Helical coils have gained increasingly interest in the field of supercritical carbon dioxide (SCCO2) Rankine cycles during the last decade due to the compact structure and high heat transfer rate. Past studies mainly focused on effects of operating conditions and the gravitational buoyancy, and are not sufficient to fully understand the behavior of SCCO2 helically coiled gas heaters. Influence of some other key factors, such as the coil orientations and internal wall roughness, on overall performance has been seldom reported and is still unclear to date. In this work we filled this gap with a solid-to-fluid conjugate heat transfer model where supercritical flow turbulence is solved by the Shear-Stress Transport kappa-omega equations. The orientation effect on performance of helical coils was revealed with the coil axis arranged in horizontal, vertically upward and downward directions, respectively. Results demonstrate that the criterion of Bo < 10(-5) for negligible buoyancy effect was applicable to coils with dimensionless curvature delta = 0.01-01 The Orientation effect only emerged when the buoyancy effect became significant with Bo > 10(-5). Deterioration and enhancement of heat transfer occurred alternatively in horizontal coils. The overall performance of horizontal coils was much worse than vertical coils, among which the downward orientation was slightly better than the upward one. The increase of delta barely improved the performance of horizontal coils. Instead, at larger delta the intensive fluctuation in heat transfer coefficients was more frequent due to the more drastic change of the flow direction. The location and extent of local impairment also varied with the inlet arrangement. Finally, the passive compound enhancement technique of helical coils in conjunction with internal helical-rib roughness was found to be very effective in mixed convection of SCCO2, especially in the horizontal coils. 2017 Elsevier Ltd. All rights reserved.