车用空调风扇的 CFD模拟
2007年 8月 6日 - 工程流体网
By Jonghyun Park, Applied Technology Research Department, Hyundai MOBIS, Korea
During the development of a new car, module designs are commonly used to simplify th
e manufacturing process and minimize problems occurring during the assembly of compon
ents. A Front End Module (FEM) is one such automotive component consisting of a cool
ing module (condenser, radiator, fan, and shroud), headlamp, bumper, and carrier. The co
oling module is a core part of the FEM because of its role in the air conditioning and e
ngine 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 w
as 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 e
xternal 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 r
pm. 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 represe
nt the heat exchanger. Using characteristic curves for the pressure drop vs. velocity, visco
us 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 frame
s (MRF) model was used, and for unsteady simulations, the sliding mesh model was use
d. 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 regi
on, the computational domain was extended upstream and downstream to minimize edge
effects. A pressure boundary condition was applied to both the inlet and outlet boundarie
s. A gauge pressure of zero was applied at the outlet, and a suitable value was determin
ed 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 st
andard k-emodel. Since the first objective of the study was to set up a process for aeroa
coustic 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 pr
eliminary results, an unsteady calculation was then performed using a sliding mesh. Durin
g the unsteady calculation, oscillating values of pressure and velocity at several monitorin
g 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 u
nsteady 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 th
at 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 upst
ream from the center of the radiator fan hub was specified as the receiver. This is a co
mmon location for microphones in a test setup. Data acquired from this receiver point w
as used to compute sound pressure fluctuations. These fluctuations, with a magnitude of a
bout 0.07 Pa, were found to be periodic, with a primary period of about 4 ms. This resu
lt 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 shr
oud. 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 Coolin
g Fan Systems Airflow; SAE 2003-01-0615, March 2003.
2. Nashimoto, A.; Akuto, T.; Nagase, Y.; Fujisawa, N. Aerodynamic Noise Reductio
n by Use of a Cooling Fan with Winglets; SAE 2003-01-0531, March 2003.