Heat transfer enhancement of laminar and transitional Newtonian and non-Newtonian flows in tubes with wire coil inserts

This work presents an experimental study on two different wire coils inserted in a smooth tube using both Newtonian and non-Newtonian fluids to characterize their thermohydraulic behavior in laminar and transitional flow. Dimensionless pitches of the wire coils were chosen as p/D = 1 and 2, whereas dimensionless wire diameter was e/D = 0.09 for both wire coils. Non-Newtonian tests considered different viscosity types with concentration of 1% of CMC (carboxyl-methyl-cellulose) solution in water at several temperatures; a wide range of flow conditions has been covered: Reynolds number from 10 to 1300 and Prandtl number from 150 to 1900. Newtonian test were carried out with propylene glycol as working fluid, covering a similar range of Reynolds number as the previously indicated for non-Newtonian fluids. Isothermal pressure drop tests and heat transfer experiments under uniform heat flux conditions were performed, and results were contrasted with own experimental data for the smooth tube and with well-know analytical solutions. At low Reynolds numbers, both wire coils behave as a smooth tube but accelerate transition to critical Reynolds numbers down to 500. Maximum augmentations of Fanning friction factor of 3.5 times and of 4.5 times of Nusselt number have been found with respect to the smooth tube.