Special Issue on Heat Transfer, February 2011
Manuscript received and revised January 2011, accepted February 2011 Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved
212
Numerical Analysis of a Two-Dimensional Thermal Model for a Car Under-Body
Antonio Di Micco
1
, Francesco Fortunato
2
, Oronzio Manca
1
, Alina A. Minea
3
, Daniele Ricci
1
Abstract – In this paper a numerical bidimensional thermal analysis of the interactions between a car underbody and the road surface is carried out. The behaviour of the exhaust system and its
thermal interface with the underbody is an important part of the thermal management of the car. In fact, the high value of temperatures along the exhaust system components such as catalytic
converter, pipes, muffler determines strong thermal interaction with the other components close to the exhaust system. In order to preserve the more sensitive components and ensure safety and
reliability, thermal shrouds are employed. Thermal design needs previous investigations on thermal and fluid dynamic behaviour by means of suitable numerical models and experimental
measurements In this paper, the investigation is lead by means of the commercial code FLUENT on a model representing a section of the underbody near to the catalyst, the most critical
component. The numerical analysis is carried out in laminar and transient state conditions, regarding the effects of natural convection in air and radiation. All thermo-physical properties of
the fluid are assumed to be constant. Results are given in terms of temperature distributions, thermal and velocity fields considering several geometric parameters of the tunnel, such as the
height, the distance from the road surface, the width and the length of the underbody. Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved.
Keywords:
Natural Convection, Thermal Management, Thermal Design
Nomenclature
D Exhaust pipe diameter m
h Tunnel
height m
H Distance from the road m
l Tunnel
length m
k Thermal
conductivity W
m
-1
K
-1
L Under-body
length m
Nu Nusselt
number q
Heat flux W m
-2
s Shield
thickness m
t Time s
T Temperature K
x, y Spatial
coordinate m Greek Symbols
ε Emissivity
ν Kinematic
viscosity m
2
s
-1
θ Dimensionless
temperature τ
Dimensionless time
Subscripts Initial condition
a Ambient
d Duct
f Fluid
I. Introduction
The thermal management plays a fundamental role in the design and optimization of automotive systems
[1],[2]. The high temperature values along the automotive exhaust systems components, such as
catalytic converter, pipes, muffler and so on, determine critical thermal interaction with the other objects close to
the exhaust system [3]. In this kind of problem the unknown variables are the heat transfer rates among the
components and their temperatures [3]-[7]. The complex geometry of the exhaust line and the special flow
conditions complicate the problem of accurately estimating several important heat transfer parameters.
Thermal design needs previous investigations on thermal and fluid dynamic behavior by means of suitable
numerical models and experimental measurements, to detect the zones at higher temperatures. The heat transfer
evaluation between the exhaust line, the underbody and the external ambient is needed [7]-[9].
The numerical studies of Kandylas and Stamatelos [10] employed different empirical convective heat
transfer correlations for the external flow in the engine exhaust. The contribution of free convection to the total
external heat transfer was neglected and the convective component was described by a forced convection heat
transfer correlation.
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
213 However, due to the high temperatures of the exhaust
components and crowded engine compartment that may impede the external forced convection, one expects free
convection heat transfer to be important, though along the external exhaust system in some stagnation zones
buoyancy force could be significant.
In this paper a two-dimensional model of a section of the car underbody near to the catalyst, the most critical
component belonging to the exhaust line [6], is proposed in order to study the interactions with the road surface
and the exhaust system. The system is represented by a partially open horizontal cavity, heated by a duct placed
in the middle. The vehicle is stationary so only the natural convection is regarded as well as the radiation
mechanism.
Manca et al. [11] studied a configuration made of two horizontal parallel plates with the upper plate heated at
uniform heat flux. Results were reported for Rayleigh numbers equal to 103 and 105 and for two aspect ratios.
Koca [12] provided a numerical study about the conjugate heat transfer in partially open square cavity
with a vertical heat source for different Rayleigh numbers, conductivity ratios, opening position and open
length. Jaluria et al. [13] provided a transient analysis of natural convection in air in a horizontal open ended
cavity for a similar configuration and presented results for several significant variables, such as the penetration
length, Rayleigh numbers and aspect ratio values.
In this work a transient analysis of the interaction of a car underbody with the exhaust system and the road
surface is carried out in order to study the behavior of the system, with or without thermal shields. The vehicle is
assumed to be stationary so natural convection is the primary heat transfer mode.
II. Description of the Geometrical and
