Heat Transfer and Pressure Drop in Single Rows

Abstract

Results of experimental investigations on the influence of turbulence intensity and pitch-to-diameter ratio on heat transfer and pressure drop in single rows of plain tubes are presented. The vertically arranged tubes with pitch-to-diameter ratios between 1.26 and 3.44 were heated by saturated steam, condensing inside, and cooled outside by air in cross flow. The turbulence intensity in the entrance cross-section was enhanced by means of different biplanar grids placed in the test section of the wind tunnel upstream before the single rows. The mean streamwise turbulence intensity behind the grids, measured with a constant temperature hot-wire anemometer, varied between 0.8% and 38.8%. Reynolds-numbers ranged from 3·10³ to 3·10⁵. The measurements show for single rows an increase of the Nusselt-number for a given turbulence intensity with increasing Reynolds-number. For constant Reynolds-number and constant turbulence intensity the Nusselt-numbers increase until a pitch-to-diameter of a=2.69. If the pitch-to-diameter ratio was enlarged the Nusselt-numbers decrease. In the investigated range of Reynolds-numbers the drag coefficient is only a weak function of the inlet turbulence intensity. The use of turbulence grids thus leads to higher efficiencies of heat exchangers.