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Inserts are used to enhance the heat transfer rates between the two fluids in heat exchanger tubes. A variety of tube inserts such as twisted tape, wire coil, swirl flow generator have been investigated for their effect on heat transfer rates and fluid friction. This paper reviews the works pertaining to the application of different class of tube inserts in order to comprehend the prevailing mechanism of fluid flow and heat transfer. An attempt has been made to elucidate the fluid flow behaviour sustained by the particular class of insert that controls the heat transfer rates across the thermal boundary layer attached to the tube wall. [ABSTRACT FROM AUTHOR]

• Performances of TD, TP, TDP, D-TD, D-TP, and D-TDP were investigated numerically. • All cases of teardrop patterns have higher Nu values than TT in the tested Re region. • All cases except TP have higher TPFs than those of TT between Re 3000 and 5500. • The D-TDP case reached a thermal performance factor of 1.517 at Re = 3000. The teardrop pattern observed to be more efficient in straight tubes was examined for the first time in a twisted tape wall. The thermo-hydraulic performances of twisted tapes having periodically teardrop-shaped dimple-protrusion patterned walls are numerically investigated. The new designs have two different densities (N=30, and 45), three alternative types of layouts (composed of only protrusions, only dimples, and a combination of dimples and protrusions) align in two different flow directions. Designs have a constant twist ratio (TR) of 3, and calculations are conducted for Reynolds numbers (Re) between 3000 and 27000 by Ansys Fluent under a constant heat flux of 50000 W/m2. All cases have higher Nusselt number (Nu) values than traditional twisted tape (TT) in the tested Re region. Besides, all arrangements except the teardrop protruded twisted tape have higher thermal performance factors than the TT in the Re range from 3000 to 5500. All dense versions of the teardrop patterned twisted tapes have higher Nu and friction factor (f) than sparse versions of them. The dense teardrop dimpled-protruded (D-TDP) case reaches TPF of 1.517, the highest value among the tested twisted tapes, at Re = 3000. [Display omitted] [ABSTRACT FROM AUTHOR]