Natural convection inside a rectangular enclosure is investigated experimentally and numerically. One of the sidewalls is heated discretely by two flush-mounted heat sources. The other sidewall is kept at a constant temperature, while the horizontal walls are unheated. Heat dissipation rates of the heat sources are equal to each other. The aspect ratio of the enclosure (AR) is 2 and the working fluid is air (Pr=0.71). The study is focused on the validation of the two and three dimensional computations under real test conditions against experiments for various modified Rayleigh number values. Experimental study is performed for various modified Rayleigh numbers in the range of 7.7x10(5) and 3.1x10(6) while numerical part covers the values between 10(4) and 5x10(6). Temperature measurements and flow visualization studies are performed in the experimental work, and streamlines, isotherms and heatlines are presented in the numerical part of the study. From the experimental and numerical studies, it is shown that two dimensional computations reflects the general characteristics of the problem, conduction and radiation heat transfer are not negligible, surface temperatures increase with the modified Rayleigh number and heatline approach is an important tool to analyze convective heat transfer.