Experimental investigation on very-high-Rayleigh-number thermal convection in tilted rectangular enclosures

•Nusselt number decreases monotonously with an increase in angle of inclination (θ).•Nusselt number decreases as aspect ratio increases for all θ and Rayleigh numbers.•The decreasing trend with aspect ratio amplifies as θ increases from 0° to 90°.•A set of correlations is proposed to estimate Nu for studied angles of inclination.•Racosθ scaling predicts the experimental data with reasonable accuracy for 0° ⩽θ ⩽60°. High Rayleigh number thermal convection in tilted rectangular enclosures of different aspect ratios (1,3,6, and 10) and angles of inclination (0°,30°,60°,90°,120°, and 150°) is experimentally studied for 1.85×106⩽Ra⩽1.04×1011 employing compressed gases. For a fixed Rayleigh number, aspect ratio is found to have a discernible effect on Nusselt number for all the cases, except for the horizontal enclosure case (0°), and this decreasing effect becomes stronger as the angle of inclination increases. Nusselt number is observed to monotonously decrease as the angle of inclination is increased, for any given Rayleigh number. A new set of correlations for Nusselt number, valid for the investigated range of studied variables, is proposed. An Racosθ scaling is also found to work reasonably well for angles of inclination less than 60°.