Saturated and superheated water vapor condensation on a custom micro-textured surface

Abstract

Condensation has a great importance for a large number of technologies such as heat exchangers, distillation columns, heat pipes, and thermal management systems. Vapor changes its phase into liquid when it contacts a surface exhibiting a lower temperature than the saturation temperature of the vapor for a given pressure. In most of those cases, systems are designed in order to achieve the phase-change process at saturated conditions over the entire heat transfer area. However, in some devices, there are sections over which prior the actual condensation occurs, the vapor flows at superheated conditions and needs to be cooled down to saturation. Thereby, one can expect that the heat transfer performance is low and the mode of condensation far away from that of saturated vapor. In this work, heat transfer and wetting characteristics during water vapor condensation at saturated and superheated conditions has been experimentally studied over a certain range of pressure and temperature. Two latter parameters were varied from 1.02 to 1.40 bar and 100{circ} mathrm{C} to 125{circ} mathrm{C}. The phase-change phenomenon has been realized on an unstructured reference surface as well as on a hexagonally micro-structured surface which was produced by laser-manufacturing. High-speed imaging technique and high-resolution temperature measurements have been adopted in order to identify the wetting behavior of the liquid on the surface and the heat transfer rate during condensation, respectively. The experiments revealed that vapor condensation on an unstructured surface at saturated conditions follows the trend of the well-known Nusselt theory for film-wise condensation. It is observed that an excess of 40 % larger heat transfer coefficients than those predicted by the Nusselt theory was achieved, and heat transfer coefficients (HTC) decrease with increasing vapor-to-surface temperature difference (T-{V}-Ts) from 4 to 24 K for both, unstructured and the micro-structured surface. In contrast, at comparable operating conditions (1.10 bar), the heat transfer coefficient for superheated (110°C) vapor condensation is remarkably lower than those for saturated vapor (sim 100{circ} mathrm{C}) and describes a considerably distinct trend. The HTC first sharply increases starting at T-{V}-T-{S}=12 mathrm{K} and shows a decreasing gradient up to a T-{V}-T-{S} text{of}22.5 mathrm{K}.