Thermal performances of tubular flows enhanced by ribbed spiky twist tapes with and without edge notches

Heat transfer enhancements (HTE) of the newly devised spiky ribbed twisted-tapes with and without edge notches are experimentally studied along with the associated pressure drop augmentations and transmitted efficiencies for heat convection. Axial Nusselt number (Nu) distributions, mean Fanning friction factors (f) and thermal performance factors (TPF) of the tubular flows enhanced by each type of present spiky ribbed twist tapes with five twist ratios (y) of 1.56, 1.88, 2.19, 2.5 and 2.82 are measured at the Reynolds numbers (Re) between 1000⩽Re⩽40,000. While these ribbed spiky twist tapes enrich the varieties of the swirl-type insert technologies, especially for retrofit applications, the favorable twisted tapes with the higher degrees of HTE benefits and/or TPF at the less expense of pressure drops from the comparative group collected by this study are disclosed. The present V-notched ribbed spiky twist tapes with forward flows considerably elevate the HTE impacts from the comparative counterparts by bursting the near-wall jets through the notches and initiating the separated vortex system from the spikes and ribs. With 1.56⩽y⩽2.82, the heat transfer ratios between present RST-V(FF), RST(FF), RST-V(BF) and RST(BF) tubes and the plain tubes are 3.92–7.86 (3.17–5.93), 3.72–6.08 (3–5), 3.52–6.24 (2.7–5.63) and 3.17–5.23 (2.75–4.38) at turbulent reference conditions; and 11.72–22.92 (9.15–17.26), 9.03–16.45 (6.51–12.85), 10.38–17.54 (8.27–14.45) and 7.69–14.02 (5.56–11.3) at laminar reference conditions. The corresponding f ratios between present RST(FF), RST-V(FF), RST(BF), RST-V(BF) tubes and the plain tubes fall in the respective ranges of 6.03–9.61, 1.42–2.94, 6.62–11.77 and 1.4–3.27. The TPF values for present RST(FF), RST(BF), RST-V(FF) and RST-V(BF) tubes are in the respective ranges of 1.86–2.57, 1.48–2.15, 2.49–3.43 and 2.07–2.7 with 1000⩽Re⩽2000; and 0.69–1.06, 0.58–0.92, 0.71–1.31 and 0.64–1.12 with 5000⩽Re⩽40,000. To assist the engineering applications, two set of empirical Nu and f correlations for the tubular flows enhanced by present two types of spiky ribbed twist tapes at forward and backward flow conditions are generated.