A typical underhood cooling air system is driven by a cooling fan. Computational Fluid Dynamics (CFD) is frequently used to simulate the underhood air system and the fan performance. Underhood CFD analysis consists of two separate cases, the fan-installed case and the system restriction case. System restriction plays an important role in the initial fan selection process for a new cooling system design.
In the current test and modeling method, the force driving the airflow is located at different locations in the fan-installed case and the system restriction case. For a system with a bypass between the inlet grill and the cooling package, due to the differences in driving force location, the air at the bypass flows in opposite directions in the two cases. Therefore, the system restriction predicted or measured using the current method is different from the restriction the fan “sees” in the fan-installed case.
This paper presents a new method for simulating the system restriction to accurately predict the restriction for systems with large bypasses. The idea is to define the driving force for the airflow at the fan location in the system restriction case, as it is in the fan-installed case. By doing so, similar flow patterns are obtained in these two cases.
The new method presented in this paper significantly improves the accuracy of system restriction and fan degradation predictions for air systems with large bypasses. Another important benefit is that the new method can also be used to study ram air effect on the system restriction.