of the venturi here the fins are confined to the u201c heat transfer u201d section 10 the pattern of fins used is quite arbitrary fig5 a and 5 b show fins extending across the venturi and intersecting to form a grid 19 within the u201c heat transfer u201d section 10 useful visualizations of the structure of such grids are provided by tennis rackets apple corers and planar tea strainers fig5 a and 5 b also serve to emphasize the arbitrariness of the cross sectional shape of the venturi many venturis possess cylindrical symmetry but this is not a requirement another aspect of the invention is the cross sectional shape of the thermally conducting fins their cross sectional shape is that of an airfoil and is designed to minimize aerodynamic drag on the working fluid flow by the fins the normally larger component of drag the so called u201c pressure u201d component is rendered negligibly small by the aerodynamic cross sectional shape of the fins unlike more common airfoils our thermally conducting fins need not provide lift and need not change their angle of attack thus they can be thin and oriented along streamlines of the working fluid flow to further reduce drag in this connection arrays of fixed airfoils are often used to suppress turbulence in duct flow another degree of design freedom with regard to the grid elements is.