3D numerical study of the effect of aspect ratios on mixed convection air flow in upward solar air heater

A. Boulemtafes-Boukadoum (1) , Cherifa Abid (2) , A. Benzaoui (2)

1 CDER - Centre de Développement des Energies Renouvelables
2 IUSTI - Institut universitaire des systèmes thermiques industriels

3D Numerical study of mixed convection air flow in upward solar air heater with large spanwise aspect ratio (A = 10 to 40) was performed using CFD commercial code Fluent14.5 (ANSYS).The main objective of this study is to investigate the channel height's effect (Aspect ratio) on flow pattern and heat transfer in upward solar air heater in the particular case of low Re and high aspect ratio. The bottom plate (absorber) was submitted to Constant Heat Flux (CHF) in the range of 200 to 1000 W/m2 and Reynolds number was varied from 50 to 1000. The simulation results of flow visualization and Nusselt number calculation show that depending on Ri*, the velocity and temperature distribution in SAH vary greatly with channel's height. Our results are in concordance with most of authors conclusions about Poiseuille-Rayleigh-Benard flows. In mixed convection, increasing heat flux enhance heat transfer unlike forced convection flows. Simulation results of flow visualization and Nusselt number calculation have shown that depending on Ri*, the velocity and temperature distribution in SAH vary greatly with the channel's height. The obtained results were different from previous studies. Indeed, our investigation of channel's height was achieved for the same heat flux but different Grashof number. For low channel's height (high aspect ratio), enhancing heat flux has not a significant effect but for higher channel's height, an augmentation of heat flux enhance buoyancy effects in the flow and causes high turbulence, Also, increasing Reynolds number in low channel's height (high A), can enhance substantially heat transfer. For higher channel's height (low A), increasing Reynolds number decrease Ri* and thus buoyancy forces. Heat transfer is reduced and so Nusselt number. Those results may be very useful for engineers in designing and testing solar collectors.

Mots clés: Unsteady; mixed convection; CFD; solar collector; PRB flow; heat transfer; duct height

Domaines: Physique [physics]

A. Boulemtafes-Boukadoum, Cherifa Abid, A. Benzaoui. 3D numerical study of the effect of aspect ratios on mixed convection air flow in upward solar air heater. International Journal of Heat and Fluid Flow, 2020, 84, pp.108570. ⟨10.1016/j.ijheatfluidflow.2020.108570⟩. ⟨hal-03658255⟩