Research On The Technology Of Ventilation Regulation In Dunhuang Mogao Grottoes

Dunhuang is located in the northwest of Gansu.It is a cold area,perennial drought and has desert climate,which is extreme cold in the winter and hot in the summer.The strong winds of the gobi desert was greatly weaken because of the valley topography which formed by the Mingsha mountain and Daquan river,and there is the static wind environment.The mogao grotto is a typical cave building,which has only one side opening,across which air is exchanged with the atmosphere.Based on the above factors,there is stable micro-environment which can provide a stable conservation micro-environment for the sculpture and murals in caves.However,it is showed that several physical environmental parameters fluctuate according to the data collected for the physical environment in the caves for several years by Dunhuang Academy.Firstly,there is the dry-wet alternate air environment in caves because of the increasing of rainfall in summer,which will increase the risk of the mural salt injury.Secondly,there is the high CO2 concentration in caves because of increased visitor numbers,which will increase the risk of mural pigment discoloration.To solve these problems,active environment controlling system will be designed and installed for the cave of Mogao Grottoes,which should be designed for the conservation of cultural relics instead of visitors' physical thermal comfort.Therefore,this paper will make exploratory research about environment control system of the Mogao grottoes.The study is mainly focused on the following three aspects:Firstly,based on analysis of the 2011-2012 meteorological data,physical environment is studied in-depth for the first time.Some physical parameters,such as wind speed,temperature and relative humidity of air,were tested by the way of field test,and the wind environment,the thermal and moisture environment and the ventilation rate in Mogao Grottoes were studied.To some physical parameters,such as different pressure between cave and atmosphere,and the internal grotto's pressure,the paper,on the one hand,discussed one sided opening natural ventilation mechanism applicable to the Mogao grottoes terrain climate and the seasonal characteristics,based on the one sided opening natural ventilation principle;on the other hand,put forward the formulas of pressure difference of the Mogao grottoes in autumn and winter under the effect of natural ventilation,which was proved by the measured data.And finally the internal grotto's pressure(5Pa)was put forward.The conservation environment of the mogao grottoes was studied by the ways of field test and the numerical simulation and the theoretical calculation.The study will provide data support and scientific basis for the manual intervention system design for cave site environment.Secondly,according to the conservation environment and the disease mechanism of mural,the design parameters of the adjusting and controlling system of cave site conservation environment was discussed for the first time.There are two parts of study,including air volume and air velocity of the supply system.Based on the natural ventilation and mechanical ventilation joint action principle,considering air tightness of the doors of caves,the calculation method of air volume of the supply system was put forward.As shown in the results of the study,as the temperature difference and the outside wind speed increased,air volume of the supply system increased.And as the air tightness of the doors of caves increased,air volume of the supply system decreased.According to the differences of grottoes natural ventilation mechanism in autumn and winter,the curve between the air volume of system and physical environment and cave doors' tightness was put forward,which will provide theoretical support and design basis for the air volume of the supply system of the cave site environment manual intervention system.The material and thickness of the original intonaco is similar to these of building envelope,based on coupling equations of heat and mass transfer of thin-wall,it was discovered that the flow of adsorption and desorption will be greater with the increase of wind speed.The flow of adsorption and desorption gradient is greatest when the wind speed is 0 to 1 m/s,and adsorption and desorption will be done in 14 hours.Based on mechanism of salt injuries,to avoid activation salt injury for adsorption and desorption over and over again,it was put forward that the internal grotto's air velocity must not be disturbed.Based on the disturbance of the internal grotto's air in the natural ventilation,the maximum of air velocity of the supply system was 0.2m/s.This study will provide theoretical support and design basis for the air velocity of the supply system of the cave site environment manual intervention system.At last,the design parameters of the cave site environment manual intervention system put forward in paper were verified by the ways of numerical simulation and field test.Based on the air volume of the supply system of the the internal grotto's environment manual intervention system,micro-positive pressure can be made to prevent moist air from permeating into the caves.The equipment selection of the internal grotto's environment manual intervention system has been completed,and the system has been installed in cave 328 in Mogao Grottoes.The air exchange rate,the time of the decline of the concentration of CO2,air age and air velocity were tested by gas tracing method.Experimental result showed that the concentration of CO2 descended,the internal grotto's air quality be raised,and the internal grotto's air disturbance didn't augment.It is proved that the system is very effective and the internal grotto's conservation environment has not be changed.