| Solar energy is omnipresent, enormous, available freely and harmless source of energy that get more and more attention. Solar air heater convert the solar energy into thermal energy and it is widely used for space heating, crop drying and industrial applications. In a conventional solar air heater, the velocity of air is relatively low in turbulent boundary layers and develops a thick viscous sub layer adjacent to the wall. The heat transfer coefficient is relatively low between absorber plate and air. In order to improve the heater transfer rate of solar air heater, an accepted method is to create artificial roughness on the absorber plate.In this study, a multi V-shaped with gap rib is chosen for the artificial roughness element. Computational Fluid Dynamics (CFD) was used to analyze the heat transfer and flow characteristics of the solar air collector having multi V-shaped with gap rib. The grid independence test, selection of turbulence model and effectiveness are conducted for the three-dimensional simulations. The optimal grid number is 6819326, and the result of RNG κ-ε turbulence model is best match up with the experimental result. The effects of rib geometrical size of gap position (d1/(w/2), d1(w2)), relative width ratio (W/w), rib width (S), rib pitch (P), rib height (e) and rib inclination angle (a) on the Nu number, friction factor, and performance factor at different Re number are obtained and analyzed.The maximum values of the average Nusselt number, friction factor and heat transfer performance occur for d1(w12)=0.33, d2/(w/2)=0.33. Heat transfer performance increases first and then declines with W/w increases. It is seen that the maximum value of heat transfer performance occurs for W/w=6, and the least value occurs for W/w=1. Heat transfer performance decreases with the increase of rib thickness S at Reynolds number equal to 6000, while for Reynolds number from 9000 to 18000, the heat transfer performance increases with the increase of rib thickness S. The heat transfer performance decreases with the increase of rib pitch for rib height equal to 1 mm,2 mm,6 mm, while for rib height equal to 2 mm, the heat transfer performance decreases first and then increases with rib pitch increases. The heat transfer performance increases with the increase of rib height and achieve a maximum value at e=2 mm. The heat transfer performance decreases with the increase of rib inclination. It is seen that the maximum value of performance factor occurs for a=30°, and the least value occurs for a=75°.The V-shaped with gap rib has a higher performance factor than that of the continous V-shaped rib at Reynolds number from 9000 to 18000.The detailed flow pattern results showed that, the fluid which passes through the gap breaks the gap downstream boundary layer and hence increases the gap leeside Nusselt number. On the sametime, the fluid passing through the gap also disturbs the secondary vortex flow at inter-rib region leading the vortex disappearing more early, which is, conversely, disadvantageous for the heat transfer. As the rib pitch decreases or rib height increases, the exchange times between the colder upper-channel fluid and the warmer near-bottom-wall fluid increase, and hence resulting in an increase of the heat transfer coefficient. The vortex diameter increases with increase of rib inclination. On the sametime, The proportion of fluid which passing into the inter-rib drawed to vortex increases. The flow direction of fluid which passing into the inter-rib changed greatly when a=75°. Since the moving velocity of the vortex along the V-rib leg direction will decrease with increasing of rib inclination. The effects of vortex diameter, flowing direction and vortex moving velocity will combine to yield the rib inclination corresponding to the maximum values of average Nusselt number at a=45°. |
