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Effect of the air outlet louver on the noise generated by the outdoor set of a split-unit air conditioner
Applied Thermal Engineering 26 (2006) 1737–1745 www.elsevier.com/locate/apthermeng
Effect of the air outlet louver on the noise generated by the outdoor set of a split-unit air conditioner Junwei Hu, Guoliang Ding
*
Department of Power and Energy Engineering, Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China Received 24 March 2005; accepted 10 October 2005 Available online 9 December 2005
Abstract In order to analyze the influence of the air outlet louver on the noise generated by the outdoor set of a split-unit air conditioner, the flow field in the outdoor set is simulated with the CFD software STAR-CD. The relative turbulent intensities are computed and the influence of the shape and the steel wire diameter of the air outlet louver on the noise generated by the outdoor set is analyzed. The results of computation and experiment show that the circular shape of air outlet louver gives benefit of noise decrease. An air outlet louver with different diameters of steel wires is designed on the basis of analysis of the influence of the steel wire diameter of the air outlet louver on the noise. The computation and experiment validate that the air outlet louver with different diameters of steel wires can improve the aerodynamic performance and reduce the noise generated by the outdoor set. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Air outlet louver; Outdoor set; Air conditioner; Simulation; Relative turbulent intensity
1. Introduction Noise is a very important performance of a split-unit air condition which consists of indoor set and outdoor set. Noise reduction of the indoor set has attracted a lot of researches. Koo [1,2] investigated the noise of crossflow fans on split-unit air conditioners and improved the structure of cross-flow fans for the noise decreasing of air-conditioners. Hayashi and Kobayashi [3] adopted frequency modification to reduce the rotating noise of cross-flow fans in split-unit air conditioners. In order to control the noise generated by the indoor set of air conditioners, Deng et al. [4] used the multifolding heat exchanger with local rewinds. *
Corresponding author. Tel.: +86 21 62932110; fax: +86 21 62932601. E-mail address: [email protected] (G. Ding). 1359-4311/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.032
But there are only a very few researches related to the noise reduction of the outdoor set, most of which are about high frequency noise generated by compressors [5,6]. The noise generated by an air conditioner outdoor set consists of three parts: (1) the aerodynamic noise; (2) the high frequency noise generated by the compressor; (3) the low frequency vibrating noise. Experiments of an air conditioner outdoor set under different working conditions, as shown in Table 1, show that the aerodynamic noise is dominant for the global sound level of the outdoor set. The aerodynamic noise of the outdoor set is related to the structure of the air duct system. The air duct system of the outdoor set consists of axial fan, heat exchanger, deflecting ring, air outlet louver, electric motor supporter and partition, as shown in Fig. 1. The shape of the outlet louver can be circular, as shown in Fig. 2(a) or square as shown in Fig. 2(b).
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
Nomenclature d k U I Cl l Ki v0 v R u q
diameter of steel wire (m) turbulent kinetic energy (m2 s2) flow velocity at the outdoor set exit (m s1) turbulent intensity coefficient of k-epsilon model characteristic length permeability velocity impulse (m s1) average velocity (m s1) radius of the air outlet louver (m) flow velocity into the heat exchanger (m s1) volume flow rate (m3 h1)
Lp f P
sound pressure level (dB) frequency (Hz) static pressure (Pa)
Greek symbols ai user-supplied coefficient bi user-supplied coefficient n relative turbulent intensity e dissipation rate (m2 s3) DR width (m) Dp pressure drop (Pa)
Table 1 Noise generated by air-conditioner outdoor set Air conditioner type (nominal cooling capacity) (W)
Sound pressure level under different working conditions, dB(A) Fan and compressor run together
Only fan runs
Only compressor runs
3500 5000 7000
52.8 57.9 58.5
50.6 56.7 56.9
47.1 51.7 54.9
Fig. 1. Configuration of an air-conditioner outdoor set.
The air outlet louver of the outdoor set acts not only to protect the axial fan, the compressor and the heat exchanger, but also to make the outflow well-ordered. Different configurations and orientations of the outlet louver will result in different outlet flow conditions and different
noise characteristics. Fig. 3 shows the 1/3 octave sound spectrums for the outdoor set. It indicates that the noise of the outdoor set with an air outlet louver is louder than that of the outdoor set without an air outlet louver. For the outdoor set with an air outlet louver the total sound
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
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in order to decrease the noise generated by the air outlet louver of outdoor set. 2. Numerical methodologies It is difficult to determine the quantitative influence of structure parameters of an air conditioner outdoor on noise by experiments accurately. Here we use STARCD, CFD software based on the finite-volume method, which has been well used in refrigeration and air conditioning fields [11]. The multiple rotating reference frames are used to simulate the axial fan rotating, and different angular velocities are assigned to different mesh blocks in the multiple rotating reference frames.
(a)
2.1. The mesh and boundary define The computational meshes of the air conditioner outdoor set are shown in Fig. 4(a) and hexahedron meshes
(b) Fig. 2. Shape of air outlet louvers.
Sound pressure level Lp /(dBA)
60
With air outlet louver Without air outlet louver
50 40 30 20 10
3k
k
25 k 1.
6.
15 3.
0 1. 6k
80
0
0
0
40
20
10
50
25
0
Frequency f / (Hz) Fig. 3. The influence of the air outlet louver on noise generated by the outdoor set.
pressure level is 57.1 dB. For the outdoor set without an air outlet louver the total sound pressure level is 55.7 dB. There are some researches on controlling of the aerodynamic noise generated by the axial fan [7–10]. But the influence of the air outlet louver is not included. In this paper, numerical simulation and experiments are done
Fig. 4. Meshes of computational domain: (a) the computational meshes of the air conditioner outdoor; (b) the computational meshes of rotating zone.
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
are adopted here. The meshes are divided into two zones: rotating zone and stationary zone. The rotating zone shown in Fig. 4(b) is a cylinder mesh zone with blades, which rotates at 801 rpm and 134,618 cells are used. The other of the outdoor set is the stationary zone without rotating and 50,046 cells are used. In order to define the boundary conveniently, the inlet and outlet of the air conditioner outdoor set should be extended, 99,239 cells are used in the extending part of inlet and 125,842 cells are used in the extending part of outlet. Considering the rotating speed of the axial fan, the airflow is assumed incompressible. The three-dimensional incompressible Navier–Stokes equations and the standard k–e model are used. The SIMPLE algorithm is employed to couple the pressure and velocity fields in the governing equations. The wall function is used for the viscous flow near the blades and the value of y+ is 100. In order to define the boundary conveniently, the computational boundaries of the inflow are set to a distance three times the impeller outer diameter from the outdoor set entrance. The computational boundaries of the outflow are set to a distance four times the impeller outer diameter from the outdoor set exit. A constant static pressure is imposed at the inflow boundary and a constant velocity or a constant volume flow rate is imposed at the outflow boundary. The k and e are determined by 2
k ¼ 1:5 ðU IÞ e ¼ C 0:75 l
k 1:5 l
ð1Þ ð2Þ
where U is the flow velocity at the outdoor set exit; I is the turbulent intensity, I = 0.1; Cl is the coefficient of kepsilon model, Cl = 0.09; l is the characteristic length, l = 0.005. In order to simulate the resistance of the heat exchanger to the air flow, the distributed resistance can be used. It is assumed that, within the volume containing the distributed resistance, there exists everywhere a local balance between pressure and resistance forces, such that: K i ui ¼ Dpi
ð3Þ
where Dpi is the pressure drop in direction i, Ki is the permeability, ui is the velocity in direction i. The permeability Ki is defined as follows: K i ¼ a i j u i j þ bi
ð4Þ
where ai and bi are user-supplied coefficients. In order to define ai and bi, the relation between the flow rate and pressure drop is analyzed by experiments. According to Eqs. (3) and (4), the coefficients ai and bi can be computed. The coefficients ai and bi of the frontal inflow are ai = 24.4 and bi = 200; and ai and bi of the side inflow are ai = 23.5 and bi = 204.
25 Experiment Computation 20 Static pressure p / (Pa)
1740
15
10
5
0 1000
1200
1400
1600
1800
2000
Volume flow rate q/ (m3s-1) Fig. 5. Comparison of flux and static pressure.
2.2. Validation of CFD model by experiment In order to validate the CFD model, the static pressure of the outdoor set exit is computed by the CFD model when the outdoor set exit has different air flow rate and the results of computation are compared with the experimental data, as shown in Fig. 5. Static pressure decreases as the flow rate increases for both computation and experiment. The average relative error between computational data and experimental data is 11.5%, and the relative error decreases as the flow rate increases. When the volume flow rate q is 1800 m3 h1, the relative error is only 6.7%. 2.3. The influence of the shape of the air outlet louver on the noise generated by the outdoor set In order to analyze the influence of the air outlet louver on the sound field of the outdoor set, it is necessary to analyze the whole flow field of the outdoor set firstly. Fig. 6 shows that air flow field when the volume flow rate of 1800 m3 h1 is imposed on the outflow boundary. Because of the fan rotating, the air flow throws off with rotating along the trailing edge of the rotating blade and mixes with the stationary air flow to make the region of rotating air flow large. If a square air outlet louver is used, the square gap of the louver will result in increase of the resistance the rotating air flow encounters. Then the instability of the air flow also increases and the turbulent noise of the outdoor set increases. When the circular air outlet louver is used, the circular gap is consistent with the air flow rotating, and the resistant decreases. Then air flow stability increases and the noise of the outdoor set decreases. In order to further investigate the influence of the shape of the air outlet louver on the noise generated
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
Square air outlet louver Circular air outlet louver
0.4
Relative turbulent intensity ξ
1741
0.3
0.2
0.1
0
0
50
100
150
200
250
Radius of outlet louver R/ (mm) Fig. 8. The influence of the shape of the air outlet louver on the relative turbulent intensity. Fig. 6. Flow field of the outdoor set.
by the outdoor set, the relative turbulent intensities at the exit zone of the outdoor set are computed for the square and circular air outlet louvers. The relative turbulent intensity n is defined as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 02 n ¼ v02 v ð5Þ i þ vj þ vk = where v0i , v0j , v0k are velocity impulse of x, y, z direction, respectively, v is the average velocity, and the coordinate system is shown in Fig. 1. Turbulent noise is closely related to velocity impulse of air flow. Big velocity impulse will result in the increase of turbulent noise. That is to
y
2.4. The influence of the air outlet louver steel wire on the noise of the outdoor set
ΔR
R
O
Fig. 7. Air outlet louver.
say, the turbulent noise will increase when the relative turbulence intensity n of air flow increases according to formula (5). Fig. 7 shows the region in which the relative turbulent intensities are computed. n is the average of the relative turbulent intensities at the dashed area with radius R and width DR. Fig. 8 shows the influence of the shape of the air outlet louver on the relative turbulent intensity at the exit zone of the outdoor set. It can be noted that the relative turbulent intensities of the outdoor set with a circular air outlet louver are smaller than those of the outdoor set with a square air outlet louver, subsequently the noise of the outdoor set with a circular air outlet louver is lower than that of the outdoor set with a square air outlet louver.
x
2.4.1. The influence of the diameter of the air outlet louver steel wire on the noise of the outdoor set In order to identify the influence of the diameter of the air outlet louver steel wire on the noise of the outdoor set, the relative turbulent intensities at the exit zone of the outdoor set are computed for circular air outlet louvers with different diameters of steel wires. The diameters include 1 mm, 2 mm, 3 mm, 4 mm and 5 mm. The volume flow rate of 1800 m3 h1 is also imposed on the outflow boundary. Fig. 9 shows the relative turbulent intensity at the exit zone of the outdoor for circular air outlet louvers with different steel wire diameters, n is the average of the relative turbulent intensities at the total exit zone of the outdoor set. It can be noted that the relative turbulent
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
Relative turbulent intensity ξ
0.206
d2
0.204 0.202 0.2 0.198 0.196 0.194 0.192
0
1
2
3
4
5
6
Steel wire diameter d / (mm)
d1
Fig. 9. The relative turbulent intensity at the exit zone of the outdoor.
intensity decreases as the steel wire diameter increases, i.e. the noise of the outdoor set decreases as the steel wire diameter increases. 2.4.2. The influence of the circular air outlet louver with different steel wire diameters on the noise of the outdoor set As analyzed above, the noise of the outdoor set decreases as the steel wire diameter increases. However the area of the air inlet becomes small as the steel wire diameter increases, the volume flow rate decreases and then the heat transfer decreases. In addition, it is difficult to process the air outlet louver with thick steel wire; especially it is more difficult to process the steel wire in the region where the radius R of the air outlet louver is small. In consideration of different velocities along radial at the exit zone of the outdoor set, the circular air outlet louver with different steel wire diameters can be designed. Fig. 10 shows the velocities of the exit zone of the outdoor set. It can be noted that the velocity
Fig. 11. Geometry of the circular air outlet louver with different steel wire diameter.
increases first and decreases afterwards as the radius of the air outlet louver increases, and the velocities are high in the middle of the air outlet louver. The circular air outlet louver with different steel wire diameters means that the steel wire diameters are big in the region the velocities are high, i.e. in the middle of the air outlet louver, however, the steel wire diameters are small in the region the velocities are low. The geometry of a circular air outlet louver with different steel wire diameter is shown in Fig. 11. Fig. 12 shows the comparison of the relative turbulent intensity n between a circular air outlet louver with different steel wire diameters and a circular air outlet louver with equal steel wire diameter. The diameters of
Louver with different steel wire diameter
Relative turbulent intensity ξ/ (m2s-2)
0.4
Louver with equal steel wire diameter
0.3
0.2
0.1
0
50
100
150
200
Radius of outlet louver R/ (mm) Fig. 10. Velocities at the exit zone of the air conditioner.
Fig. 12. Comparison of the relative turbulent intensity.
250
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
the circular air outlet louver with different steel wire diameters are 4 mm (d1) and 1 mm (d2). The diameter of the air outlet louver with equal steel wire diameter is 1 mm. The volume flow rate of 1800 m3 h1 is also imposed on the outflow boundary. Fig. 12 indicates that the relative turbulent intensity n of the air outlet louver with different steel wire diameters is smaller than that of the air outlet louver with equal steel wire diameter, i.e. the air outlet louver with different steel wire diameters can reduce the noise of the outdoor set. Because the steel wire diameters are big only in the middle of the air outlet louver, the changing of the volume flow rate becomes small and it is also easy to process.
3. Experimental analyses In order to validate the computational results, the noise of the outdoor set should be tested. It is necessary to ensure that the system performances of the air-conditioner will not be changed strongly while the noise can be reduced. Therefore the system energy efficient performances of the air-conditioner are also tested. 3.1. Testing device The static pressures and other system performances are tested in a calorimeter with air-enthalpy test method. When the static pressures of the exit are tested, the rotating velocity of the axial fan of the outdoor set can not be changed and the volume flow rate at the exit will be changed by adjusting the rotating velocity of the auxiliary fan. The probes of the static pressure are distributed in the inlet of the calorimeter. When the system performances are tested, the standard work condition is pulseon. The system performances are tested according to GB/T 7725-1996 [12]. The testing of noise is operated in the semianechoic room and the equipment of noise testing is the B&K2236 precise sound level meters and filters. The A-weighted noise and the spectrum of 1/3 octave are tested at the exit of the outdoor set, according to GB/ T 7725-1996.
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3.2. Analysis of testing results The testing results of the system performance are summarized in Table 2. The parameters of the system performances include volume flow rate, heat transfer and power consumption. Table 2 shows that the heat transfer and the flow volume of the outdoor set with a circular air outlet louver are larger than those of the outdoor set with a square air outlet louver and the power consumption of the outdoor set with a circular air outlet louver is smaller than that of the outdoor set with a square air outlet louver. When the circular air outlet louver with equal steel wire diameter is used, the volume flow rate and the heat transfer decrease and the power consumption increases as the steel wire diameter increases. When the air outlet louver with different steel wire diameters is used, the volume flow rate and the heat transfer increase and the power consumption decreases. The relative variation of the power consumption, the volume flow rate and the heat transfer between the air outlet louver with different steel wire diameters and the air outlet louver with equal steel wire diameters is 0.6%, 1.5% and 1.1%, respectively. So the system performances have few changes. Fig. 13 shows the 1/3 octave sound spectrums for the circular air outlet louver and the square air outlet louver. It can be noted that the sound pressure levels reduce when the circular air outlet louver is used instead of the square air outlet louver. Especially for the frequencies between 800 Hz and 2500 Hz, the reduction is obvious. In fact, the noise decreasing of frequency region between 800 Hz and 2500 Hz is related to the frequency of votex shedding when air flow goes through the outlet louver. The frequency of votex shedding is ranging between 800 Hz and 2500 Hz when air flow goes through the outlet louver and the noise will vary obviously in the frequency of votex shedding. Fig. 14 shows the testing results of noise (the total sound pressure level) generated by the outdoor set for circular air outlet louvers with different steel wire diameters. It can be noted that the noise generated by the outdoor set reduces as the steel wire diameter increases. Fig. 15 shows the 1/3 octave sound spectrums for circular air outlet louvers with different steel wire diameter
Table 2 System performance tested Serial number
Voltage (V)
Power consumption (W)
Volume flow rate (m3/h)
Heat transfer (W)
Square louver d = 1 mm Circular louver d = 1 mm Circular louver d = 2 mm Circular louver d = 3 mm Circular louver d = 4 mm Circular louver d1 = 4 mm, d2 = 1 mm
220.4 220.9 219.8 221 220.4 220.8
2018 1986 1994 2010 2024 1998
1675 1718 1695 1685 1673 1692
2355 2398 2379 2363 2348 2372
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
for the different steel wires. It can be noted that the sound pressure levels reduce when the air outlet louver with different steel wire diameter is used instead of the air outlet louver with equal steel wire diameter. Especially for the frequencies between 800 Hz and 2500 Hz, the reduction is obvious.
Sound pressure level Lp /(dBA)
50 40 30 20 Circulation air outlet louver d=1mm Square air outlet louver d=1mm
10
4. Conclusions The effect of the air outlet louver on the noise generated by an air conditioner outdoor set is investigated by the CFD simulation and experimental analysis. Following conclusions are gotten.
63 10 0 16 0 25 0 40 0 63 0 1k 1. 6k 2. 5k 4k 6. 3k 10 k 1. 6k
25 40
0
Frequency f / (Hz) Fig. 13. The influence of shape of air outlet louver on noise generated by the outdoor set.
Sound pressure level Lp /(dBA)
56.5 56 55.5 55 54.5 54
0
1
2
3
4
(1) The interaction between the air outlet louver and the axial fan can increase the noise generated by the outdoor set. (2) Compared with the square air outlet louver, the circular air outlet louver can decrease the noise generated by the outdoor set. (3) The noise generated by the outdoor set decreases as the steel wire diameter increases. However, the volume flow rate and the heat transfer decrease, and the power consumption increases. (4) Increasing the diameter of parts of steel wires on air outlet louver can decrease the noise generated by the outdoor set while the system performances have few changes.
5
Steel wire diameter d / (mm) Fig. 14. The influence of steel wire diameter of air outlet louver on noise generated by the outdoor set.
Sound pressure level Lp /(dBA)
50
40
30
Lover with different steel wire diameter Lover with equal steel wire diameter
20
10
5k .2
k
3k
1.
6.
6k
15 3.
1.
80 0
0
0
40 0
20
10
50
25
0
Frequency f / (Hz) Fig. 15. Sound spectrums of the outdoor set.
and equal steel wire diameter. The diameter is 1 mm for the equal steel wire. The d1 is 4 mm and the d2 is 1 mm
References [1] H.M. Koo, Experimental study of the noise and the performance of cross-flow fans in room air-conditioning systems, Noise Control Engineering Journal 48 (2) (2000) 41–47. [2] H.M. Koo, Discrete frequency noise reduction of the cross-flow fan of the split type room air-conditioners using the skewed stabilizers, JSME International Journal 43 (1) (2000) 104– 109. [3] T. Hayashi, Y. Kobayashi, Low-noise design of cross-flow fan based on frequency modulation, Transactions of the Japan Society of Mechanical Engineers 62 (601) (1996) 3446–3451. [4] M.Y. Deng, M. Meng, J. Wang, et al., Influence of multi-folding heat exchanger and local rewinding on the aeroacoustic of the air conditioner indoor set, Fluid Machinery (China) 9 (10) (2001) 58– 60. [5] Z.Q. Bao, Z.Z. Yang, Researches of the fault diagnosis and noise control on the air conditioner, Fluid Machinery (China) 29 (9) (2001) 41–43. [6] J.W. Lu, W.X. Wu, T.N. Chen, et al., Fault diagnosis of the noise of a split-unit air conditioner, Machine Design and Research (China) 19 (3) (2003) 65–67. [7] T. Fukano, Y. Kodama, Y. Takamatsu, The effect of the tip clearance on the noise of low pressure axial and mixedflow fans, Journal of Sound and Vibration 105 (1986) 299–308. [8] D.A. Quinlan, P.H. Bent, High frequency noise generation in small axial flow fans, Journal of Sound and Vibration 218 (2) (1998) 177–204.
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745 [9] A. Maaloum, S. Kouidri, F. Bakir, et al., Effect of inlet duct contour and lack thereof on the noise generated of an axial flow fan, Applied Acoustics 64 (2003) 999–1010. [10] T. Fukano, C.M. Jang, Tip clearance noise of axial flow fans operating at design and off-design condition, Journal of Sound and Vibration 275 (2004) 1027–1050.
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[11] G.L Ding, H.T. Qiao, Z.L. Lu, Ways to improve thermal uniformity inside a refrigerator, Applied Thermal Engineering 24 (13) (2004) 1827–1840. [12] Q.P. Shi, J.M. Jiang, Z.Q. Qian, et al., GB/T 7725-1996 Room Air Conditioners, State Bureau of Technical Supervision, Peking, 1996.
Effect of the air outlet louver on the noise generated by the outdoor set of a split-unit air conditioner Junwei Hu, Guoliang Ding
*
Department of Power and Energy Engineering, Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, China Received 24 March 2005; accepted 10 October 2005 Available online 9 December 2005
Abstract In order to analyze the influence of the air outlet louver on the noise generated by the outdoor set of a split-unit air conditioner, the flow field in the outdoor set is simulated with the CFD software STAR-CD. The relative turbulent intensities are computed and the influence of the shape and the steel wire diameter of the air outlet louver on the noise generated by the outdoor set is analyzed. The results of computation and experiment show that the circular shape of air outlet louver gives benefit of noise decrease. An air outlet louver with different diameters of steel wires is designed on the basis of analysis of the influence of the steel wire diameter of the air outlet louver on the noise. The computation and experiment validate that the air outlet louver with different diameters of steel wires can improve the aerodynamic performance and reduce the noise generated by the outdoor set. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Air outlet louver; Outdoor set; Air conditioner; Simulation; Relative turbulent intensity
1. Introduction Noise is a very important performance of a split-unit air condition which consists of indoor set and outdoor set. Noise reduction of the indoor set has attracted a lot of researches. Koo [1,2] investigated the noise of crossflow fans on split-unit air conditioners and improved the structure of cross-flow fans for the noise decreasing of air-conditioners. Hayashi and Kobayashi [3] adopted frequency modification to reduce the rotating noise of cross-flow fans in split-unit air conditioners. In order to control the noise generated by the indoor set of air conditioners, Deng et al. [4] used the multifolding heat exchanger with local rewinds. *
Corresponding author. Tel.: +86 21 62932110; fax: +86 21 62932601. E-mail address: [email protected] (G. Ding). 1359-4311/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.032
But there are only a very few researches related to the noise reduction of the outdoor set, most of which are about high frequency noise generated by compressors [5,6]. The noise generated by an air conditioner outdoor set consists of three parts: (1) the aerodynamic noise; (2) the high frequency noise generated by the compressor; (3) the low frequency vibrating noise. Experiments of an air conditioner outdoor set under different working conditions, as shown in Table 1, show that the aerodynamic noise is dominant for the global sound level of the outdoor set. The aerodynamic noise of the outdoor set is related to the structure of the air duct system. The air duct system of the outdoor set consists of axial fan, heat exchanger, deflecting ring, air outlet louver, electric motor supporter and partition, as shown in Fig. 1. The shape of the outlet louver can be circular, as shown in Fig. 2(a) or square as shown in Fig. 2(b).
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
Nomenclature d k U I Cl l Ki v0 v R u q
diameter of steel wire (m) turbulent kinetic energy (m2 s2) flow velocity at the outdoor set exit (m s1) turbulent intensity coefficient of k-epsilon model characteristic length permeability velocity impulse (m s1) average velocity (m s1) radius of the air outlet louver (m) flow velocity into the heat exchanger (m s1) volume flow rate (m3 h1)
Lp f P
sound pressure level (dB) frequency (Hz) static pressure (Pa)
Greek symbols ai user-supplied coefficient bi user-supplied coefficient n relative turbulent intensity e dissipation rate (m2 s3) DR width (m) Dp pressure drop (Pa)
Table 1 Noise generated by air-conditioner outdoor set Air conditioner type (nominal cooling capacity) (W)
Sound pressure level under different working conditions, dB(A) Fan and compressor run together
Only fan runs
Only compressor runs
3500 5000 7000
52.8 57.9 58.5
50.6 56.7 56.9
47.1 51.7 54.9
Fig. 1. Configuration of an air-conditioner outdoor set.
The air outlet louver of the outdoor set acts not only to protect the axial fan, the compressor and the heat exchanger, but also to make the outflow well-ordered. Different configurations and orientations of the outlet louver will result in different outlet flow conditions and different
noise characteristics. Fig. 3 shows the 1/3 octave sound spectrums for the outdoor set. It indicates that the noise of the outdoor set with an air outlet louver is louder than that of the outdoor set without an air outlet louver. For the outdoor set with an air outlet louver the total sound
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
1739
in order to decrease the noise generated by the air outlet louver of outdoor set. 2. Numerical methodologies It is difficult to determine the quantitative influence of structure parameters of an air conditioner outdoor on noise by experiments accurately. Here we use STARCD, CFD software based on the finite-volume method, which has been well used in refrigeration and air conditioning fields [11]. The multiple rotating reference frames are used to simulate the axial fan rotating, and different angular velocities are assigned to different mesh blocks in the multiple rotating reference frames.
(a)
2.1. The mesh and boundary define The computational meshes of the air conditioner outdoor set are shown in Fig. 4(a) and hexahedron meshes
(b) Fig. 2. Shape of air outlet louvers.
Sound pressure level Lp /(dBA)
60
With air outlet louver Without air outlet louver
50 40 30 20 10
3k
k
25 k 1.
6.
15 3.
0 1. 6k
80
0
0
0
40
20
10
50
25
0
Frequency f / (Hz) Fig. 3. The influence of the air outlet louver on noise generated by the outdoor set.
pressure level is 57.1 dB. For the outdoor set without an air outlet louver the total sound pressure level is 55.7 dB. There are some researches on controlling of the aerodynamic noise generated by the axial fan [7–10]. But the influence of the air outlet louver is not included. In this paper, numerical simulation and experiments are done
Fig. 4. Meshes of computational domain: (a) the computational meshes of the air conditioner outdoor; (b) the computational meshes of rotating zone.
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
are adopted here. The meshes are divided into two zones: rotating zone and stationary zone. The rotating zone shown in Fig. 4(b) is a cylinder mesh zone with blades, which rotates at 801 rpm and 134,618 cells are used. The other of the outdoor set is the stationary zone without rotating and 50,046 cells are used. In order to define the boundary conveniently, the inlet and outlet of the air conditioner outdoor set should be extended, 99,239 cells are used in the extending part of inlet and 125,842 cells are used in the extending part of outlet. Considering the rotating speed of the axial fan, the airflow is assumed incompressible. The three-dimensional incompressible Navier–Stokes equations and the standard k–e model are used. The SIMPLE algorithm is employed to couple the pressure and velocity fields in the governing equations. The wall function is used for the viscous flow near the blades and the value of y+ is 100. In order to define the boundary conveniently, the computational boundaries of the inflow are set to a distance three times the impeller outer diameter from the outdoor set entrance. The computational boundaries of the outflow are set to a distance four times the impeller outer diameter from the outdoor set exit. A constant static pressure is imposed at the inflow boundary and a constant velocity or a constant volume flow rate is imposed at the outflow boundary. The k and e are determined by 2
k ¼ 1:5 ðU IÞ e ¼ C 0:75 l
k 1:5 l
ð1Þ ð2Þ
where U is the flow velocity at the outdoor set exit; I is the turbulent intensity, I = 0.1; Cl is the coefficient of kepsilon model, Cl = 0.09; l is the characteristic length, l = 0.005. In order to simulate the resistance of the heat exchanger to the air flow, the distributed resistance can be used. It is assumed that, within the volume containing the distributed resistance, there exists everywhere a local balance between pressure and resistance forces, such that: K i ui ¼ Dpi
ð3Þ
where Dpi is the pressure drop in direction i, Ki is the permeability, ui is the velocity in direction i. The permeability Ki is defined as follows: K i ¼ a i j u i j þ bi
ð4Þ
where ai and bi are user-supplied coefficients. In order to define ai and bi, the relation between the flow rate and pressure drop is analyzed by experiments. According to Eqs. (3) and (4), the coefficients ai and bi can be computed. The coefficients ai and bi of the frontal inflow are ai = 24.4 and bi = 200; and ai and bi of the side inflow are ai = 23.5 and bi = 204.
25 Experiment Computation 20 Static pressure p / (Pa)
1740
15
10
5
0 1000
1200
1400
1600
1800
2000
Volume flow rate q/ (m3s-1) Fig. 5. Comparison of flux and static pressure.
2.2. Validation of CFD model by experiment In order to validate the CFD model, the static pressure of the outdoor set exit is computed by the CFD model when the outdoor set exit has different air flow rate and the results of computation are compared with the experimental data, as shown in Fig. 5. Static pressure decreases as the flow rate increases for both computation and experiment. The average relative error between computational data and experimental data is 11.5%, and the relative error decreases as the flow rate increases. When the volume flow rate q is 1800 m3 h1, the relative error is only 6.7%. 2.3. The influence of the shape of the air outlet louver on the noise generated by the outdoor set In order to analyze the influence of the air outlet louver on the sound field of the outdoor set, it is necessary to analyze the whole flow field of the outdoor set firstly. Fig. 6 shows that air flow field when the volume flow rate of 1800 m3 h1 is imposed on the outflow boundary. Because of the fan rotating, the air flow throws off with rotating along the trailing edge of the rotating blade and mixes with the stationary air flow to make the region of rotating air flow large. If a square air outlet louver is used, the square gap of the louver will result in increase of the resistance the rotating air flow encounters. Then the instability of the air flow also increases and the turbulent noise of the outdoor set increases. When the circular air outlet louver is used, the circular gap is consistent with the air flow rotating, and the resistant decreases. Then air flow stability increases and the noise of the outdoor set decreases. In order to further investigate the influence of the shape of the air outlet louver on the noise generated
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
Square air outlet louver Circular air outlet louver
0.4
Relative turbulent intensity ξ
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0.3
0.2
0.1
0
0
50
100
150
200
250
Radius of outlet louver R/ (mm) Fig. 8. The influence of the shape of the air outlet louver on the relative turbulent intensity. Fig. 6. Flow field of the outdoor set.
by the outdoor set, the relative turbulent intensities at the exit zone of the outdoor set are computed for the square and circular air outlet louvers. The relative turbulent intensity n is defined as follows: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 02 02 n ¼ v02 v ð5Þ i þ vj þ vk = where v0i , v0j , v0k are velocity impulse of x, y, z direction, respectively, v is the average velocity, and the coordinate system is shown in Fig. 1. Turbulent noise is closely related to velocity impulse of air flow. Big velocity impulse will result in the increase of turbulent noise. That is to
y
2.4. The influence of the air outlet louver steel wire on the noise of the outdoor set
ΔR
R
O
Fig. 7. Air outlet louver.
say, the turbulent noise will increase when the relative turbulence intensity n of air flow increases according to formula (5). Fig. 7 shows the region in which the relative turbulent intensities are computed. n is the average of the relative turbulent intensities at the dashed area with radius R and width DR. Fig. 8 shows the influence of the shape of the air outlet louver on the relative turbulent intensity at the exit zone of the outdoor set. It can be noted that the relative turbulent intensities of the outdoor set with a circular air outlet louver are smaller than those of the outdoor set with a square air outlet louver, subsequently the noise of the outdoor set with a circular air outlet louver is lower than that of the outdoor set with a square air outlet louver.
x
2.4.1. The influence of the diameter of the air outlet louver steel wire on the noise of the outdoor set In order to identify the influence of the diameter of the air outlet louver steel wire on the noise of the outdoor set, the relative turbulent intensities at the exit zone of the outdoor set are computed for circular air outlet louvers with different diameters of steel wires. The diameters include 1 mm, 2 mm, 3 mm, 4 mm and 5 mm. The volume flow rate of 1800 m3 h1 is also imposed on the outflow boundary. Fig. 9 shows the relative turbulent intensity at the exit zone of the outdoor for circular air outlet louvers with different steel wire diameters, n is the average of the relative turbulent intensities at the total exit zone of the outdoor set. It can be noted that the relative turbulent
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
Relative turbulent intensity ξ
0.206
d2
0.204 0.202 0.2 0.198 0.196 0.194 0.192
0
1
2
3
4
5
6
Steel wire diameter d / (mm)
d1
Fig. 9. The relative turbulent intensity at the exit zone of the outdoor.
intensity decreases as the steel wire diameter increases, i.e. the noise of the outdoor set decreases as the steel wire diameter increases. 2.4.2. The influence of the circular air outlet louver with different steel wire diameters on the noise of the outdoor set As analyzed above, the noise of the outdoor set decreases as the steel wire diameter increases. However the area of the air inlet becomes small as the steel wire diameter increases, the volume flow rate decreases and then the heat transfer decreases. In addition, it is difficult to process the air outlet louver with thick steel wire; especially it is more difficult to process the steel wire in the region where the radius R of the air outlet louver is small. In consideration of different velocities along radial at the exit zone of the outdoor set, the circular air outlet louver with different steel wire diameters can be designed. Fig. 10 shows the velocities of the exit zone of the outdoor set. It can be noted that the velocity
Fig. 11. Geometry of the circular air outlet louver with different steel wire diameter.
increases first and decreases afterwards as the radius of the air outlet louver increases, and the velocities are high in the middle of the air outlet louver. The circular air outlet louver with different steel wire diameters means that the steel wire diameters are big in the region the velocities are high, i.e. in the middle of the air outlet louver, however, the steel wire diameters are small in the region the velocities are low. The geometry of a circular air outlet louver with different steel wire diameter is shown in Fig. 11. Fig. 12 shows the comparison of the relative turbulent intensity n between a circular air outlet louver with different steel wire diameters and a circular air outlet louver with equal steel wire diameter. The diameters of
Louver with different steel wire diameter
Relative turbulent intensity ξ/ (m2s-2)
0.4
Louver with equal steel wire diameter
0.3
0.2
0.1
0
50
100
150
200
Radius of outlet louver R/ (mm) Fig. 10. Velocities at the exit zone of the air conditioner.
Fig. 12. Comparison of the relative turbulent intensity.
250
J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
the circular air outlet louver with different steel wire diameters are 4 mm (d1) and 1 mm (d2). The diameter of the air outlet louver with equal steel wire diameter is 1 mm. The volume flow rate of 1800 m3 h1 is also imposed on the outflow boundary. Fig. 12 indicates that the relative turbulent intensity n of the air outlet louver with different steel wire diameters is smaller than that of the air outlet louver with equal steel wire diameter, i.e. the air outlet louver with different steel wire diameters can reduce the noise of the outdoor set. Because the steel wire diameters are big only in the middle of the air outlet louver, the changing of the volume flow rate becomes small and it is also easy to process.
3. Experimental analyses In order to validate the computational results, the noise of the outdoor set should be tested. It is necessary to ensure that the system performances of the air-conditioner will not be changed strongly while the noise can be reduced. Therefore the system energy efficient performances of the air-conditioner are also tested. 3.1. Testing device The static pressures and other system performances are tested in a calorimeter with air-enthalpy test method. When the static pressures of the exit are tested, the rotating velocity of the axial fan of the outdoor set can not be changed and the volume flow rate at the exit will be changed by adjusting the rotating velocity of the auxiliary fan. The probes of the static pressure are distributed in the inlet of the calorimeter. When the system performances are tested, the standard work condition is pulseon. The system performances are tested according to GB/T 7725-1996 [12]. The testing of noise is operated in the semianechoic room and the equipment of noise testing is the B&K2236 precise sound level meters and filters. The A-weighted noise and the spectrum of 1/3 octave are tested at the exit of the outdoor set, according to GB/ T 7725-1996.
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3.2. Analysis of testing results The testing results of the system performance are summarized in Table 2. The parameters of the system performances include volume flow rate, heat transfer and power consumption. Table 2 shows that the heat transfer and the flow volume of the outdoor set with a circular air outlet louver are larger than those of the outdoor set with a square air outlet louver and the power consumption of the outdoor set with a circular air outlet louver is smaller than that of the outdoor set with a square air outlet louver. When the circular air outlet louver with equal steel wire diameter is used, the volume flow rate and the heat transfer decrease and the power consumption increases as the steel wire diameter increases. When the air outlet louver with different steel wire diameters is used, the volume flow rate and the heat transfer increase and the power consumption decreases. The relative variation of the power consumption, the volume flow rate and the heat transfer between the air outlet louver with different steel wire diameters and the air outlet louver with equal steel wire diameters is 0.6%, 1.5% and 1.1%, respectively. So the system performances have few changes. Fig. 13 shows the 1/3 octave sound spectrums for the circular air outlet louver and the square air outlet louver. It can be noted that the sound pressure levels reduce when the circular air outlet louver is used instead of the square air outlet louver. Especially for the frequencies between 800 Hz and 2500 Hz, the reduction is obvious. In fact, the noise decreasing of frequency region between 800 Hz and 2500 Hz is related to the frequency of votex shedding when air flow goes through the outlet louver. The frequency of votex shedding is ranging between 800 Hz and 2500 Hz when air flow goes through the outlet louver and the noise will vary obviously in the frequency of votex shedding. Fig. 14 shows the testing results of noise (the total sound pressure level) generated by the outdoor set for circular air outlet louvers with different steel wire diameters. It can be noted that the noise generated by the outdoor set reduces as the steel wire diameter increases. Fig. 15 shows the 1/3 octave sound spectrums for circular air outlet louvers with different steel wire diameter
Table 2 System performance tested Serial number
Voltage (V)
Power consumption (W)
Volume flow rate (m3/h)
Heat transfer (W)
Square louver d = 1 mm Circular louver d = 1 mm Circular louver d = 2 mm Circular louver d = 3 mm Circular louver d = 4 mm Circular louver d1 = 4 mm, d2 = 1 mm
220.4 220.9 219.8 221 220.4 220.8
2018 1986 1994 2010 2024 1998
1675 1718 1695 1685 1673 1692
2355 2398 2379 2363 2348 2372
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745
for the different steel wires. It can be noted that the sound pressure levels reduce when the air outlet louver with different steel wire diameter is used instead of the air outlet louver with equal steel wire diameter. Especially for the frequencies between 800 Hz and 2500 Hz, the reduction is obvious.
Sound pressure level Lp /(dBA)
50 40 30 20 Circulation air outlet louver d=1mm Square air outlet louver d=1mm
10
4. Conclusions The effect of the air outlet louver on the noise generated by an air conditioner outdoor set is investigated by the CFD simulation and experimental analysis. Following conclusions are gotten.
63 10 0 16 0 25 0 40 0 63 0 1k 1. 6k 2. 5k 4k 6. 3k 10 k 1. 6k
25 40
0
Frequency f / (Hz) Fig. 13. The influence of shape of air outlet louver on noise generated by the outdoor set.
Sound pressure level Lp /(dBA)
56.5 56 55.5 55 54.5 54
0
1
2
3
4
(1) The interaction between the air outlet louver and the axial fan can increase the noise generated by the outdoor set. (2) Compared with the square air outlet louver, the circular air outlet louver can decrease the noise generated by the outdoor set. (3) The noise generated by the outdoor set decreases as the steel wire diameter increases. However, the volume flow rate and the heat transfer decrease, and the power consumption increases. (4) Increasing the diameter of parts of steel wires on air outlet louver can decrease the noise generated by the outdoor set while the system performances have few changes.
5
Steel wire diameter d / (mm) Fig. 14. The influence of steel wire diameter of air outlet louver on noise generated by the outdoor set.
Sound pressure level Lp /(dBA)
50
40
30
Lover with different steel wire diameter Lover with equal steel wire diameter
20
10
5k .2
k
3k
1.
6.
6k
15 3.
1.
80 0
0
0
40 0
20
10
50
25
0
Frequency f / (Hz) Fig. 15. Sound spectrums of the outdoor set.
and equal steel wire diameter. The diameter is 1 mm for the equal steel wire. The d1 is 4 mm and the d2 is 1 mm
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J. Hu, G. Ding / Applied Thermal Engineering 26 (2006) 1737–1745 [9] A. Maaloum, S. Kouidri, F. Bakir, et al., Effect of inlet duct contour and lack thereof on the noise generated of an axial flow fan, Applied Acoustics 64 (2003) 999–1010. [10] T. Fukano, C.M. Jang, Tip clearance noise of axial flow fans operating at design and off-design condition, Journal of Sound and Vibration 275 (2004) 1027–1050.
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