Condensation in Mini and Microchannels: Effect of Diameter, Shape, Inclination and Fluid Properties.
The main results of an experimental and numerical investigation on condensation heat transfer inside circular and square cross section mini/microchannels are hereafter presented. The experimental investigation is performed inside a circular and a square cross section channel with hydraulic diameter of around 1 mm: the local heat transfer coefficients are measured during condensation in both channels at varying mass flux, vapour quality, channel inclination and fluid properties. Pure halogenated fluids have been tested. These experimental data are used to compare the heat transfer characteristics of the two minichannels during condensation and to highlight the effect of the cross sectional shape. Ad-hoc experimental tests have been performed to show the effect of inclination in the same 1 mm diameter minichannel. The experimental results are compared against three-dimensional simulations for a better understanding of the forces governing condensation inside the present geometry. The VOF (Volume Of Fluid) method is used to track the vapour-liquid interface and the effects of interfacial shear stress and surface tension are both taken into account. Depending on the mass flux values, the liquid film is assumed laminar or turbulent while the vapour flow is turbulent for all the simulated mass velocities. At low mass flux, when the effect of shear stress diminishes and surface tension forces compete with gravity force to determine the shape of the liquid film at the wall, the square shape displays better performance than the circular one. Ad-hoc simulations have also been performed with a lower channel diameter to compare with the 1 mm diameter geometry.