Characterisation and validation of a pulsating jet erosion
test
Abstract: This study focuses on the further development of a new method of liquid
erosion testing by means of a pulsating jet erosion test (PJET). The method
was validated in comparison to existing test procedures.
Parameters regarding velocity, length and diameter of the jet were evaluated
by optical observation through a real-time triggered camera with a high-speed
shutter. For the method of the optical parameterisation, good conditions for
correct framing of the water jets, were achieved by high illumination and
short shutter times.
The influence on the jet’s front shape and rounding was included in the
parameterisation and investigated in terms of air resistance, diameter and
shape of nozzles, the propulsion system of the water, the pump, the hose, and
the rotating disc. The jet front plays an important role for the simulation
of raindrops by means of water jets.
A preliminary study on the measurement of the water hammer pressure of the
water jet segments was carried out with a piezo-sensor. The results showed a
rapid rise of the pressure, corresponding to the impact pressure and
duration. The results presented 100 times lower values when compared to the
theoretical water hammer pressure, indicating a lack of water hammer pressure
at the moment of impact. However, it is more likely that the piezo-sensor
was unable to capture the fast appearing and disappearing water hammer
pressure, thus measuring only the transmitted impulse.
The optical observation showed the presence of a water-film on the samples.
The theory states a diminishment of erosion damage due to such film, which
was verified by the material test. The problem was solved by installing an
air nozzle to blow-off the water-film from the samples with compressed air.
The first step in the validation of the test procedure was taken by comparing
the apparent failure mechanisms of the PJET with those obtained with the
Multiple Impact Jet Apparatus (MIJA) of the Cavendish Laboratory, University
of Cambridge. A comparability of the damage mechanisms was possible to
observe regarding both non-destructive surface deformations and destructive
damage to the coatings. The second step in the validation was the actual
material tests of coatings, which were used to evaluate the specific
materials. The coatings examined were polysiloxane coatings, applied to an
aluminium substrate, with a CAA surface treatment, and an epoxy primer as
adhesion promoter. The tests were also the base for the verification of the
PJET procedure. The material evaluation was correlated to parameters of the
samples such as hardness, coating thickness and erosion resistance.
From the results and the validation of the material testing it can be
concluded that the PJET is a working test procedure for liquid impact. The
erosion resistance was evaluated with both the PJET and the MIJA by two
independent methods. The results of the evaluations presented a practically
complete coherence in question of the mutual erosion resistance between the
samples pertaining to each evaluation method. It was seen that the harder the
sample, the better the erosion resistance, at the same time as the higher the
cross-linking, the better the erosion resistance, although a higher
cross-linking decreases the hardness.
As conclusion it can be stated that the PJET complies with existing test
standards. The erosion damage obtained is comparable within proper test
series and with other erosion test methods and finally that the validation of
PJET test procedure was successful.
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