Theoretical and experimental study of wavy flow during R134a condensation flow in symmetrically and asymmetrically cooled microchannels

The paper presents experimental and theoretical studies of wavy flows during R134a condensation in oval parallel microchannels with a hydraulic diameter of 301.6 µm for mass fluxes from 60 to 250 kg/(m2s) and vapor mass qualities from 0.1 to 0.9. The inlet saturation temperature of R134a is 31.3°C. Waves were seen in the thin liquid film and in the channel corners with measurements of their wave lengths and velocities. The results show that both the film wave length and the corner wave length increase with decreasing mass flux and decreasing vapor mass quality. Additionally, the film wave velocity and corner wave velocity both increase with increasing mass flux. The cooling method, either asymmetric cooling or symmetric cooling, only affected the corner wave length and velocity along the channel except near the inlet. A two-dimensional theoretical model was developed to predict the liquid-vapor interface instability conditions during condensation flow by neglecting the liquid inertia and vapor flow viscous terms. The normal modes method was used to analyze the system instability reactions to various perturbation wave lengths. Two modes were obtained with one giving the greatest instability wave length which agrees well with the measured wave length data.