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Fluid Flow and Heat Transfer in a Heat Exchanger Channel with Short-Length Twisted Tape Turbulator Inserts

Abstract

The thermohydraulic characteristics of turbulent flow of air (Pr 0.7) through circular tube with short-length and full-length twisted-tape turbulators inserts have been studied experimentally. Also, heat transfer performance for different values of the twist ratio, tape thickness ratio and diameter ratio are investigated for Reynolds numbers within the range 6000–20,000. The full-length twisted tape enhances heat transfer more than those of short-length twisted tapes. Heat transfer and pressure drop tests were carried out in brass channels. The channel with twisted tape inserts specifies that the use of tapes augments heat transfer mostly, which is complemented by a greater friction penalty. The thermohydraulic performance of the problem under study has been assessed. It is observed that by using full-length twisted tape at constant pumping power, up to 27% heat duty rises in comparison with short-length twisted tape. Similarly, by using full-length twisted tape at heat duty, up to 33% pumping power increases in comparison with short-length twisted tape.

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Abbreviations

A :

Heat transfer area [πDL (m2)]

A 0 :

Plain duct flow cross-sectional area (m2)

Cp:

Constant pressure specific heat (J/kg K)

D :

Internal diameter of the plain channel (m)

D h :

Hydraulic diameter of the test duct = 4A0/P (m)

f :

Fully developed Fanning friction factor (dimensionless)

FLTT:

Full-length twisted-tape

FLHT:

Full-length helical tape

g :

Gravitational acceleration (m/s2)

Gr :

Grashof number = gβρ2Dh3ΔTw/μ2 (dimensionless)

H :

Pitch for 180° rotation of twisted tape (m)

h z :

Axially local heat transfer coefficient [W/(m2 K)]

k :

Fluid thermal conductivity [W/(mK)]

L :

Length of twisted-tape (m)

m :

Mass flow rate (kg/min)

Nu m :

Axially averaged Nusselt number = \(\frac{1}{L}\int\limits_{0}^{L} {\frac{{h_{\text{z}} D_{\text{h}} {\text{d}}z}}{k}}\) (dimensionless)

ΔPz :

Pressure drop (mm)

P :

Wetted perimeter in the particular cross section of the duct (m)

Pr :

Fluid Prandtl number = \({\raise0.7ex\hbox{${\mu C_{\text{p}} }$} \!\mathord{\left/ {\vphantom {{\mu C_{\text{p}} } k}}\right.\kern-0pt} \!\lower0.7ex\hbox{$k$}}\) (dimensionless)

Ra :

Rayleigh number = Gr·Pr

Re :

Reynolds number based on plain channel diameter (dimensionless)

T :

Temperature (K)

SLTT:

Short-length twisted tape

Tw :

Wall to fluid bulk temperature difference (K)

X :

Prn, the value of n depends on the exponent of Pr in the correlation

Y :

\(\left( {\frac{{\mu_{\text{b}} }}{{\mu_{\text{w}} }}} \right)^{ - 0.14} \times \frac{1}{5.172}\)

y :

Twist ratio (dimensionless)

β :

Coefficient of isobaric thermal expansion (K−1)

δ :

Tape thickness ratio [t/D (m)]

µ :

Fluid dynamic viscosity (kg/ms)

ρ :

Density of the fluid (kg/m3)

ax:

At axial flow condition

b :

At bulk fluid temperature

m :

Axially averaged

sw:

At swirl flow condition

w :

At duct wall temperature, with

z :

Local value

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Correspondence to Suvanjan Bhattacharyya.

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Bhattacharyya, S. Fluid Flow and Heat Transfer in a Heat Exchanger Channel with Short-Length Twisted Tape Turbulator Inserts. Iran J Sci Technol Trans Mech Eng 44, 217–227 (2020). https://doi.org/10.1007/s40997-018-0251-0

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Keywords

  • Turbulent flow
  • Twisted tape
  • Swirl flow
  • Heat transfer
  • Heat exchanger
  • Forced convection