Two-dimensional laminar natural convection in air contained in a long horizontal rectangular enclosure with isothermal walls at different temperatures has been investigated using numerical techniques. The time-dependent governing differential equations were solved using a method based on that of Crank and Nicholson. Steady-state solutions were obtained for height to width ratios of 1, 2.5, 10, and 20, and for values of the Grashof number, GrL, covering the range 4 × 103 to 1.4 × 105. The bounds on the Grashof number for H/L = 20 is 8 × 103 ≤ GrL ≤ 4 × 104. The results were correlated with a three-dimensional power law which, yielded  
H/L=1
 
Nu¯L=0.0547(GrL)0.397
 
2.5H/L20
 
Nu¯L=0.155(GrL)0.315(H/L)0.265
The results compare favorably with available experimental results.
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