Thermal design evaluation of ribbed/grooved tubes: An entropy and exergy approach

Abstract

The smooth circular tubes are typically used in air-cooled heat exchangers systems. In literature, various research toward improving the hydraulic/thermal performance by changing the tubes' inner surface topology is found. In the current work, fluid friction and heat transfer characteristics of air-cooled tube heat exchangers with installed ribs and grooves have been numerically compared. Initially, the numerically calculated data of the ribbed tubes are validated /compared to the performance results in the literature. Performance evaluation criteria (PEC), entropy and exergy are numerically investigated for a tube with a group of ribs and grooves over the range of Reynolds number Re = 6700–20,000. Rib radius (r), number of ribs in the same group (Ng), the distance between two adjacent ribs (SL), and pitch between two adjacent groups (p) are the ribs parameters selected to study their effects on flow and heat transfer criteria. Also, the same evaluation criteria are performed on grooved tube with optimal parameters to make a comparison between results. Moreover, streamline patterns and temperature/pressure/velocity/turbulence kinetic energy contours are visually introduced to explicate the physics of heat transfer and pressure drop phenomena and to assess the flow structures inside different ribbed/grooved tubes. For the tubes with high-performance evaluation criteria (PEC), lower and higher entropy and exergy efficiency are determined. Recapitulation results from the technical comparison supported the use of grooved-tubes instead of ribbed-tubes. This is mainly due to lower flow irreversibility accompanied by higher PEC and exergy efficiency than those of ribbed tubes at the same flow condition.