Laminar thermal and fluid flow characteristics in tubes with sinusoidal ribs

The article presents the effect of novel sinusoidal ribs transversely mounted in a tubular heat exchanger on convective heat transfer and flow pressure loss characteristics. The numerical study was performed under a constant heat flux condition for laminar water flow with Reynolds number (Re) ranged from 400 to 1800. The employment of sinusoidal ribs in the tube was aim to induce longitudinal vortex streams to give more intense flow mixing and to interrupt thermal boundary layer at the tube wall. The sinusoidal rib tubes (SRTs) were investigated at four rib height to diameter ratios (HID = 0.026, 0.042, 0.058 and 0.074), three rib amplitude to diameter ratios (AID = 0.211, 0.316 and 0.421), four rib width to diameter ratios (MD = 0.158, 0.263, 0.368 and 0.474), three rib pitch to diameter ratios (PID = 1.053, 1.316 and 1.579) and six circumferential rib numbers (N = 1, 2, 3, 4, 5 and 6). The obtained results reveal that increasing rib height, decreasing rib amplitude, decreasing rib width and decreasing rib pitch at most of Re lead to increase of both Nusselt number (Nu) and overall thermal performance evaluation criterion (PEC) while Nu and PEC increase from N = 1 to 3 and then decrease from N = 3 to 6. The enhanced heat transfer rate up to 4.89 times above that of the plain tube is found in the SRTs and the augmented friction loss is up to 5.62 times. The PEC is gained in the range of about 1.03-2.75 in the case studies and can be further improved by combined employing of the optimized individual geometric parameters in the preliminary trend analysis with the maximum value being about 3.64. The well organized flow patterns in the SRTs imply the probability in active controlling heat transfer characteristics by geometries. In addition, the obtained results indicate that the dimensionless absolute vorticity analysis, the entransy dissipation extremum principle and the field synergy theories could well depict and evaluate the heat transfer behaviors. At last, by comparing the present work with previous studies, the SRTs possess a potential promise in laminar heat transfer enhancement. (C) 2017 Elsevier Ltd. All rights reserved.