UTMsim™

MIRA’s Thermal Management Simulation Process

MIRA’s UTMsim™ underbonnet thermal management simulation process offers the real prospect of starting to tackle complex thermal management issues from much earlier in a vehicle programme. The process can start right at the early concept stage of the design programme using FLOWMASTER 1D network software. This software is used to create a representative baseline model of a vehicle’s cooling system.

The initial results from the 1D network model are used to size the required cooling system components, which are then incorporated in to the early vehicle-packaging model in CAD. This is when 3D modelling using CFD begins in earnest. This is a particularly intensive part of the process that requires software with a good balance of ease of use, speed of model set-up, numerical model availability as well as numerical stability. We find that FLUENT software for the CFD simulation coupled with ICEM software for mesh generation fits this requirement well.

The interface between the vehicle CAD model and the meshing software is a key area of the process. At MIRA we have developed procedures and protocols to ensure the seamless transfer of data between CAD and the CFD meshing software in order to reduce CAD preparation and hence mesh generation timescales. In the same way that bucks and mule vehicles are used for development test work we do the same with early CFD models by making use of the parts bin from other similar vehicles or previous models. During the programme the CFD model becomes more refined as more design surfaces are released thus ensuring that the model tracks the design but also that we don’t have to wait until detailed CAD is available before applying this process (as by that time it would be too late).

Boundary conditions for the CFD model are derived from a combination of manufacturers’ data and our own rig tests on components. The 3D analysis with FLUENT has two main objectives. Firstly to calculate the amount of cooling air passing through the cooling pack assembly in its installed condition and secondly to provide information on the engine bay airflow and temperatures.

Cooling Airflow Through Radiator

The cooling pack airflow results are used to identify any dead zones where there is low airflow and also any zones of re-circulation. Re-circulation of cooling air can be a significant source of cooling system problems. This occurs where the cooling fan is operating and, due to poor sealing, lack of air ducting and generally poor packaging design, some of the hot air leaving the cooling pack is actually drawn back through the radiator thus elevating the effective ambient temperature. These sorts of problems can be minimised at an early stage.

The airflow data from the CFD analysis is then used to derive effective radiator velocities and temperatures that are fed in to the 1D cooling system model. This model can then be used to predict rise-over-ambient and safe-limiting-ambient temperatures that are next compared to cooling system performance targets to identify whether further changes to the vehicle CAD model are needed.

Evaluation of Heat Shields

The engine bay CFD results are also used to identify likely hot spots in order to direct the packaging of thermally sensitive components and the design of heat shields. RADTHERM heat transfer software is used for performing rapid radiation calculations to guide heat shield design. As heat shield design is a largely parametric optimisation exercise we recently developed a large database of component plus heat shield scenarios based on our extensive experience in this area. This allows heat shield design proposals to be formulated very quickly.

Results

Overall, the MIRA process ensures that an integrated simulation approach is used throughout the prototype design phase. MIRA’s process has been developed around the provision of an integrated mix of simulation, rig test and technical know-how to enable design problems to be highlighted early and solutions implemented quickly.

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