The interaction phenomena between laminar forced convection and thermal radiation in a participating medium inside a helical pipe were studied numerically. The P-l radiation model was applied to the combined convection-radiation simulations. The three-dimensional governing equations for laminar flow and heat transfer were solved with a control-volume finite difference method (CVFDM) with second-order accuracy, and the O-type structure grid was adopted in this study. The effects of thermal radiation on the convective flow and heat transfer were measured by comparing the numerical results with and without thermal radiation. The comparison showed that under the conditions examined in this paper, although the thermal radiation had no significant influence for flow and temperature fields, especially in a fully developed region, it substantially enhanced the total heat transfer in the helical pipe. (C) 2000 Elsevier Science Ltd. All rights reserved.