| Abstract: | It is shown experimentally that flattened tubes give higher laminar-flow heat transfer coefficients than tubes of circular cross-section. Part of the increase is due to an increase in the tube-wall shear rate; the magnitude of this effect is predicted and checked by reference to the behaviour of Newtonian liquids. The other part is due to secondary flow patterns which develop in viscoelastic liquids; this effect is at a maximum when the aspect ratio of the tube cross-section is of the order 1.5:1. The first effect may increase heat transfer coefficient by over 90% and the latter by over 45%. |
