Effect of curvature ratios on the heat transfer and flow developments in the horizontal spirally coiled tubes are investigated. The spirally coiled tube is fabricated by bending a 8.00 mm diameter straight copper tube into a spiral-coil of five turns. The spirally coiled tube with three different curvature ratios of 0.02, 0.04, 0.05 under constant wall temperature are tested. Cold water entering the innermost turn flows along the spiral tube and flows out at the outermost turn. The turbulent flow and heat transfer developments are simulated by using the k-epsilon standard turbulence model. A finite volume method with an unstructured nonuniform grid system is employed for solving the model. The simulated results are validated by comparing with the present experiment. The predicted results for the convective heat transfer and flow characteristics are reasonable agreement with the experiments. The centrifugal force has significant effect on the enhancements of heat transfer and pressure drop. In addition, due to this force, the heat transfer and pressure drop obtained from the spirally coiled tube are higher than those from the straight tube. (c) 2006 Elsevier Ltd. All rights reserved.