Flow patterns and heat transfer in a square cross-section micro condenser working at low mass flux

摘要

Flow patterns and heat transfer were investigated during condensation of n-pentane in an air-cooled square cross-section micro condenser. The test section consisted of a borosilicate square microchannel, of inner and outer edges 553 μm and 675 μm, respectively, and of a length 208 mm. The transparency of the microchannel walls allowed the visualization of the phases distribution and the different condensation flow regimes. Different mass velocities ranging between 3 and 15 kg m~(-2) s~(-1) were imposed. Three main flow regimes were identified: annular regime, intermittent regime, and spherical bubbles regime. A specific experimental procedure was developed, based on bubbles tracking, in order to determine accurately the hydraulic and thermal parameters profiles in the isolated bubbles zone, such as the time-averaged void fraction profile α(z) and the time-averaged vapour quality profile x(z), according to the axial position in the microchannel. Thanks to energy balance, the time-averaged liquid temperature profile T_l(z) in the isolated bubbles zone was determined. A significant temperature difference between the liquid and vapour phases was highlighted. Therefore, the latent and total heat fluxes released in this zone were quantified and compared to each other. Besides, the relationship between the void fraction and the vapour quality in the spherical bubbles zone was determined and compared to existing void fraction models. Finally, the bubbles detachment frequency was determined. A relationship between this frequency and the mass velocity was proposed.