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CFD modeling of rectangular solar air heater featuring curved flow passage in turbulent flow

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Abstract

This research presents a numerical modeling of the turbulent flow characteristics and thermal behavior in convex and concave curved solar air heater duct having a curved flow passage. The governing equations were solved using the computational fluid dynamics software ( Ansys Fluent 15.0). The RNG k–ɛ turbulence model has been used for modeling turbulent flow. Different geometric and operating parameters such as curvature angle (\(\theta\)) value in the range of 15°–90°, Reynolds number (Re) between 3,800 and 18,000 and insertion of semi-circular artificial roughness were considered. The effects of these parameters on Nusselt number (Nuc), friction factor (frc), thermal performance (\({\varepsilon }_{th}\)) and thermo-hydraulic performance parameter (THPP) were studied. Then, the optimum configuration of the curvature angle for smooth and roughened curved solar air heater was evaluated. For smooth design, there was a heat transfer enhancement when higher values for the curvature angle were used accompanied by increase of the friction losses. Moreover, the maximum thermal performance values were more enhanced with convex curved flow passage basically for low curvature angle values as compared to concave flow passage. The best Thermo-Hydraulic Performance Parameters (THPP) for smooth curved solar air heater were computed to be 2.07 and 1.45 respectively for convex curvature angle \(\uptheta =30^\circ\) and concave curvature angle \(\uptheta =15^\circ\) with Re = 3,800. This above optimum value of concave design was improved by 20% using artificial semi-circular ribs while in convex design the optimum THPP was grown by 7.24%.

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Abbreviations

C p :

Heat capacity of air, J/kg K

D h :

Hydraulic diameter of curved SAH, mm

e :

Height of rib, mm

D :

Depth of curved SAH, mm

h :

Coefficient of heat transfer, W/m2K

k :

Thermal conductivity of air, W/m K

L :

Curved SAH length, mm

L 1 :

Entrance length, mm

L 2 :

Tested zone length, mm

L 3 :

Exit zone length, mm

L c :

Curved tested zone length, mm

I :

Solar radiation intensity, W/m2

I T :

Turbulence intensity, %

k air :

Thermal conductivity of air, W/m K

m :

Mass flow rate, kg/s

P :

Pitch: \(P={R}_{c}\times {\theta }_{p}\), mm

\(\Delta\) p :

Pressure drop, Pa

q :

Heat flux of curved absorber, W/m2

R c :

Curved flow passage radius, mm

T :

Temperature, K

T am :

Mean air temperature, K

T in :

Air inlet temperature, K

T out :

Air outlet temperature, K

T pm :

Mean plate temperature, K

T w :

Wall temperature, K

u :

Velocity of air in the (xx’) direction, m/s

U :

Mean velocity of air, m/s

v :

Velocity of air in the (yy’) direction, m/s

W :

Width of curved SAH, mm

s :

Surface of absorber plate

HTEF :

Heat transfer enhancement factor

HTER :

Heat transfer enhancement Ratio

FLEF :

Friction loses enhancement factor

THPP :

Thermohydraulic performance parameter

ɛ th :

Thermal performance

Nu :

Nusselt number

Nu c :

Nusselt number for curved SAH

Nu f :

Nusselt number for flat SAH

Nu r :

Nusselt number for roughened curved SAH

P/e :

Relative roughness pitch

e/D :

Relative roughness high

Pr :

Prandtl number

W/D :

Rectangular duct aspect ratio

fr c :

Friction factor for curved SAH

fr f :

Friction factor for flat duct

θ :

Curvature angle, degree

\({\theta }_{p}\),:

Angular pitch, radium

\(\Gamma\) :

Molecular thermal diffusivity, m2/s

\({\Gamma }_{t}\) :

Turbulent thermal diffusivity, m2/s

\(\rho\) :

Density of air, kg/m3

µ :

Dynamic viscosity, Ns/m2

µ t :

Turbulent viscosity, Ns/m2

\({\mu }_{\tau }\) :

Frictional velocity

ɛ :

Dissipation rate

\({C}_{\mu }\) :

Closure coefficient, 0.0845

\({C}_{1\varepsilon }\) :

Coefficient in turbulent model, 1.42

\({C}_{2\varepsilon }\) :

Coefficient in turbulent model, 1.68

\({\alpha }_{\varepsilon }\) :

Coefficient in turbulent model, 1.39

\({\alpha }_{k}\) :

Coefficient in turbulent model, 1.39

a :

Ambient

am :

Air mean

f :

Flat

in :

Inlet

m :

Mean

max :

Maximum

min :

Minimum

pm :

Plate mean

r :

Roughened

c :

Curved

w :

Wall

out :

Outlet

fm :

Fluid mean

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Authors and Affiliations

  1. Mechanical Engineering Department, National Engineering School of Sfax, ENIS, 3038, Sfax, Tunisia

    A. Ben Mabrouk, H. Djemel, M. Hammami & M. Baccar

Authors
  1. A. Ben Mabrouk
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  2. H. Djemel
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  3. M. Hammami
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Correspondence to A. Ben Mabrouk.

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Highlights

• Adopting of curved flow passage on solar air heater improves the heat transfer.

• The convex flow passage brings more efficiency than concave design in turbulent flow.

• Numerical study of the effect of convex and concave curvature angle on the thermohydraulic performance.

• The low curvature angle and insertion of ribs increase the performance of the solar air heater.

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Mabrouk, A.B., Djemel, H., Hammami, M. et al. CFD modeling of rectangular solar air heater featuring curved flow passage in turbulent flow. Heat Mass Transfer 59, 1–20 (2023). https://doi.org/10.1007/s00231-022-03239-6

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  • DOI: https://doi.org/10.1007/s00231-022-03239-6

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