A numerical study has been carried out to investigate thc problem of convective heat transfer in a gap formed by vertical concentric cylinders. Numerical results of a finite difference scheme of velocities and temperature distribution are presented here. The study covers the case of stationary cylinders and the influences of the flow direction and the inlet temperature when both walls are at constant temperatures. The results were obtained for axial Reynolds number Re(a)) less-than-or-equal-to 250 and 2.5 x 10(4) less-than-or-equal-to Gr less-than-or-equal-to 4 x 10(4).