The effects of Prandtl numbers on local and average convective heat transfer characteristics in helical pipes

To investigate the effects of the Prandtl number and geometric parameters on the local and average convective heat transfer characteristics in helical pipes, experiments with three different fluids-air, water, and ethylene glycol-were carried out on five uniformly heated helical pipes. The test sections were made from 22.9 mm I.D. and 10.2 mm I.D. 304 stainless steel pipes. The ratios of the pipe diameter and pitch to coil diameter (d/D and b/D) ranged from 0.0267 to 0.0884 and 0.20 to 2.56, respectively. The peripheral and average, fully developed Nusselt numbers were evaluated in the experiments. Experimental findings indicate that after two turns (X > 2) the temperature distributions along the wall are almost parallel to the linear fluid bulk temperatures, and all dimensionless peripheral wall temperatures are nearly identical, implying that both the flow and temperature distribution within the helical pipes are fully developed. These results reveal that the peripheral Nusselt number varies significantly for higher Prandtl numbers and Dean numbers in the laminar flow region. A new set of empirical expressions for the average fully developed Nusselt number has therefore been regressed based on the present data and some data from previous investigations. No obvious effects of the coil pitch or torsion were observed in the scope of this investigation.