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

•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.