Natural convection heat transfer from horizontal concentric cylinders is studied experimentally and numerically. Concentric cylinders were formed with two test cylinders made of copper, and the annulus was filled with water. Concentric cylinder system was located in the ambient air and the inner horizontal cylinder was kept at a constant temperature. Experimental study was carried out at different ambient temperatures in a conditioned room which can be maintained at a stable required value and inside a sufficiently designed test cabin. The ambient and inner copper cylinder surface temperatures (T(infinity) and T(c)) varied between 20 degrees C-30 degrees C and 30 degrees C-60 degrees C respectively. Also, experiments were performed for bare cylinder at the same conditions as concentric cylinders to compare results. On the basis of the experimental data average Nusselt numbers for the air side of the concentric cylinders were calculated and compared with numerical results. The effective thermal conductivity(k(eff)) of the annulus was calculated by using the experimental data and numerical solution results and also compared with the well known correlation. Isotherms and streamlines are presented in the annulus and the air side for Ra(L) = 9 x 10(5)-5 x 10(6) and Ra = 2 x 10(5)-7 x 10(5) respectively. It is seen that numerical and experimental results are in a good agreement. Heat transfer rates under steady-state conditions from bare and concentric horizontal cylinders were compared and heat transfer enhancement was determined. Also the effect of the decrease in the temperature of the inner copper cylinder surface (condensation temperature) on COP was investigated considering an ideal Carnot refrigeration cycle. It is found that the enhancement in COP of a Carnot refrigeration cycle is 42.6% under steady-state conditions. (C) 2011 Elsevier Masson SAS. All rights reserved.