Falling-Film Evaporation On Tube Bundle With Plain And Enhanced Tubes-Part I: Experimental Results

The complexities of two-phase flow and falling-film evaporation on a tube bundle present important challenges in the design of heat exchangers and the understanding of the physical phenomena taking place. The development of structured surfaces to enhance nucleate boiling heat transfer and, thus, reduce the size of evaporators adds another level of complexity, requiring the characterization of the effect of enhanced surfaces on the boiling phenomena occurring in a falling-film evaporator. In this article, falling-film evaporation measurements on a single tube row bundle and a three-row tube bundle were performed to obtain local heat transfer coefficients. Four types of tubes were tested: a plain tube, an enhanced condensing tube (Gewa-C+LW, Wieland Werke, Ulm, Germany)and two enhanced boiling tubes (Turbo-EDE2, Wolverine, Hunstville, Alabama, USA; Gewa-B4, Wieland Werke, Ulm, Germany). Two refrigerants, R134a and R236fa, were tested at a saturation temperature of 5 degrees C, liquid film Reynolds numbers ranging from 0 to 3,000, and heat fluxes between 20 and 60 kW/m(2) in pool boiling and falling-film configurations. Finally, the understanding of the physical phenomena governing the falling-film evaporation of liquid refrigerants has been investigated with visualization of the process.