Experimental studies on local and average heat transfer characteristics in helical pipes with single phase flow

Highlights: • Centrifugal force and buoyancy on fluid in the helical tube are investigated. • Heat transfer in helical coil tube can be categorized by a non-dimensional number. • The wall temperature varies drastically between different regions at cross section. • Average Nusselt number fits well with Hardik, Xin & Ebadian, and Mori & Nakayama. • Hardik's correlation can't predict local circumferential heat transfer. - Abstract: In order to investigate the effects of centrifugal force and buoyancy on the local and average convective heat transfer characteristics of single phase, experiments were carried out on uniformly heated helical pipes. The local wall temperature is measured along the length of coil as well as in the circumferential direction using eight thermocouples at each cross section, which is made of INCONEL alloy 625 tubes with an inner diameter of 15.26 mm and a thickness of 1.8 mm. The ratio of the pipe diameter to coil diameter is 0.0436 and coil pitch is 194 mm. It is shown that the circumferentially local heat transfer coefficient vary drastically between different regions at cross section in helical coil tubes. A non-dimensional number which represents the ratio of centrifugal force and buoyancy could predict heat transfer characteristic in helical coil tubes well. It is also demonstrated that experimental data of the average Nusselt number agrees well with existing correlations. However, the local circumferential average Nusselt number is not well predicted by existing correlation.