Using a two-phase method for numerical natural convection simulation in a cavity containing multiwalled carbon nanotube/water

In the present study, using two-phase mixing method, the natural heat transfer (HT) in a nanofluid cavity is investigated by numerical method. The first phase is the water, and the second phase is the multiwalled carbon nanotubes. To investigate the various factors, numerical study on Rayleigh numbers (Ra) between 10 3 and 10 6 and nanofluid volume fraction (VF) range of 0–3% is performed. The effect of porous media is also investigated at Darcy numbers between 0 and 60. Results were presented through temperature contour, velocity contour, and mean Nusselt number (Nu) diagram. The results show that increasing the nanofluid VF will result in an improvement in HT and an increase in average Nu. As the Ra number increases and the convective term dominates, the nanofluid flow becomes turbulent, resulting in an increase in the average Nu number. At low Ra, due to the weak buoyancy force, the temperature lines will be almost the same shape as the cavity, but at Ra of 10 6 , an increase in buoyancy forces will result in disrupting the uniform arrangement of the temperature lines. Using Taguchi method and signal-to-noise ratio, the order of importance of the parameters is investigated. Among the three different input parameters (Ra, VF, and porosity), the porosity or non-porosity of the medium is the most effective parameter on the average Nu number. Based on results, increasing Ra number will increase the average Nu number by 2300% for non-porous media and about 34–140% for porous mediums. Also, the maximum vertical velocity increases by about 249 times because of the increase in the Ra number. Graphic abstract