Air exchange within the Buddhist cave temples at Yungang, China

The Buddhist cave temples at Yungang, China, are subjected to rapid soiling due to the deposition of airborne particles onto the thousands of statues in those caves. 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 ms^(−1), amounting to an air exchange rate of 121 m^3 min^(−1), which achieves one complete air change within Cave 9 in only 4.3 min 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