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Analysis of the impact of flow characteristics on the separation efficiency and pressure drop of a cyclone-type oil separator


The effects of flow structure on the separation efficiency and pressure drop in a cyclone-type oil separator were investigated via Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES), and the results were compared with experiments. Compared with the RANS simulations, the LES results were more similar to the experimental data as they simulated the complex flow structure more realistically. Swirling flow with strong turbulent kinetic energy (TKE) at the top section hindered the flow of particles toward the separator wall. In addition, a decrease in tangential velocity along the wall at the bottom reduced the centrifugal force, resulting in a decreased separation efficiency as particles were able to flow directly toward the outlet. The LES also predicted the pressure drop slightly better than the RANS simulation did, due to increased pressure drop caused by collision of the flow with the helix and outlet tube, which led to the formation of vortical flow structures with strong TKE.

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C D :

Drag coefficient

d p :

Particle diameter

\({{\bar d}_p}\) :

Mean particle diameter

E s :

Separation efficiency

f i :

Body force

F D :

Drag force

F c :

Centrifugal force

\({{\bar F}_{c,inlet}}\) :

Area-averaged centrifugal force at inlet

k :

Turbulent kinetic energy

\({{\bar k}_<Subscript>let</Subscript>}\) :

Area-averaged turbulent kinetic energy at inlet


Large eddy simulation

m p :

Mass of the particle

p :

Mass flow rate of the particles

w,p :

Mass flow rate of the particle trapped to the wall

n :

Spread parameter of the particles

P local :

Local pressure in oil separator

P exit :

Exit pressure of oil separator

r :

Radial direction in cylindrical coordinate system

R :

Radius of oil separator


Reynolds-averaged Navier-Stokes

Re :

Relative Reynolds number


Reynolds stress transport


Sub-gird scale


Turbulent kinetic energy

Δt :

Time step

u :

Velocity of the main flow

u p :

Velocity of a particle

u p,t :

Tangential velocity of a particle

U :

Velocity vector


Velocity magnitude

Ū inlet∣:

Average velocity magnitude at inlet

U axial∣:

Axial velocity magnitude

U tangential∣:

Tangential velocity magnitude

y :


Y d :

Droplet mass fraction with diameter greater than dp

θ :

Azimuthal direction in cylindrical coordinate system

μ :

Dynamic viscosity of the main flow

ρ p :

Density of the particle


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This work has been supported by the UAV High Efficiency Turbine Research Center program of the Defense Acquisition Program Administration and Agency for Defense Development.

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Correspondence to Joon Ahn or Wontae Hwang.

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Wontae Hwang received his B.S. from Seoul National University (1997), and M.S. (1999) and Ph.D. (2005) from Stanford University. He worked as a Lead Engineer at GE Research (2008–2016) and is now an Associate Professor at Seoul National University. His research interests include multiphase flow, turbulence, and gas turbines.

Joon Ahn received his B.S. (1997), M.S. (1999), and Ph.D. (2003) from Seoul National University, Korea. He worked as a Senior Researcher at Korea Institute of Energy Research (KIER) (2006–2010) and is now a Professor at Kookmin University. His research interests include heat transfer, refrigeration and air conditioning, and combustion problems in energy systems.

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Seon, G., Ahn, J. & Hwang, W. Analysis of the impact of flow characteristics on the separation efficiency and pressure drop of a cyclone-type oil separator. J Mech Sci Technol 36, 273–283 (2022).

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  • Oil separator
  • Reynolds averaged Navier Stokes (RANS)
  • Large eddy simulation (LES)
  • Separation efficiency
  • Pressure drop