b in each of these groups extending parallel to the direction of overall extension thus the passage follows a convoluted path with many changes of direction the coolant flow within the passage has to negotiate these changes of direction and consequently the flow is continually separating and reattaching within the passage ensuring that a thin boundary layer is maintained and that high levels of turbulence occur as a consequence high levels of heat transfer can be achieved however because the passage sections repeat themselves every eight passage sections i e they have a motif consisting of eight passage sections the heat transfer characteristics of the passage are fairly uniform along the length of the passage fig5 shows schematically a different configuration for the internal cooling passage in this configuration every group of three successive end to end connected passage sections has one passage section extending along each of the axes x y z the passage sections repeat themselves in this case every six passage sections further the passage sections 50 c which extend parallel to the z axis and perpendicular to the planes a and b in fig5 are shorter than the passage sections 50 a 50 b by a ratio of 2 3 indeed it is possible for the passage sections to have different lengths depending on which axis they extend along fig6 shows schematically a passage that is similar to that of fig5 except that in this case the shorter passage sections 50 a lie in the planes.