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Investigation of the fluid temperature field inside a flat-plate solar collector

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Abstract

An experimental study was conducted to investigate fluid temperature fields inside a flat-plate solar collector tube. The results show the highest fluid temperature at the upper end of the tube which decreased gradually to the lowest value at the bottom end of the tube, whereas, the temperature field in the horizontal plane is symmetric about the centerline. The vertical temperature gradients vary with the axial distance. The local fluid temperature increased nonlinearly along the collector length and its magnitude decreased with an increase in the Reynolds number. The local Rayleigh number increased with the axial distance and at a given location, its magnitude increased with a decrease in the Reynolds number, whereas, the local Nusselt number trends in flat-plate collector tube are in general similar to that in the conventional laminar channel flows. The local fluid temperature increased with an increase in the incident heat flux at a given collector orientation but decreased for the inclined collectors. The results show that over the given Reynolds number range, the fluid in a flat-plate collector tube is stably stratified over most of the fluid cross-sectional domain and the convective currents are suppressed and restricted to a thin layer adjacent to the lower tube wall. The results from the present study provide the physical explanation for the heat transfer enhancement by insert devices. That is, the insert devices disrupt the stably stratified layer and induce mixing which enhances the heat transfer.

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Abbreviations

Ac :

Curved surface area of collector tube (m2)

Cp :

Specific heat of working fluid (J/kgK)

D:

Inner diameter of the collector tube (m)

g:

Acceleration due to gravity (m/s2)

Gr:

Grashof number

\( \dot{m} \) :

Mass flow rate (kg/s)

kf :

Fluid thermal conductivity (W/mK)

L:

Total length of the heat absorbing area (m)

Nu:

Local Nusselt number

Pr:

Prandtl number

Ra:

Local Rayleigh number

Re:

Local Reynolds number

q″:

Wall heat flux (W/m2)

\( \dot{Q} \) :

Heat transfer rate (W)

Tp :

Panel heater temperature (°C)

Tf :

Local mean fluid temperature (°C)

Tf(th) :

Theoretical local fluid temperature for a round tube with uniform heat flux (°C)

Tw :

Local mean wall temperature (°C)

x:

A variable vector along the tube axis lying within the absorbing region and starting from the inlet towards the outlet (m)

μ:

Dynamic viscosity (kg/ms)

ρ:

Density of fluid (kg/m3)

θ:

Angle of inclination of the collector (°)

f:

Bulk fluid

i:

Inlet

m:

Mean

o:

Outlet

w:

Tube wall

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Acknowledgments

Authors would like to thank Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of Western Ontario for providing the support.

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Authors and Affiliations

  1. Department of Mechanical and Materials Engineering, University of Western Ontario, London, Canada

    Gurveer Sandhu & Kamran Siddiqui

Authors
  1. Gurveer Sandhu
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  2. Kamran Siddiqui
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Correspondence to Kamran Siddiqui.

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Sandhu, G., Siddiqui, K. Investigation of the fluid temperature field inside a flat-plate solar collector. Heat Mass Transfer 50, 1499–1514 (2014). https://doi.org/10.1007/s00231-014-1348-7

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  • DOI: https://doi.org/10.1007/s00231-014-1348-7

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