Influence of different base angle and equilateral triangular surface roughness on performance of isosceles trapezoids duct solar air heater: Numerical investigation and its optimization

Abstract

The thermal energy available in the solar radiation can be utilized in the drying application with the help of solar air heater. Most of the existing designs of the solar air heater consists of triangular or rectangular duct, but in the proposed design an isosceles trapezoidal duct has been proposed for the solar air heater. Six distinct SAH duct model withdifferent base angles i.e. 45⁰ (i.e. triangular), isosceles trapezoids (i.e. 50⁰, 60⁰, 70⁰, and 80⁰) to 90⁰. These models include ducts with cross-sectional shapes of a rectangle (90⁰), a triangle (45⁰), and four different isosceles trapezoids with base angles of 50⁰, 60⁰, 70⁰, and 80⁰ with equilateral artificial roughness with relative rib-height of 0.042 and relative pitch of 7.14, 10.71. and 14.29. Analysis is carried out by maintaining the constant duct height (0.08 m) and absorber plate width of 0.16 m for all the developed models to create the same operating conditions in the considered ducts. The simulations are performed using the computational fluid dynamics approach by assuming heat flux of 1000 W/m² on the absorber plate for the Reynolds number range of 2800 to 29000. The authenticity of the proposed simulation is validated with the existed results and it is concluded that the RNG k–ɛ turbulence model is suited best for modelling turbulence. The augmentation of heat takes place with the increase of base angle from 45⁰ to 90⁰ and the duct with angle of 90⁰ gives best results with 33.45 % higher magnitude at a Reynolds number of 5000. However, the frictional factor curve follows a trend as that of heat transfer and the highest frictional losses exhibited for the duct with angle of 90⁰ i.e. 6.54 % higher in comparison to the duct with base angle of 45⁰, at a Reynolds number of 5000. Thus, it is concluded that the trapezoidal SAH duct with the widest base angle (namely the rectangular) exhibits the most optimal performance. The results of heat transfer and friction factor are generalized in the form of mathematical expression with the help of regression analysis and the results estimated using the developed expression predicts results with an error of ±2.48 % and ±3.45 % for Nusselt number and friction factor, respectively.