Modeling of Pressure Drop During Condensation in Circular and Non-Circular Microchannels

This paper presents a multiple flow-regime model for pressure drop during condensation of refrigerant R134a in horizontal microchannels. Condensation pressure drops measured in two circular and six non-circular channels ranging in hydraulic diameter from 0.42 mm to 0.8 mm are considered here. For each tube under consideration, pressure drop measurements were taken over the entire range of qualities from 100% vapor to 100% liquid for five different refrigerant mass fluxes between 150 kg/m2-s and 750 kg/m2-s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to assign the applicable flow regime to the data points. Garimella et al. [1] reported a comprehensive model for circular tubes that addresses the progression of the condensation process from the vapor phase to the liquid phase by modifying and combining the pressure drop models for intermittent [2] and annular [3] flows reported earlier by them. In this paper, the multiple flow regime model of Garimella et al. [1] for circular microchannels has been extended to horizontal non-circular microchannels of a variety of cross-sections. This combined model accurately predicts condensation pressure drops in the annular, disperse wave, mist, discrete wave, and intermittent flow regimes for both circular and non-circular microchannels of similar hydraulic diameters. Overlap and transition regions between the respective regimes are also addressed using an appropriate interpolation technique that results in relatively smooth transitions between the predicted pressure drops. The resulting model predicts 80% of the data within ±25%. The effect of tube shape on pressure drop is also demonstrated.