Heat transfer and flow visualization of falling film condensation on tube arrays with plain and enhanced surfaces

The tubes in shell-and-tube condensers, widely used in refrigeration and chemical process industries, are subjected to condensate inundation from the neighboring tubes. The aim of the present investigation is to study the effect of condensate inundation on the thermal performance of an vertical array of horizontal tubes with plain and enhanced surfaces. The experimental approach is split in two parts: measurement of the heat transfer coefficients and visualization of the flow patterns of the condensate falling between the tubes. Refrigerant R-134a was condensed at a saturation temperature of 304K on tube arrays with up to ten tubes at pitches of 25.5, 28.6, and 44.5mm. Four commercially available copper tubes with a nominal diameter of 19.05mm and 544mm in length were tested: a plain tube, a 26 fpi / 1024 fpm low finned tube (Turbo-Chil) and two tubes with three-dimensional enhanced surface structures (Turbo-CSL and Gewa-C). Measurements were performed at three nominal heat flux levels up to 60kW/m2 with liquid overfeed corresponding to film Reynolds numbers up to 3000. The test section offers full visual access to study the flow patterns of the condensate. Furthermore, the large experimental database is unique in that true local heat transfer coefficients were measured as opposed to tube length averaged values in previous studies. With little liquid inundation the tubes with 3D enhanced surface structures outperform the low finned tube. Increasing liquid inundation deteriorates the thermal performance of the 3D enhanced tubes, while it has nearly no affect on the low finned tube, resulting in a higher heat transfer coefficient for the low finned tube at high film Reynolds numbers. Large differences in condensate flow patterns were observed. For the 3D enhanced tubes the ideal flow modes (droplet, column and sheet mode) were observed, while the flow was very unstable for the other two types of tubes. For the 3D enhanced tubes oscillations occurred in the sheet mode at film Reynolds numbers such that liquid left the array of tubes sideways. A heat transfer model for an array of 3D enhanced tubes based on these visual observations was proposed, including the effects of liquid lost by the sideways slinging phenomenon. The measurements were predicted within a mean error of 3% and a standard deviation of 13% by this model.