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TMF Lite Process Development
Qingzhong Li, Dong Wei, Bill Moser, Don Sit, Scott Thompson
Caterpillar Inc.
Abstract: A simplified manifold transient thermal analysis method has been developed and used in
engine design and validations. Traditionally full-blown manifold transient thermal analysis has
been performed using CFD and it will take weeks for completion. In this simplified method, a user
subroutine film.f is used to define the htc and gas temperature at manifold surfaces so that the
transient thermal analysis can be performed in Abaqus quickly. The method is designed to reduce
the simulation time significantly by sacrificing some accuracy. A comparison study showed that
the simplified simulation results can capture the general trend in temperature distribution and can
be used in thermal stress and fatigue analysis for quick design evaluations during the early design
stage when many design versions need to be compared in a short time as in the case for Tier 4
manifold design. The simulation time has been reduced from weeks to days. It proved to be a good
alternative to full scale CFD analysis when many design versions need to be compared at the
early design stage.
1. Introduction
TMF represents thermal mechanical fatigue analysis which is mainly used in engine components
which experience significant high temperature in service like in manifolds. An accurate TMF life
prediction is based on accurate transient temperature predictions. Currently, the best method for
predicting these temperatures is a conjugate (i.e., fluid and metal) transient CFD analysis of the
exhaust manifold system. This analysis incorporates a portion of the cylinder head, the exhaust
manifold, fasteners, and turbine housing, and it accounts for spatial variation in metal-fluid
boundary conditions as well as temporal variation through the cycle. The typical TMF thermal
cycles and standard TMF process are illustrated in Figure 1 and 2.
The drawback to an exhaust manifold CFD is that it is quite time-consuming. A model of this
complexity typically takes two to five weeks to complete. Such time duration is not quick enough
for rapid design iterations in the early phase of manifold design process. The purpose of the TMF
“Lite” process is to simplify the TMF analysis of an early manifold design such that the analysis
completion time is one week or less. A key to this compressed time schedule is bypassing the
CFD by using a simplified FE-based thermal transient analysis.
An Abaqus user subroutine film.f has been developed to define the heat transfer boundary
conditions in the center section of the manifold. It reduces the heat transfer analysis time and
overall TMF analysis time significantly. Fatigue analysis results comparison showed that the Lite
process can capture the general pattern of fatigue life in the center section and can be used in the
early design stage.
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Cylinder Head
Center section
Exhaust Manifold
Turbo-housing
Temperature cycles at different locations
Gas temperature
1100
Rated gas temp
1000
tc11
tc10
tc9
tc8
tc7
tc6
tc5
tc4
tc3
tc2
tc1
900
(full load)
Idle gas temp
Temperature(K)
800
700
600
500
400
300
t1
t2
t3
t4
Time (s)
200
100
0
0
t0
Typical Test Cycle
200
400
t4
600
800
Time(Sec)
t81000
Figure 1. Typical TMF Thermal Cycles
CFD Analysis
FEA thermal stress Analysis
-Create CFD model
-Run Thermal Cycle Analysis
-CFD/FEA –Post Process
-Create FEA model
-Abaqus Assembly/ Verify Contacts
-Thermal stress analysis
Fatigue Analysis
-Convert Stress format (skin
mesh result)
-Run in-house code for TMF
- Post Processing
Figure 2. Overview of Standard TMF Process
2. Lite Model Development
The procedure was specifically developed and validated for exhaust manifold center sections of
mid-range and heavy-duty diesel engines with in-line cylinder design. As shown in Figure 3, only
center section is analyzed for thermal analysis and structural analysis instead of full CFD analysis
and full structural analysis. A transient heat transfer analysis is performed in Abaqus to replace
CFD analysis to save simulation time. An Abaqus user subroutine film.f has been developed
specifically to define the heat transfer boundary conditions for the internal ports in the center
section of the manifold.
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Full
Structural
(ABAQUS)
Full CFD
(Fluent)
Lite transient heat transfer
analysis model (ABAQUS)
Lite structural model (ABAQUS)
Same mesh
Figure 3. Full TMF and Lite TMF Model
2.1
Structural Boundary Conditions
Figure 4. Full TMF and Lite TMF Model
Figure 4 shows simplified structural boundary condition for the center section.
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2.2
Thermal Boundary Conditions
2.2.1
External Thermal Boundary Conditions
The thermal boundary conditions are in the form of a heat transfer coefficient (HTC) and fluid
temperature (T). The external boundary conditions are fixed (as shown in Figure 5). The values
for the external surface, for the manifold-to-head joint surfaces and for the slip joint surface were
determined by matching results to a full TMF analysis of a manifold center section.
* head-manifold: htc=1.0 Htc unit: mw/mm^2-C
•Outer surface: Htc=0.05
Internal surfaces: constant htc 0.70
•Same as internal surface/
or no BC
HTC
Abaqus usage:
*sfilm,amplitude=temp_amp_2, film amplitude=htc_int_long2
Full load htc
internal_long_surf_b, F, 127,0.70
Idle load htc
t1
t2 t3
t4
Step Time (s)
Figure 5. Initial Lite Model Assumption
2.2.2
Internal Thermal Boundary Conditions
The internal boundary conditions vary with time, and they are meant to be simplification of the
Diesel Engine Exhaust System Durability Test. Different methods have been proposed and
evaluated. The initial method was to assign a fixed value (an average value based on CFD
analysis) for the entire internal port. The comparison of temperature contour plots between CFD
results and initial simplified method shows that while the initial simplified method can capture the
general trend, there are obvious temperature differences at some thermal couple locations.
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In order to further improve the simplified method, the method of defining the internal htc with
Abaqus user subroutine film.f was developed. It was designed in a way that the internal passage is
divided into different regions and each region is assigned to a different htc based on the exhaust
air flow as shown in Figure 7.
ABAQUS simplified model
CFD
v Idle temperature contour from CFD
v The goal is to map CFD steady results
with a set of heat transfer BC
t0
0
200
400
t4
600
t81000
800
Time (Sec)
Figure 6. Comparison with CFD Results
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The head ports see 1 exhaust pulse
every 2 engine revolutions. Physics
says this area HTC “should” be ~1/3
of the “high” value.
All cylinders have
1 exhaust pulse
every 2 revs of the
engine.
This area sees 2
exhaust pulses every 2
engine revolutions.
Physics says this area
HTC “should” be ~2/3
of the “high” value.
HTC
Full load htc
Idle load htc
t0 t1
t2 t3
This area sees 3
exhaust pulses every 2
engine revolutions.
Assume this area gets
the “high” HTC value.
t4
Step Time (s)
Figure 7. Abaqus User Subroutine film.f Development
In Abaqus user subroutine film.f, the first task is to divide the internal ports into different region
automatically. This was done by asking FEA analysts to enter some coordinates at specific nodes
like in the OD of the port interface as shown in Figure 8. With these nodal coordinate inputs, the
internal passage was divided into several regions and assigned different htc values.
In order to verify whether the right htc was assigned to the right region, the htc values assigned
were written into user variables using Abaqus user subroutine UVARM and displayed in
Abaqus/Viewer. In this way it was verified that intended htc values were assigned to the right
regions.
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A1
B1
A2
B2
C2
C1
Head Flange Surface 3
A3
C3
B3
Abaqus usage:
*sfilm
internal_long_surf, FNU, 127, 0.61
This should be the
high htc of the port
Figure 8. Input for Abaqus User Subroutine film.f
Figure 9. Verification of HTC Using User Subroutine UVARM
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2.3 Temperature and Fatigue Results Comparison
Temperature comparison between CFD and Lite Models (fixed htc method and user subroutine
method) is shown in Figure 10. It is shown that temperature difference between CFD results and
Lite model is reduced significantly with the use of Abaqus user subroutine. The max temperature
difference at key thermocouple locations is less than 20C.
80
old- h=0.70
60
New - h=0.61
40
user subroutine
20
s
iu
ra
d
ed
ge
to
p
TC
11
TC
8
TC
TC
-20
Thermal Couple and Other
Critical Locations Used for
Comparison
3
0
1
Temperature Comparison
between Full Model and
Lite Models
Temperature Diff (C)
Temperature Difference
between CFD and Lite Models
TC6, 7, 8
Top edge
TC3
TC3
TC9
TC4, 5
TC11
TC11
TC10
TC1
TC1
TC3
TC3
Radius
TC2
Figure 10. Temperature Comparison
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Thermal fatigue analyses were performed also based on thermal stresses from both full TMF and
TMF-Lite. The fatigue life results were compared at critical locations (shown in Figure 11).
Comparison of the fatigue results is shown in Figure 12. The X axis shows the life predicted by
using full TMF while y axis represents life predicted using TMF-Lite procedure. It is shown that
most data fall in the range of one order of magnitude, and that more than 70% of data are below
Full TMF results. It also shows that the data which exceed full TMF results are less than 2x of the
CFD results. Therefore the model is generally conservative.
TMF Comparison Locations
Left View
Right View
R1
R3
L3
L1
L2
R2
Top View
Bottom View
T4
B1
T3
T2
B2
B3
T1
Head Flange View
B4
B6
B5
H1
H2
Figure 11. Fatigue Results Comparison Locations
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1.E+07
10x
2x
1.E+06
0.5x
Cycles - Lite
0.1x
1.E+05
1.E+04
1.E+03
Lite Analysis -User subroutine
1.E+02
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
Cycles - Full CFD
Figure 12. Fatigue Results Comparison
3. Conclusion
A. A TMF Lite Procedure has been developed and verified; Its advantages / limitations
defined;
B. Lite Process significantly reduces the TMF analysis time to 3 ~ 5 working days
(Transient FEA + Stress FEA +TMF) from about more than 3 weeks for the full TMF;
C.
This process has been incorporated as an important part of the overall manifold
validation strategy;
D. TMF Lite procedure has been used in the validation of Tier 4 exhaust manifold designs.
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