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AIR FLOW AND CHARGE MOTION STUDY OF ENGINE INTAKE PORT

ABSTRACT

Design evaluation of engine port using virtual flow bench “CFD” is significantly improved by identifying best practices which reduces turn around time for product realization. Compressible steady state CFD calculation was performed to emulate conventional test rig “Flow Bench” with close attention to three dimensional flow field including design-imposed pressure losses. The calculation was carried out at discrete valve lifts to obtain air flow rate and rotational speed for predefined constant pressure differential between the atmosphere and the system. CFD and experimental results were compared for its accuracy and to develop a standard methodology for future iterations. Both flow coefficient and swirl ratio calculated through CFD show good agreement with experiments.

Keywords: Flow Bench, CFD, Flow Coefficient, Swirl Ratio, UDF. INTRODUCTION

One of the main problems encountered in the design investigation of inlet port using steady state

test rig, “Flow Bench” is that the time it took for realizing the final design. The cost and time involved in building a new design renders the designers to evaluate more and more designs. With the reduced lead time for design cycle, this would lead to compromised design.

Also, flowbench testing does not provide a very efficient path to the final design because the designers do not have an insight on the details of recirculation areas, turbulence and design-imposed pressure losses.

CFD (Virtual Flow Bench), an analysis tool used to improve the port design by simulating the flow in alternative port designs. CFD simulation provides fluid velocity and pressure throughout the solution domain with complex geometries and boundary conditions. With this efficient and effective tool, designers can evaluate the effect of various design modifications and boundary conditions on the alternative port designs. This reduces the amount of experimentation necessary to develop a new product, which ultimately reduces cost and time.

In the current practice it takes considerable time to prototype the port configuration and run a test that provides no information on internal flow patterns.

Vinodh Kumar B Larsen and Toubro Limited, IES

Sivagaminathan N Larsen and Toubro Limited, IES

Gopalakrishnan N Larsen and Toubro Limited, IES

Scott Morton Mercury Marine

Paul Radavich Mercury Marine

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CFD simulation makes it possible to analyze a new design in less time and provide complete information on flow velocity and pressure throughout the model.

CFD analysis was carried out to aid the design of effective air intake system. An extensive study was conducted to convert the physical model to comparable computational model in order to reduce the modeling/numerical error with fewer numbers of iterations. WORK FLOW CHART

The work flow chart used for both flow bench measurement and CFD calculation is shown in the Fig. 1.

Start

Head Preparation

Clear Tube Preparation

Paddle Wheel Preparation

Fixture Preparation

Setup & Measurement

Data Acquisition

Flow Path Extraction

Geometry Cleanup

Meshing

Problem Setup/ Simulation

Post Processing Angular Momentum Flux Calculation

Correlation Study

Documentation

End

Flow Bench CFD

Clear Tube Preparation

Fixture Preparation

Setup & Measurement

Data Acquisition

Flow Coefficient Swirl Ratio

Fig. 1 Work Flow Chart

CORRELATION STUDY MODEL Flow bench

Figure. 2 shows the actual flow bench set up used to perform the steady state testing measurement

for flow coefficient. Calibrated dial gauge is used to adjust the valve lift. Clay bell mouth is used to reduce the entrance losses. Compressor unit is used to provide sufficient vacuum (suction pressure) inside the cylinder. Pressure sensor is used to monitor both the atmospheric and system pressure.

There is a compensating tank with pressure sensor for system pressure measurement at the bottom of the clear tube. This tank is used to recover the dynamic head and thereby maintaining the system pressure equal to stagnation pressure. Temperature sensor is also present to record the atmospheric temperature. Air flow sensor is used to measure the flow rate across the system.

Fig. 2 Experimental set up for flow coefficient measurement

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Fig. 3 Experimental set up for swirl measurement

Figure. 3 shows the paddle wheel set up for swirl ratio measurement. Clear tube/cylinder contains the paddle wheel. Speed sensor is used to measure the paddle wheel speed which in turn is used to calculate the circumferential velocity. Computational model Air flow model

The model consists of intake port, valve stem, valve, valve seat, spark plug, exhaust valve, combustion chamber and cylinder. Hemispherical inlet with bell mouth entrance is created to model the atmospheric condition without entrance loss and the cylinder bottom is closed to define enclosed volume.

Figure.4 shows the computational model used

for the CFD calculation.

Charge motion model

Unlike flow bench (where there is a compensating tank underneath the cylinder bottom which acts as outlet), cylinder bottom is used as outlet for the CFD calculation. 2D interior plane is created to replicate the paddle wheel in flow bench for swirl ratio calculation. Fig. 5 shows the location of interior plane used to extract angular momentum flux in CFD.

Fig. 4 Computational model of intake port

Fig. 5 Computational model of intake port with swirl

monitoring plane

Cylinder

Combustion chamber

Bell mouth

Hemispherical inlet

Swirl monitoring plane

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Computational grid Computational grid used for the calculation is

shown in Fig. 6. Unstructured tetrahedral elements with two layers of inflation are used to discretize the flow domain. Fig. 7 shows the close view of inflation created along the walls of intake valve and throat region. Note here that the mesh is designed to capture the jet coming through the throat.

Fig. 6 Computational grid of intake port

Fig. 7 Close view of inflation on valve and throat CALCULATION PARAMETERS Flow coefficient

The flow coefficient ( kα ) is defined as the ratio of the actual or measured mass flow rate at

standard condition and the theoretical mass flow rate [9]. Cylinder bore diameter is used as characteristic length for calculating the theoretical mass flow rate.

theor

stdk m

m=α

The actual or measured mass flow rate at standard condition is calculated using the following expression.

std

stdstd TR

PVm×

×=

The theoretical mass flow rate is calculated using the following expression.

sstheor CAm ××= ρ.

Piston area is calculated from cylinder bore diameter

A 2

4 cylDπ=

Density is calculated for isentropic flow conditions

k

stds P

PTR

P1

1

21⎥⎦

⎤⎢⎣

⎡×

=ρ

Flow velocity is calculated for isentropic flow

PPP

mNP

PP

TRk

kCk

k

stds

∆−=

=

⎥⎥⎥

⎦

⎤

⎢⎢⎢

⎣

⎡

⎟⎟⎠

⎞⎜⎜⎝

⎛−×××

−×

=

−

12

21

1

1

2

/101325

11

2

Gauge static pressure P2 measured in the compensating tank can be considered as stagnation pressure for ∆P calculation as the dynamic component is fully recovered.

(1)

(2)

(3)

(4)

(5)

(6)

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Swirl ratio Swirl ratio is defined as the ratio of

circumferential air speed in the cylinder to the axial speed of the air flow in the cylinder [9].

)(VelocityAxial

)(VelocityntialCircumfereRatioSwirl

A

u

C

C=

Circumferential air speed in the cylinder is calculated using,

nDC MFLu ××=π Axial speed of the air flow in the cylinder is calculated using,

42 π×

=cyl

realA D

VC

Theoretical volumetric flow rate across the system is calculated using orifice meter equation as,

KT

mNP

PTTPVVV

std

std

ambstd

ambstd

real

stdreal

7.288

/100000 2

=

=

××

××= =ρρ

CALCULATION METHOD Experimental measurement

Flow bench is used for cylinder head steady

flow measurements. Measurements are conducted at a constant pressure difference between tube and atmosphere. The test pressure is measured and adjusted automatically by the flow bench controller, which also measures air flow. Ambient pressure and ambient temperature are being recorded. Swirl ratio is obtained by evaluating the rotational speed of the paddle wheel.

The table below shows discharge coefficient and swirl ratio obtained from flow bench.

Lift Flow Coefficient (/) Swirl Ratio (/)

Low 0.2484 0.900 Medium 0.4506 0.775

High 0.5517 0.779 Steady state CFD calculation

Steady state air flow calculations are

performed for three different intake valve lifts viz. low lift, medium lift and high lift to investigate the flow features. Sufficient mesh refinement has been provided near the throat area because the flow velocity changes rapidly in this region and capturing the gradients is key for an accurate simulation. Governing equation The calculations are performed by solving compressible Navier-Stokes equation for mass, momentum and energy. Also two equation turbulence model, Realizable εκ − is used to capture the flows involving rotation, boundary layer under strong adverse pressure gradients, separation and re-circulation. To accurately represent the flow in the near wall region, Non-equilibrium wall function is used to predict the wall bounded turbulent flows since walls are the main source of mean vorticity and turbulence. Solver

Pressure based segregated solver is used to solve the transport equation for mass, momentum and energy. Pressure field is obtained using SIMPLE algorithm for Pressure-Velocity coupling. Spatial discretization Control volume technique is used as numerical scheme for solving mass, momentum and energy. Second order upwind scheme is used as discretization scheme for convective terms of each governing equation.

(7)

(8)

(9)

(10)

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Boundary condition Stagnation pressure at the hemispherical inlet and static pressure at the cylinder bottom outlet (which is same as test pressure) is given as boundary condition and the walls are assumed as adiabatic and no-slip. The table below shows flow coefficient and swirl ratio obtained from steady state CFD calculation.

Lift Flow Coefficient (/) Swirl Ratio (/)

Low 0.2431 1.360 Medium 0.4575 0.903

High 0.6025 0.716 Swirl ratio UDF

A User Defined Function (UDF) is developed to calculate the angular momentum flux at the swirl monitoring plane shown in Fig. 5. The various inputs like mass flux, cell centroid from the swirl axis and also velocity magnitude at each cell face in the monitoring plane is obtained from the solver by the UDF at the end of the calculation. The collected inputs were used to calculate angular velocity/circumferential velocity and axial velocity which are needed to calculate swirl ratio.

RESULTS AND DISCUSSION

The results obtained from CFD analysis are

correlated with experimentally measured values from flow bench. Mass flow rate and swirl ratio were compared between CFD and flow bench. General flow field

Much of the flow coming from the inlet goes directly into the chamber but some hugs the outer diameter of the port around the valve stem. Especially, the flow coming from the lower part of the port (flow from inner diameter) exits in the same direction whereas the flow from upper part (flow from outer diameter) of the port exits in the opposite direction.

V (m/s)

V (m/s)

V (m/sec)

Fig. 8 Streamlines colored by velocity magnitude

Low Lift

Medium Lift

High Lift

V, m/s

V, m/s

V, m/s

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The flow from upper part is mainly responsible for swirl inside the engine cylinder. Streamlines released from inlet shows this flow feature and is shown in Fig.8. Swirl created is stronger and visible near top of the cylinder during lower lifts but as the valve opens up, the swirl location moves down and gets weaker.

Fig. 9 Static pressure contours

Pressure and velocity distribution Strong pressure differential exists near the

throat which resulted in flow acceleration through the throat and hence high velocity. Fig. 9 and 10 shows the static pressure and velocity across the throat for various valve lifts.

Fig. 10 Velocity magnitude vectors

Low Lift

Medium Lift

High Lift

Low Lift

Medium Lift

High Lift

Ps, Pa

Ps, Pa

Ps, Pa

V, m/s

V, m/s

V, m/s

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At lower lifts the flow is attached to the wall causing less turbulence near the throat. The jet of flow coming out of the throat causes recirculation below the valve head. A portion of high velocity flow out from throat creates strong swirl inside the engine cylinder at low lifts compared to high lifts. Flow coefficient comparison

Mass flow rate matches closely with flow bench at lower lifts and starts to over predict during higher valve lifts. This could be because at higher lifts the flow starts to separate from the wall creating turbulence near the throat. The flow thus rushing into the chamber creating more suction upstream of the throat which resulted in more air flow at high lift position of the valve and this flow behavior is clearly visible in Fig. 9 and Fig. 10.

The comparison of flow coefficient between flow bench and CFD is shown in Fig. 11.

Flow Coefficient Comparison Flow Bench Vs CFD

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 3 6 9 12

Valve Lift (mm)

Flow

Coe

ffici

ent (

/)

FB 62mbarCFD 62mbar WRH (Profilometer)

Low Medium High

Fig. 11 Flow coefficient comparison Swirl ratio comparison

CFD was over predicting the swirl ratio in all the valve lifts but the magnitude is higher at lower lift (48%) and as the lift increases the % deviation is getting smaller (+/-15%), Fig.12. This could be because at lower lift CFD was over predicting the circumferential velocity and under predicting the mass flow rate. When the lift increases the deviation starts to come down.

Overall, CFD was exhibiting the same trend as observed in flow bench but on the higher side. This agrees with the anticipation because of the drag on the physical paddle wheel in the flowbench test

and uncertainty in the measurements like the difference in the outlet pressure measurement location (cylinder bottom in CFD and compensating tank in Flow bench) and 2D monitoring plane in CFD instead of paddle wheel in test. Moreover, the flow is highly three dimensional, so it is very difficult to accurately match the circumferential velocity calculated from CFD with flow bench.

Swirl Ratio Comparison Flow Bench Vs CFD

0.000

0.200

0.400

0.600

0.800

1.000

1.200

1.400

1.600

0 3 6 9 12

Valve Lift (mm)

Swirl

Rat

io

FBCFD

Low Medium High

Fig. 12 Swirl ratio comparison CONCLUSION The following conclusions were made with regard to flow parameters:

The CFD predicted values are close to flow bench measured values for all the valve lifts except high lift.

Maximum % deviation is occurring at the high lift because (i) CAD geometry might be a little different compared to flow bench, (ii) Error in dial gauge indicator for valve lift measurement, (iii) Type of turbulence model used to resolve boundary layer at the throat.

Discrepancy in the suction pressure measurement location between CFD and Flow bench.

The following conclusions were made with regard to charge motion:

Even though the paddle wheel speed is different between Flow bench and CFD, the trend exhibited by CFD is closely matching with Flow bench. The main reason for this difference could be (i) 2D monitoring plane instead of a paddle wheel, (ii) the paddle wheel speed calculation between CFD and Flow bench.

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NOMENCLATURE Symbols

nditionscoambientstandardunderDensityconditionsisentropicunderDensity

onditionscambienttestrundeDensityrateflowvolumealTheroretic

benchflowinmeasuredrateflowVolumeetemperaturambientStanandard

etemperaturambientTestconstantgasUniversal

conditionambientStandardpressureambientTest

tankngcompensatiinpressureStaticvalveofupstreampressureStagnation

(Swirl)speedwheelPaddlen1.40ratioheatSpecificdiameterwheelpaddleMean

diameterboreCylindervelocityFlowareaPiston

2

1

std

s

real

real

std

amb

std

amb

MFL

cyl

s

VVTTR

PPPP

kDDCA

ρρρ

=

Subscripts s Isentropic condition Notation ∆ Difference Abbreviation cyl Cylinder bore MFL Mean paddle wheel std Standard condition real Theoretically calculated amb Ambient condition

REFERENCES 1. Padmesh Mandloi, Nidhesh Jain and Laz Foley.

Port Flow Meshing Guidelines, ANSYS FLUENT Technical Document.

2. Laz Foley. Solution Strategies for Port Flow Analysis, ANSYS FLUENT Technical Document.

3. Xiao Hu. Angular Momentum Flux UDF, ANSYS FLUENT User Defined Function Database.

4. Andras Horvath, Zoltan Horvath., 2003. Application Of CFD Numerical Simulation For Intake Port Shape Design Of A Diesel Engine, Journal of Computational and Applied Mechanics, Vol. 4, No. 2, 129-146.

5. Takenaka, Y., Yabe, M., Aoyagi, Y., and Shiozaki., T., 1990. Three Dimensional Computation of In-Cylinder Flow with Intake Port in DI Diesel Engine, International Symposium COMODIA 90: 425-430.

6. Befrui, B.A., 1994. CFD Simulation and Comparison with Measurement Steady Flow in Intake Ports and Combustion Chambers, International Symposium COMODIA 94: 535-540.

7. Kang, Kern Y., and Reitz, R.D., 1999. The effect of intake valve and alignment on swirl generation in a DI diesel engine, Experimental Thermal and Fluid Sciences, 20, 94-103.

8. John B. Heywood. Text book on Internal Combustion Engine Fundamentals, McGraw-Hill Book Company.

9. Frank M. White. Text book on Fluid Mechanics, Fourth Edition, McGraw-Hill Book Company.