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Research Papers: Flows in Complex Systems

Mixing and Entrainment Characteristics in Circular Short Ejectors

[+] Author and Article Information
Ju Hyun Im

Agency for Defense Development,
4th R&D Institute
Yuseong, P.O. Box 35,
Daejeon 305-600, South Korea
e-mail: juhyunim@add.re.kr

Seung Jin Song

Department of Mechanical and
Aerospace Engineering,
Seoul National University,
Gwanak-ro 1, Gwanak-gu,
Seoul 151-744, South Korea
e-mail: sjsong@snu.ac.kr

The separation bubble has been verified via flow-visualization.

The temperature profiles at the mixing duct exit have not been measured in this research because both primary and secondary flow temperatures were the same. However, the temperature scaling approximations can be found in Munk and Prim [23].

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received March 18, 2014; final manuscript received December 13, 2014; published online February 2, 2015. Assoc. Editor: Kwang-Yong Kim.

J. Fluids Eng 137(5), 051103 (May 01, 2015) (10 pages) Paper No: FE-14-1140; doi: 10.1115/1.4029412 History: Received March 18, 2014; Revised December 13, 2014; Online February 02, 2015

Analytical and experimental investigations have been conducted to characterize the performance of “short” ejectors. In short ejectors, the core of primary (motive) flow still exists at the mixing duct exit, and nonuniform mixed flow is discharged from the mixing duct. Due to incomplete mixing, short ejector pumping performance is degraded and cannot be predicted by the existing “long” ejector models. The new analytical short ejector model presented in this paper is based on the control volume analysis and jet expansion model. The secondary (entrained) flow velocity and the corresponding shear layer (between the primary and the secondary flows) growth rate variations along the mixing duct are taken into account. In addition, measurements have been made in ejectors with length ratios (LRs) of two and three for an area ratio (AR) of 1.95; and a LR of two for an AR of 3.08. Velocity profiles at the mixing duct inlet and exit, and static pressure distribution along the mixing duct have been measured with pitot probes and pressure taps. All of the tests have been carried out at a Reynolds number of 420,000. Comparison shows that the new ejector model can accurately predict flow characteristics and performance of short ejectors. For all of the test cases, the velocity profiles at the mixing duct inlet and exit are well predicted. Also, both predictions and measurements show pumping enhancement with increasing mixing duct length. The pumping enhancement is due to the increase in the static pressure difference between the mixing duct inlet and atmosphere as the mixing duct is lengthened. Furthermore, both measured and predicted static pressure distributions along the mixing duct show a kink. According to the analysis, the kink occurs when the outer shear layer reaches the mixing duct wall, and the secondary flow velocity decreases along the mixing duct upstream of the kink and increases downstream of the kink. Thus, the new ejector model can accurately predict not only the integral performance but also different mixing regimes in short ejectors.

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References

Figures

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Fig. 1

Geometry of ejector

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Fig. 2

Control volume applied to long ejector model

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Fig. 3

Classification of ejectors based on mixing duct length and flow regime: (a) short ejector, (b) medium ejector, and (c) long ejector

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Fig. 4

Control volume of short ejector

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Fig. 5

Schematic of discrete calculation method for predicting shear layer in ejector

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Fig. 6

Calculation process in short ejector model

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Fig. 7

Schematic of ejector test section and measurement locations

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Fig. 8

Test section and experimental equipment

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Fig. 9

Measurement grid (AR = 1.95): (a) mixing duct inlet and (b) mixing duct exit

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Fig. 10

Comparison of predicted and measured velocity profiles at the mixing duct inlet of SNU short ejectors at Re = 420,000: (a) AR = 1.95, LR = 2, (b) AR = 1.95, LR = 3, and (c) AR = 3.08, LR = 2

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Fig. 11

Comparison of predicted and measured velocity profiles at the mixing duct exit of SNU short ejectors at Re = 420,000: (a) AR = 1.95, LR = 2, (b) AR = 1.95, LR = 3, and (c) AR = 3.08, LR = 2

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Fig. 12

Velocity profiles inside mixing duct at x/L = 0, 0.33, 0.67, and 1 for SNU short ejectors at Re = 420,000: (a) AR = 1.95, LR = 2, (b) AR = 1.95, LR = 3, and (c) AR = 3.08, LR = 2

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Fig. 13

Comparison of measured pumping from SNU, Toulmay [18], McBean and Birk [24], and predictions

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Fig. 14

Analytical and experimental pumping performance of SNU short ejectors at Re = 420,000

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Fig. 15

Schematic of static pressure rise with mixing progress in the mixing duct

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Fig. 16

Experimental and analytical static pressure distribution along the mixing ducts at Re = 420,000 (AR = 1.95)

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Fig. 17

Schematic of static pressure rise patterns in ejector

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Fig. 18

Regimes of secondary velocity variation relative to the kink: (a) upstream of kink and (b) downstream of kink

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