PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

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Seo-Yoon Ryu1; Cheolung Cheong1 ;Jong Wook Kim2; Byung Il Park2; and Se Min Park2;
1

Pusan National University, Busan 609-735, Republic of South Korea
2 LG Electronics, Changwon 642-713, Republic of South Korea

ABSTRACT
It is well known that the flow performance, efficiency, and aerodynamic noise of axial-flow fans heavily
depend on the gap distance between the fan blade tip and the fan shroud. As the blade tip clearance becomesless, the flow performance increases, and the aerodynamic noise decrease. These results can be attributed tothe decrease of tip vortex strength. However, the gap clearance cannot lessen below a certain critical value
because of the manufacturing, installation, and operational constraints. Besides, the fan shroud generally
covers the fan blade tip partially because of its installation environment. Recently, as an alternative, the tiprake shape is introduced to solve these issues, which is generally known to weaken the tip vortex strength as
the wing tip fence does in the airplane. However, the effectiveness of tip-rake is not apparent because of theadditional interaction of tip rake with the fan shroud. In this paper, the effects of blade tip shape on tip vortex
structure and aerodynamic noise of a cooling fan, which are used as a cooling fan in an outdoor unit of airconditioners, are numerically and experimentally investigated. Two types of axial fans are considered: one iswith blade tip-rake, and the other is without blade tip-rake. First, the flow field around the fans was predictedby solving the three-dimensional unsteady incompressible RANS equations using computational fluid
dynamics techniques. Then, the validity of the numerical results is confirmed by comparing the predictedflow rates with the measured ones. The characteristics of flow field driven by the fans with and without blade
tip rake are analyzed in detail, especially with an emphasis on the coherent vortex structure. Finally, the flow
performance, efficiency and aerodynamic noise of axial fans are characterized according to the vortex
structure whose characteristics are also closely related to blade tip shape as well as the fan shroud.
Keywords: axial-flow fans, tip-rake, tip vortex, shroud-tip vortex, aerodynamic noise, air-conditioner

1. INTRODUCTION
Air conditioners are used to control air quality in a confined space by removing heat and moisturefrom it. The market demands for air conditioners have continuously increased. The split type air
conditioner generally consists of two units: outdoor and indoor units. The performance of the air
conditioner is dominantly affected by that of the outdoor unit, which is closely related to theperformance of cooling fans as well as of compressors. As the performance of the air conditioner
increases, the higher cooling capacity by a fan is required, which in turn requires higherair flow andccheong@pusan.ac.kr

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higher rotational fan speed. However, the increase of a fan speed may emphasize the relative
importance of aerodynamic noise. The high performance and low noise fans are, therefore, essential
for the air-conditioners.
The axial-flow fan is generally used as a cooling fan for the outdoor unit of an air-conditioner.
Compared with the centrifugal fan, the flow structure driven by the axial -flow fan is more complex,
which implies that the various design parameters are related to high performance and low noise axial flow fans: airfoil geometry (chamber line, thickness), pitch angle, solidity, tip geometry and so on.
Fukano et al. (1) showed that the ratio of the wake width to the pitch of the blade row was thedominant factor of the turbulent noise. Fukano et al. (2) also showed that the less tip clearance
improved the fan performance and reduced the noise level, effectivel y. Ren et al. (3) investigated theeffects of shroud structure designs on a volume flow rate of axial -flow fan system in a machine roomby varying its length and showed that there was the optimum length of housing for maximum volume
flow rate. Heo et al. (4) improved aerodynamic and aeroacoustic performances of an axial-flow fanunit in the mechanical room of a household refrigerator by modifying its shroud structure withoutchanging the fan blade shape. Zhao et al. (5) investigated the shapes of the tip and trailing edge ofaxial fan blades to reduce fan noise in an outdoor unit. Jiang et al. (6) investigated the aeroacoustics
of an axial fan in an outdoor unit. Fukano’s model, combined with typical computational fluid
dynamics (CFD) simulation, is used to predict the broadband noise. After the predicted results werecompared with the measured data, the distance to blade trailing edge is proposed as an importantparameter to improve the accuracy of noise predictions in the Fukano’s model. Wright et al. (7)presented the aerodynamic and acoustic properties of axial-flow fans with swept blades based on thetheory of Kerschen and Envia for swept cascades. The experimental results confirmed that the sweptbladed fans show noise savings compared to the zero-sweep baseline model. Jin et al. (8) investigatedthe effect of sweep on the aerodynamic stall limit and aerodynamic noise source. The swept blade wasfound to be effective in controlling unsteady flow to improve aerodynamic stall limit and reduce noise
source in rotors at off-design points. Ye et al. (9) investigated the effects of various tip structures on
the flow field, losses distribution, and noise characteristics and improved fan performance bychanging the blade tip structure. It was shown that the grooved blade tips improved the efficiency byreducing mixing losses between the leakage flow and mainstream, but increased the fan noise.
These studies showed that the flow performance, efficiency and aerodynamic noise of axial-flowfans heavily depend on the gap clearance between the fan blade tip and the fan shroud; as the gap
distance becomes less, the flow performance increases and the aerodynamic noise decrease. These
results are generally attributed to the decrease of tip vortex streng th. However, it is difficult todecrease the gap clearance without limit because of manufacturing, installation, and operational
constraints. Besides, the fan shroud generally cannot cover the fan blade tip fully because of its
installation environment. Recently, as an alternative, the tip-rake shape is introduced to solve theseissues, which is generally known to weaken the tip vortex strength as the wing tip fence does in theairplane (10). Representatively, winglet developed by Richard T. Whitcomb (11) has been used in anaircraft to reduce drag and to save fuel and cost. Also, in a marine propulsion system, the KAPPEL
propeller using the tip rake was developed to increase efficiency (12).
However, the effectiveness of tip-rake is not clear because of the inevitable interaction of tip rakewith the fan shroud. In this paper, the effects of blade tip shape on tip vor tex structure, flow
performance, efficiency and aerodynamic noise of a cooling fan, which are used as a cooling fan in
an outdoor unit of air-conditioners, are numerically and experimentally investigated. Two types ofaxial fans are considered: one is with blade tip-rake, and the other is without blade tip-rake. First, theflow field around the fans was predicted by solving the three-dimensional unsteady incompressible
RANS equations using computational fluid dynamics techniques. Then, the validity of the n umericalresults is confirmed by comparing the predicted flow rates with the measured ones. The characteristicsof flow field driven by the fans with and without blade tip rake are analyzed in detail, especially with
an emphasis on the coherent vortex structure. Finally, the flow performance, efficiency andaerodynamic noise of axial fans are characterized according to the vortex structure whosecharacteristics are also closely related to blade tip shape as well as the fan shroud.

2. TARGET AXIAL-FLOW FANS
The two types of axial fans considered in this study are shown in Fig. 1: one is with blade tip-rake(Model A), and the other is without blade tip-rake (Model B). These fans are used as cooling fans for5376

an outdoor unit of split-type air-conditioners. The tip shapes between two fans are compared in detailat the three cross-sectional planes of constant azimuthal angles that are denoted by the circled numbers£ ¤and ¥ in Figs. 1 and 2.

Figure 1 – Model A

Figure 2 – Model B

3. GOVERNING EQUATIONS AND NUMERICAL METHODSTo predict the flow performance of the target blades, the following three -dimensional
incompressible unsteady Reynolds-Averaged Navier-Stokes (RANS) equations are used as thegoverning equations:߲൫‫ ݑ‬൯ ൌ Ͳ߲‫ݔ‬௝ ௝(1)߲ͳ ߲‫݌‬߲߲‫ݑ‬௜ ߲‫ݑ‬௝߲ሺ‫ ݑ‬ሻ ൅ሺ‫ ݑ ݑ‬ሻ ൌ െ൅ቆߥ ቆ൅ቇ െ ‫ݑ‬௝′ ‫ݑ‬௜′ ቇ߲‫ݔ‬௝ ߲‫ݔ‬௜߲‫ݔ‬௝ ௝ ௜ߩ଴ ߲‫ݔ‬௜ ߲‫ݔ‬௝߲‫ ݐ‬௜

(2)Eqs. (1) and (2) are numerically solved using the finite volume method based on the unstructured
meshes. The k-ƃ RNG turbulence model is used as a closer. Numerical computations are realizedusing the ANSYS FLUENT. For more detailed numerical schemes used in this study, the studies (3,4)
can be referred to.

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4. NUMERICAL RESULTS AND DISCUSSION
4.1 Validation of numerical methods
The virtual fan performance tester is designed, as shown in Figure 3, to investigate the detailed
characteristics of the flow field driven by axial fans. The rectangular ducts are set on upstream anddownstream of the axial-flow fan. To simulate the experimental conditions of actual fan performancetester and thus to predict the P-Q curve of the target fan unit, the pressure difference between inlet
and outlet boundary are varied.
The numerical results predicted using the virtual fan performance tester are compared with themeasured one using the actual fan performance tester, which satisfies the regulations of AMCA 21007. To improve measurement accuracy, a large chamber of 600(W) ൈ 600(D)ൈ 1500(L) mm, fivedifferent-sized nozzles and two screens were installed. The more detailed information about the fanperformance tester was provided in the preceding studies (3,4).
Figure 4 shows a comparison between two results for Model A. It is seen that the range of relative
error in the volume flow rate for the two fixed pressure differences at two rotational speeds spans
between 0.6% and 6.6%. This result confirms the validity of the current numerical method.

Figure 3 – Computational domain for virtual fan performance tester

Figure 4 – Comparison of predicted volume flow rate at a given pressure difference withmeasured P-Q curves

4.2 Detailed flow field analysis
The predicted flow fields driven by the target fans are investigated in detail to assess the effects of
blade tip rake on the flow structure. Figure 5 shows the snapshots of the velocity vectors near theblade tip on the horizontal cross-sectional plane passing the rotating center of the fan. It can be seenthat the tip vortex around Model B is stronger than Model A. This result confirm that the blade tip-5378

rake geometry reduces the tip vortex strength. Further analysis is carried out to identify the detailed
vortex structure around the tip and to assess the vortex strength quantitatively.

Figure 5 (a) – Model A

Figure 5 (b) – Model B
To identify the coherent vortex structure, Q-criterion is used as a vortex-identification criterion.
Figure 6 shows the iso-contours of Q-criterion between the blade tip and the fan shroud for the twomodels. Two different large coherent vortex structures can be found in both models: one is well-known
blade tip vortex, and the other is shroud-tip vortex which is formed along with the shroud in the tip
clearance region. The blade tip vortex is generated from the leading edge, develops along with thechord of the blade tip, and interacts with the shroud-tip vortex in the gap region between the blade tip
and the blade shroud. For a more quantitative comparison of vortex strength between two models, thepressure distribution around the vortex core is compared in Figure 7. It can be seen that the minimumpressure at the center of vortex core of Model B is lower than that of Model A . This implies that thestronger tip vortex forms inthe flow driven by the fan of Model B. However, the pressure distributionsin the gap region between the blade tip and the fan shroud are similar in two models, which meansthat the tip-shroud vortex is not affected by the tip rake. Figure 7 also compares the su rface pressure
distribution of the fan blades where the stronger tip vortex shadow can be identified in Model B thanin Model A. From the fact that the surface pressure fluctuations play a role of dipole sources for
aerodynamic noise, it can be inferred that the aerodynamic noise of Model B is higher than that ofModel A. The averaged thrust and torque of a fan blade are computed as overall flow performanceindex and are listed in Table 1. Model B shows higher thrust and torque than Model A. From the factthat the thrust is directly related to the volume flow rate and the torque is related to the energy
consumption, it can be expected that the volume flow rate of Model B is higher than Model A, but theefficiency of Model B is lower than Model A.

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(a) – Model A(b) – Model B
Figure 6. Iso-contours of Q criterion (Green: Tip vortex /Blue: Tip-shroud vortex)

Model A(b) – Model B

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Figure 7. Comparison of pressure distributions between two models
Table 1 – Comparison of thrust and torque between two models
ModelThrust
TorqueAſ

ƀ

B

1.11ſ

1.13 ƀ

5. EXPERIMENTAL ANALYSIS
To confirm the numerical results, the proto-types of Model A and Model B were manufactured, andtheir performances are experimentally evaluated. The results are shown in Figure 8. The volume flow
rate, the sound pressure level, and the power consumption of Model A are lower than Model B. Theseresults closely follow the numerical results presented in Section 4.

Figure 7 – Experimental results

6. CONCLUSIONSIn this study, the flow performance, efficiency, and noise of the axial fans used in the outdoor unitof the split-type air-conditioner were numerically and experimentally investigated with emphasis on
the effects of blade tip-rake in association with the fan shroud. Two types of fans were considered:
Model A is with tip-rake, and Model B is without tip-rake. The blade tip vortex and the shroud-tip
vortex were found as the most massive coherent vortex structure in the flow driven by both fans. As
expected, the strength of tip vortex was weakened by the use of blade tip rake, but the strength ofshroud-tip vortex was similar in both models. The latter fact implies that the role of blade tip-rake is
not effective in the blade tip clearance region between the blade tip and the fan shroud. Although thevolume flow rate of Model B was higher than the Model A, the aerodynamic noise and the power
consumption of Model A are lower than Model B. All of these results are confirmed by the measured
data obtained from the prototypes.
This study showed that the shape of the blade tip rake located in the blade tip clearance region
needs to be optimized to more improve the performance of the axial fan, which will be investigatedin the next study.

ACKNOWLEDGMENTS
This study was carried out through the research projects supported financially by LG Electronics
and by Hyundai Motor Company.

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