BEHR, Stuttgart has developed a completely closed chain of simulation models, that allows the virtual aerothermodynamic and structural assessment of charge air cooler designs with respect to transient temperature loads. The method consists of a transient CFD prediction of structural temperatures, then mapping onto a FEA mesh and the subsequent FEA prediction of stresses and strains in all points and at any time of a transient operation mode. Charge air and cooling air flow, heat transfer to, from and in all solids, including the fins, are fully represented. A special two step procedure, utilizing different time step sizes for solving flow quantities and energy equation makes the methodology computationally very efficient. The methodology may be used in tightly scheduled design processes, particularly in early development phases. A full validation has been performed, based on measurements on a large truck charge air cooler in a predefined test cycle, representing typical field load. The methodology is applicable also for in truck/in car application, e.g. based on data derived from an underhood simulation. Furthermore, investigations of combined charge air cooler loads like that of temperature transients and inner pressure are feasible. Work on using the results for virtual life time assessments, e.g. based on component Woehler lines or based on fatigue software is under way and will be reported in future papers. In addition, there are several other heat exchangers that experience severe temperature transients during operation. Here, the methodology may also be transferred beneficially. The basic idea of the presented methodology is to perform a transient CFD analysis including all structural parts of the charge air cooler and use the resulting structural temperatures as loads in a subsequent FEA simulation. The CFD analysis is based on a full representation of the charge air and cooling air flow, the convective and conductive heat transfer in the fluid streams, the conjugate heat transfer between the fluids and the structure as well as the heat conduction in all solid parts of the cooler. To achieve high efficiency in the model setup process, the FEA-modeled solid parts are re-used in the CFD to a large extend. Standards for the FEA meshing and the CFD meshing used at BEHR have been defined accordingly. The FEA results meet the qualitative characteristics of the measured curves in all cases. Deviations in the absolute values are relatively small, given the level of accuracy that can be reached in a strain gage measurement under the harsh conditions of a hot air test stand. In addition, it has been shown that the positions of the strain gages are in areas with strong spatial strain gradients. Therefore, even slight deviations in the positioning can lead to significant differences. In view of these inherent difficulties, the agreement is excellent.
Simulation of the aerothermodynamic and structural behavior of charge air coolers during transient operation modes
2007
11 Seiten, 6 Bilder, 4 Quellen
Conference paper
English
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