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Research Papers

Characterization of Twisted-Tape-Induced Helical Swirl Flows for Enhancement of Forced Convective Heat Transfer in Single-Phase and Two-Phase Flows

[+] Author and Article Information
Raj M. Manglik

Fellow ASME
Thermal-Fluids & Thermal Processing Laboratory,
University of Cincinnati,
Cincinnati, OH 45221
e-mail: Raj.Manglik@uc.edu

Arthur E. Bergles

Honorary Mem. ASME
Department of Mechanical Engineering,
University of Maryland,
College Park, MD 20742

Interestingly, contrary to expectations of a numerical study and perhaps due to their methodological inadequacies, neither the earliest attempt [36] nor a recent one [37] has shown the flow circulation or swirl behavior.

In most practical applications, twisted tapes are snug-to-loosely fitted inside the tubes of a heat exchanger, because of the mechanics of inserting and removing them to facilitate cleaning and/or retrofitting. Also, even with metallic tapes of very high conductivity and good tube-tape contact, fin effects would be very small in the presence of very high convective heat transfer coefficients [52].

1Corresponding author.

Manuscript received November 9, 2012; final manuscript received February 28, 2013; published online May 17, 2013. Assoc. Editor: Srinath V. Ekkad.

J. Thermal Sci. Eng. Appl 5(2), 021010 (May 17, 2013) (12 pages) Paper No: TSEA-12-1204; doi: 10.1115/1.4023935 History: Received November 09, 2012; Revised February 28, 2013

By generating helical swirling motion inside a tube with a twisted-tape insert, forced convective heat transfer is significantly enhanced. The primary mechanism entails imparting a centrifugal force component to the longitudinal fluid motion, which superimposes secondary circulation over the main axial flow to promote cross-stream mixing. Based on experimental flow visualization and computational modeling of single-phase laminar flows, a fundamental scaling of the cross-sectional vortex structure and a parametric analysis of the primary enhancement mechanisms in single-phase flows are delineated. Heat transfer coefficient and friction factor correlations for both laminar and turbulent regimes are presented, and the damping effect of swirl on the transition region is highlighted. In flow boiling with net vapor generation, tape-twist-induced helical swirl pushes liquid droplets from the core to the wall to enhance heat transfer and delay dryout. In subcooled boiling, the radial pressure gradient due to the swirl promotes vapor removal from the heated surface to retard vapor blanketing and accommodate higher heat fluxes. The scaling and phenomenological descriptions of the underlying vapor-liquid transport in these different boiling modes and regimes are presented along with any available predictive correlations.

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Figures

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Fig. 1

(a) Twisted-tape inserts of different helical pitch, (b) use in a typical shell-and-tube heat exchanger, and (c) geometrical attributes (images (a) and (b) courtesy of Brown Fintube Co.)

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Fig. 2

Twisted-tape induced swirl flow patterns characterized by smoke streaks in laminar flows in circular tubes [26] (clockwise direction of tape-twist; δ/d = 0.021): (a) y = 4.32 and (b) y = 3.53

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Fig. 3

Computational characterization of swirl in circular tubes with twisted-tape inserts of negligible thickness (δ = 0) [35]: (a) variation with Re for fixed twist ratio (y = 3) and (b) variation with y for fixed Re = 800

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Fig. 4

Different regimes in the development of twisted-tape-induced swirl in laminar tube flow friction factor variation

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Fig. 5

Comparison of experimental data for diabatic Fanning friction factor in laminar flows with the predictions of correlation in Eq. (4); the agreement is within ±10%

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Fig. 6

Comparison of experimental data for diabatic friction factor and other predictions for turbulent flows with the predictions of correlation in Eq. (5); the agreement is within ±5%

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Fig. 7

Twisted-tape-induced swirl “damping” and smooth transition from laminar to turbulent flow in typical f – Re data

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Fig. 8

Swirl-flow effects on the variation of Nu in the thermal entrance region of tubeside laminar flows with twisted-tape inserts

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Fig. 9

Influence of free convection on swirl-flow heat transfer in circular tubes with twisted-tape inserts

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Fig. 10

Comparison of experimental Nu data in the turbulent regime with predictions of correlation in Eq. (9)

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Fig. 11

The evolution of fluid bulk and tube-wall temperatures along the length of a tube, with and without a twisted-tape insert, in forced convection boiling

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Fig. 12

Experimental data [57] indicating influence of twisted-tape generated swirl on fully developed subcooled boiling heat transfer

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Fig. 13

Influence of twisted-tape-generated swirl on critical heat flux in subcooled flow boiling

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Fig. 14

Comparison of swirl-flow CHF predictions of the correlation in Eq. (10) with experimental data

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Fig. 15

Subcooled boiling pressure drop data from the Tong et al. [7] experiments

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Fig. 16

Pabisz and Bergles [71] correlation of subcooled boiling pressure drop

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Fig. 17

Two-phase flow pressure drop showing minimum that leads to hydrodynamic CHF, as developed in Ref. [71]

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