Control of air exchange and particle deposition within the Buddhist cave temples at Yungang, China

Citation

Christoforou, Christos S. (1995) Control of air exchange and particle deposition within the Buddhist cave temples at Yungang, China. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/myqd-8n20. https://resolver.caltech.edu/CaltechETD:etd-09252007-094025

Abstract

The Yungang Grottoes are a collection of man-made Buddhist Cave Temples in the northern part of Shanxi Province in the People's Republic of China. These cave temples were carved into the face of a cliff in the 5th century AD and contain more than 50,000 Buddhist sculptures, some of which retain their historical coloring. The sculptures in the caves are soiled at a rapid rate by the deposition of airborne particles, generated in part by activities in one of China's largest coal mining regions. During the month of April, 1991, an extensive environmental monitoring experiment was conducted there, and additional measurements were made through the following year. The overall purpose of that work was to characterize the exposure of the grottoes to air pollutants in a manner that will establish a basis for the future protection of the grottoes from air pollution damage.

First, measurements were made in order to quantify the particulate air pollution levels at Yungang, and to gather data necessary for verification of models that predict the fate of airborne particles as they deposit onto surfaces inside the caves. Average mass deposition rates to horizontal surfaces of 13.42 [mu]g m[superscript -2] s[superscript -1] outdoors and 5.23 [mu]g m[superscript -2] s[superscript -1] inside Cave 6 were measured over a one year period in 1991-92. These rates are comparable to the rates inferred by examination of historically accumulated deposits within the caves. The surface area coverage by coarse particles is dominated by particles larger than about 10-20 [mu]m in diameter, while the mass flux is dominated by even larger particles greater than 20-30 [mu]m in diameter. Comparison of the deposition rate in Cave 6, which retains a wooden temple structure in front of the cave entrances, to that in Cave 9 which is open to the outdoors shows that the wooden shelter does provide some protection. During April 1991, the deposition rate to horizontal surfaces in Cave 6 was 4.5 [mu]g m[superscript -2] s[superscript -1] compared to 13.4 [mu]g m[superscript -2] s[superscript -1] in Cave 9 and 21.5 [mu]g m[superscript -2] s[superscript -1] outdoors.

Also during April, 1991, temperatures and air exchange rates were measured at Caves 6 and 9 at Yungang in order to establish baseline parameters necessary for modeling the air flow that carries air pollutant particles into and out of the caves. Air flow through the caves was found to be governed by a natural convection flow pattern that is driven by the difference between the temperature of the outdoor air and the temperature of the interior walls of the caves. During the day, warm outdoor air enters the upper entrances to the caves, is cooled by the cave walls and flows out through the ground level exits from the caves, while during the night the situation is reversed. The average air velocity at the entrance of Cave 9 during the course of the experiment was 0.274 m s[superscript -1], amounting to an air exchange rate of 121 m[superscript 3] min[superscript -1], which achieves one complete air change within Cave 9 in only 4.3 minutes on average. Cave 6 is larger than Cave 9, and air flow through Cave 6 is restricted by the presence of the wooden temple structure that is built over the entrances to Cave 6, yielding times to achieve a complete air exchange within Cave 6 that are typically 4 times longer than at Cave 9 under the April conditions studied. A theoretical model has been developed that takes as input cave wall and outdoor air temperatures and then predicts indoor air temperatures as well as air velocities at the entrance to the caves. The model can be used to predict air flows through the caves in the presence of increased resistance to air flow such as may occur following the future installation of filtration systems for particle removal at the caves.

Next, computer-based models have been constructed that simulate the particle deposition processes within the caves. The evolution of the airborne particle concentration and size distribution is tracked as outdoor air is drawn into the caves by a natural convection flow that is driven by the temperature difference between the outdoor air and the interior walls of the caves. Particle deposition rates are computed from the boundary layer flows along the surfaces within the caves. Predicted aerosol size distributions and particle deposition fluxes within Caves 6 and 9 at Yungang are compared to experimental observations made during the period April 15-16, 1991. It is found that horizontal surfaces within Caves 6 and 9 at Yungang would become completely covered by a full monolayer of deposited particles in only 0.3-0.5 years under the April 15-16 conditions studied here, and will be soiled even more rapidly under annual average conditions.

Finally, alternative approaches to the control of the particle deposition problem within the Yungang Grottoes are examined using the computer models described earlier. Two general approaches are possible: (1) control of particle concentrations in the outdoor air through reduction of particle generation at local sources, and (2) removal of particles from the air entering the caves by filtration or similar technical means. Both mechanical filtration and passive filtration systems are examined. It was found that mechanical filtration using high efficiency filters will reduce the mass of particles deposited onto upward facing surfaces within the caves by more than 99% provided that the infiltration of unfiltered air into the caves is eliminated. Passive filtration, where the existing natural convection flow is utilized instead of mechanical fans to drive air through low pressure drop filters, was also examined. It was found that about a 97% reduction of the total mass of particles deposited on horizontal surfaces caves can be achieved by passive filtration if filters are placed in the surface of newly reconstructed shelters in front of the caves that have been designed specifically for that purpose.

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