Investigation of the main factor affecting the NOx distribution in the street canyon with the photo-catalytic wall.

To find out the most contaminated street region and protect the pedestrian with the photo-catalytic equipment to decrease the hazard of oxynitride (NOx), Computational Fluid Dynamics (CFD) simulation could be used to research the main factor affecting the statistical characteristics of the oxynitride distribution in the urban street canyon with the photo-catalytic building walls. Additionally, the connection was investigated and focused on the swirling flow and oxynitride concentration to find out the root of the main factor affecting oxynitride distribution. The simulation results showed that there was one three-dimensional swirling flow in the whole canyon and the statistical concentration was straightforwardly related to the swirling or whirling flow structure (such as eddy). The characteristics had been confirmed that the whirling flow structure affected the complex oxynitride distribution in the street canyon with the photo-catalytic building walls. Furthermore, one formula was found which described the oxynitride concentration constrained by the street canyon. This study illustrated that different sections in the canyon had various patterns of the whirling flow structure (swirling flow) and oxynitride. In the symmetrical portion of the street canyon (in the middle of the street length), there is one concise equation to describe the NOx concentration affected by the turbulence intensity. Moreover, the equation was presented as CR = 1.094 + 0.11e-I, where I was the turbulence intensity and CR was the oxynitride relative concentration in the street canyon.