Experimental study of heat transfer in a non-uniformly heated horizontal tube with wire-coil insert under single-phase flow

Despite the prevalence of non-uniform heat flux in applications like concentrated solar power, the combined effects of non-uniform heating and wire-coil inserts on single-phase heat transfer coefficients remain scarcely investigated. Consequently, this work experimentally characterises the effects of circumferentially non-uniform heating coupled with wire-coil inserts on the circumferential local heat transfer coefficients, average heat transfer coefficients and frictional pressures drop for R245fa, covering Reynolds numbers from 1500 to 4000. A stainless-steel horizontal tubular test section with a wall thickness of 2.11 mm, an internal diameter of 17.12 mm, and a heated length of 1000 mm was considered for cases with and without wire-coil inserts. Three wire-coil diameters (1.25, 1.50, and 1.80 mm) were tested at a constant pitch of 6 mm, corresponding to pitch-to-wire-diameter ratios of 4.80, 4.00, and 3.33, respectively. The heat flux conditions included uniform (as reference), and circumferentially non-uniform profiles (the latter representing a parabolic solar trough collector case at a concentration ratio of 18) at an average heat rate of 2.5 kW/m 2 . It was found that the non-uniform heating regime with wire-coil inserts significantly enhanced both local and average heat transfer coefficients relative to the uniform-heating smooth tube. Specifically, the local circumferential heat transfer coefficient increased by a factor of 1.1 to 4.5, and the average heat transfer coefficient improved by between approximately 20% to 80%. Furthermore, the thermal performance factor, expressing the combined impact of the thermal and frictional pressure drop, peaked at 1.21 at higher Reynolds numbers. The friction factor for the wire-coil inserts ranged between approximately 0.04 to 0.07 compared to approximately 0.01 for the reference case. It was concluded that wire-coil-induced swirl, together with secondary flow potentially arising from circumferentially non-uniform heating, may have contributed to enhanced heat transfer. Among the tested configurations, the 1.25 mm diameter wire-coil insert exhibited the most favourable thermal performance factor.