Mechanism of Heat Transfer Enhancement by Secondary Flow in a Helically Coiled Circular Tube

Helically coiled tubes have been widely used as heat exchange equipment. Therefore, the heat transfer mechanism inside the helically coiled tube was studied under two boundary conditions of uniform wall temperature and uniform heat flux. The contribution of velocity and velocity gradient to heat transfer was explored using the heat flux convection transfer equation. The study revealed that heat transfer at the outer side of the helically coiled tube was enhanced by secondary flow, while heat transfer at the inner side was weakened, resulting in local heat exchange performance inferior to that of a straight tube of the same size. Under uniform wall temperature boundary conditions, the combined effect of velocity and velocity gradient enhanced heat transfer inside the helically coiled tube, whereas under uniform heat flux boundary conditions, the velocity gradient has a greater contribution to heat flux transfer inside the tube.