1996
SAE International Congress & Exposition
Feb. 26-29, 1996,
Detroit, MI
AIR
FLOW AND HEAT TRANSFER ANALYSIS OF AN AUTOMOTIVE ENGINE RADIATOR TO
CALCULATE AIR-TO-BOIL TEMPERATURE
A.
Ecer, C. Toksoy, V. Rubek, R. Hall & G. Gezmisoglu, Technalysis
V. Pagliarulo, S. Caruso,
J. Azzali, Fiat Auto
Introduction &
Background
The use of higher output
engines with tightly compacted underhood packaging, the addition of new
emission components, and aerodynamic front end styling with narrower
openings to increase the fuel efficiency are creating a hostile thermal
environment in the engine compartment by decreasing the volume of underhood
cooling air. At the same time, automotive companies are striving to shorten
design cycles and to cut engineering as well as prototype costs. These
conditions demand a better understanding of the complex cooling air flow
characteristics and resulting thermal performance of the radiator and other
heat generating components in the engine compartment. One solution is to
utilize cost effective numerical tools such as Computational Fluid Dynamics
(CFD) as part of the systems engineering of the underhood package.
PASSAGE®,
a CFD software developed by Technalysis, was used to calculate the air
velocity distribution over the radiator and the resulting Air-To-Boil
temperature (ATB). The objective was to show how CFD can be used as a
practical engineering tool to complement and enhance the design process. The
procedure described in this paper can provide answers to questions by the
cooling system engineer.
The objective of the project
was to establish CFD tools and methodologies to:
1) predict velocity
distribution over the radiator
2) predict ATB temperature
3) validate the analysis
results
The methodology was
demonstrated for a sample car. Graphical outputs from the simulation
included the velocity vectors and the contour plots detailing the flow
characteristics around the front end of the car and over the radiator.
A good correlation with
experimental results was achieved for the predictions of the amount of air
flow going through the radiator and the ATB temperature.
DESCRIPTION OF THE PROBLEM -
The fluid flow around the front end of an Alfa Romeo automobile was analyzed
using one-dimensional and three-dimensional finite element analysis methods.
These analyses required a numerical model of the flow domain around the
front end of the car, as well as information relating to the underhood
components and the underhood layout.
The thermal performance
of the engine was evaluated under the given car operating conditions:
- car speed (wind
velocity) 34.5 km/h
- upstream air
pressure 100,450 Pa
- engine rpm 4390
rpm
- engine load 33.3
kW
- amount of engine
heat rejection to coolant 25.4 kW
- coolant mass flow
rate into the radiator 6300 lt/hr
METHOD OF SOLUTION - A
numerical model that combines one-dimensional system modeling together with
three-dimensional CFD techniques was applied to predict the engine thermal
performance at low car speeds. A multi-step modeling effort, as described
below, was used to predict the complex flow field at the front end of the
vehicle. Several computer models were developed to predict various
components of the system resistance, air mass flow rate, flow through the
fan and radiator heat rejection.
Modeling Steps
- System flow
analysis
- Overall flow
analysis around the front end
- Detailed flow
analysis of the front end
- System heat
transfer analysis
A one-dimensional flow
analysis model of the engine cooling air was developed to estimate the mass
flow rate going through the radiator.
A two step flow analysis
procedure was used to estimate the velocity distribution over the radiator.
First, a three-dimensional finite element overall flow analysis model of the
entire front end of the car was developed. This coarse flow analysis
together with particle tracing analysis predicted flow streamlines leading
into the radiator. Later, a detailed flow analysis model of the upstream air
bounded by the flow streamlines leading into the radiator was generated and
the velocity distribution over the radiator was predicted.
A system heat transfer model
of the radiator was developed by dividing the radiator into a 10x10 mesh.
With the known velocity distribution over the radiator and heat rejection
characteristics of the radiator, the ATB temperature was estimated.