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Research Papers: Flows in Complex Systems

A Performance Prediction Model for Low-Speed Centrifugal Fans

[+] Author and Article Information
Tristan Wolfe

Carderock Division,
Naval Surface Warfare Center,
Ships Systems Engineering Station,
Philadelphia, PA 19112
e-mail: tristan.wolfe@navy.mil

Yu-Tai Lee

Carderock Division,
Naval Surface Warfare Center,
West Bethesda, MD 20817
e-mail: yu.lee@navy.mil

Michael E. Slipper

Carderock Division,
Naval Surface Warfare Center,
Ships Systems Engineering Station,
Philadelphia, PA 19112
e-mail: michael.slipper@navy.mil

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received August 18, 2014; final manuscript received December 12, 2014; published online February 2, 2015. Assoc. Editor: Bart van Esch. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Fluids Eng 137(5), 051106 (May 01, 2015) (9 pages) Paper No: FE-14-1457; doi: 10.1115/1.4029397 History: Received August 18, 2014; Revised December 12, 2014; Online February 02, 2015

A generalized model for mapping the trend of the performance characteristics of a double-discharge centrifugal fan is developed based on the work by Casey and Robinson (C&R), which formulated compressor performance maps for tip-speed Mach numbers ranging from 0.4 to 2 using test data obtained from turbochargers with vaneless diffusers. The current paper focuses on low-speed applications for Mach number below 0.4. The C&R model uses four nondimensional parameters at the design condition including the flow coefficient, the work input coefficient, the tip-speed Mach number, and the polytropic efficiency, in developing a prediction model that requires limited geometrical knowledge of the centrifugal turbomachine. For the low-speed fan case, the C&R formulas are further extended to a low-speed, incompressible analysis. The effort described in this paper begins by comparing generalized results using efficiency data obtained from a series of fan measurements to that using the C&R model. For the efficiency map, the C&R model is found to heavily depend on the ratio of the flow coefficient at peak efficiency to that at the choke flow condition. Since choke flow is generally not applicable in the low-speed centrifugal fan operational environment, an alternate, but accurate estimation method based on fan free delivery derived from the fan test data is presented. Using this new estimation procedure, the modified C&R model predicts reasonably well using the double-discharge centrifugal fan data for high-flow coefficients, but fails to correlate with the data for low-flow coefficients. To address this undesirable characteristic, additional modifications to the C&R model are also presented for the fan application at low flow conditions. A Reynolds number correction is implemented in the work input prediction of the C&R model to account for low-speed test conditions. The new model provides reasonable prediction with the current fan data in both work input and pressure rise coefficients. Along with the developments for the efficiency and work input coefficient maps, the use of fan shut-off and free delivery conditions are also discussed for low-speed applications.

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References

Figures

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Fig. 1

Centrifugal fan geometry used for the current study (only half of the fan is shown and arrows indicate the flow directions)

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Fig. 2

Centrifugal fan test rig

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Fig. 3

Comparison of fan #1 data, C&R turbocharger test data at the lowest M (=0.41), and C&R theoretical curve for low Mach numbers

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Fig. 4

Comparison of theoretical low-speed curve from C&R model to fan #1 data at varying Mach numbers with a non-normalized flow coefficient (same data as plotted in Fig. 3)

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Fig. 5

Comparison of theoretical low-speed curve from C&R model to fan #2 test data from Fig. 4

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Fig. 6

Comparison of ϕ'c between all fan data (using ϕf), turbocharger data and C&R and Swain models

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Fig. 7

Linear fit of all fan data (also presented in Fig. 6) used for the determination of ϕ'f as a function of the ratio of impeller outlet width to impeller tip diameter

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Fig. 8

Comparison of theoretical low-speed curve with the optimal peak to choke flow coefficient ratio from C&R model (same data as in Fig. 5 for fan #2)

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Fig. 9

Fan #1 test data and curves of theoretical efficiency at varying Mach number and flow coefficient (same measured data as in Figs. 3 and 4, note the efficiency is not normalized)

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Fig. 10

Linear fit of all fan data (also presented in Fig. 6) used for the determination of J as a function of the ratio of impeller outlet width to impeller tip diameter

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Fig. 11

Comparison of theoretical low-speed curve with correction for ϕ < ϕ' from C&R model to fan #1 test data (same measured data as in Figs. 3, 4, and 9)

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Fig. 12

Fan #2 geometry total efficiency test data with Reynolds number effects and curves of theoretical efficiency at varying Mach number and flow coefficient (same data as in Figs. 5 and 7)

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Fig. 13

Comparison of total efficiency model (M = 0.26) derived from peak efficiency predicted by Strub correction for Reynolds number based on M = 0.28 data and derived from peak efficiency test data of fan #1

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Fig. 14

Velocity triangle at impeller outlet

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Fig. 15

Theoretical and measured comparisons of efficiency, work input coefficient, and pressure coefficient at varying flow coefficient of fan #1 (a) and fan #2 (b) at M = 0.28

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