Heatline concept has been used in order to estimate thermal performance of various energy related systems. Heatlines and streamlines are found to be adequate to visualize and understand heat distribution and thermal mixing occurring inside a inclined porous square cavity. At high Darcy number, Da(m) (Da(m) = 10(-2)), heatlines take the shapes of streamlines at the central core of the cavity resulting in enhanced thermal mixing as seen from closed convective heatline cells with high magnitudes. Heat transfer rates are discussed based on local and average Nusselt numbers and further, these are adequately explained based on heatlines. The inclined cavity with higher inclination angle (phi = 75 degrees) provides the higher (Nu) over bar (BC) whereas higher (Nu) over bar (DA) is observed for the small inclination angles (phi = 15 degrees) at Da(m) = 10(-2). Note that, BC and DA denote the right and left walls, respectively. The larger inclination angle may be optimal for the energy efficient processes involving inclined enclosures due to larger heat flow circulations with enhanced thermal mixing. (C) 2015 Elsevier Ltd. All rights reserved.