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<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.

[1]  M. W. Wambsganss,et al.  Small circular- and rectangular-channel boiling with two refrigerants , 1996 .

[2]  M. Pate,et al.  Heat transfer of R-134a in single-tube spray evaporation including lubricant effects and enhanced surface results , 1995 .

[3]  L. Falk,et al.  Falling liquid film stability in microgas/liquid absorption , 2010 .

[4]  J. Thome.  Boiling in microchannels: a review of experiment and theory , 2004 .

[5]  P. Kew,et al.  Correlations for the prediction of boiling heat transfer in small-diameter channels , 1997 .

[6]  M. Jakob,et al.  Der Wärmeübergang von einer waagerechten Platte an siedendes Wasser , 1933 .

[7]  K. Stephan,et al.  Heat-transfer correlations for natural convection boiling , 1980 .

[8]  S. Hasegawa,et al.  An Analytical Model for Displacement Velocity of Liquid Film on a Hot Vertical Surface , 1975 .

[9]  Michel Cabassud,et al.  Heat exchanger/reactors (HEX reactors) : Concepts, technologies: State-of-the-art , 2008 .

[10]  R. Shah,et al.  Laminar Flow Forced Convection in Ducts: A Source Book for Compact Heat Exchanger Analytical Data , 2014 .

[11]  L. Falk,et al.  Intensification of G/L absorption in microstructured falling film. Application to the treatment of chlorinated VOC's - part II: Modeling and geometric optimization , 2011 .

[12]  K. Sefiane,et al.  Bubble confinement in flow boiling of FC-72 in a "rectangular" microchannel of high aspect ratio , 2010 .

[13]  John R. Thome,et al.  Falling Film Evaporation: State-of-the-Art Review of Recent Work , 1999 .

[14]  Satish G. Kandlikar.  Heat Transfer Mechanisms During Flow Boiling in Microchannels , 2004 .

[15]  R. A. Seban,et al.  Heat Transfer to Evaporating Liquid Films , 1971 .

[16]  K. Stephan.  Heat Transfer in Condensation and Boiling , 1992 .

[17]  A. Bergles,et al.  Horizontal-tube falling-film evaporation with structured surfaces , 1989 .

[18]  P. A. Pilavachi.  Energy Efficiency in Process Technology , 1993 .

[19]  M. Urbicain,et al.  Determination and correlation of heat transfer coefficients in a falling film evaporator , 2006 .

[20]  Gherhardt Ribatski,et al.  Modeling and experimental study of nucleate boiling on a vertical array of horizontal plain tubes , 2008 .

[21]  Albert Renken,et al.  Microstructured reactors for catalytic reactions , 2005 .

[22]  V. Sernas,et al.  EVAPORATION FROM THIN WATER FILM ON HORIZONTAL TUBES , 1974 .

[23]  J. Thome,et al.  Convective Boiling and Condensation , 1972 .

[24]  I. Morioka,et al.  Absorption of Water Vapor into a Film of Aqueous Solution of LiBr Falling along a Vertical Pipe. , 1993 .

[25]  L. Falk,et al.  Intensification of the G/L absorption in microstructured falling film application to the treatment of chlorinated VOC's. Part III: Influence of gas thickness channel on mass transfer , 2011 .

[26]  Fujita Toshihiko,et al.  Heat transfer to falling liquid films and film breakdown—II: Saturated liquid films with nucleate boiling , 1978 .

[27]  Uwe Dingerdissen,et al.  Ozonolysis of Acetic Acid 1-Vinyl-hexyl Ester in a Falling Film Microreactor , 2007 .

[28]  A. Boyde,et al.  Characterisation of liquid film in a microstructured falling film reactor using laser scanning confocal microscopy , 2006 .

[29]  Don W. Green,et al.  Perry's Chemical Engineers' Handbook , 2007 .

[30]  S. Ishigai,et al.  Hydrodynamics and Heat Transfer of Vertical Falling Liquid Films : Part 1, Classification of Flow Regimes , 1971 .

[31]  J. Vasseur,et al.  Bibliographic analysis of predicting heat transfer coefficients in boiling for applications in designing liquid food evaporators , 2008 .

[32]  Stephen Whitaker,et al.  Some Theoretical and Experimental Observations of the Wave Structure of Falling Liquid Films , 1977 .

[33]  Anthony M. Jacobi,et al.  Falling-film evaporation on horizontal tubes—a critical review , 2005 .

[34]  Amir Faghri,et al.  Transport Phenomena in Multiphase Systems , 2006 .

[35]  W. Nusselt.  Die Oberflachenkondensation des Wasserdampfes , 1916 .

[36]  J. C. Chen.  Correlation for Boiling Heat Transfer to Saturated Fluids in Convective Flow , 1966 .

[37]  R. Phillips,et al.  Microchannel heat sinks , 1988 .

[38]  Guangwen Chen,et al.  Hydrodynamics and mass transfer of gas–liquid flow in a falling film microreactor , 2009 .

[39]  V. Rifert,et al.  Heat transfer in thin film-type evaporator with profile tubes , 1989 .

[40]  P. Wayner.  Effect of interfacial phenomena in the interline region on the rewetting of a hot spot , 1979 .

[41]  Zhen-hua Liu,et al.  Enhanced evaporation heat transfer of water and R-11 falling film with the roll-worked enhanced tube bundle , 2001 .

[42]  Je-Chin Han,et al.  Heat Transfer Through Falling Film Evaporation and Boiling on Horizontal Tubes , 1990 .

[43]  Keith Cornwell,et al.  BOILING IN SMALL PARALLEL CHANNELS , 1993 .

[44]  John R. Thome,et al.  Heat Transfer Model for Evaporation in Microchannels, Part I: Presentation of the Model , 2004 .

[45]  W. Nakayama,et al.  Evaporative heat transfer from horizontal enhanced tubes in thin film flow , 1990 .

[46]  S. Nukiyama.  The maximum and minimum values of the heat Q transmitted from metal to boiling water under atmospheric pressure , 1966 .

[47]  Thomas E. Diller,et al.  A Convection Calibration Method for Local Heat Flux Gages , 1984 .

[48]  Wolfgang Ehrfeld,et al.  Direct fluorination of toluene using elemental fluorine in gas/liquid microreactors , 2000 .

[49]  S. Garimella,et al.  Prediction of the onset of nucleate boiling in microchannel flow , 2005 .

[50]  M. Matlosz,et al.  Gas-phase mass-transfer measurements in a falling-film microreactor , 2011 .

[51]  Mohamed S. El-Genk,et al.  Minimum thickness of a flowing down liquid film on a vertical surface , 2001 .

[52]  M. W. Wambsganss,et al.  Two-phase pressure drop of refrigerants during flow boiling in small channels : an experimental investigation and correlation development. , 1999 .

[53]  John R. Thome,et al.  Heat Transfer Model for Evaporation of Elongated Bubble Flows in Microchannels , 2002 .

[54]  N. Zuber,et al.  Dynamics of vapor bubbles and boiling heat transfer , 1955 .

[55]  Arthur E. Bergles,et al.  AN ANALYTICAL AND EXPERIMENTAL STUDY OF FALLING – FILM EVAPORATION ON A HORIZONTAL TUBE , 1987 .

[56]  E. Ganić,et al.  An Experimental Study of Falling Liquid Film Breakdown on a Horizontal Cylinder During Heat Transfer , 1980 .

[57]  L. Falk,et al.  Capability of a falling film microstructured contactor for the separation of binary mixtures , 2011 .

[58]  J. Thome,et al.  Boiling of multicomponent liquid mixtures , 1984 .

[59]  Qi Wang,et al.  Falling film evaporation of binary mixtures , 1994 .

[60]  Y. Jaluria,et al.  An Introduction to Heat Transfer , 1950 .

[61]  Bogdan Z. Dlugogorski,et al.  Catalysis Today: Preface , 2004 .

[62]  Asterios Gavriilidis,et al.  Catalyst preparation and deactivation issues for nitrobenzene hydrogenation in a microstructured falling film reactor , 2003 .

[63]  X. Zeng,et al.  Evaporation heat transfer performance of nozzle-sprayed ammonia on a horizontal tube , 2000 .

[64]  Johann Gottlob Leidenfrost.  On the fixation of water in diverse fire , 1966 .

[65]  L. Falk,et al.  Intensification of G/L absorption in microstructured falling film. Application to the treatment of chlorinated VOC’s – part I: Comparison between structured and microstructured packings in absorption devices , 2010 .

[66]  V. L. Podberezny,et al.  Evaporation heat transfer in liquid films flowing down horizontal smooth and longitudinally profiled tubes , 1996 .

[67]  Manfred Groll,et al.  Experimental results of flow boiling of water in a vertical microchannel , 2006 .

[68]  R. Shah.  Laminar Flow Forced convection in ducts , 1978 .

[69]  Tatsuhiro Ueda,et al.  Heat transfer to falling liquid films and film breakdown—I: Subcooled liquid films , 1978 .