch
effective power, and the maximal value exceeds
by 2.5 – 3 times the heat flux density in the first
and final reactor sections, and ranges from 25
kW/m2 to 45 kW/m2. The heat flux density
distribution related to the maximum value stays
virtually unchanged for all values of input heat
power, gas flow rate, and initial gas enthalpy
considered by experiments. Hydrogen contained
in nitrogen brought about virtually no changes of
measured values of heat fluxes in the reactor.
After the plasma nozzle decreased from 10 mm to
6 mm, the area where the high-temperature flux
Figure 1. Calculated values of the temperature distribution and
flux lines in the plasma reactor.
The rate profile computations suggest that the
reactor flux is quite complicated and has
stationary vortexes in various temperature zones
(Fig. 1). Such vortexes are conditioned with
sudden expansion of the plasma flow while
enteringd into the reactor, and localized gas outlet
from the reactor. The nanoparticles involved into
vortex flow areas will stay longer in respective
temperature zones which may considerably
change their properties and in particular increase
dimensions as the result of growth by coagulation
3.
EXPERIMENTAL
DISCUSSION
RESULTS
AND
This research covers the heat flux density
distribution on the surface of reactor sections 200
Figure 2. Heat flux density distribution to the wall along the
reactor with various effective power of plasma torch and plasmaforming gas flow rate. 1 — 12.3 kW, 1.7 m3/h; 2 — 10.4 kW, 1.6
m3/h; 3 — 8.8 kW, 1.2 m3/h; 4 — 6.7 kW, 0.84 m3/h.
connects with the reactor wall and consequently
the heat flux maximum moved away from the
reactor entrance. The experiments suggest that the
reactor with confined plasma jet is characterized
with considerable unevenness in distribution of
the heat flux on its surface. In the area of the
maximal heat flux, the processes of nanoparticles
degradation forming the deposited layer are
maximal.
The research also covers generation of the
nanosized copper particle layer as the copper
powder re-condenses in the nitrogen plasma. The
experiments were conducted for the following
range of parameter changes: plasma-forming gas
flow rate 1.3 – 1.6 m3STP/h, plasma flux enthalpy
at the reactor entrance: 6.0 – 7.1 kWh/nm3,
plasma torch effective power: 8 – 11 kW, copper
powder flow rate 0.7 – 3 g/min. It was found that
the heat flux density distribution along the reactor
just as in the above-mentioned case with the flow
without nanoparticles is not monotonous, and its
maximum lies in the area where the hightemperature flow connects with the reactor wall.
As the thickness of deposited nanoparticles layer
grows, the thermal resistance of this layer
increases to reduce to a certain degree the heat
flux to the reactor wall.
Distribution of the deposited nanoparticles flux
density along the reactor just as the energy flux
density distribution is not monotonous, and its
Figure 3. Distribution of Cu nano-particle mass flux density to the
wall along the reactor with various process durations.
1 — 5 min, 2 — 20 min.
maximum lies in the area where the hightemperature flux connects with the reactor wall
(Fig. 3). The longer the process is, and the greater
the thickness of the deposited nano-particle layer
is, the less is the mass flux of nano-particles on
the wall. The mass flux density value is
determined by the mass concentration of the
condensed phase in the gas-nanoparticles flow,
and the maximal value exceeds by 2 – 3 times the
mass flux density in the first and final reactor
sections.
Figure 4. Distribution of Cu nano-particle specific surface along
the reactor with various process duration.
1 — 20 min, 2 — 35 min, 3 — 80 min
The fact that the nanoparticles mass flux density
maximum lies near the point where the gasdispersed flux connects, can be explained by
maximal values of turbulent pulsations in this
area which bring about an increase in the
turbulent transfer of particles to the reactor wall.
Depending on plasma process parameters, the
average size of produced copper nanoparticles
ranged from 35 to 120 nm, and the local mass
flux density for copper nanoparticles deposited on
the reactor surface came to 3 – 8.5 g/m2 min. The
average size of nanoparticles in layers depositing
on various reactor sections increased as the layer
thickness grew which is conditioned by an
increase in the layer temperature and sintering of
nanoparticles (Fig. 4).
The research also covers generation of the
tungsten nanoparticles layer obtained by the
reduction of tungsten oxide powder WO3 in
hydrogen-nitrogen plasma (22 vol.% Н2). The
experiments were conducted for the following
range of parameter changes: plasma-forming gas
flow rate 1.5 – 1.6 m3STP/h, plasma enthalpy at
the reactor entrance: 5.4 – 5.8 kWh/nm3, plasma
torch effective power: 8 – 10 kW, WO3 powder
flow rate 1.5 g/min. The average size of obtained
tungsten nanoparticles ranged from 40 to 45 nm.
It was found that the heat flux density and mass
flux distribution to the wall along the reactor
corresponds to that obtained during copper recondensation experiments. The local mass flux
density for tungsten nanoparticles depositing on
the reactor surface ranged from 2.1 to 5.1 g/m2
min. In contrast to the experiments with copper
the average size of particles was unchanged in the
layers depositing on various reactor sections. As
the layer thickness grew, no increase in the
average size of particles was noted. This is
conditioned by a considerable difference in
melting temperatures and consequently copper
and tungsten sintering temperatures. As it was
found during tungsten nanopowder heat treatment
experiments, nanoparticles grow at temperatures
above 600 С. With tungsten nanoparticles layer
thicknesses obtained during experiments, this
temperature was not reached, so there was no
upsizing registered with nanoparticles.
4. CONCLUSIONS
Distribution of the heat and nanoparticles mass
flux density to the reactor wall is investigated in
the reactor with confined plasma jet.
It was found that the density distribution
maximum for these fluxes along the reactor lies in
the area where the high-temperature flow
connects with the reactor wall. The maximal
value exceeds by 2.5 – 3 times the flux densities
in the first and final reactor sections.
As the deposited nanoparticles layer thickness
grows, the heat and mass fluxes to the reactor
wall decrease.
In case of nanoparticles with a relatively low
melting temperature, the layer thickness and
location of the deposit area has considerable
effect on the average nanoparticles size.
This research was conducted with support of the
Russian Fundamental Research Foundation
(Grants 10-03-00462-a and 11-08-00516-а).
5. REFERENCES
1.
RF patent No. 2311225, 2007.