Natural convection heat transfer inside a vertical rectangular enclosure with four two-dimensional discrete flush-mounted heaters is investigated numerically and experimentally to unveil primarily the influence of aspect ratio of the enclosure. Numerical simulations for the problem have been conducted for the aspect ratio varying from 1 to 10 and the modified Rayleigh number in the range between 10(3) and 10(7) with a given relative heater size and location. Numerical results reveal that the increase of the aspect ratio leads to substantial degradation of convective dissipation from the discrete heaters. The highest heater surface temperature occurs usually at the top heater for Ra* greater-than-or-equal-to 10(4) regardless the aspect ratio of enclosure. Correlation in terms of the modified Rayleigh number and the aspect ratio of the enclosure has been generated for the average Nusselt numbers for each heater as well as the maximum surface temperature at the discrete heaters. Holographic interferometry and a smoke flow visualization are employed to map the temperature and flow fields within an enclosure of aspect ratio 10. The predicted temperature and flow fields are found to be in good agreement with the experiments.