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Comparative study of solar air heater performance with various shapes and configurations of obstacles

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Abstract

An investigation is performed to find an optimum shape of obstacles attached to a solar air heater using three-dimensional Reynolds-averaged Navier–Stokes analyses of heat transfer and fluid flow. The Reynolds number, which is based on the hydraulic diameter of the channel, is in the range of 6800–10,000. The Nusselt number and friction factor are used to measure the thermal and aerodynamic performances of the solar air heater, respectively. Four different obstacle shapes (U-shaped, rectangular, trapezoidal, and pentagonal) and three arrangements of obstacles were tested to determine their effects on performance of the solar air heater. The results show that the performance factor (defined by a ratio of thermal to aerodynamic performance) was above unity for all the cases tested, and the pentagonal obstacle shape indicates the highest performance regardless of the Reynolds number. Detailed analyses of the thermal and flow fields are performed in order to obtain a better understanding of the heat transfer characteristics.

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Abbreviations

A :

Area of the absorber plate (m2)

e :

Base of obstacles, rib height (M)

c p :

Specific heat of air (J/kg K)

D h :

Equivalent hydraulic diameter of the air passage (m)

h 1 :

Obstacle height (m)

h 1/H :

Height of relative obstacle

f :

Friction factor

f o :

Friction factor in an obstacle duct

f s :

Friction factor in a smooth duct

g :

Groove distance from center line

h :

Convective heat transfer coefficient (W/m2 K)

H :

Duct height (m)

k air :

Thermal conductivity of air (W/m K)

L :

Test section duct length (m)

\( \dot{m} \) :

Mass flow rate of air (kg/s)

Nu :

Nusselt number

Nu o :

Nusselt number of the obstacle duct

Nu s :

Nusselt number of the smooth duct

ΔP :

Pressure drop across the test section (Pa)

PF :

Performance factor

p L :

Longitudinal space between rows of obstacles (m)

p T :

Transverse distance between two obstacles (m)

p L /h 1 :

Relative obstacle longitudinal pitch

p T /e :

Relative obstacle transversal pitch

Q air :

Rate of heat transfer to air

Re :

Reynolds number

T am :

Average temperature of air (K)

T i :

Bulk mean temperature of air at inlet (K)

T o :

Bulk mean temperature of air at outlet (K)

T pm :

Average temperature of plate (K)

U :

Average velocity of air (m/s)

W :

Duct width (m)

W/H :

Aspect ratio

ϕ :

Chamfer angle (°)

θ :

Angle of attack (°)

ρ:

Density of air (kg/m3)

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Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by Korea government (MSIP) (No. 2009-0083510). The authors gratefully acknowledge this support.

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Authors and Affiliations

  1. Department of Mechanical Engineering, Inha University, 253 Yonghyun-Dong Nam-Gu, Incheon, 402-751, Republic of Korea

    Kishor Kulkarni & Kwang-Yong Kim

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  1. Kishor Kulkarni
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Correspondence to Kwang-Yong Kim.

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Kulkarni, K., Kim, KY. Comparative study of solar air heater performance with various shapes and configurations of obstacles. Heat Mass Transfer 52, 2795–2811 (2016). https://doi.org/10.1007/s00231-016-1788-3

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