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

Abstract

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 5kW/m2. Various differences of temperatures between wall and saturation temperature were tested. Feed flow rate ranged between 1 and 5g/min (“smooth” laminar flow: 2<ReL<5). Experiments were performed at atmospheric pressure. The heat transfer rate is studied with respect to the liquid flow rate and the electrical heat flux. Results show that it is possible “to optimize” the geometry of the apparatus in order to intensify the heat transfer. At low difference of temperatures between the wall and the saturation temperature, the boiling heat transfer is characterized by nucleate boiling where the liquid phase wets the solid surface. At high difference of temperatures, a thin layer of vapour is formed between the wall and the liquid, named calefaction phenomenon, resulting in the important diminution of the heat transfer coefficient. Furthermore experiments allow to show a flow regime from confined-bubble: using of microchannels and too high heat flux can generate fluctuating of wall temperature and dry zones which reduce the heat transfer coefficient, and decrease the performance of micro-exchanger. By using a binary mixture, experiments show a variation of hL. It is due to the chemical and physical properties of the mixture which change according to compounds, composition, temperatures (wall and saturation) and also involves mass and heat transfer within the liquid film. To show a possible intensification of the heat transfer, results have been represented by a dimensionless correlation using dimensionless numbers as Nusselt, Reynolds, Kapitza and Prandtl. According to comparison with other relations found in the literature for micro- and macro-exchangers, intensification can occur thanks to a judicious choice of the operating conditions.