An experimental study of the inundation effect on filmwise condensation heat transfer over horizontal smooth and enhanced tubes

•Inundation liquid enhances condensation heat transfer only at low qo and ReΓ for 3D and C9+ tube.•Low fin tube is hardly affected and even enhanced by inundation due to the axial restriction of inundation liquid by 2D fins.•Smaller fins on 3D fins of C9+ tube make the C9+ tube perform best at small ReΓ or without inundation. Inundation effect on condensation heat transfer is ubiquitous and a crucial factor in designing a large-scale condenser. In the present study, the effect of condensation liquid inundation on heat transfer is experimentally studied for R134a on a plain tube and three kinds of enhanced tubes. A specially designed test method with a liquid distributor for studying the effect of inundation liquid has been adopted, by which the inundation liquid flow rate can be adjusted easily within a wide range with high accuracy of inundation liquid flow rate. It is found that the inundation liquid may enhance the heat transfer at low heat flux of 20 kW/m2 when ReΓ is less than 600 and 800 for the 3D finned tube and C9+ tube (a kind of 3D finned tube with smaller fins on each fin to break the liquid film), respectively, and for the 2D low fin tube within the entire test range of ReΓ, with the maximum enhancement factor being 1.21. At high heat flux of 80 kW/m2 the inundation liquid always deteriorate heat transfer for the three enhanced tubes tested. At middle heat flux of 40 kW/m2, the inundation usually has negative effect but for C9+ tube heat transfer is enhanced when ReΓ is less than 1100. It is also found that the advantage of C9+ tube in enhancing condensation heat transfer over the finned tubes is not so appreciable under the inundation case as it is for non-inundation case. Some flow visualization pictures are provided to deepen the understanding of physical process.