Heat transfer characteristics of decaying swirl flow through a circular tube with co/counter dual twisted-tape swirl generators

The influence of co/counter dual-twisted tapes (CoT/CT) on heat transfer rate in a circular tube has been investigated experimentally. In the experiment, the dual-twisted tapes are placed at the entry of the test tube in two arrangements: (1) each of dual twisted tape was twisted in the same direction that can produce co-swirl flow at the entry and (2) each of dual twisted tape was twisted in the opposite direction that can produce counter-swirl flow. Dual tapes were twisted in three different twist ratios (y/w = 3, 4, and 5) for generating different swirl intensities at the entry of the test section while the single twisted tape (ST) was also the test for comparison. The aim at using the dual twisted tapes is to create co/counter-rotating swirl flows having a significant influence on the flow turbulence intensity at the entry section leading to higher heat transfer enhancement. Average Nusselt numbers of CoT/CT are determined and also compared with those obtained from other similar cases, i.e., ST. The experimental results on the heat transfer rates indicated that the tubes with the dual twisted tapes (CoT/CT) are higher than those with the single tape at the entry section (x/D = 0 to 10). The heat transfer rates at longer distance became lower due to high interaction of each swirl. In addition, the mean Nusselt number and friction factor for the swirl generator created by the CT is nearly similar to CoT results.     

Silver–water nanofluid flow and convective heat transfer in a microfin tube equipped with loose-fit twisted tapes

Heat transfer enhancement and performance of compact heat exchangers have been extensively studied in the past century for the purpose of promoting energy efficiency. Microfin tubes in single/two/multiple-phase flow heat exchangers into which twisted tape swirl generators are installed can promote heat transfer with a moderate pressure loss penalty. This article reports on the enhanced heat transfer of silver–water nanofluids in a microfin tube into which loose-fit twisted tapes are installed in a counter-flow arrangement. The experiments were carried out using nanofluids with various silver concentrations (0.007–0.03 vol%), loose-fit twisted tapes with clearance ratios (c/D) of 0.0 (tight-fit), 0.05, 0.075 and 0.1, for a twist ratio, y/W, of 2.0. The results indicate that the heat transfer rate (Nu) and pressure drop (f) increase with a decrease in clearance ratio (c/D) and increase in silver (Ag) nanoparticle concentration. Additionally, the thermal performance factor tends to increase with the decrease in Reynolds numbers.