Intensification of heat transfer during evaporation of a falling liquid film in vertical microchannels—Experimental investigations

The aim of this article is to show the different boiling heat transfer regimes identified by Nukiyama (1934) in a micro-exchanger during a falling film evaporation. This study allows establishing operating conditions to develop the micro-evaporation or the micro-distillation where heat transfer has to be intensified. Indeed, the behaviour of binary mixtures on heat transfer is sometimes different than with a pure compound. The present study investigates the evaporation of ethanol in a sandwich plate micro-heat exchanger. Ethanol is a chemical test. It streams by gravity in falling film on two symmetrical heated micro-structured plates where micro-straight vertical channels have been machined (Kane et al., 2011). Electrical heat fluxes ranged from 2 to 5 kW/m2. Various differences of temperatures between wall and saturation temperature were tested. Feed flow rate ranged between 1 and 5 g/min (“smooth” laminar flow: 2

► Operating conditions avoiding confined-bubble to develop the micro-evaporation.
► The surface tension effects are large towards the convective effects.
► The influence of hydrodynamic regime: (NuL=0.012ReL0.46PrL0.33).
► Confined-bubble phenomena reduce the heat transfer coefficient (<300 W/m2 °C).
► The behaviour of binary mixture on heat transfer is different than a pure compound.