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°.