Drainage control and diffusion resistance in dropwise condensation in a compact heat exchanger

Abstract

Condensation of a vapor in the presence of non-condensable gas occurs frequently in process industry. For example in compact condensers for heat recovery, in extraction of toxic components from exhaust gases, in cooling systems of nuclear power plants, seawater desalination systems, air conditioning and petrochemical industry. It is well known that even small concentrations of non-condensable gas have a detrimental effect on condensation heat transfer rates. The key difference between the condensation of pure vapor and vapor in the presence of non-condensable gas compounds is that mass diffusion in the gas phase instead of heat diffusion in the condensate layer dominates heat transfer in the heat exchanger in the latter case. In condensation heat transfer with non-condensable gases, vapor-sided heat and mass transfer are essential. If possible, the diffusion resistance in the gas-liquid boundary layer should be reduced in order to enhance heat and mass transfer. A way to augment heat transfer is by introducing dropwise condensation instead of filmwise condensation. Heat transfer by dropwise condensation is possibly a factor 8 to 10 higher than filmwise condensation [115]. The exact reasons for this difference are still not fully understood. Where filmwise condensation is characteristic of metal heat exchangers with clean uncontaminated surfaces, dropwise condensation is, for example, achieved by applying a fluoropolymer heat exchanging surface. This study aims to elucidate the importance of the initial phase of dropwise condensation after drainage on heat transfer, when diffusion is not yet limiting. The effects of growth, coalescence and drainage of droplets with surface refreshing on airsteam condensation heat transfer enhancement are quantified. For this reason, a new small scale condenser setup is designed and applied. To supply a gas flow at well defined conditions, existing infrastructure is combined with an acoustic relative humidity sensor tailored to the required flow conditions. Also other measures are taken to increase accuracy of the heat exchanger test rig. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and applied. The dropwise condensation process is frequently interrupted upon which nucleation restarts upon each sweep. Condensate growth and surface temperatures are assessed by simultaneous video and infrared recordings. Software is developed to automatically extract the positions and radii of condensate droplets from images quickly and reliably. Cold wakes downstream of big drops on the condenser plate were observed. A single controlled droplet removal action enables a ’reset’ of the condenser surface. This allows measurement of droplet growth histories. It is found that droplet growth follows a power law with the exponent increasing with increasing inlet vapor mass fraction. Direct contact condensation on drops at condenser plate dominates drop growth. The main finding is that that the total heat transfer resistance decreases with increasing droplet removal frequency, while two measures for mass transfer simultaneously increase. Increasing diffusion limitation is one explanation for the observed decreasing mass transfer rate with time. After initial fast growth of drops, the slight increase in interfacial temperature observed offers another explanation. Furthermore, the effect of a structured heterogeneous plate surface on droplet drainage and heat transfer in dropwise condensation is investigated. A structured coating of the condenser plates is applied to create two coexisting dropwise condensation patterns. The structured coating constrains drainage and introduces directed surface energy gradients. The condenser with the structured coating is compared with two equally sized condensers: a non-coated pvdf and a fully coated pvdf condenser. It is found that drop drainage is promoted by oriented Ti-coated tracks to such a degree that the maximum obtainable heat transfer performance is practically reached. Design recommendations are given.