Drag reduction and heat transfer characteristics of water flow through the tubes with superhydrophobic surfaces

In the present study, the drag reduction and heat transfer characteristics of water flowing through the tubes with superhydrophobic surfaces were investigated. The tubes with inner superhydrophobic surfaces in diameters of 4.0, 8.0 and 12.0 mm were fixed along the center axis of an outer tube to form the counter-current double-tube heat exchanger with a length of 1000.0 mm. Hot and cold water were used as the working fluids flowing through the inner tube and annular space of the heat exchanger, respectively. The experiments were performed at the Reynolds numbers ranging from 3000 to 11,000. The results showed that the drag reduction ranged from 8.3% to 17.8% for superhydrophobic surfaces. The effect of superhydrophobic surface on drag reduction increased with the decrease of tube diameter, and it also decreased with the increase of the Reynolds number. The friction factors of superhydrophobic tubes were smaller than those of smooth tubes at the same Reynolds number. The Colebrook equation was not applicable to estimate the friction factors of superhydrophobic surfaces, and the modified equation was proposed to describe the experimental results. The heat transfer performance was suppressed by the air cavities on superhydrophobic surfaces. The Gnielinski equation also failed to predict the heat transfer coefficients of superhydrophobic surfaces, and the calculated results by the modified equations were consistent with the experimental results. The PECs of combined performance of the drag reduction and heat transfer for superhydrophobic tubes with inner diameter of 8.0 mm was relatively larger than those of superhydrophobic tubes with inner diameters of 4.0 and 12.0 mm.