Effects of inlet radius and bell mouth radius on flow rate and sound quality of centrifugal blower

Transcription

1 Journal of Mechanical Science and Technology 26 (5) (2012) 1531~ DOI /s Effects of inlet radius and bell mouth radius on flow rate and sound quality of centrifugal blower Pham Ngoc Son 1,*, Jae Won Kim 1, S. M. Byun 1 and E. Y. Ahn 2 1 Department of Mechanical engineering, Sunmoon University, Asan , Korea 2 Department of Multimedia, Hanbat National University, Daejon , Korea (Manuscript Received September 18, 2011; Revised January 30, 2012; Accepted February 7, 2012) Abstract The effect of inlet radius and bell mouth radius on flow rate of centrifugal blower were numerically simulated using a commercial CFD program, FLUENT. In this research, a total of eight numerical models were prepared by combining different values of bell mouth radii and inlet radii (the cross section of bell mouth was chosen as a circular arc in this research). The frozen rotor method combined with a realizable k-epsilon turbulence model and non-equilibrium wall function was used to simulate the three-dimensional flow inside the centrifugal blowers. The inlet radius was then revealed to have significant impact on flow rate with the maximum difference between analyzed models was about 4.5% while the bell mouth radius had about 3% impact on flow rate. Parallel experiments were carried out to confirm the results of CFD analysis. The CFD results were thereafter validated owning to the good agreement between CFD results and the parallel experiment results. In addition to performance analysis, noise experiments were carried out to analyze the dependence of sound quality on inlet radius and bell mouth radius with different flow rate. The noise experiment results showed that the loudness and sharpness value of different models were quite similar, which mean the inlet radius and the bell mouth radius didn't have a clear impact on sound quality of centrifugal blower. Keywords: Bell mouth; Centrifugal blower; CFD; Fan performance; Sound quality Introduction Centrifugal blower is a member of the turbo machinery family. With the ability to provide moderate to high pressure rise and flow rate, they are broadly used in many AHU (air handling unit) or HVAC (heat ventilation and air conditioning) systems, such as air-conditioning systems in buildings, blowers in automotive cooling units and so on. A large number of researches were carried out to optimize the geometries of centrifugal blower, which included scroll casing [1], impeller [2], cut-off [3], inlet [4] and bell mouth [5], using both experiments and CFD analysis. For CFD analysis, several methods were suggested to simulate the flow inside turbo machinery, from 2D analysis [6] to 3D analysis [7], from steady stage analysis [8] to un-steady analysis [9] and from using turbulence models [10] to using LES (large eddy simulation) [11]. The selection of analysis method was problem dependence and purpose of analysis dependence. Son et al. [5] used 3D steady stage analysis with realizable * Corresponding author. Tel.: , Fax.: address: Pham_ngoc_son@yahoo.com Recommended by Associate Editor Byeong Rog Shin. KSME & Springer 2012 k-ε turbulence model [12] to numerically study the effect of bell mouth geometries on flow rate of centrifugal blower. It was figured out that the bell mouth radius had major impact on flow rate while the gap between bell mouth and upper fan case only had minor impact on flow rate. However, as the bell mouth radius became bigger, the inlet radius also became smaller. Therefore, it was not clear whether the effect came from the change of bell mouth radius or came from the change of inlet radius. This research focused on clarifying the effect of bell mouth radius and inlet radius on flow rate of centrifugal blower. In addition to performance analysis, noise analysis of the fan was also an interest for many researchers [13, 14]. Therefore, noise experiments were carried out to reveal the effect of bell mouth radius and inlet radius on sound quality, which included loudness and sharpness value. 2. Numerical analysis 2.1 Assumptions and governing equations: In order to simplify the problem, several assumptions were proposed. At first, the flow was assumed to be incompressible, which was validated because of the relatively low speed of the flow compared with the speed of sound (Mach number was

2 1532 P. N. Son et al. / Journal of Mechanical Science and Technology 26 (5) (2012) 1531~1538 Table 1. Cases of CFD simulation. r/r 6% 9% 11% D/R (10/155) (14/155) (17.5/155) 91% (140/155) Case 1 (*) Case 2 NA (**) 86% (133/155) Case 3 Case 4 Case 5 82% (126/155) Case 6 Case 7 Case 8 Table 2. Impeller specifications. Impeller OD (mm) Impeller ID (mm) Blade number Blade outer angle (degree) * Running model in market. ** Not available: The outer part of the bell mouth will be out of the scroll casing. Fig. 2. Blower's components. Fig. 1. Parameters of bell mouth. about 0.05). Second, so as to deal with the turbulence flows inside the centrifugal blower, the equations of motion for fluid flow were time-averaged, which is well-known as Reynolds Averaged Navier-Stokes equations (RANS). The continuity and momentum equations for three dimensional incompressible flow thereafter could be written as the following using Einstein notation: Continuity Momentum To close the RANS equations, realizable k-ε turbulence model was selected. Non-equilibrium wall function [15] was used to treat the near wall region. 2.2 Geometrical models and grid generation The main parameters consist of: - r/r: ratio between bell mouth radius and impeller radius - D/R: ratio between inlet radius and impeller radius.. (1) (2) Fig. 3. Full model meshing (include extended inlet and outlet). These parameters are illustrated in Fig. 1. Total 8 cases were simulated as Table 1 (all units in mm). Parameters of impeller were shown in Table 2. The model in market was designed to run at average rotation speed of 500 rpm with flow rate around 10.3 cubic meter per minute and static pressure of 69 Pa. The geometrical model of the centrifugal blower was designed using Catia V5 R17 and then meshed using Gambit V Hexahedra elements were chosen for the meshing process. A meshing number of 1,400,000 elements were chosen as the result of the meshing refinement test. The setting of solver and boundary conditions were similar to Ref. [5], part 2.3 "Solver setting and boundary conditions". The convergence criterion was set at 10-3 for all residuals. In addition to residuals monitoring, the static pressure at inlet and outlet were checked every cycle. To ensure the true convergence of solution, the simulation was only finished when the change in static pressure was less than 0.1% or a periodical change in pressure was observed. 3. CFD results 3.1 Non-dimensional parameters All parameters of centrifugal blowers performance were normalized as following: - Flow rate coefficient (3)

3 P. N. Son et al. / Journal of Mechanical Science and Technology 26 (5) (2012) 1531~ Fig. 4. Performance curves in case of r/r = 6%. Fig. 6. Performance curves in case of r/r = 9%. Fig. 5. Efficiency curves in case of r/r = 6%. Fig. 7. Efficiency curves in case of r/r = 9%. - Pressure coefficient (4) - Power coefficient (5) - Static efficiency (6) Fig. 8. Performance curves in case of r/r = 11%. 3.2 Effect of inlet radius on centrifugal blower's performance To study the effect of inlet radius on flow rate, we fixed the bell mouth radius value and changed the inlet radius value as below: - 1st case: r/r fixed at 6% As can be seen from Fig. 4, the 86% inlet ratio (D/R) had the highest flow rate while the 91% inlet ratio (the largest inlet radius) had the lowest flow rate. The difference in flow rate between the 91% inlet ratio and the 86% inlet ratio was about 4.5% (The flow rate differences between cases were defined as the flow rate differences at maximum flow rate (free flow condition)). - 2nd case: r/r fixed at 9% Similar trend was observed in Fig. 6, in which the flow rate of the 86% inlet ratio was higher than the flow rate of the 91% and 82% inlet ratio. Fig. 9. Efficiency curves in case of r/r = 11%. - 3rd case: r/r fixed at 11% Only two models existed in figured 8 but the flow rate of the inlet ratio of 86% was still the highest one. However, the difference of flow rate between 82% and 86% was quite small compare to two previous cases. From three above cases, it's concluded that inlet ratio had major effect on flow rate of centrifugal blower. A mistake in selecting the inlet ratio, either too small value or too large

4 1534 P. N. Son et al. / Journal of Mechanical Science and Technology 26 (5) (2012) 1531~1538 Fig. 10. Performance curves with respect to bell mouth radius (D/R = 91%). Fig. 14. Performance curves with respect to bell mouth radius (D/R = 82%). Fig. 11. Efficiency curves with respect to bell mouth radius (D/R = 91%). Fig. 15. Efficiency curves with respect to bell mouth radius (D/R = 82%). radius value as below: - 1st case: D/R fixed at 91% As shown in Fig. 10, among the two models, the 6.5% radius ratio (r/r) (the smaller one) had lower flow rate. However, the flow rate improvement was not so great in this case. Fig. 12. Performance curves with respect to bell mouth radius (D/R = 86%). Fig. 13. Efficiency curves with respect to bell mouth radius (D/R = 86%). value, would result in a noticeable loss in the flow rate. 3.3 Effect of bell mouth radius on performance of centrifugal blower In order to study the effect of bell mouth radius on flow rate, we fixed the inlet radius value and changed the bell mouth - 2nd case: D/R fixed at 86% As can be seen from Fig. 12, the smallest bell mouth radius (6.5%) had the lowest flow rate. The flow rate of the 9.0% and 11.3% bell mouth radius were quite similar. - 3rd case: D/R fixed at 82% A similar trend was observed in Fig. 14. The smallest bell mouth radius had the lowest flow rate while the flow rate difference between the other two models was quite small. The difference in flow rate between the 6.5% model and the other two models was about 3%. The effect of bell mouth radius on flow rate was clearly observed in all three above cases. Too small radius would have negative impact on flow rate. The recommended value of bell mouth radius ratio was about 9% (in case of bigger radius, the change in flow rate was quite small). 3.4 Details of flow field in typical cases In order to explain the effect of inlet radius and bell mouth radius on flow rate, we'll compare the detail flow field close to the bell mouth region of the two following cases:

5 P. N. Son et al. / Journal of Mechanical Science and Technology 26 (5) (2012) 1531~ Fig. 16. Cross-section plane. Fig. 19. Flow detail in case of r/r 6% D/R 86% (case 1). Fig. 17. Flow detail in case of r/r 6% D/R 91% (case 1). Fig. 20. Contours of Vorticity in case of r/r 6% D/R 86% (case 1). which the flow just circulated and thus less flow moved out of the blowers. That should explain the reason why this model had low flow rate. The picture in Fig. 20 was totally different compare to that in Fig. 18. Under the bell mouth region, it existed only a very weak vortex. The flow field in Fig. 19 also was not disturbed as the flow field in Fig. 17. With the appropriated value of inlet radius, we could avoid creating a strong vortex under the bell mouth region and improve the flow rate of centrifugal blower. Fig. 18. Contours of vorticity in case of r/r 6% D/R 91% (case 1). Case 1 : (r/r 6% D/R 91%) and (r/r 6% D/R 86%) Case 2 : (r/r 6% D/R 86%) and (r/r 9% D/R 86%) Cross section plane 1 and 2 (Fig. 16) were chosen to illustrate the flow field of Case 1 and Case 2, respectively. Case 1: (r/r 6% D/R 91%) and (r/r 6% D/R 86%) The effect of inlet radius on flow rate would be explained in this case. As shown in Fig. 4, the flow rate difference between these two models were 4.5%. It's very easy to see a strong vortex which appeared under the bell mouth in Fig. 18. Looking carefully at Fig. 17 at the same region, a disturbed flow field could also be observed. This formation of vortex create a "dead zone" in that area in Case 2: (r/r 6% D/R 86%) and (r/r 9% D/R 86%) The effect of bell mouth radius would be explained in this case. Since the effect of bell mouth radius on flow rate was not strong as shown in Fig. 12 (only 3%), it's expected that the flow field would not be much different between the two cases. In Fig. 22, we could see the formation of two strong vortices and one mild vortex but its size was quite large. The flow field in Fig. 21 was also quite disturbed in these regions. Similar to the previous case, the vortex formation under the bell mouth was the cause of decreasing flow rate of this model. As shown in Fig. 24, one weak vortex, on strong vortex and one moderate vortex but has medium size existed under the bell mouth. Compare to Fig. 22, the vortex close to the blade in Fig. 24 was weaker and the size of the moderate vortex under bell mouth was also smaller. Since this difference was not very clear, the flow rate difference between these two models should not be very different. The result in Fig. 12 was correctly proved in this case.

6 1536 P. N. Son et al. / Journal of Mechanical Science and Technology 26 (5) (2012) 1531~ Model 1: r/r 6%, D/R 91% (original model) Fig. 25. Performance curves of experiment and CFD (model 1). Fig. 21. Flow detail in case of r/r 6% D/R 86% (case 2). - Model 2: r/r 9%, D/R 86% (best model) Fig. 26. Performance curves of experiments and CFD (model 2). Fig. 22. Contours of Vorticity in case of r/r 6% D/R 86% (case 2). Fig. 23. Flow detail in case of r/r 9% D/R 86% (case 2). Fig. 24. Contours of Vorticity in case of r/r 9% D/R 86% (case 2). 4. Performance experiments The experiments were carried out using fan tester system. Experiment set-up was similar to Ref. [5], part 4 "Experiments". The original model (inlet 91%, radius 6%) and the best model (inlet 86%, radius 9%) were chosen for performing experiments to validate the CFD result. As illustrated in Fig. 25 and Fig. 26, the CFD results were validated by the corresponding experiment results. It can be seen that the CFD results were less accurate in the low flow rate region (stall region). 5. Noise experiments Noise experiments with model 1 and model 2 were carried out using "Anechoic chamber with wind tunnel" system to verify the effect of inlet radius and bell mouth radius on sound quality. The experiment setup was illustrated in Fig. 27. The distance between the microphone and the fan was 2m. The sound signal was processed using PIMENTO software. Loudness and sharpness value were then calculated using Zwicker method [16] (ISO 532B, DIN 45631). The experiment results were shown in Fig. 28 and 29. As shown in Fig. 28, the loudness value was quite similar between the two models. Both models showed a very sharp rise of loudness value after BEP (best efficiency point) (φ ~0.3). At high flow rate, the noise was almost three times louder than the noise at BEP. The sharpness value was also not different between the two model as illustrated in Fig. 29. As the flow rate increased, the sharpness value remained stable for both models.

7 P. N. Son et al. / Journal of Mechanical Science and Technology 26 (5) (2012) 1531~ Fig. 27. Anechoic chamber with wind tunnel system. centrifugal blower. Too small or too large of a radius will result in a noticeable loss in the flow rate due to the appearance of a strong vortex under the bell mouth region. An inlet radius ratio about 86% is recommended. (2) The bell mouth radius has moderate impact on performance of centrifugal blower. Too small of a radius will have negative impact on flow rate. However, bell mouth with bigger radius will have higher cost and sometimes it's impossible to use a bell mouth with big radius due to the limit on the size of the blower. Anyway, it's best to have the bell mouth radius ratio at the value around 9%. (3) The inlet radius and the bell mouth radius don't have much impact on the sound quality of centrifugal blower. Therefore, it's not necessary to optimize the bell mouth geometries and the inlet radius if one only needs to improve the sound quality of a blower system. Acknowledgement This work has been supported by CAERIS company and Sunmoon University. The first author also wishes to thank all members of fluid lab for their great help during this research. Nomenclature Fig. 28. Dependence of loudness value on flow rate. Fig. 29. Dependence of sharpness value on flow rate. P ε D d 2 η k μ M N Q r R : Static pressure rise : Turbulence dissipation rate : Inlet radius : Outer diameter of impeller : Static efficiency : Turbulence kinetic energy : Dynamic viscosity : Power coefficient : Total moment acting on impeller : Impeller rotation speed in rpm : Flow coefficient : Pressure coefficient : Volume flow rate of centrifugal blower : Bell mouth radius : Outer radius of impeller : Specific weight It's concluded that the inlet radius and the bell mouth radius didn't have much impact on the sound quality of centrifugal blowers. Since this was only the result of experiments, CFD simulation should be carried out to clearly explain the sound quality of centrifugal blower in future work. 6. Conclusions The effect of bell mouth radius and inlet radius on flow rate and sound quality of centrifugal blower was clarified by using CFD simulation and parallel experiments. Here below were some conclusions: (1) The inlet radius has major impact on the flow rate of the References [1] P. Gasparovic and M. Carnogurska, Aerodynamic optimisation of centrifugal fan casing using CFD, Journal of applied in the thermodynamics and fluid mechanics, 2 (1) (2008). [2] S.-C. Lin and C.-L. Huang, An integrated experimental and numerical study of forward-curved centrifugal fan, Experimental thermal and fluid science, 26 (5) (2002) [3] S.-Y. Han and J.-S. Maeng, Shape optimization of cut-off in a multi blade fan/scroll system using neural network, International journal of heat and mass transfer, 46 (1) (2003) [4] C. N. Montazerin, A. Damangir and H. Mirzaie, Inlet induced flow in squirrel-cage fans, Proc. Instn Mech. Engrs

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