Effects of hygrothermal properties and boundary condition uncertainties in heat and mass transfer simulations of cave walls

In the Mogao Grottoes, hygrothermal cycling is the main cause for the observed mural deterioration development. A coupled heat and mass transfer (HMT) model in cave walls is constructed; it describes the hygrothermal behavior in porous wall construction and materials. This model allows us to evaluate their abilities to regulate microclimate conditions in caves and ensure a suitable protective environment. In this paper, finite variation effects in boundary conditions and material characteristics on the HMT model response are investigated. To determine the most influential hygrothermal parameters, further uncertainties in the parameters (density, heat capacity, thermal conductivity, adsorption isotherm and vapor permeability) and boundary conditions of the model are discussed. Additionally, both quantitative local sensitivity analysis (LSA) and global sensitivity analysis (GSA) was conducted to analyze the input parameters of the numerical model to identify the primary input parameters influencing the model's results. The results show that density and heat capacity do not significantly influence the simulated results when these properties are changed to values 2.5 times greater than those of the reference case. A lower vapor permeability or thermal conductivity and an improved adsorption isotherm are better for maintaining a stable humidity environment and reducing the mural salt disease risk. Note that the boundary conditions significantly affect the simulated results, especially reducing the average air relative humidity value; the amplitude air temperature value seems more beneficial for suppressing the salt damage risk to mural paintings.