本文主要針對垂直型橫流扇之性能提升進行改良，首先依據原型橫流扇之幾何參數建模，並利用CFD 模擬軟體進行數值分析，透過流場可視化來分析並討論其設計缺失，再藉由上述缺失提出垂直型橫流扇之改良設計方案，改良方案依序為上、下背板以及內部參數，最後將其改良方案做整合，找出最佳之設計參數。模擬結果顯示，改善下背板對於流量有明顯的變化，其流量增幅21.5%(由262.2 CFM提升至318.7 CFM)；改變上背板之參數則是對扭矩有明顯的變化，在相同的流量下，其扭矩下降13.6%；在優化參數組合參數後流量為347.2cfm，扭矩為0.271N-m，其整體流量相較於原型橫流扇上升32.4%(85 cfm)，扭矩上升14.8%(0.035 N-m)。
接著探討改良型之垂直型橫流扇的噪音特性，與原型橫流扇相比是否有得到改善，並加入亥姆霍茲共振器以期對噪音性能有進一步的改善。模擬結果顯示，在最佳化流道後整體噪音值與第一特徵頻率之噪音相較於原型橫流扇並無太大的差異，而其餘特徵頻率相較原型橫流扇皆下降並接近寬頻帶噪音。在瞭解流場與噪音之連動關係後，將亥姆霍茲共振器裝於噪音源處進行降噪，模擬結果顯示第一特徵頻之聲壓強度有得到改善，其中最佳之方案可降低約5dB，而整體噪音皆並無太大的變化。本文所探討的亥姆霍茲共振器應用於垂直式橫流扇確實有效果，但應針對共振器之設計參數和安裝位置作更多探討，以確保此減噪方案之實用性。

This CFD analysis is aiming to improve the aerodynamic and acoustic performances of a vertical cross-flow fan equipped inside a portable air conditioner. At first, CFD software Fluent is utilized to simulate flow field associated with this reference cross-flow fan; then, the numerical flow visualization is observed and analyzed carefully for identifying the reversed and circulation flows. Thereafter, several modifications are proposed to diminish these adverse flow patterns for enhancing the aerodynamic and acoustic performances of this cross-flow fan. Furthermore, the corresponding parametric study is imposed in sequence to decide the proper settings of these variables, which can be classified into three categories regarding the rear wall, the vortex wall, and two cut-off structures. Consequently, fan design with the best rear wall yields an apparent 21.5% growth on the airflow (increasing from 262.2 CFM to 318.8 CFM) with an extra 17.6% torque requirement (increasing from 0.24 N-m to 0.28 N-m). Also, the main influence caused by the revised vortex wall is a 13.6% reduction on torque requirement while no significant change on flow rate is observed. Regarding variations on the inlet and the outlet cut-offs, the optimized seating of cut-off results in a 3.7% rise on flow rate and a 3% drop on torque demand. Evidently, the energy efficiency can be effectively upgraded for the redesigned cross-flow fan with the optimal rear wall, vortex walls, and cut-off structures.
With regard to the noise-reduction effort, the acoustic fields associated with the reference and the redesigned fans are calculated and compared via the transient CFD simulation. It is found that the improved fan generates similar overall sound pressure level like the reference fan while an apparent 5 dB decrease is identified on its 2nd harmonics. Subsequently, the acoustic characteristics of this new fan is utilized to design two Helmholtz resonators, which are placed at the outlet cut-off (noise source) and the highest-pressure fluctuating location on the rear wall as illustrated in the numerical outcomes. Consequently, according to the CFD simulation, the maximum noise reductions is 3.8 dB at the 1st harmonic frequency when 5 Helmholtz resonators are installed near the outlet side of rear wall. In conclusion, this study establishes a systematic and reliable scheme for performance enhancement and acoustic analysis of the cross-flow fan within the framework of CFD technology. Also, a superior fan is successfully attained by using this design scheme to deliver the flowrate at 345 CFM (31.7% increase) with a torque demand at 0.27 N-m (14.5% increase), which results in a better energy efficiency. Moreover, the annoying noise at the 1st harmonic has been reduced by an impressive 5 dB with the installation of 5 Helmholtz resonators on the rear wall of this improved cross-flow fan.

[1]Eck, I. B., “Fans: Design and Operation of Centrifugal, Axial-Flow and Cross-Flow Fans,” Pergamon Press, New York, 1973.
[2]Raj, D. and Swim, W. B., “Measurements of the Mean Flow Velocity and Velocity Fluctuations at the Exit of on F-C Centrifugal Fan Rotor,” Journal of Engineering for Power, Vol. 103, pp. 393-399, 1981.
[3]Lin, S. C., “A Novel F-C Centrifugal Fan Design for Improved Performance,” Department of Mechanical Engineering, Technical Report, Tennessee Technological University, 1982.
[4]吳慶財，“前傾式離心扇之實驗設計”，國立台灣工業技術學院，機械工程技術研究所碩士論文，1993 年。
[5]Morinushi, K., “The Influence of Geometric Parameters on F. C. Centrifugal Fan Noise,” Journal of Vibration, Acoustics, Stress and Reliability in Design, Vol. 109, pp. 227-234, 1987.
[6]Lui, C. H., Vafidis, C., and Whitelaw, J. H. “Flow Characteristics of a Centrifugal Pump,” ASME Journal of Fluids Engineering, Vol. 116, pp. 303-309, 1994.
[7]Huang, J., Yin, M., and Yao, X. J., “Effects of Concealed Motor’s Shape on Performance of Forward Multi-Blade Centrifugal Fans,” Journal of Shanghai Jiaotong University, Vol. 35, No. 8, pp. 1196-1199, 2001.
[8]許宏祺，“Pentium 4 筆記型電腦冷卻風扇之實驗研究”，國立台灣科技大學，機械工程技術研究所碩士論文，2001年。
[9]Neise, W., “Noise Reduction in Centrifugal Fans: a Literature Survey,” Journal of Sound and Vibration, Vol. 45, No. 3, pp. 375-403, 1976.
[10]Neise, W., “Review of Noise Reduction Methods for Centrifugal Fans,” Journal of Engineering for Industry, Vol. 104, pp. 151-161, 1982.
[11]Mortier, P., DRP 146464, 1892.
[12]Darlin, DPR 471551, 1929.
[13]Murata, S. and Nisnihara, K., “An Experimental of Cross Flow Fan- 1st Report, Effects of Housing Geometry on the Fan Performance,” Bulletin of the JSME, Vol. 19, pp. 314-321, 1976.
[14]Tanaka, S. and Murata, S., “Scale Effects in Cross Flow Fans (Effects of Fan Dimensions on Performance Curves),” JSME, International Journal Series, Vol. 37, No. 4, pp. 844-852, 1994.
[15]Matsuki , K., Shinobu, Y., Takushima, A., and Tanaka, S., “Experimental Study of Internal Flow of a Room Air Conditioner Incorporating a Cross Flow Fan,” ASHARE Transactions, Vol. 94, Part 1, pp. 330-364, 1994.
[16]林進坤，“風道舌部幾何形狀對橫流風扇性能曲線與噪音的影響”，國立台灣大學機械工程研究所碩士論文，1995年。
[17]Lazzarotto, L., Lazzaretto, A., Macor, A., and Martegani, A. D., ‘‘On Cross-Flow Fan Similarity: Effect of Casing Shape,” ASME Journal of Fluids Engineering., Vol. 123, pp. 1-9, 2001.
[18]Lazzaretto, A., Toffolo, A., and Martegani, A. D., ‘‘A Systematic Experimental Approach to Cross-Flow Fan Design,’’ ASME Journal of Fluids Engineering., Vol. 125, pp. 684-693, 2003.
[19]Lazzaretto, A., ‘‘A Criterion to Define Cross-Flow Fan Design Parameters,” ASME Journal of Fluids Engineering., Vol. 125, pp. 680-683, 2003.
[20]施智達，“橫流冷卻風扇運用於筆記型電腦之實驗研究”，國立台灣科技大學，機械工程研究所碩士論文，2004年。
[21]莊惠婷，“新型複合式風扇以及流線型鰭片散熱座脂數值與實驗整合研究”，國立台灣科技大學，機械工程研究所碩士論文，2005年。
[22]許建德，“水平型橫流扇之數值模擬與實驗的整合研究”，國立台灣科技大學，機械工程研究所碩士論文，2011年。
[23]侯宏奇、施陽正、劉正熙、江旭政，“舌部間隙與舌部開孔對分離式空調室內機橫流扇之效能提昇研究”，機械月刊，第二十九卷第十期，80-94頁，2003年
[24]Zeller, W. and Stang, H., “Predetermination of the Axial-Flow Fans,” Heizung-Luftung-Haustchnik, Vol. 9, No. 12, 1957.
[25]Zeller, W., “Conerning Mathematical Treatment of Noise Behaviour in Fans for Air Conditioning Plants,” VID-Berichte, No. 38, 1959.
[26]Hüebner, G., “Noise Generated by Centrifugal Fans,” Siemens-Zeitschrift, Vol. 8, pp. 145-155, 1959.
[27]Schlichting, H., “Application of the Boundary Layer Theory to Flow Problems of Turbo Machines,” Siemens-Zeitschrift, Vol. 33, No. 7, pp. 74-82, 1959.
[28]Powell, A., “Theory of Vortex Sound”, The Journal of the Acoustical Society of America, Vol. 36, pp. 177-195, 1964.
[29]Smith, W. A., O’Malley, J. K. and Phelps, A. H., “Reduction Blade Passage Noise in a Centrifugal Fan,” American Society of Heating, Refrigeration and Air Condition Engineer Transactions, Vol. 80, pp. 45-51, 1974.
[30]李達生，“風機道舌部間隙、風道轉子型式、風道舌部構型以及風道整體構型對線流扇風扇性能曲線與噪音之影響”，國立台灣大學，機械工程研究所碩士論文，1996 年。
[31]Koo, H. M., “An Experimental Study of the Performance of Cross-Flow Fans in Room Air-Conditioning Systems,” Institute of Noise Control Engineering, Vol. 48, No. 2, pp. 41-47, 2000.
[32]游昌隆，“橫流式風扇之流場與其引發噪音之研究”，國立清華大學，動力機械工程研究所碩士論文，2001年。
[33]Cho, Y. and Moon, Y. J., “Discrete Noise Prediction of Variable Pitch Cross-Flow Fans by Unsteady Navier-Stokes Computations,” ASMEJournal of Fluids Engineering, Vol. 125, pp. 543-550, 2003.
[34]Li, Y., Ouyang, H., Tian, J. Du, Z., and Zheng, Z., ‘‘Experimental and Numerical Studies on the Discrete Noise about the Cross-Flow Fan with Block-Shifted Impellers,’’ Applied Acoustics, Vol. 71, pp. 1142-1155, 2010.
[35]Patankar, S. V. and Spalding, D. B., “A Calculation Procedure forHeat Mass and Momentum Transfer in Three-Dimensional ParabolicFlows,” International Journal of Heat and Mass Transfer, Vol. 15, pp.1787-1806, 1972.
[36]Fluent, Version 13.0.0, Methodology, 2010.
[37]NSI/AMCA Standard 210-99 (ANSI/ASHRAE 51-1999), “Laboratory Method of Testing Fans for Aerodynamic Performance Rating,” Air Movement and Control Association, Inc., 1999.
[38]CNS 8753, “Determination of Sound Power Level of Noises for Fan,Blower and Compressors,” Chinese National Standard, 1982.
[39]Porter, A. M. and Markland, E., “A Study of the Cross-Flow Fan,” Journal of Mech. Eng. Sci., Vol. 12, No. 6, pp. 421-431, 1970.
[40]Kondo, L. and Aoki, Y., “Noise Reduction in Turbo Fans for Air Condictioners,” Technical Review-Mitsubishi Heavy Industries, Vol.26, No. 3, pp.173-179, Octorber 1989.
[41]Launder, B. E. and Spalding, D. B., “Lectures in Mathematical Models of Turbulence”, Academic Press, London, England, 1972.
[42]Smirnov, R., Shi, S. and Celik, I., “Randon Flow Generation Technique for Large Eddy Simulations and Particle-Dynamics Modeling,” Journal of Fluids Engineering, Vol. 123, pp. 359-371, 2001.
[43]Uranga, A., Persson, P., Drela, M. and Peraire, J., “Implicit Large Eddy Simulation of Transitional Flows over Airfoils and Wings,” 19th AIAA Computational Fluid Dynamics Conference, San Antonio, Texas, 2009.