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Experimental investigation of the effect of variously-shaped ribs on local heat transfer on the outer wall of the turning portion of a U-channel inside solar air heater

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Abstract

In the present work, an experimental investigation of convective heat transfer and pressure drop was carried out for the turning portion of a U-channel where the outer wall was equipped with ribs. The shape of the ribs was varied. The investigation aims to give guidelines for improving the thermo-hydraulic performance of a solar air heater at the turning portion of a U-channel. Both the U-channel and the ribs were made in acrylic material to allow optical access for measuring the surface temperature by using a high-resolution technique based on narrow band thermochromic liquid crystals (TLC R35C5 W) and a CCD camera placed to face the turning portion of the U-channel. The uncertainties were estimated to 5 and 7 % for the Nusselt number and friction factor, respectively. The pressure drop was approximately the same for all the considered shapes of the ribs while the dimpled rib case gave the highest heat transfer coefficient while the grooved rib presented the highest performance index.

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Abbreviations

A c :

Cross section area of U-duct (m2)

A s :

Surface heat transfer area (m2)

c p :

Specific heat (J/kg K)

D h :

Hydraulic diameter (m)

E :

Rib height (m)

F :

Friction factor

f 0 :

Friction factor in a smooth straight duct

h :

Heat transfer coefficient (W/m2 K)

k :

Thermal conductivity (W/m K)

k f :

Thermal conductivity of air (W/m K)

L :

Distance between pressure taps (m)

m :

Mass flow rate of air (kg/s)

N :

Pixel number

Nu :

Local Nusselt number

Nu 0 :

Nusselt number in a smooth straight duct

Nu av :

Average Nusselt number

ΔP :

Pressure drop (Pa)

PI :

Performance index

Pr :

Prandtl number

Q el :

Electrical power supply (W)

Q loss :

Heat loss (W)

Re :

Reynolds number

ΔT :

Temperature difference (°C)

T bulk :

Air bulk temperature (°C)

T bulkiin :

Bulk inlet air temperature (°C)

T bulkiout :

Bulk outlet air temperature (°C)

T s :

Surface temperature (°C)

u m :

Mean velocity (m/s)

V :

Volumetric flow rate (m3/s)

X :

Streamwise direction (m)

Y :

Transverse direction (m)

Z :

Spanwise direction (m)

ɛ :

Emissivity

ν :

Air kinematic viscosity (m2/s)

ρ :

Air density (kg/m3)

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Acknowledgments

This work was carried out at the Division of Heat Transfer, Department of Energy Sciences, Faculty of Engineering, Lund University, Sweden. Financial support was provided by the Swedish Energy Agency and the National Scientific Council. Special thanks go to our technician Mr. Ingjald Andréasson, who carefully manufactured the experimental apparatus.

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Authors and Affiliations

  1. Department Sustainable and Renewable Energy Engineering, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates

    Tareq Salameh & Abdul Hai Alami

  2. Department of Energy Sciences, Division of Heat Transfer, Lund University, P.O. Box 118, SE-221 00, Lund, Sweden

    Bengt Sunden

Authors
  1. Tareq Salameh
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  2. Abdul Hai Alami
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  3. Bengt Sunden
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Correspondence to Bengt Sunden.

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Salameh, T., Alami, A.H. & Sunden, B. Experimental investigation of the effect of variously-shaped ribs on local heat transfer on the outer wall of the turning portion of a U-channel inside solar air heater. Heat Mass Transfer 52, 539–546 (2016). https://doi.org/10.1007/s00231-015-1541-3

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  • DOI: https://doi.org/10.1007/s00231-015-1541-3

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