Laminar flow heat transfer of dilute viscoelastic solutions in flattened tube heat exchangers

Heat transfer studies of non-Newtonian liquids in non-circular exchangers are not many, and much less on viscoelastic liquids in flattened tubes. Heat transfer studies of dilute viscoelastic liquids in flattened tubes with 0.635 cm and 1.27 cm original diameters and 50 cm–76 cm lengths and aspect ratios ranging from 1.4 to 5.7 were carried out. Five flattened tube heat exchangers with four thermocouples soldered at regular intervals on the outside wall were placed in turn in the experimental circuit to determine the heat transfer coefficients. Hot water was used as the heating medium; and dilute solutions of polyacrylamide in water and in water/glycerol mixtures were used as the viscoelastic solutions. Heat transfer increase as a result of flattening the tubes could be 101% higher while the effect due to secondary flow had a maximum additional increase compared to that for water of 40% for the 250 ppm solution at an aspect ratio of 1.6. Corresponding values for the 500 ppm solutions in water and in water/glycerol mixture were about 53% and 55% respectively at an aspect ratio of 1.8. Increased polymer concentration had only a marginal effect on heat transfer performance.

► Increased aspect ratio increases heat transfer performance.
► Increased heat transfer coefficient due to flattening could reach 100%.
► Secondary flow could enhance heat transfer performance up to 55%.
► Increased viscoelasticity increases heat transfer coefficients.