pressure drop evaluation in a pilot plant hydrocyclone drop evaluation in a pilot plant hydrocyclone...

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Pressure Drop Evaluation in a Pilot Plant HydrocyclonePressure Drop Evaluation in a Pilot Plant HydrocycloneFabio Kasper, M.Sc.Emilio Paladino, D.Sc.Marcus Reis, M.Sc.ESSS

Carlos A. Capela Moraes, D.Sc.Dárley C. Melo, M.Sc.Petrobras Research Center


Outline

• Introduction• Objectives• Physical Model• Numerical Model• Computational Details• Results• Discussions• Main Conclusions• Future Work


Introduction

• Hydrocyclones– Extensively used in the oil

industry to separate oil-water mixtures

• High efficiency• High flow rates• Compact equipment

– Challenging CFD test case• High Re number• Known turbulence

modeling issuesCourtesy of PETROBRAS S.A.


Objectives

• Compare the numerical results with field data obtained in a pilot offshore plant– PDC (Pre-de-oiler cyclone) Kvaerner

hydrocyclone

• Evaluate the overall pressure drop given by different modeling approaches

Pilot plant facility – Courtesy of PETROBRAS S.A.

PDC Kvaerner hydrocyclone


Objectives

• Understand the flow pattern – Analysis of velocity profiles and its

influence on the pressure drop and efficiency

• Develop a methodology to perform parametric analysis– Model needs to be robust and fast– Help at pre-design stages


Physical Model

• PDC Kvaerner hydrocyclone• Geometric parameters

– Dcylinder = 70 mm– Total height ≈ 1.8 m

• Operating Conditions• Fluid: Water

– Inlet mass flow rate• 93.1 kg / min

– Overflow split• 41%

– Re ≈ 2.105

Inlets

Underflow

Overflow


Numerical Model

• Numeric details– Strong gradients

• Numerical dissipation– Highly swirling flow

• Physical instabilities• Transient simulations

– General case: • Multiphase with different oil concentrations• On this study we will focus on the single phase

(water) problem


Numerical Model

• Meshing– Hexa elements– Use of 2 GGI’s to save on the total

number of nodes• Overflow and Underflow regions• Pressure tap points

– Grids built in ANSYS ICEM HEXA

• Grid 1: 1.3mi nodes• Grid 2: 2.5mi nodes• Grid 3: 4.2mi nodes• Maximum aspect ratio < 160 (axial

refinement needed)


Numerical Model

• Grid 1 details

Overflow region GGI

Underflow region GGIPDC body


Numerical Model

• All runs were carried out with the ANSYS CFX-10 solver

• Boundary conditions– 1 inlet and 2 outlets

• Specified mass flow boundaries– Smooth walls

• Modeling approach– “Laminar”– RSTM – SSG– LES Smagorinsky (CS = 0.1)


Numerical Model

• Numerical details– 2nd order transient scheme– Convergence criteria

• 5.10-5 RMS within each time step– Time step size

• Adaptive time step based on the maximum CFL number

• Usually between 2-4 internal loops required– Higher order advection schemes

• All runs with β=1, except LES where CDS was used

– Transient statistics• Time averaged variables were calculated after

flow was “established”


Computational Details

• Performed analysis– CPU: Intel Xeon 3.20 GHz– Parallel runs with MPI

20-30121311“Laminar”< 1202115LES

10-20455SSGCPU cost (Days)

16108Number of CPU’s CFL

range *

Grid 3Grid 2Grid 1Turbulence modeling approach

* set to converge within 2-4 loops per time step


Results

• Pressure drop comparison

Targets


Results

• Grid sensitivity analysis

0.120m


Results

• Tangential velocity profiles on Grid 3

0.160m0.150m0.140m0.130m


Results

• Tangential velocity profiles on Grid 3

0.120m0.110m0.100m0.090m


Results

• Axial velocity profiles on Grid 3

0.160m0.150m0.140m0.130m


Results

• Axial velocity profiles on Grid 3

0.120m0.110m0.100m0.090m


Results

• Eddy viscosity ratio comparison (µt / µ)More than 100 times

bigger

SSG LES


Results

• Turbulence model smearing effect– Starting from a “Laminar” result as initial guess


Results

• Tangential velocity on Grid 2 (“Laminar”)


Results

• Pressure on Grid 2 (“Laminar”)


Results

• Axial velocity on Grid 2 (“Laminar”)


Discussions

• Runs were long enough to get statistical established flow conditions– *LES runs were likely stopped prematurely

• Discrepancies still found for the underflow pressure drops even in the fine mesh (Grid 3)– Apparently because of not enough

axial mesh refinement• Grid independent solution still not reached in

the LES and Laminar models– Might be already reached for the SSG model


Main Conclusions

• Turbulence modeling for this specific Hi-Re hydrocyclone– RSTM-SSG seems to be too diffusive

• Smears the velocity profiles under predicting the experimental pressure drop

– LES is too expensive (high Re number)• Fine mesh resolution requirement• CDS scheme used• Requires very low time steps

– CFL < 1


Main Conclusions

• The “Laminar” approach was the one who predicted best results whemcompared to operational conditions– “Conceptually” inconsistent – It seems that numerical diffusion stabilizes

the solution preventing the solver to diverge

– Cheaper when compared to the other “turbulent” approaches


Future Work

• Research is still going on– Search for some systematic and error and

sensibility (e.g. BC, fluid properties) analysis is now being done.

THANK YOU !

pressure drop evaluation in a pilot plant hydrocyclone drop evaluation in a pilot plant hydrocyclone...

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