# 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.

[Examensarbete på avancerad nivå]

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 ÅF. At ÅF’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. v 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.

**Nyckelord: **CFD, CFX, Fluent, k − ! SST, Multiphase, gas-liquid flow, Lagrangian Particle Tracking, Coefficient of restitution, Wall film, one way coupling, spherical droplets

Publikationen registrerades 2015-07-03. Den ändrades senast 2015-07-06

CPL ID: 219400

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