Coarse Mesh CFD: Trend Analysis In a Fraction of the Time

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1 Coarse Mesh CFD: Trend Analysis In a Fraction of the Time Y. He, C. J. Rutland, Z. Nagel, R. P. Hessel, R. D. Reitz, D.E. Foster Engine Research Center, University of Wisconsin-Madison In an effort to decrease concept evaluation time in engine design and analysis, a lite suite of kivav-erc CFD codes has been developed. The term lite is used to imply ease of use, faster run-times and less accuracy, as compared to the standard version of kivav-erc. Initial stages of concept evaluation are performed using the lite suite, whose grid uses either a flat piston, or one with an on-axis cylindrical bowl. The mesh resolution is coarse, as found by studies done by He and Rutland, which yields run times on the order of 1 to 1 minutes on a MHz PC. Only the closed portion of the cycle is modeled. Once an initial engine concept has been studied and formulated using the lite suite, the user transitions to the standard kivav-erc for final validation. The suite includes the grid generator, kprep_lite, the solver, kivav_lite, and the postprocessor, kpost_lite. All of these codes are based on the original Los Alamos versions, (Amsden, 199) but include many enhanced subroutines developed at the Engine Research Center (ERC) (Patterson, 199). This paper covers, the intended use of the lite suite, in general, how it was made from the standard kivav-erc code and results. 1. The Intended Use of the lite Suite The kivav_lite suite has two main purposes. The first is to decrease run times so that CFD can be used as a practical analysis tool in the engine development process. Typical CFD run times for simulating the closed portion of the engine cycle are -1 hours. kivav_lite can run in approximately 1 minutes on a MHz PC with a Mb RAM. Of course run time will vary depending on the CPU speed, RAM, the number of jobs running simultaneously and the size of the combustion chamber. It is hoped that kivav_lite will provide accurate trend-wise results, not necessarily be correct in an absolute sense. After an analysis is done with kivav_lite, running the problem in kivav_erc is suggested for validation. The second reason for developing kivav_lite is to provide a learning tool for those new to in-cylinder CFD modeling. The number of user inputs has been greatly reduced for all three lite codes. This makes the kivav_lite suite relatively easy to learn, compared to other kiva versions. And, since the code runs so much faster, new and experienced users alike can increase the rate at which they gain knowledge about in-cylinder processes, by running many simulations in a short period of time and observing calculated trends. Of course the user must still use good engineering judgment and base their model inputs on good engine data (Yi, ) so as not to generate false results at an alarming rate. That is, the kivav_lite must still be calibrated to good engine data.

2 . How the lite Suite was Made As background, the kivav-erc CFD code is based on the Los Alamos kivav program (Amsden, 199). Some of the Los Alamos subroutines have been replaced with those developed at the ERC. Some new ERC subroutines have also been added to account for physical processes that the Los Alamos version does not account for. In-turn, kivav_lite is based on kivav-erc. Many changes were made to make kivav_lite relatively easy to use. Most of the user inputs that a typical user does not change in kivav-erc have been hard coded. This greatly reduced the number of inputs required, as seen in table 1. Table 1. Compares number of inputs required by kivav-erc and kivav_lite. kivav-erc kivav_lite kprep (+ tabular data, nbo) 1 kivav 1 kpost 1 variables, 1 array 1 array The filename convention has been changed to be more intuitive, as seen in table. Input and output files end with the extensions.input and.output respectively. The base name matches either the program used (ex: kprep_lite), or gives a description of what the file contains (ex: graphics). Table. Comparison of filenames for kivav-erc and kivav_lite. kivav-erc kivav_lite kprep iprep otape11 otape1 gridfile.input kivav kpost itape otape1 itapeerc itape1 otape9 ipost otape1 itape9 kprep_lite.input kprep_lite.output kivav_lite.input kivav_lite.output erc_lite.input gridfile.input graphics.input kpost_lite.input kpost_lite.output graphics.input.1 kprep_lite Figure 1 shows a sample kprep_lite.input file. Simplifying the input, as was done here, imposes limitations on the type of combustion chambers that can be built. Only flat-top pistons, or those with on-axis cylindrical shaped bowls are allowed. The compression ratio is a new input and is used to size bowl depth for with-bowl pistons, or calculate squish height for flat-top pistons. Postscript output of the mesh is automatically output for error checking.

3 Figure 1. Example kprep_lite input file.. kivav_lite Most kivav-erc features are still available with kivav_lite, except for the following: Only sector grids (< 1 o ) with 1 nozzle are allowed, Only the closed portion of the engine cycle is modeled, Crevice model is not active, Spark ignition is not allowed. Enhancements include: Injection spray cone angle and injector discharge coefficient are calculated internally rather than being model inputs (Schmidt, 199), Initial species concentrations are estimated using a simple emptying/filling model, which accounts for internal and external EGR (Keith Richards, private communications), Brake quantities are estimated and output for BSFC predictions, Grid is automatically converted from D to D at start-of-injection.. kpost_lite kpost_lite reads in the graphics file created by kivav_lite. The user has access to many variables, but can choose which ones to view. Postscript files are automatically generated at the y= plane.. Results A grid resolution study was done (He, Rutland, private communications) to determine how coarse the mesh could be before results became unacceptable. It was found that the grid could be made coarse in the Y and Z directions (azimuthal and axial respectively), but not in the X direction (radial). The normal grid and the coarse grid used the same model constants in the study. Those results are summarized in Figure.

4 Figure. a) Pressure and b) heat release rate results with normal coarse and normal grids. 1 1 solid line = normal grid dashed = coarse grid -1 1 Crank Angle (deg) Heat Release (J/deg) a) b) 1 solid line = normal grid dashed = coarse grid -1 1 Crank Angle (deg) These pressure and heat release rate results were considered acceptable for their intended use. kivav_lite was not used for these calculations, but the grid resolution results were transferable to the kivav_lite project. The results of figure are from kivav_lite. They were done on the same engine that was used for the grid resolution study (single cylinder CAT 1 engine, bore = 1 mm, stroke = 1 mm). The timing swing shown was done at 1 RPM, % load and 9 MPa injection pressure. Trends are predicted adequately. Figure. Pressure traces and emissions results using kivav_lite at various SOI. 1 1 Model Expt. - Crank Angle, ATDC kivav_lite kivav-erc Expt.. -9 NOx (g/kg-fuel) kivav_lite was also used in calculating US FTP modes 1 through (see Table ). Figure shows how kivav-erc and kivav_lite compare to experimental data at these modes and at baseline conditions. Model constants that required changes in transitioning from kivav-erc to kivav_lite are also given. kivav- ERC results are closer to experimental values than kivav_lite results, but the kivav_lite results are considered acceptable for their purpose. Model constants had to be adjusted in transitioning to the coarse grid for NOx (rsc), soot (asf), ignition delay (af), and droplet breakup (cnst). Once determined, all these constants were held constant for all seven simulations. The only constants that were slightly modified between simulations were tnparc, which specifies the number of droplets to represent the spray and dtmax, the maximum time step. Changing these two inputs is normal, since tnparc will change with the amount of fuel injected and dtmax is problem specific.

5 Figure. Pressure comparison between experiments, kivav-erc and kivav_lite, and constants used., Kivav, Kivav_Lite: Baseline, Kivav, Kivav_Lite: Mode Kivav Kivav_Lite Kivav Kivav_Lite Baseline Mode 1 - -, -1 Kivav, 1 Kivav_Lite: Mode Kivav Kivav_Lite Mode Kivav Kivav_Lite Mode Kivav Kivav_Lite Mode Kivav Kivav_Lite Mode Kivav Kivav_Lite Mode Model constants comparison. kivav-erc kivav_lite rsc asf af 1.e 1.e cnst tnparc 1 1- most=1 dtmax,sec e- e- e- most=e- Crank Shaft Angle [Degrees After Top Dead Center]

6 Table. Mode definitions (baseline case: 1 RPM, % load) Mode 1 RPM % Load Emissions comparisons for these same cases are given in figure. For NOx, both kivav-erc and kivav_lite match experimental trends and absolute values quite well. For soot, kivav_lite results are very similar to kivav-erc results. Both deviate from experimental values, partially due to the fact that the experiments present measured particulate whereas the computations present dry soot. Still, trends are followed in most cases. Figure. NOx (left) and soot (right) comparison between experiments, kivav- ERC and kivav_lite. NOx vs. Mode Results, Kivav and Kivav_Lite Predictions Soot vs. Mode Results, Kivav and Kivav_Lite Predictions. NOx (g/kgf) 1 Soot (g/kgf) Mode 1 Mode Mode Mode Mode Mode Base Mode Mode 1 Mode Mode Mode Mode Mode Base Mode Kivav kltv Kivav kltv. Conclusion A version of kivav-erc has been developed, kivav_lite, which is relatively easy to use, runs relatively fast and has demonstrated good calculation ability in both performance and emissions. For these reasons, kivav_lite has the possibility of making CFD more of an integral part of the engine design and development process and can be used instead of simple global simulation models, which run in about the same CPU time.. Acknowledgements: Caterpillar, International Truck and Engine Company, and TACOM for their support of this project. Intelligent Light for granting use of their Fieldview postprocessing software.. References Amsden, A.A., "KIVA-V: A Block-Structured KIVA Program for Engines with Vertical or Canted Valves," Los Alamos National Labs., LA-11-MS, 199. Patterson, M. A., Kong, S.-C., Hampson, G. J. and Reitz, R.D., "Modeling the Effects of Fuel Injection Characteristics on Diesel Engine Soot and NOx Emissions," SAE Paper 9, 199. Yi, Y., Hessel, R.P., Zhu, G.-S., Reitz, R.D., The Influence of Physical Input Parameter Uncertainties on Multidimensional Model Predictions of Diesel Engine Performance and Emissions, SAE Paper Schmidt, D.P., Rutland, C.J., Corradini, M.L., A Numerical Study of Cavitating Flow Through Various Nozzle Shapes, SAE Paper 919, 199.