Thermal and moisture characteristics of Takamatsuzuka tumulus mound and its cooling

Transcription

1 Thermal and moisture characteristics of Takamatsuzuka tumulus mound and its cooling Takeshi Ishizaki 1, Sadatoshi Miura 1, Masahide Inuzuka 1, and Mamoru Mmiura 2 Introduction There are many important mural paintings in temples and tumuli in Asian countries. Among them, the stone chamber of the Takamatsuzuka tumulus is well known for its beautiful paintings. The Tumulus was found on 21 March 1972 in Asuka village, Nara prefecture, and was soon registered as a national treasure. The Tumulus is a circular mound with a diameter of 18m and a height of 5m (Photo 1). The interior length, width, and height of the stone chamber are 2.65m, 1.03m, and 1.13m, respectively. The stone chamber, about 3 m below ground level, is made of tuff-breccia. A committee of specialists was organized to consider the conservation of the mural paintings. After extensive discussion, the committee decided to preserve the mural paintings at the site and monitor the conditions of the interior of the stone chamber carefully. The completed conservation facilities have three anterooms (two antechambers and a vestibule) in front of the stone chamber and one room beneath the chamber to house the air-conditioning machinery (Fig.1). In September 2001, fungi were found on lime wallplaster both inside and outside the stone chamber. Various investigations were carried out to determine the cause of this problem and to develop protective measures against it. Since the temperature and humidity conditions and the water content in the stone chamber are important factors for fungi growth, we observed the water content profile in the lime plaster wall and water content distribution in the mound of tumulus. We also made a microclimate study around the tumulus. An extensive microbiological study of the stone chamber under high humidity conditions showed that it is difficult to stop the microbiological activity. The special committee devoted to conservation of the Takamatsuzuka tumulus wall paintings decided to excavate the tumulus mound and remove the stone chamber, with the goal of restoring the wall paintings over a 10-year period beginning in The committee also discussed how to minimize the biological activity in the stone chamber. They discussed several options, including lowering the temperature, decreasing the oxygen concentration, and decreasing the humidity. A cooling method was preferred because cooling would not have a bad effect on the wall paintings. However, as cooling cannot completely stop the microbiological activity inside the stone chamber, so this method could not be a long-term protective method. Rather, cooling could be a temporary protective measure against fungi until the start of the excavation of the mound. Here we report our observations and describe the proposed method adopted to cool the stone chamber until the excavation commenced for the dismantling of the stone chamber. Photo 1 Overview of Takamatsuzuka tumulus 1 National Research Institute for Cultural Properties, Tokyo 2 Kyoto University 17

2 Fig.1 Longitudinal section of the conservation facilities at Takamatsuzuka tumulus Moisture characteristics of the mound soil The undisturbed Takamatsuzuka soil samples were investigated and analyzed experimentally to study the relationship between the relative humidity and volumetric water content. Three experiments were carried out, using the water hanging method, the plate pressure apparatus method, and the chemical solution method. From these experimental results, the relationship between the relative humidity and volumetric water content was established (Fig.2). These results showed that when the volumetric water content in the soil around the Takamatsuzuka tumulus is equal to 30%, the relative humidity is around 100%. This result corresponded well with the measured relative humidity inside the stone chamber. Methods for cooling the stone chamber The following five methods were proposed for cooling the stone chamber: Sending cool air into the stone chamber from the south side; Cooling the area between the stone chamber and antechamber A; Installing cooling pipes around the stone chamber; Installing a cooling sheet on the mound; Installing cooling pipes on the mound surface and under the stone chamber; We evaluated the effectiveness and feasibility of each method. Sending cool air into the stone chamber from the south side Real scale model experiments were performed to evaluate the cooling effect on the inside temperature of the stone chamber caused by installing a cooling apparatus on the south side of the chamber. When the cooling capacity of the heat exchanger was 100W, the inside temperature decreased by 2.5 C. Therefore, we might have concluded that if we could use a thermomodule of 200W, we could expect a 5 C decrease of inside temperature. However, to increase the heat exchange rate, it would have been necessary to increase the rate of air flow inside the stone chamber. We felt that the increased air flow would have a bad effect on the fragile lime wallplaster in the stone chamber, so we did not adopt this cooling method. Cooling the area between the stone chamber and antechamber A This cooling method is performed by sending cool air into the area between the stone chamber and Fig.2 Relationship between volumetric water content and the relative humidity of the tumulus mound 18

3 install more than 30 cooling pipes of 5-cm diameter around the stone chamber. As the boring required for this would damage the compacted mound (hanchiku), this method was not adopted. Fig.3 Installing cooling pipes around the stone chamber Installing a cooling sheet on the mound In this method, the tumulus mound would be cooled by installing a cooling sheet on the mound. Numerical simulations were performed to calculate the temperature change in the stone chamber caused by lowering the temperature of the cooling sheet by 3 C or 5 C. The calculated results showed that the cooling effect was most effective when the cooling area was 8m from the top of the mound and the temperature decrease was 5 C. Since the temperature decrease of the stone chamber was only 2 C and it was not possible to cool the stone chamber lower than 15 C, this method was not adopted. Installing cooling pipes on the mound surface and under the stone chamber Fig.4 Installing cooling pipes on the mound and under the stone chamber antechamber A, as shown in Fig.1, using an air conditioner. To evaluate this effect, we did a numerical simulation to obtain a temperature profile of the stone chamber. The simulation results showed that even if the temperature inside the area was lowered to 15 C or 18 C, an effective temperature decrease was not obtained. In addition to that, use of this method causes a large temperature difference in the horizontal direction and increases the risk of water condensation inside the stone chamber. For these reasons, we did not adopt this method. Photo 2 Cooling pipes on the mound Installing cooling pipes around the stone chamber Figure 3 shows the proposed locations of cooling pipes around the stone chamber. The stone chamber would be cooled by circulating coolants in the pipes. Using this system, it would be possible to cool the stone chamber uniformly thereby avoiding a sudden temperature change occurring. However, to cool the stone chamber uniformly, it would be necessary to Photo 3 Insulation sheet on the mound 19

4 In this method, the tumulus mound is cooled by installing cooling pipes on the mound (Photo 2), covered by an insulation cover (Photo 3), and installing cooling pipes beneath the stone chamber, as shown in Fig.4. Simulation tests showed that it would be possible to decrease the temperature inside the stone chamber to less than 10 C without causing heavy damage to the tumulus mound. For these reasons, in May 2005 the special committee on the conservation of the paintings in the national treasure Takamatsuzuka tumulus, adopted this cooling method, as a protective measure against fungi in the stone chamber. Following this decision, installation of the cooling pipes and cooling facilities commenced in July After the completion of the installation, mound cooling was started in September Thermal properties of soil of the Takamatsuzuka tumulus mound To predict the temperature change inside the stone chamber due to the cooling of the mound, we measured physical properties such as the thermal conductivity and dry density of undisturbed soil samples obtained by boring at three locations (B-1, B-2 and B-3). The physical properties such as dry density, particle density, void ratio, gravimetric water content, volumetric water content, and saturation degree of the mound soil located 1m below the compacted soil surface (elevation 111m) at site B-3, which is 2m east of the outside wall of the stone chamber, were 1.25 g/cm 3, 2.62 g/cm 3, 52.2%, 21.6%, 27.1%, and 56.7%, respectively (Ishizaki et al. 2006). The thermal conductivity of the soil sample was measured with KD2 (Decagon Co., Ltd.) by the thermal probe method. The relationship between the thermal conductivity and the degree of saturation is Fig.5 Thermal conductivity of the mound soil shown in Fig.5. The thermal conductivity increased with increasing degree of saturation. This is due to the fact that the thermal conductivity increases with increasing water content. Numerical simulation of the temperature change of the mound due to cooling Numerical simulation method The temperature changes in the mound were calculated with software using a two-dimensional explicit differential equation developed by Matsuoka (Seiken. Co., Ltd). A schematic diagram of the northsouth cross section of the mound is shown in Fig.1. The mesh size is m. The time span of calculation is 0.05 h. The lower boundary of the simulation was located 5m below the cooling pipes. The north end of the simulation point was located 5m to the north of the surface cooling pipes, and the south end of the simulation point was located at the south end of the cooling pipes. As boundary conditions, the south and north end surfaces and the inside surface of the stone chamber were set to be insulated. The lower boundary temperature was set to 17 C. The thermal permeance κ of the mound surface with 30-mm-thick insulation was set to 1.62 W/m 2 K. Physical properties and boundary condition Void ratio and degree of saturation Based on the measurement of the physical properties of the mound soil, the average void ratio and degree of saturation of the mound soil were set to n = 53% and Sr = 51%, respectively. The average void ratio and degree of saturation of the stone chamber were set to n = 20% and Sr = 100%, respectively. Initial ground temperature profile and air temperature The initial ground temperature profile was set to the measured data of August 30, As for the outside air temperature, meteorological data at Nara city in 2002 was used for this simulation study. Thermal conductivity Based on the measured relationship between the thermal conductivity and degree of saturation (Fig.5), the thermal conductivity of the mound soil 20

5 2. S02 (0.9 m): 90 cm depth from the mound surface 3. S02 (1.75 m): 1.75 m depth from the mound surface 4. S01 (center): center of the two cooling pipes under the stone chamber Fig.6 Location of temperature sensors ( south-north cross-section) was determined. Planning of optimum cooling operation To avoid vapor condensation inside the stone chamber, it was necessary to decrease the temperature around the stone chamber uniformly by decreasing the temperature of the cooling pipes located on the mound and under the stone chamber. Numerical simulations were repeated to obtain suitable temperature settings for the coolants that circulated inside the cooling pipes. These simulation studies showed that when the cooling pipe temperatures were changed in the appropriate way, the stone chamber temperature would decrease to 10 C in 240 days and the maximum temperature difference would be 1.8 C. Comparison between the calculated temperature changes and measured ones Temperature sensors were placed at more than 20 points in the mound, and data were collected for storage on a computer every 30 minutes. The simulation results were compared with the measured data. Locations of the temperature sensors Figure 6 shows the locations of the temperature sensor compared with the simulation results in the north-south cross-section. Details of the locations of the temperature sensors are as follows: 1. Mound surface: 40 cm depth from the mound surface Comparison between the measured data and calculated data Figure 7 shows a comparison of the calculated temperature changes with the measured changes at the north-south cross-section. The measured temperature at the mound surface was lower than the calculated temperature at the beginning of cooling, but gradually the measured values came close to the calculated ones. The measured temperatures at S02 (0.9 m), S02 (1.75m), and S01 (center) decreased gradually and corresponded well with the calculated ones. These results showed that the measured mound Fig.7 Calculated and measured temperature changes in the mound temperature was lower than 10 C by the beginning of March Conclusion High humidity in the stone chamber of the Takamatsuzuka tumulus and the high water content of its lime wallplaster have caused fungi to appear on the walls inside the stone chamber of the tumulus. The characteristic moisture curves of Takamatsuzuka mound soil samples were obtained. These measurements showed that when the volumetric water content in the soil around the Takamatsuzuka tumulus is equal to 30%, the relative humidity is around 100%. This result corresponded well with the 21

6 measured relative humidity inside the stone chamber. Protective measures for the conservation of the mural paintings of the Takamatsuzuka tumulus were studied extensively for several years by the specialists of a committee organized by the Agency for Cultural Affairs. The committee members decided finally to remove the stones for the restoration of the paintings on the lime wallplaster. As a preliminary protective measure, it was decided to cool the stone chamber to reduce fungal activity. Five methods were proposed for this, including sending cool air into the stone chamber. Among the methods studied, that of installing cooling pipes on the mound and below the stone chamber was selected, because it allowed uniform cooling and would not damage the mound so severely. Undisturbed soil samples were obtained by boring, and the soil s thermal properties were measured. Based on these measurements, temperature changes were calculated by numerical simulation. After the cooling system was completed, cooling of the mound began in September The measured temperature inside the stone chamber reached 10 C at the beginning of March 2006 and was kept constant. The measured temperature profile corresponded well with the simulated results. Due to the cooling of the stone chamber, fungal activities were reduced. Acknowledgements The authors are very grateful to Mr. Satoshi Yoshida and Mr. Hisao Izuta of Seiken Co., Ltd. for their support in conducting numerical analysis. References Ishizaki, T., Sano, C. and Miura, S., Study on the water regime around tumulus with mural paintings, Proc. of the 10th International Congress on the Deterioration and Conservation of Stone, Stockholm, ICOMOS SWEDEN pp (2004) Ishizaki, T., Inuzuka, M. and Mimura, M., Study on geotechnical properties and moisture regime of Takamatsuzuka Tumulus (in Japanese), Science for Conservation, pp (2006) Miura, S., Ishizaki, T. and Akamatsu, S., Temperature change in 30 years at Takamatsuzuka Tumulus (in Japanese), Science for Conservation, pp (2005) 22