By Jonghyun Park, Applied Technology Research Department, Hyundai MOBIS, Korea

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During the development of a new car, module designs are commonly used to simplify the manufacturing process and minimize problems occurring during the assembly of components. A Front End Module (FEM) is one such automotive component consisting of a cooling module (condenser, radiator, fan, and shroud), headlamp, bumper, and carrier. The cooling module is a core part of the FEM because of its role in the air conditioning and engine cooling systems. The acoustic performance of the cooling fan is important as well, when considering human sensitivity to noise.

Velocity (left) and pressure (right) contours on the mid-plane of the radiator fan

Geometry of the FEM cooling fans and shroud

Close-up view of the cooling module

The component that was expected to be the main noise source in the FEM consists of a radiator fan and a cooling fan, both enclosed in a shroud. The present work considered only the radiator fan operating with the condenser fan fixed. Condenser and radiator heat exchangers were included in the simulation, although the radiator heat exchanger tank was not. Three-dimensional laser scanning equipment was used to obtain a digital model of the radiator fan. From the laser scanner, a cloud of points with the coordinates of the external surface of the fan blade was generated. The resulting geometry was used to build a hybrid mesh of about 2 million cells. The rotation speed of the fan was set at 1875 rpm. This speed was chosen so that the period of one blade passing would be about 4ms for convenience of checking the simulation results. A porous media was used to represent the heat exchanger. Using characteristic curves for the pressure drop vs. velocity, viscous and inertial loss coefficients were calculated and then used in the numerical simulation as boundary conditions.

The computational domain consisted of a rotational zone containing the radiator fan, and large stationary zones elsewhere. For steady-state simulations, the multiple reference frames (MRF) model was used, and for unsteady simulations, the sliding mesh model was used. Two partition walls at the front and rear faces of the radiator were used so that air is drawn into the radiator fan through the heat exchanger. Beyond the fan and shroud region, the computational domain was extended upstream and downstream to minimize edge effects. A pressure boundary condition was applied to both the inlet and outlet boundaries. A gauge pressure of zero was applied at the outlet, and a suitable value was determined for the inlet. Stationary side walls were used with a no-slip condition to minimize the wall interference effect. The turbulent nature of the flow was incorporated through the standard k-emodel. Since the first objective of the study was to set up a process for aeroacoustic simulations of FEM cooling fans, the more costly large eddy simulation (LES) or detached eddy simulation (DES) models were not used. These models will be considered separately in the future, however.

The CFD simulation process began with a steady flow analysis using MRF. Using the preliminary results, an unsteady calculation was then performed using a sliding mesh. During the unsteady calculation, oscillating values of pressure and velocity at several monitoring points located behind the rotating fan were checked. Because of passing fan blades, the periodic time histories of pressure and velocity values were used to indicate when the unsteady flow calculation was fully-developed. Only after this stage had been reached was an unsteady acoustic analysis performed.

Flow pathlines through the radiator fan

The unsteady flow results were found to be similar to actual flow through the fan, with a predicted flow rate of approximately 1200 m3/hr. A periodic steady-state condition was reached about 10 ms after the unsteady calculation was launched. During this stage, the oscillations in the monitored variables had a period of about 4 ms, which is equal to that of blade passing in the radiator fan.

Time history of velocity fluctuations at the monitoring point

Time history of sound pressure fluctuations calculated at the receiver position

Sound Pressure Level (SPL) prediction graph

Starting 40 ms after the start of the unsteady calculation, the aeroacoustic calculation was begun. The fan and shroud were treated as the main noise source and a point 1 m upstream from the center of the radiator fan hub was specified as the receiver. This is a common location for microphones in a test setup. Data acquired from this receiver point was used to compute sound pressure fluctuations. These fluctuations, with a magnitude of about 0.07 Pa, were found to be periodic, with a primary period of about 4 ms. This result indicates that the blade of the radiator fan is the main contribution to the aeroacoustic characteristics of the flow field. A graph of the sound pressure level (SPL) suggests that the dominant mode occurs at 250 Hz, which corresponds to the blade passing frequency. Other peaks in the spectrum are due to interference between the rotating blades and shroud. The overall SPL value calculated from the CFD simulation is 60.0 dB.

References:

1. Henner, M.; Levasseur, A.; Moreau, S. Detailed CFD Modeling of Engine Cooling Fan Systems Airflow; SAE 2003-01-0615, March 2003.
2. Nashimoto, A.; Akuto, T.; Nagase, Y.; Fujisawa, N. Aerodynamic Noise Reduction by Use of a Cooling Fan with Winglets; SAE 2003-01-0531, March 2003.

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