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.