Differentiation characteristics and attribution analysis of natural ventilation performance of Mogao Caves

Natural ventilation performance plays a key role in air quality of Mogao caves. As a consequence, it also has an important effect on the conservation of cultural relics and climate comfort degree of tourists in caves. This study aims to investigate the natural ventilation performance of two types of caves, one with truncated pyramidal ceiling and the other with central pillar, by means of tracer gas concentration decay method and air changes per hour (ACH) is used as the evaluation index. Additionally, the spatial difference, quantitative characteristics and factors of ACH in caves are well discussed. The results show that the ACH in caves with truncated pyramidal ceiling declines successively in the order of corridor, center of main chamber, southwest corner and northeast corner. While the caves with central pillar exhibit a different situation and its order of ACH from high to low is east side of pillar, corridor, south and north side of pillar, southwest corner and west side of pillar. Furthermore, it is also found that the overall ACH in both types of caves decreases significantly with the reduction of door opening degree. ACH ranges from 1.7 to 13.3 h^-1 for open caves, 1.5 to 5.2 h^-1 for caves only with gauze door closed and 0.4 to 1.3 h^-1 for closed caves, and their average value are 7.0, 2.8 and 0.8 h^-1 respectively. In terms of the factors that affects the ACH obviously, this study finds that ACH has a positive correlation with door opening degree, temperature difference between inside and outside and door area and a negative relationship with cave volume. Besides, the form of door and window also influences the ACH in caves dramatically. In conclusion, ACH in caves depends on the additive effect of multiple factors mentioned above. The results of this study are meant to provide a scientific basis for designing and evaluating the strategy of natural ventilation in Mogao Caves.