Axial-vapor-flow induced, low-liquid-saturation heat-pipe effect in wet insulation: Simulation and experiment

In field applications, segmented pipe thermal insulation wrappings are susceptible to inter-segment water penetration and its axial transport, which degrades the insulation thermal performance and causes pipe corrosion under insulation (CUI). Through CFD simulation, this axial transport, its local condensation/evaporation and liquid/vapor movement, and the temperature and liquid saturation distributions are predicted. The accompanying experiment (mineral wool insulation) conditions are a horizontal pipe with a length of 70 cm and a diameter of 9 cm, covered with 5 cm of mineral wool insulation and jacketed with an impermeable aluminum sheet under surface-convection and radiation heat transfer to the surroundings (27°C). The simulated 100°C vapor supply (0.013 g/s baseline) is prescribed at the axial inlet. The simulation results show the formation of an entrance region with radial condensation and evaporation, a dry region adjacent to the pipe, and low liquid saturation adjacent to the outer boundary. The heat loss in this entrance region is 4 - 7 folds larger than the dry insulation, depending on the inlet moisture flow rate. This increase is predominantly due to the radial evaporation/condensation, i.e., the heat-pipe effect, since the liquid saturation is too small to substantially increase the apparent thermal conductivity. Thus, the radial temperature distribution is noticeably altered by this heat-pipe effect. The entrance region is marked by complete condensation of the injected vapor and has a length and excess heat loss proportional to the vapor injection rate. The test results confirm the entrance region heat-pipe effect signified by large heat loss, temperature rises as a function of angle, and the vapor penetration depth.