https://orcid.org/0000-0002-9411-6551
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ABSTRACT
In the current study, numerical investigations are carried out to enhance the convective heat transfer rate in a solar air heater embedded with multiple boomerang-shaped roughness. The boomerang roughness is attached to the absorber plate which is supplied with uniform heat flux. The boomerang roughness geometry is defined by the ratios of height to hydraulic diameter (e/Dh) at 0.09, pitch space to height (P/e) of 10.5 to 18, and the gap between the multiple boomerang arm (g) at 8.5 to 13.1 mm. These parameters are examined for turbulent flow conditions having Reynolds number (Re) ranging from 4000 to 22,000. Heat transfer and friction characteristics in the present work are defined and examined by nondimensional parameters Nusselt number (Nu) and friction factor (fr), respectively. The study compared Nusselt number and friction factor readings to measurements of a smooth channel under similar flow conditions. It compared friction and the Nusselt number coefficient to a smooth rectangular channel and a rough, purposely rough channel. The aim was to determine better heat transfer and reduced friction. The study found that the average Nusselt number (Nu/Nus) and convective heat movement were 3.39 times higher at g = 8.5 mm (θ = 90º) compared to g = 10.9 mm (θ = 80º) and 13.1 mm (θ = 70º) at p = 71. mm and Re = 22,000. The lowest Nu/Nus was 1.75 times for g = 13.1 mm and p = 71.3 mm at Re = 4000. Thinner boundary layers increased the average Nusselt number and convective heat transfer coefficients in systems with smaller gaps. The design model has a maximum THPP = 1.93 at a pitch space between roughness (p = 71.3 and P/e = 15.5, where e = 4.6 mm), gap (g = 8.5), and Re = 22,000.
Nomenclature
| L | = | Length of the rectangle-shaped channel (m) |
| W | = | Rectangle channel width (m) |
| H | = | Width of the rectangular channel (m) |
| e | = | Roughness in the shape of a multiple boomerang’s height (m) |
| Dh | = | Dimensions of hydraulics (m) |
| Re | = | Reynolds number |
| Nus | = | Smooth channel Nusselt number |
| Nu | = | Nusselt number of multiple boomerang-shaped SAH |
| frs | = | Smooth channel coefficient of friction |
| fr | = | Coefficient of friction for multiple boomerang-shaped SAH |
| g | = | Gap between each arm of multiple boomerang-shaped roughness (m) |
| T | = | Temperature (K) |
| P | = | Space between multiple boomerang-shaped roughness as pitch in the x-direction (m) |
| P/e | = | Ratio of pitch space and height of multiple boomerang-shaped roughness |
| e/Dh | = | Relative relation between height of multiple boomerang-shaped and hydraulic diameter |
| v | = | Air velocity (m.s−1) |
| T1 and T2 | = | Intake airflow and outlet air flow temperature of air, respectively |
| Cp | = | Specific heat (J.kg−1.K−1) |
| K | = | Conduction coefficient (W.m−1.k−1) |
| h | = | Coefficient of heat transfer (W.m−2.K) |
| SAH | = | Solar air heater |
| CFD | = | Computational fluid dynamics |
| Pr | = | Prandtl Number |
| RNG | = | Re-normalizing group |
| THPP | = | Thermohydraulic performance parameter |
| ρ | = | Air density (kg.m−3) |
| µ | = | Viscosity, N-s/m2 |
| g | = | Space between multiple boomerang-shaped roughness in y-direction (m) |
| ϴ | = | Central angle (°) |
| I | = | Uniform heat flux (W.m−2) |
| m | = | Mass flow rate of air (Kg. s−1) |
| t | = | Thickness of multiple boomerang-shaped roughness (m) |
| Tair and Tplate | = | Average temperature of ambient air and temperature of the absorber plate, respectively (K) |
| Aplate | = | Heat transfer area, m2 |
Disclosure statement
No potential conflict of interest was reported by the author(s).