Influence of inlet contraction ratios on the heat transfer and pressure drop characteristics of single-phase flow in smooth circular tubes in the transitional flow regime

Abstract

An experimental investigation of the single-phase heat transfer and pressure drop characteristics in the transitional flow regime has been performed using flow-calming sections with different inlet geometries and section-to-tube inlet contraction ratios. Four different types of inlets were investigated namely a square-edged, re-entrant, a hydrodynamically fully developed inlet, and a 90° bend inlet. The contraction ratios for the squared-edged and re-entrant inlets were 5, 11, 15 and 33. For the hydrodynamically fully developed inlet and the 90° bend inlet, a contraction ratio of one was used. Experiments were conducted using a circular test section with an inner diameter of 5.1 mm, at a Reynolds number range of 1000–6000 to ensure that the laminar, transitional, quasi-turbulent and turbulent flow regimes were covered. Water was used as the working fluid and the test section was heated at constant wall heat fluxes of 4, 6 and 8 kW/m2. It was found that for a square-edged inlet geometry, the transition from the laminar to the turbulent flow regimes occurred earlier as the inlet contraction ratio increased, while for the re-entrant inlet, transition was delayed. The transitional flow regime was significantly affected by smaller contraction ratios and this effect increased with increasing heat flux. However, it was found that the critical Reynolds numbers were independent of inlet geometry for contraction ratios larger than 33. For the 90° bend inlet, transition occurred earlier than all the other inlet geometries and contraction ratios.