DEVELOPMENT OF A HELICAL COORDINATE SYSTEM AND ITS APPLICATION TO ANALYSIS OF POLYMER FLOW IN SCREW EXTRUDERS .1. THE BALANCE-EQUATIONS IN A HELICAL COORDINATE SYSTEM

Two sets of the balance equations for a compressible and non-Newtonian fluid in helical coordinate systems are developed. They are associate d with two different ways of decomposing a velocity vector; one with t hree non-orthogonal velocity components in the normal, tangential and helical directions and the other with three orthogonal velocity compon ents in the radial, cross-channel and down-channel directions. It is s hown that that compared with classical rectangular and cylindrical coo rdinate systems, a helical coordinate system is most suitable for desc ribing flow in a screw extruder, in which the flow path is characteris ed by two inherent geometrical features: curvature and helicity. (C) 1 997 Elsevier Science B.V.