Pressure drop study of two-phase flows under condensation within capillary tubes

In the present study, an experimental investigation of pressure drops in two-phase flows within capillary tubes operating at low mass flow rates under condensation conditions was conducted. The primary objective was to understand the mechanics driving such flows to support the design of condensers for small two-phase devices, such as miniature loop heat pipes. A simple yet innovative experimental setup was developed, wherein the flows, similar to those in heat pipe technologies, were passively induced by vapor pressure. Pressure drops in two-phase flows under diabatic conditions were measured. Water and ethanol were tested, with mass flow rates below 6 × 10⁻⁵ kg/s and mass fluxes ranging from 15.5 to 209.3 kg/m²s. Copper and high-quality PVC capillary tubes with diameters between 0.446 mm and 1.010 mm were employed. Under diabatic conditions, the vapor entered the tubes with an initial vapor quality of one and condensed as it traveled through the tubes, leaving with vapor qualities between approximately 70 % and 90 %. In some cases, the condensed liquid coalesced into large plugs that completely filled the tube's cross-section, leading to confined flow conditions. These liquid plugs required higher vapor pressures to be pumped along the tube. Consequently, the pressure drop behavior was highly dependent on the flow regime. Lower pressure drops were observed when the vapor flowed at high velocities, causing the liquid to disperse into small droplets. Conversely, higher pressure drops occurred when the condensate formed blockages within the tube. The pressure drops in the flow ranged from 958 Pa to 75.4 kPa for water and 524 Pa to 59.8 kPa for ethanol. These values were consistently between the single-phase vapor and liquid pressure drops, typically closer to the latter. Notably, the observed pressure drops were significantly lower than those predicted by conventional literature correlations for two-phase flows. Furthermore, the study discusses the critical roles of pressure drops induced by fluid phase changes and the influence of capillary forces due to the confined liquid flows. © 2025 The Author(s)