This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Accurate predictions of two-phase pressure drop in small to micro diameter passages are necessary for the design of compact and ultra-compact heat exchangers which find wide application in process and refrigeration industries and in cooling of electronics. A semi-mechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach of Thome et al. (2004) for heat transfer. The total pressure drop is calculated by time-averaging the respective pressure drop values of single-phase liquid, elongated bubble with a thin liquid film and single-phase vapour. The model results were compared with experimental data collected for a wide range of diameter tubes (4.26, 2.88, 2.02, 1.1 and 0.52 mm) for R134a at 6 – 12 bar
