Evaporation of sessile droplet is a significant common phenomenon. A three-dimensional numerical analysis was carried out to investigate the flow structure inside the sessile evaporating droplet. The present model developed in this study takes buoyancy, vapor diffusion, and evaporation cooling into account. Different flow structures are present inside the droplet evaporating on the hydrophobic and super hydrophobic substrates. On the hydrophobic substrate, the axisymmetric vortex appears in the droplet. The fluid at the bottom of the droplet flows upward along the axis direction, and then flows downward along the droplet surface. On the super hydrophobic substrate, a single non-axis-symmetric vortex is present inside the droplet, the fluid at the bottom of the droplet flows upward along one side of the droplet surface and downward along the other side. The influence of droplet volume and substrate temperature on the flow velocity and temperature distribution inside droplets is discussed quantitatively. The origin of the non-axisymmetric Raleigh convection inside the droplet on the heated superhydrophobic substrate is discussed. We found that the axis-symmetric flow in the droplet is not stable on heated superhydrophobic substrate. Any perturbation in the system can get enlarged and drive the instable axis-symmetric flow turns to a non-axis-symmetric rolling flow.

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