A duct (10) for transporting a two phase mixture has a length portion of circular cross-section (12) leading, via a transition zone (14) in which the duct cross-section changes from circular to lobed, to a zone in which its cross-section exhibits at least one lobe (16) extending helically around and along the duct. The duct has a cross-sectional area which is substantially constant, or which progressively increases, over the transition zone. The transition from circular to lobed cross-section is alternatively smooth and continuous, and the transition zone occupies a duct length in which the or each lobe becomes relocated by at least one position.