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Research Project
2014 - Strategic Project
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Publications
Self-organization in dc glow microdischarges in krypton: modelling and experiments
Publication . Zhu, W.; Niraula, P.; Almeida, P. G. C.; Benilov, M. S.; Santos, D. F. N.
Self-organized patterns of cathodic spots have been observed in microdischarges operated in
xenon, but not in other gases. However, modelling has indicated that it is, in principle, possible
to observe the patterns of spots in discharges operated in other gases provided that
experimental conditions, in particular pressure, are right. In this work, self-organized patterns
of cathodic spots are for the first time observed in dc glow microdischarges operated in a gas
other than xenon: krypton. The experiments have been guided by the modelling. According to
both the experiment and the modelling, patterns in krypton are similar to those found earlier in
xenon, however occur at higher pressures.
Energy harvesting from hydroelectric systems for remote sensors
Publication . Azevedo, Joaquim Amândio Rodrigues; Lopes, Jorge
Hydroelectric systems are well-known for large scale power generation. However, there are virtually no studies on energy harvesting with these systems to produce tens or hundreds of milliwatts. The goal of this work was to study which design parameters from large-scale systems can be applied to small-scale systems. Two types of hydro turbines were evaluated. The first one was a Pelton turbine which is suitable for high heads and low flow rates. The second one was a propeller turbine used for low heads and high flow rates. Several turbine geometries and nozzle diameters were tested for the Pelton system. For the propeller, a three-bladed turbine was tested for different heads and draft tubes. The mechanical power provided by these turbines was measured to evaluate the range of efficiencies of these systems. A small three-phase generator was developed for coupling with the turbines in order to evaluate the generated electric power. Selected turbines were used to test battery charging with hydroelectric systems and a comparison between several efficiencies of the systems was made.
Keywords
Stability of stationary solutions in the theory of cathode spots in arcs in vacuum and ambient gas
Publication . Benilov, M. S.; Cunha, M. D.; Hartmann, W.; Wenzel, N.
Stability of stationary spots on cathodes of arcs
in vacuum and ambient gas has been investigated by
means of simulation of development in time of
perturbations imposed over steady-state solutions. Two
cases of loading conditions have been considered, namely,
spots operating at fixed current (the case typical of
small-scale experiments) and spots operating at fixed
voltage (the case typical of high-power circuit breakers).
Results are reported on spots on large copper cathodes of
vacuum arcs and on spots on tungsten cathodes of
high-pressure argon arcs. It is shown, in particular, that if
the ballast resistance in a small-scale laboratory
experiment with a high-current arc is insufficient,
potential consequence may be a thermal explosion of a spot,
if the arc burns in vacuum, and massive melting of the
cathode surface, if the arc burns in ambient gas. This
conclusion conforms to trends observed in the experiment.
Numerical investigation of the stability of stationary solutions in the theory of cathode spots in arcs in vacuum and ambient gas
Publication . Benilov, M. S.; Cunha, M. D.; Hartmann, W.; Wenzel, N.
The stability of stationary spots on cathodes of arcs in vacuum and ambient gas is investigated
by means of the simulation of the temporal evolution of perturbations imposed over
steady-state solutions. Two cases of loading conditions are considered, namely, spots
operating at a fixed current (the case typical of small-scale experiments) and spots operating at
a fixed voltage (the case typical of high-power circuit breakers). Results are reported on spots
on large copper cathodes of vacuum arcs and on spots on tungsten cathodes of high-pressure
argon arcs. It is shown, in particular, that if the ballast resistance in small-scale laboratory
experiments with a high-current arc is insufficient, the potential consequence may be a thermal
explosion of a spot, if the arc burns in vacuum, and massive melting of the cathode surface, if
the arc burns in ambient gas. This conclusion conforms to trends observed in the experiment.
Analyzing spotless mode of current transfer to cathodes of metal-vapor arcs
Publication . Benilov, M. S.; Benilova, L. G.
The diffuse, or spotless, mode of current transfer
has been observed on cathodes of vacuum arcs under
conditions where the average cathode temperature was
high enough, about 2000 K. It has been known for many
years that none of the known mechanisms of current
transfer to cathodes of vacuum arcs and, in particular, of
the electron emission is capable of producing the current
densities of the order of 105
- 106
Am-2 deduced from the
experiment. A fresh attempt to clarify this question is
made in this work. Cathodes made of chromium are
considered, on which the most of the experiments have
been performed. It is shown that an account of the
difference between values of thermionic and photoelectric
work functions given in the reference literature allows one
to significantly reduce the deviation between the theory
and the experiment. Unfortunately, data on thermionic
work function available in the literature refer to the
cathode surface temperatures below 1400 K, which is
significantly smaller than measured temperatures of the
chromium cathodes of vacuum arcs operating in the
spotless mode. Therefore, further experimental data are
needed in order to clarify this effect.
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Funders
Funding agency
Fundação para a Ciência e a Tecnologia
Funding programme
6817 - DCRRNI ID
Funding Award Number
PEst-OE/MAT/UI0219/2014