CFD simulation of rain water runoff from porous building façades for implementation of self-cleaning

Wind-driven rain (WDR) is one of the most important moisture sources for a building façade. This study presents the implementation and application of a rainwater runoff model based on the Nusselt solution on coupled to a 2D Heat-Air-Moisture (HAM) model. Commercial software WUFI 2D is used to calculate the absorption on a porous façade based on heat, air and moisture transfer inside porous façade during rain. The calculation is performed with a realistic distribution of the impinging WDR intensity, based on CFD simulations, and with meteorological data, on a 10 hours basis. The flow of the liquid film is essential for self-cleaning. To clean a surface, liquid has to transport along the impurities and run off them. It is the flowing liquid film that carries away the impurities. At the macroscale, our simulation results demonstrate that the façade surface coated with nano-TiO2 show features of more pronounced in increasing runoff water film thickness and velocity comparing to the uncoated façade during raining event. Therefore suggesting better effect of self-cleaning on superhydrophilic surface. The dust particle-wall interaction was analyzed in this study and particle detachment model was established. At the microscale, our simulation results demonstrate that under high relative humidity (>50%), the deposited dust at different sizes on the superhydrophilic wall surface can be easily detached by the runoff during raining.