Evaporation Heat Transfer of Falling Films on Horizontal Tubes : 3rd Report, Heat Transfer of Tube Array

We present the results of experimental investigation of heat transfer during evaporation of thin films flowing over horizontal tubes. Experiments were conducted using 25 mm o.d. copper tubes heated by internal electrical cartridge heaters so that uniform heat flux was generated on the tube outer wall. Five heated tubes were arrayed on a vertical plane with a pitch of 50 mm. Freon R 11 preheated to the saturation temperature at 0.2 MPa was supplied to the topmost heated tube through feeding devices. Heat transfer characteristics of each heated tubes were clarified in the range of film Reynolds number from 10 to 2000. The topmost tube showed 10 to 20% lower heat transfer coefficients than the other four tubes arrayed below it. Tubes except for the topmost one gave the same heat transfer characteristics. Heat transfer coefficients for those tubes were correlated to within 15% accuracy using an empiricalformula : Num=(Ref-2/3+0.010Ref0.3Pr0.25)1/2 Falling film turned into rivulets at flow rates below a certain value. This film breakdown caused the deterioration of heat transfer. A relationship between heat flux and film flow rate was given for the film breakdown.