Computational Fluid Dynamics for Engineers: Preface

CFD simulation of flashing jet applied to area classification

Subsea is a term that refers to drilling and processing of gas and oil in underwater
locations. One example of a subsea technology is a wet gas compressor which is
used to compress fluids that consists of multiple phases. By compressing the wet
gas the recovery of unprocessed streams can be increased and the investment cost
reduced. The Norwegian company OneSubsea has designed and manufactured a
wet gas compressor, first of its kind, and is developing the next generation of the
compressor with assistance from the technical consultancy company AF. At AF’s
department for technical analysis in Gothenburg simulations of the compressor with
pure gas flow are performed. To compliment these simulations a separate project is
performed to evaluate the effects of a flow that is multiphase. Therefore the aim of
this project is to study the effect of different droplet sizes on a gas flow around a
compressor blade in a wet gas compressor.
Multiphase flow, consisting of natural gas and oil droplets, around one blade in the
first step of the wet gas compressor is considered. Computational fluid dynamic
simulations of one way coupled multiphase flow are solved using the conservation
equations of mass and momentum, Lagrangian particle tracking and the k −! SST
turbulence model. The range of the droplet size and volume fraction evaluated are
1-200 μm and 1-2%, respectively.
Several different studies were performed. The results are characterised by flow properties
outputted just after the blade, at the start of the next blade row, and with
visualisations of the particle tracks around the blade. The main study, the Base
case, consisted of 22 different case studies where the droplet size was held constant
for each case, but varied within the size range between the cases. A coefficient of
restitution (COR) was used to model the droplet wall interaction and the results
showed that the droplets have an effect on the outflow from the first compressor
step. The droplets decrease the average velocity angle at the axial clearance for all
droplet sizes. The decrease is low, at a relatively constant value, for droplet sizes
up to around 80 μm. For droplets larger than 80 μm, velocity angle decreases with
increasing droplet size. By studying the particle tracks around the blade the droplet
flow could be divided into three characteristic regions, according to the importance
of wall interaction and effect of gas flow on the droplet.
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After analysing the results from the Base case the importance of wall interaction
was studied further. Simulations showed that the majority of the droplets are colliding
with the wall. A sensitivity study for the COR was performed which showed
that the droplet flow is independent of COR for droplet sizes up to 50 μm, almost
independent up to 100 μm, and strongly dependent for the rest of the size range.
A case study where the droplets were trapped at the wall was performed, but the
reliability of these results are questionable since the data is based on a small fraction
of droplets that pass the blade. For the final wall interaction study a liquid wall film
at the blade was modelled. According to the theory this should be the most realistic
way to model a droplet wall interaction. Due to lack of time this case study could
not be fully completed and only an idea of the result is presented. The result shows
that a thin film will cover the blade which has an effect on the particle tracks.
The conclusion from this project is that the droplets will effect the flow around
the blade by decreasing the average velocity angle for the flow entering the next
blade row; the magnitude of the effect is increasing with increased droplet size. The
droplet wall interaction is important for the particle tracks, thus it is recommended
to further evaluate this aspect.

/pdf/effect-of-different-oil-droplet-sizes-in-a-flow-of-natural-3panvf2c07.pdf

Effect of Different Oil Droplet Sizes in a Flow of Natural Gas around a Compressor Blade. Numerical Simulations of Multiphase Flow using Computational Fluid Dynamics.

Signý Tryggvadóttir

Papers

Effect of Different Oil Droplet Sizes in a Flow of Natural Gas around a Compressor Blade. Numerical Simulations of Multiphase Flow using Computational Fluid Dynamics.