A study of flow patterns in a thermosyphon for compact heat exchanger applications

Recently, thermosyphons have attracted interest in the design of smaller, lighter and cheaper heat exchangers, because of their compactness, low thermal resistance, high heat recovery effectiveness, safety and reliability. In order to understand the effects of the angle of inclination on heat transfer characteristics of a thermosyphon, a dedicated flow visualization study of flow patterns in a transparent two-phase thermosyphon was conducted. Heat flux and angle of inclination were varied in wide ranges. The thermosyphon is made of glass with an inner diameter of 16 mm and a total length of 290 mm. Acetone is the working fluid at a filling ratio of 80%. The results show that at all angles of inclination, ß: (1) vapor plugs exist at heat fluxes less than 14 kW/m2; (2) an annular condensate film flow with a wavy structure exists at heat fluxes between 14 kW/m2 and 32 kW/m2; (3) waves from condenser to evaporator propagate faster with increasing heat flux. These waves explain the corresponding heat transfer enhancement. The whole perimeter is wetted for ß<20º, and probably for all ß<80º. This explains the proper functioning for each orientation of a R- 134a filled copper thermosyphon found in a previous study.