Experimental Analysis of the Improvement of Heat Transfer in Tube Heat Exchangers Via Passive Flow Generators Using Wire Coil Inserts

Abstract Due to their high performance and low-cost demands, internally treated tube heat exchanger surfaces are one of the passive heat transfer enhancements that have caught the industry's attention. At bulk temperatures of 30 °C, an experiment for the insertion of 1 mm and 0.5 mm wire coils with a constant pitch length of 8 mm was carried out in this study. The results on the improvement of heat transfer, including the velocity profile, Nusselt number (6000 < Re < 20,000), friction factor, and thermal enhancement efficiency, were significant. Based on a lower surface temperature recorded beyond the uncertainty value, the results demonstrated an improvement in heat transfer for smaller diameter of wire coil inserts. It's interesting that this improvement is concentrated at low Reynolds numbers, indicating that there may be a point at which an increase in wire thickness does not necessarily result in an equivalent improvement in heat transfer. For both wire thicknesses, a Nusselt number increase of up to five times was visible. The friction factor penalty, however, varies depending on the wire thickness, with a higher magnitude (3.2-fold increase) obtained for 1 mm as opposed to a 1.8-fold increase for the lower counterpart. This distinction results in the 0.5 mm coil insert gaining better overall performance with an average of 2.2 for the thermal performance ratio, further solidifying the advantage of this technique for enhancing heat transfer in conduits. The diameter of the wire coil is found to be a key factor in improving heat transfer and convection on the boundary layer surface.