The influence of surface roughness on the pressure drop and non-fully-developed heat transfer for water in subcooled vortex flow

The characteristics of fluid friction and non-fully-developed nonboiling heat transfer, from tubes with various roughnesses, to water in vortex flow were investigated. Axial flow values were obtained for comparison. The vortex was maintained by a full length copper tape and the roughness was generated by various internal V-threads. The test sections were electron beam heated over a 0.64 cm by 6.99 cm beam spot. Local heat transfer coefficients were determined for non-fully-developed conditions, 6.3 diameters from the tube entrance and 3.1 diameters from the thermal entrance, for nonboiling forced convection heating. Reynolds numbers were between 8 x 10³ and 8 x 10⁴ and Prandtl numbers were from 7.9 to 10.0.

The nonboiling heat transfer coefficients for both axial and vortex flow were found to increase substantially with an increase in surface roughness. For very rough surfaces an increase of up to 3.9 times for axial flow and up to 7 times for vortex flow was observed. At equal Reynolds numbers, the heat transfer coefficients of a very rough tube in vortex flow were as much as 9 times higher than a smooth tube in axial flow. On a constant pumping power basis, the heat transfer coefficients flow. of tubes in both axial and vortex flow, increased as the surface roughness increased. Improvements in heat transfer of up to 2.6 times for axial flow and 5 times for vortex flow were observed, for very rough surfaces.

Sounds associated with boiling were observed for tubes in both axial and vortex flow. Curves of the input heat flux at which these sounds began, versus Reynolds numbers, were determined.