Experimental investigation into two-phase flow patterns inside a herringbone microfin tube

During in-tube microfinned enhanced tubes show a heat transfer enhancement, as compared to equivalent smooth tubes under the same operating conditions, that is partly due to the mere increase in the effective exchange area and additionally to the turbulence induced in the liquid film by the micro fins and to the surface tension effect on the liquid drainage. Furthermore there is agreement in the literature that the mechanisms of heat transfer and pressure drop are intimately linked with the prevailing two-phase flow regime. In the recent open literature evidence is given to the effect of fins orientation on flow patterns in herringbone tubes (Miyara et al., 2003). In particular, at the same operating conditions, it was pointed out that when the fins convergences are positioned at the top and bottom of the tube (dubbed here as Position-I), the occurring flow pattern can be completely different from the tube arrangement with the fins convergences at both sides (Position-II). In a previous paper by the present authors, the \u201cPosition-II\u201d arrangement was investigated with three different refrigerants for a saturation temperature of 40\ub0C and mass velocities 100\uf7800 kg m-2s-1. In this paper the \u201cPosition- I\u201d arrangement is now investigated for the fluid R134a and a comparison with the available visualization data for \u201cPosition-II\u201d is presented. In order to investigate the two phase flow pattern during condensation a specific test section was built up. For the study of the main flow patterns, in particular focusing on the stratified/annular mode transition, the visualisation experimental data are analysed with reference to the dimensionless vapour velocity and the Martinelli parameter