Sokkuram grotto, a UNESCO cultural heritage in Kyongju Korea, was repaired into a concrete dome structure during the early 20th century to protect from total collapse. Since then, dew formation on the granite surface began to occur and it damaged the grotto significantly. It is thought that heavy dew forms during sunny day after long rainy days than during continued sunny days or rainy days. It is also thought that while it rains, granite surface in the grotto gets cold and as it becomes sunny, evaporation from surrounding wet ground increases absolute humidity. So dew forms at the yet cold surface. In the early 1960's, another dome was built outside the inner dome and air conditioning facilities were added to keep the grotto dry. Dew formation has stopped at the expenses of noise, vibration and entrance blockage. Before repaired by Japanese, Sokkuram was covered with crushed rocks. It makes air flow from inside to outside or from outside to inside of Sokkuram depending on the unsteady change of ambient air temperature. Experimental results by simulation of porous media structure show that this air flow prevents granite surface from forming dew. If it gets sunny for a while in rainy season, as the worst dew formation condition, granite surface has been cooled during the rainy days. With sunny weather with a relatively large daily temperature change, air flows by natural convection from outside to inside during day and from inside to outside during night. When warm and humid air flows into inside, it is dehumidified in the upper porous layer and remains dry thereafter. In the night, inside cold air rises through warm porous layer. Therefore, with porous dome structure, dew never forms on the granite surface. The employed temperature/humidity controlled room simulates temperature and humidity near Sokkuram. To measure temperature distribution of concrete and porous specimens, K-type thermocouples are set up along the vertical axis. VCR, capable of interval recording, is used to observe dew formation and a hot-wire anemometer and a smoke generator are used to measure air velocity and to view flow direction driven by natural convection. The experimental results show that dew does not form during continued sunny days or rainy days at both specimens. Dew forms severely at concrete specimen surface on sunny days after long rainy days, but it does not form at porous media specimen. Air velocity in porous layer is about 2cm/s with continued sunny days, and about 5cm/s with sunny day after long rainy days. The direction of air flow is in agreement with the above reasoning and thus confirms the proposed mechanism of dehumidifying characteristics of the porous layer.
