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Heat flux: thermohydraulic investigation of solar air heaters used in agro-industrial applications

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Abstract

A new design of solar air heater simulator is presented to comply with the extensive applications inagro-industry. A wise installation of increased heat transfer surface area provided uniform and efficient heat diffusion over the duct. Nusselt number and friction factor have been investigated based on the constant roughness parameters such as relative roughness height (e/D), relative roughness pitch (P/e), angle of attack (α) and aspect ratio with Reynolds numbers ranging from 5000 to 19,000 in the fully developed region. Heat fluxes of 800, 900 and 1000 Wm−2 were provided. The enhancement in friction factor is observed to be 3.1656, 3.47 and 3.0856 times, and for the Nusselt number either, augmentation is calculated to be 1.4437, 1.4963 and 1.535 times, respectively, over the smooth duct for 800, 900 and 1000 Wm−2 heat fluxes. Thermohydraulic performance is plotted versus the Reynolds number based on the aforementioned roughness parameters at varying heat fluxes. The results show up that thermohydraulic performance is found to be maximum for 1000 Wm−2 at the average Reynolds number of 5151. Based on the results, we can verify that the introduced solar simulator can help analyzing and developing solar collector installations at the simulated heat fluxes.

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Abbreviations

\({\text{A}}_{\text{duct}}\) :

Cross sectional area of duct \((A_{duct} = WH)\) (m2)

\({\text{A}}_{\text{p}}\) :

Surface area of collector absorber plate (m2)

\({\text{c}}_{\text{p}}\) :

Specific heat of air at constant pressure (kJ/kg K)

\({\text{D}}_{\text{h}}\) :

Equivalent diameter of the air passage = 2WH/(W + H) (m)

b:

Width of the rib (m)

e:

Roughness height (m)

e/Dh :

Relative roughness height

P/e:

Relative roughness pitch

d/W:

Relative gap position

f s :

Experimental friction factor for smooth duct

f Blasius :

Predicted friction factor for smooth duct by Blasius equation

f r :

Experimental friction factor for roughened duct

ΔP:

Pressure drop through the test section of the absorber plate (Pa)

Re:

Reynolds number

ρ:

Density of air (kg/m3)

η:

Thermohydraulic performance

ν:

Kinematic viscosity (m2/s)

W:

Duct width (m)

H:

Duct height (m)

α:

Angle of attack (°)

g:

Gap width (m)

d:

Center distance of the gap from the side of the duct (m)

m:

Mass flow rate (kg/s)

Nu:

Nusselt number of roughened duct

Nus :

Nusselt number of smooth duct

Qu :

Useful heat gain (Wm−2)

k:

Thermal conductivity of air (W/mK)

P:

Pitch of the rib (m)

Tf :

Bulk mean temperature of the air through the duct (°C)

Ti :

Inlet temperature of the air (°C)

To :

Outlet temperature of the air (°C)

Tp :

Mean temperature of the absorber plate (°C)

V:

Velocity of air (m/s)

μ:

Dynamic viscosity (Pa s)

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Acknowledgments

The authors would like to acknowledge the support of Hadi Javidi, Arman Gahremani and Sajjad Baheri in construction and mechanization of this project. Also particular thanks are due to Vice Chancellor of Research and Technology, Urmia University.

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Authors and Affiliations

  1. Mechanical Engineering of Biosystems, Urmia University, P.O. Box: 165, Urmia, Iran

    H. Rahmati Aidinlou & A. M. Nikbakht

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  1. H. Rahmati Aidinlou
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  2. A. M. Nikbakht
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Correspondence to A. M. Nikbakht.

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Rahmati Aidinlou, H., Nikbakht, A.M. Heat flux: thermohydraulic investigation of solar air heaters used in agro-industrial applications. Heat Mass Transfer 53, 917–928 (2017). https://doi.org/10.1007/s00231-016-1864-8

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