finite element thermomechanical analysis of an exhaust manifold system of a diesel engine for...
TRANSCRIPT
Finite Element Thermomechanical Analysis
of an Exhaust Manifold System of an
Engine for Automotive Applications
Matteo Giacopini, Roberto Rosi, Simone Sissa
Outline
• Aim and scope of the activity
• Modeling strategy, based on thermal and structural decoupled simulations
• Thermal analysis: gas/solid heat transfer
• Mechanical analysis: thermostructural loading FEM analysis
• Fatigue analysis: energy based LCF criterion
• Conclusions
Aim and scope
The present activity take its bases from some crack propagations that actually have
been observed during engine bench tests. In particular, cracks have been detected in
the region of the manifold bellows:
This activity aims at evaluating the mechanical behaviour of the exhaust manifold
system and at investigating if low cycles fatigue phenomena could be identified as the
reason behind these crack propagations.
Modeling strategy
Non-linear Finite Element models have been employed to mimic the manifold system
behaviour when subjected to the exhaust gases thermal loading.
Engine Head Interfaces
The modeling strategy developed in this study consists in separated thermal and
mechanical simulations, performed using the commercial Finite Element software
Altair HyperMesh and RADIOSS (Bulk Data Format) solver v12.0.
Engine Block Interfaces
Turbine
Flange Exhaust Manifold
Bellows
Modeling strategy
A model of the bellows alone is then employed to better investigate the mechanical
behaviour of the bellows in the crack regions.
Double layer bellows
Modeling strategy
Particular care has been devoted to the mesh generation process. A uniform
thickness boundary layer has been created on the manifold surface.
Advantage:
1. Better contact detection
2. Optimal surface resolution in stress and strain calculation
Thermal analysis
The thermal analysis is based on a previous CFD 1D-3D coupled simulation of a
whole engine cycle.
Due to high thermal inertia of metals, the instantaneous gas temperatures and heat
transfer coefficients have been cycle-averaged and they have been mapped on the
exhaust system FEM model with an ad-hoc developed fortran routine.
HTC [W/mm2/K]
External surfaces h = 5∙10-5 [W/mm2/K]
T = 60 [°C]
Engine Head T = 170 [°C]
Thermal analysis
Exhaust gases h = mapped from previous CFD simulations
T = 820 [°C]
Thermal boundary conditions:
• Heat transfer coefficient and reference temperatures on external surfaces and
internal surfaces touched by exhaust gases.
• Fixed temperature on the engine head interfaces.
Thermal analysis
Thermal contacts are modeled with CGAP elements and the contact thermal
resistance for closed gap imposed in PGAPHT card is estimated with the Yovanovic
expression for thermal resistance of conforming rough contacts as follow:
where:
𝑘𝑠: thermal conductivity [W/m/K]
𝑚: mean absolute surface slope
𝜎: rms surface roughness [μm]
𝑃: pressure [Pa]
𝐻𝑚𝑖𝑐: microhardness of the softer material in contact [GPa]
𝐴: contact area [m]
𝑛_𝐶𝐺𝐴𝑃 : number of CGAP element involved in contact surface
ℎ𝑠 = 1.25𝑘𝑠
𝑚
𝜎
𝑃
𝐻𝑚𝑖𝑐
0.95
𝐾𝐴𝐻𝑇 =ℎ𝑠𝐴
𝑛_𝐶𝐺𝐴𝑃
Thermal analysis results
Nodal Temperatures
Bellows T = 365 [°C]
Exhaust manifold T = 765 [°C]
Inner tube T = 770 [°C]
Mechanical analysis
The mechanical model consists in a subsequent applying and removal of the
temperature distribution. The aim is to evaluate if some point eventually undergoes a
plastic hysteresis cycle.
Bolt Tightening 1 I Warming 2
I Cooling 3
II Warming 4 II Cooling 5
Mechanical analysis
Bolt tightening F = 22000 [N]
Engine head/block interfaces dof1 = 0 dof2 = 0 dof3 = 0
Engine head/block interfaces dof1 ≠ 0 dof2 ≠ 0 dof3 ≠ 0
Mechanical boundary conditions:
• Fixed zero displacements are applied on the engine head and engine block
interfaces during bolt tightening.
• Displacements evaluated with a previous engine head/block thermomechanical
FEM analysis are superimposed to the model during warming/cooling loadings.
Mechanical analysis
The non-linear temperature-dependent mechanical behaviour of all the materials
involved in the analysis has been considered to carry out plastic strain by simulations.
Mechanical analysis
Due to the high temperatures and to the particular bellows geometry, thermal low
cycle fatigue phenomena could be considered as a possible reason of crack initiation.
The employed approach consists in the computation of the plastic energy dissipated
per cycle at a certain location, whose value can be directly correlated to the low cycle
fatigue life of the component.
Thermal Load History
Plastic Strain/Stress
Hysteresis Cycle
Low Cycle Fatigue
-300
-200
-100
0
100
200
300
-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08
str
ess [
MP
a]
plastic strain
strass - plastic strain curves
20 °C
250 °C
Mechanical analysis results
403 [MPa]
Displacements
Tmax
377 [MPa]
323 [MPa]
von Mises Stress
Mechanical analysis results
0.01394
0.01449
Plastic strain
Tmax
Tmin
Mechanical analysis results
Plastic strain
0.01113
0.01184
Tmax
Tmin
Mechanical analysis results
Displacements
Mechanical analysis results
Results obtained by the previous whole model analysis show two critical areas
located at the bellows extremities. Therefore, a model of the bellows alone has been
created to better evaluate stress and strain in these regions.
Mechanical analysis results
Displacements previously evaluated on the welded joints at the both sides of the
bellows have been applied to the corresponding nodes of the bellows alone model.
REB2 used to impose global displacements
Mechanical analysis results
Plastic strain-stress curves
Mechanical analysis results
Plastic strain-stress curves
Conclusions
• A thermomechanical analysis has been performed in order to evaluate the
thermomechanical behaviour of an exhaust system;
• First, decoupled thermal and mechanical FEM models have been developed of the
whole exhaust system. Secondly, a mechanical model of the bellows alone has been
introduced to better compute stress and strain in this critical region;
• An energetic LCF criterion based on the plastic dissipated energy per cycle has
been employed in order to rationalise some crack propagations observed in the
bellows during engine bench tests;
• The methodology has been shown to be able to correctly locate the most critical
areas in terms of Low Cycle Fatigue life and it constitutes a valid instrument for future
design optimizations of the bellows geometry.
Thank you for your attention!