cfd modeling of crankcase inertial oil separators - ansys · cfd modeling of crankcase inertial oil...

© 2011 ANSYS, Inc. September 6, 20111

CFD Modeling of Crankcase Inertial Oil Separators

Arun P. Janakiraman Anna Balazy Saru Dawar


Emission regulations are becoming stringent and will include blowby emissions. There is a need to vent gases from the crankcase and remove liquid oil and filter aerosol from the blowby gases before its either vented or send back to the turbo. Two technologies engineered at Cummins Filtration Inc. for this purpose are

Inertial Oil Separators Coalescing Oil Separators

Inertial Impactors are non serviceable parts which usually last the life of the engine.

Background


Impactor Nozzles

Impactor media

Aerosol accelerated by the flow

Impactor Nozzle

Some amount of flow which enters the media and particles it carries are filtered.

Porous fiber impaction zone

Background (contd.)Inertial Impactor designed and manufactured at CF Inc. Impactors accelerate the aerosol stream and makes it turn by 90 degrees at the impaction surface. Particles with enough inertia are captured as they cannot make the turn.

3D CAD model of a typical impactor section


Background (contd.) The purpose is to understand the performance of the inertial impactor using CFD. Parameters which constitute performance index are

Pressure drop across the device Particle Separation Efficiency ( Fractional and Gravimetric)

The flow field and dP can be easily predicted by CFD. Ansys Fluent used for this purpose. Complexity arises in particle separation efficiency prediction due to the media patch. Not all of the flow enters the media. Non uniform velocity distribution through the media . Aerosol transport predicted using Fluent discrete phase model. Filtration inside the media patch needs to be accounted for evaluating overall performance. Strong coupling between flow and particle trajectories.


Constituent EquationsReynolds Averaged Navier Stokes solved using realizable k-ε closure

model.

Continuity:

Momentum:

Turbulence Closure Equations

k:

ε:

Media patch represented by a porous zone with momentum sink term.

Darcy’s law used for the sink term:

Equation of motion for particles/parcels:


Mechanisms of single fiber theory particle capture considered are

Diffusion:Natanson (1957)

Stechkina and Fuchs (1966)

Interception:

Stenhouse A

Stenhouse B

Impaction:

3/23/1 PeKu32.2 DE

31 1

2211111ln12

Ku21

RRRRRR NNNNNE

22.0Stk77.0StkStk

23

3

IE

Constituent Equations (contd.)

13/23/1 Pe62.0PeKu9.2 DE

ln5.0Kn996.1ln5.075.02

1ln1Kn996.11211 1

F

RRFRRR

NNNNE

Overall Efficiency based on Perfectly mixed flow model taking into account fiber diameter distribution

Where m =1…3 represents various mechanisms of particle capture, NF is the number of fibers considered.


Computational DetailsOnly a single nozzle was considered for the analysis

Fluent Pressure based Segregated solver used. 2D Axisymmetric assumption for nozzle geometry. Flow assumed to be steady. Pressure Staggering option and 2nd order upwind schemes used for discretization. SIMPLE algorithm used for pressure velocity coupling. Under relaxation at Fluent default values. Mesh has around 75K cells. Production of k turned off inside the porous zone. Media porosity is constant.Droplet injection done from a rake surface

Fluent UDFs incorporating all the single fiber physics inside the porous zone hooked to the simulation. UDFs generalized to run on 2D,3D geometries serial and parallel Fluent.


Grid Independence and DPM parameters Study (4 Factor 2 level factorial DOE)

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tiona

l Effi

cien

cy %

√Stk#

Fine mesh 120537 cellsMedium mesh 29890 cellsCoarse mesh 8507 cells

76.6%

76.7%

76.8%

76.9%

77.0%

77.1%

77.2%

77.3%

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vim

etric

Effi

cien

cy %

No. of Rake Points

Effect of Number of Rake Injection Points


Grid Independence and DPM parameters Study (4 Factor 2 level factorial DOE)-contd.

76.5%

77.0%

77.5%

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78.5%

79.0%

79.5%

1.00E-11 1.00E-10 1.00E-09 1.00E-08 1.00E-07 1.00E-06 1.00E-05

Gra

vim

etric

Effi

cien

cy %

Tolerance

Effect of Tolerance on Gravimetric Efficiency

Chosen for subsequent studies

Main DPM parameter affecting the particle capture efficiency

Fluent default - 1e-5


Experimental Setup for measuring Pressure drop and Fractional efficiency

Aerosol generator

Part under test

Isokinetic Sampling

probes

Welas particle Counter

Upstream

dP

Welas particle Counter

Downstream

Absolute Filter

Flow meterPump


kg

0.5 micron diesel liquid aerosol.Amount injected = 1gThe below plot shows how the particle stream losses mass as it goes through the media based on single fiber theory

Media zone in CFD

Efficiency = Mass captured/Total Mass Injected

4" Thick MediaFiber Diameter = 4e-5 mPacking Density = 0.1713

Idealized Case to understand whether CFD has captured all the single fiber equations correctly.

Results and Discussion


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Effic

ienc

y

Particle Diameter (m)

Theory CFD Model

Equations have been captured correctly in the UDFs

Results and DiscussionCFD vs. Single Fiber Theory –Idealized case

Depth Filtration


Results and DiscussionNon Dimensionalized Velocity magnitude contours

Flow pathlines colored by non-dim velocity magnitude

Non uniform flow distribution through the media.

Strong coupling between the flow inside and outside the media.

Not all of the flow enters the media.

Flow makes an 90 degree turn as the media acts like a pseudo wall.


y = 1.1569x1.9903

R² = 0.9999

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Non

Dim

ensi

onal

ized

dP

Non Dimensionalized Flowrate

Non Dimensionalized dP versus flow.

Results and DiscussionContours of Non dimensionalized static pressure.

Typical 2nd order behavior in pressure drop of an inertial loss device captured here by CFDTypical rise in static pressure seen close to the

impaction surface


Aerosol trajectories colored by parcel mass variationTotal mass injected = 1 g- 200 aerosol streams

Mass per stream = 5e-3 gSize of Aerosol Injected = 0.5 micron

Aerosol streams losses a lot more mass here due to capture than through the rest of the media. This is due to the flow distribution and various mechanisms contributing differently based on the local flow conditions. Very strong coupling evidenced between flow and aerosol trajectories

kg


Not all the aerosol streams enter the media

The CFD model captures all the interaction effects as evidenced in this slide

Due to low Stk# the aerosol follows the flow for most part.


kg

Results and DiscussionAerosol trajectories colored by parcel mass variation

Total mass injected = 1 g- 200 aerosol streamsMass per stream = 5e-3 g

Size of Aerosol Injected = 0.8 micron

With increase in particle size and Stk# more particles penetrate the media and are filtered based on single fiber theory





kg

Aerosol filtration inside the media visually seen with the color change





kg

Particle size at a point were all the aerosol streams enter the media due to their inertia.





kg

Further penetration of the aerosol streams into the media as the particle size goes up.





kg

Some of the particle streams are completely captured inside the media patch





kg

Complete capture of all aerosol streams within the media patch. Criterion for capture depends upon the parcel mass.


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ficie

ncy

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√ Stk #

Q1 Q2

Fractional efficiency curve constructed based on simulation results at two different flow rates


Improvement in efficiency with increase in flowrate typical of inertial separation device captured here by the CFD model.

Q2>Q1


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dP/d

P max

V/Vmax

CFD Model Test

Results and DiscussionComparison with test data

Agreement in pressure drop between CFD and test data is very good.

Pressure Drop


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Test CFD Model


Very close agreement between CFD and test data for fractional efficiency at this flow rate. Quite surprising result considering the simplicity of the model.

Flow Rate = Q1Comparison with test data - Efficiency


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CFD Model Test


Higher efficiency predicted by CFD than test at this flowrate. Due to the high velocities seen through the nozzle it might be quite possible for aerosol re-entrainment into the air stream. This is not captured here by the CFD model.

Flow Rate = Q2

Comparison with test data - Efficiency


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Particle Size (microns)

ISO Ultrafine Aerosol DistributionRosin Rammler Distribution

Ultra Fine Aerosol Particle Size Distribution entered in Fluent using Rosin Rammler Distribution


In order to understand gravimetric efficiency performance ultra fine aerosol distribution used in lab tests was injected in Fluent.


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)

Flow Rate (m3/hr)

Test CFD


Overall good agreement with test data

Range of operation


The single fiber theory has been successfully incorporated into CFD

through the use of Fluent UDFs.

The model was tested against the single fiber theory and was found

to be spot on indicating that all the equations have been correctly

captured in the Fluent UDF.

Reasonably good agreement between CFD and test for Fractional

Efficiency.

Gravimetric efficiency prediction by the model is found to be

reasonably close to test data for impactors.

Model can be used with enough confidence for optimization studies.

Conclusions


AcknowledgementThe authors gratefully acknowledge the support of Cummins Filtration Inc. in this work. They would like to personally thank Bryan Steffen, and Shiming Feng for their assistance.


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