TMF Lite Process Development

Transcription

1 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 SIMULIA Customer Conference 1

2 Cylinder Head Turbo-housing Gas temperature Rated gas temp (full load) Idle gas temp t1 t2 t3 t4 Time (s) Typical Test Cycle Temperature(K) Center section Exhaust Manifold Temperature cycles at different locations t0 t Time(Sec) t8 tc11 tc10 tc9 tc8 tc7 tc6 tc5 tc4 tc3 tc2 tc1 Figure 1. Typical TMF Thermal Cycles CFD Analysis -Create CFD model -Run Thermal Cycle Analysis -CFD/FEA Post Process FEA thermal stress Analysis -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 2. Lite Model Development Figure 2. Overview of Standard TMF Process 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 SIMULIA Community Conference

3 Full CFD (Fluent) Full Structural (ABAQUS) 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 SIMULIA Customer Conference 3

4 2.2 Thermal Boundary Conditions 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. Outer surface: Htc=0.05 * head-manifold: htc=1.0 Htc unit: mw/mm^2-c Same as internal surface/ or no BC HTC Full load htc Internal surfaces: constant htc 0.70 Abaqus usage: *sfilm,amplitude=temp_amp_2, film amplitude=htc_int_long2 internal_long_surf_b, F, 127,0.70 Idle load htc t1 t2 t3 t4 Step Time (s) Figure 5. Initial Lite Model Assumption 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 SIMULIA Community Conference

5 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 0t t t81 Time(Sec) Figure 6. Comparison with CFD Results 2010 SIMULIA Customer Conference 5

6 All cylinders have 1 exhaust pulse every 2 revs of the engine. The head ports see 1 exhaust pulse every 2 engine revolutions. Physics says this area HTC should be ~1/3 of the high value. This area sees 2 exhaust pulses every 2 engine revolutions. Physics says this area HTC should be ~2/3 of the high value. t0 HTC Full load htc Idle load htc t1 t2 t3 t4 Step Time (s) This area sees 3 exhaust pulses every 2 engine revolutions. Assume this area gets the high HTC value. 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 SIMULIA Community Conference

7 A1 B1 A2 B2 C1 C2 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 2010 SIMULIA Customer Conference 7

8 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. Temperature Comparison between Full Model and Lite Models Thermal Couple and Other Critical Locations Used for Comparison Temperature Diff (C) TC6, 7, 8 Temperature Difference between CFD and Lite Models TC 1 TC 3 TC8 TC11 top edge radius old- h=0.70 New - h=0.61 user subroutine TC9 TC4, 5 TC11 TC11 TC1 TC1 TC3 TC3 TC2 TC10 TC3 Top edge Radius Figure 10. Temperature Comparison SIMULIA Community Conference

9 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. Left View TMF Comparison Locations R1 Right View L3 R3 L2 L1 Top View R2 Bottom View T4 B1 T3 B3 B2 B4 T1 T2 Head Flange View B5 B6 H1 H2 Figure 11. Fatigue Results Comparison Locations 2010 SIMULIA Customer Conference 9

10 1.E+07 10x 2x 1.E x Cycles - Lite 1.E+05 1.E x 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 SIMULIA Community Conference