The investigation of thermal radiation and free convection heat transfer mechanisms of nanofluid inside a shallow cavity by lattice Boltzmann method

This paper aims to simulate the interaction between thermal surface radiation and nanofluid free convection in a two dimensional shallow cavity by lattice Boltzmann method. The supposed nanofluid is generated by a homogeneous mixture of water and nanoparticles of Al2O3. The upper and lower walls of cavity are maintained at cold and hot temperature, respectively; while the side walls are kept thermally insulated. The cavity aspect ratio is chosen as 5 which indicates a shallow one. The cavity all inner surfaces are considered as the gray diffuse emitters and reflectors of radiation. The computations are performed for the wide range of parameters as Ra=10⁴ and Ra=10⁵; ε=0.5 and ε=0.9 while nanoparticles volume fraction changes between 0.0≤$$$\le$0.04 at each case. As a result, the effects of emissivity and Rayleigh number are studied on the total heat transfer of radiation and free convection of nanofluid. The suitable validations are examined beside the useful grid study procedure. The results are presented as the profiles of velocity and temperature and also the streamlines and isotherms. Moreover the local and averaged Nusselt numbers are provided for the coupled and uncoupled states of radiation and free convection heat transfer mechanisms. It is seen that Nu_m of total free convection and radiation would be more at higher Ra and ε; which indicates that radiation heat transfer coupled with free convection might affect the flow field and improve the Nusselt number.