The Cooling Airflow of Heavy Trucks - a Parametric Study

Thomas Hällqvist, Scania CV AB

Company Logo

2008-01-1171

2

Introduction Study of the influence on the cooling airflow

from various installation parameters on a Heavy duty truck.

Analysis performed by means of 3D CFD. The focus of the paper is on the system

pressure loss, flow distribution and cooling capacity.

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Physical Model

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Complete 2-axle tractor. Air enters in the front via mesh

screens. Cooling package includes:

Condensor Oil cooler EGR cooler CAC cooler Radiator

Pictures show the surface mesh

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Physical Model

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Fan diameter of 750 mm (default). Fan placement depends on engine

type. Both V8 and inline six engines are

considered. High level of details in the engine

compartment.

Pictures show the surface mesh

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Simulation Technique 3D isothermal CFD simulation. LBM solver by EXA corp. Coupling to 2D heat exchanger calculation. Fan modeled via MRF. Heat exchangers and mesh screens modeled as

porous media.

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Model Size and Accuracy Statistical convergence within <1 %. Absolute accuracy within 6 % for massflow (rel. MP). 40-50·106 volume elements. Simulated on 128 cpu’s Linux cluster. Total runtime of approx. 22-30 h.

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data sampling interval

MP: Micro Probe measurements

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General Boundary Conditions

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Virtual windtunnel with moving ground. Windspeed of 30 km/h. Ambient temperature of 25°C. Fan speed of 1700 rpm.

inletoutlet

L = 170 mW = 60 mH = 45 m

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Parameter Variations

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Front opening area. Fan-to-radiator spacing. Fan-to-engine spacing. Width of cooling module. Fan diameter. Fan projection into shroud.

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Results

Study of the impact from various parameter settings on: the flow character, the total pressure loss, the flow distribution through the radiator, the cooling capacity.

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Results: general flow character

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The underhood includes different subsystems. Subsystems installed in serial or in parallel. The fan shroud has large influence on the pressure loss. All subsystems, but the HX’s, must be optimized w.r.t dP. A HX with large dP generally comes with large heat transfer

capacity.

Fan shroudCooling pacakge

RAD

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Results: general flow character

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Airflow enters via the front. Static pressure decreases

until the fan, where the pressure is build up to Pamb + dPrear underhood.

Three main flow directions below the cab.

Flow also underneath the engine.

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Results: general flow character

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V8: fan in high position S6: fan in low position Fan placement and engine type

influences the flow distribution.

V8: Fan on top of crossmember. S6: Fan in front of crossmember. Strong influence on dP below the

engine.

V8 setup Inline-six setup (S6) V8 setup Inline-six setup

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0.98

1

1.02

1.04

1.06

0 20 40 60 80 100dx (mm)

ma

ssf

low

/ m

as

sflo

w (

Ca

se-R

EF

)

V8 cases

S6 cases

Results: fan-to-radiator spacing

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V8 setup

Inline-six setup

Fan shrouds with different depths tested. Default setup V8 has a deeper shroud. dx more critical for S6-cases. At same depth the V8 setup features higher

dPtot than S6.

Case REF*

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Results: fan-to-radiator spacing

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N

i

i

U

u

Nuniformity

1

12

11

0.86

0.87

0.88

0.89

0.9

0.91

0.92

0.93

0.94

Case REF* Case FS Case dx60+

un

ifo

rmit

y

dx also influences the flow distribution. So also the shape of the shroud. Bad uniformity for RAD higher dPRAD.

V8 setup

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Results: fan diameter / width of RAD

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The geometrical shape of the fan shroud influences the flow distribution. A wide cooler gives low flow rates in the outer regions. A larger fan improves the uniformity. A larger fan can geometrically be compared to a deeper fan shroud.

V8 default case setup 20 % wider cooling package 20 % larger fan

uniformity = 0.87 (Case NF) uniformity = 0.88 uniformity = 0.91

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Results: fan projection into shroud

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FPiS determines the flow direction behind the fan. FPiS should be tuned for each specific installation. Large FPiS axial fan behavior, high dP for large engine silhouette. Small FPiS radial fan behavior, high rates of leak flows. The smaller fan-tip to fan-ring spacing the smaller FPiS is possible.

axial fan behavior

leak flows

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1

1.01

1.02

1.03

1.04

1.05

0.86 0.88 0.9 0.92 0.94 0.96 0.98 1uniformity

CC

/CC

*

0.95

1

1.05

1.1

1.15

1.2

1.25

1 1.05 1.1 1.15 1.2 1.25

normalized massflow

no

rma

lize

d c

oo

ling

pe

rfo

rma

nc

e

Results: cooling performance (1/CC)

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Influence from flow uniformity Influence from massflow

The flow uniformity has some effect on the cooling performance.

The character of the flow distribution is also relevant.

Within the present interval the cooling performance has a linear relation to the massflow.

The non-uniform and the uniform flow show the same trends.

non-uniform flow

uniform flow

),()(

HXuniformitykUhA

QCC

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Conclusions The underhood involves several subsystems. The design of the fan shroud is crucial. The flow distribution is important w.r.t. to dP. For the cooling performance the massflow is

of main importance, uniformity of less.

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Future Work Additional parameter settings. Extend the study w.r.t. fan configuration. Study the effect from fan modeling. Extend the thermodynamic analysis.