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

Abstract

Flow patterns and heat transfer in an air-cooled square cross-section micro condenser were investigated. The test section consisted of a borosilicate square micro channel, of inner and outer hydraulic diameters of 0.49 mm and 0.6 mm respectively, and a length of 100 mm. The transparent material of the micro channel allowed the visualization of the different condensation flow patterns. The imposed mass velocities were ranging between 1 and 10 kg m-2 s-1. In this range of mass fluxes, three main flow regimes were identified: Annular regime, intermittent regime, and spherical bubbles regime. Then, the isolated bubbles zone (the end of the intermittent zone + the spherical bubbles zone) was particularly studied. A specific experimental procedure was developed, basing on bubble tracking, in order to determine accurately the hydraulic and thermal parameters profiles in this zone according to the axial position in the micro channel, such as the vapour quality profile x(z). Thanks to energy balance, the liquid temperature profile Tl(z) in the isolated bubbles zone was determined for different initial values. A thermal non-equilibrium between the liquid and vapour phases was identified. Therefore, the latent heat flux was then quantified and compared to the total heat flux in this zone.