In this paper the development of mixed convection in a helically coiled beat exchanger for Re = 500, PI = 5 and delta = 1/14 is studied. The influence of buoyancy forces (Gr = O (10(5))) on heat transfer and secondary how is analyzed. In the method used the parabolized equations are solved using a finite difference discretization. The code is tested on mixed convection flow in a 90 degrees curved tube of which the results are compared to the results obtained with an elliptical code. For the helically coiled tube a constant wall temperature is considered. It appeared that heat transfer is highly influenced by secondary flow induced by centrifugal and buoyancy forces. For low Grashof numbers a splitting phenomenon of the temperature field is observed due to large secondary velocities, resulting in two separated areas of fluid. For high Grashof numbers the fluid in the coiled pipe becomes almost linearly startified which results in small secondary velocities. A wavy behaviour in the Nusselt number is observed for medium Grashof numbers. (C) 1997 Elsevier Science Ltd.