UWE Bristol Engineering showcase 2015

Thomas Gabriel
BEng Mechanical Engineering
Project Supervisor
Mike Ackerman
To Assess and Compare Mechanical Presses and Hydraulic Presses
Introduction
Aquaponics is a relatively new farming method
that utilises nutrient rich fish waste water to fuel
the growth of crops. The nutrient fish water is
pumped up from a fish tank to a header tank full
of a porous medium that acts as an anchorage
point for the roots of the crops on the surface
above. The nutrients within the waste water are
absorbed by the nitrifying bacteria on the base of
the roots, resulting in the cleansing of the waste
water and the uptake of nutrients into the crop
roots.
Currently the construction of a functioning autosiphon
involves a trial and error method so that a
30 minute siphon trigger time is achieved. This
process can be time and labour intensive, reducing
the accessibility of Aquaponics to new,
inexperienced practitioners.
As a result, it would be beneficial if a
mathematical model could be constructed that
could take system input parameters, such as
geometry of the grow bed, volumetric inlet of fish
wastewater and maximum root depth and
produce the required geometry for a specific
siphon design.
The first step to the construction of such a model
would be to determine the pressure drop of a
liquid-gas fluid flow within a small siphon driven
drainage system.
Aims of invesitgation
The aims of the investigation are as follows:
-Construct a Mathematical Model that accurately
predicts the total pressure drop experienced by
the an auto-priming siphon system.
-Design and Test a prototype of the chosen autopriming
siphon.
-Predict the Flow Regimes present throughout all
stages of the siphon cycle and at what point they
move from one regime to another.
Test Setup
Pressure (Pa)
1800
1600
1400
1200
1000
800
600
400
200
0
Results
The Pressure drop was found to be 1400 Pa, as
shown in the graphs below.
The Mathematical Model predcited a drop of 4800
Pa. This was due to the unmodelled error of the
two stream interactions as depicted in the figure
below
Schematic Highlighting the Influence of Annular Layer Thickness on Effective Gas
Outlet Diameter
Pressure Drop
0 100 200 300 400 500 600
Time (s)
Pressure Drop
Mark 1
Mark 2
Mark 3
Mark 4
Mark 5
Mark 6
Full Flow Begins
Full Flow Terminates
Project Conclusion
As it currently stands, the mathematical model using
the Brill and Beggs method does not accurately
predict the test geometry. The largest source of the
inaccuracy has been hypothesised to occur due to the
unanticipated effects of the second inlet stream on
the behaviour of the EZ-T siphon system as a whole.
The addition of the second stream has been
hypothesised to lower the pressure gradient between
the inlet of pipe one and the inlet of pipe three,
resulting in a reduced total pressure drop.
This hypothesis was reached after an initial analysis
of the mathematical model produced erroneous
results due to an incorrect gas fraction reading
generated by the balloon testing method described in
section. The balloon method returned a value of
volumetric flow rate for the gaseous phase of
0.944x10E-03 m3/s. This equated to a gas content of
42.7% within the flow mixture. The error in the gas
fraction measurement was found after the
construction of the transparent PVC prototype.
During the documenting of the flow regimes within
the transparent PVC model it was observed that there
were no bubbles in the horizontal or vertical outlet
pipes. As a result it was hypothesised that the gas
fraction measurement had been erroneous. This led
to running the mathematical model again with a nonexistent
gas fraction. This lead to a pressure drop that
was further away from the values tested than the
initial result using the erroneous gas fraction.