Study of the optimal pitch angle of jet fans in road tunnels based on turbulent jet theory and numerical simulation

Abstract

The diffusion angle of the air jet and the optimal pitch angle (OPA) of the jet fan are studied and compared through turbulent jet theory and FLUENT numerical simulation in this paper. The optimal design pitch angle of jet fans with different outlet diameters and velocities is systematically provided, rather than based on a specific outlet diameter and velocity of the jet fan or only put forward a rough range. The results show that, tilting the fan outlet with a certain angle can reduce the wall shear stress (WSS) and improve the pressure-rise coefficient. The full diffusion angle (FDA) is greatly influenced by the outlet diameter and velocity, while the semi-diffusion angle (SDA) is relatively steady and fluctuates around 6°. Meanwhile, the OPA is hardly affected by the outlet velocity and mainly influenced by the outlet diameter. The OPA is 8° for the jet fan with the diameter less than or equal to 900 mm and 6° for the jet fan diameter more than 900 mm. Moreover, the pressure-rise coefficient will be improved under the OPA and the relative increase rate is between 5.9% and 44.5% compared with that of no pitch angle. This study can provide reference and guidance for the performance research of alternative jet fans in fire case and the optimization design of ventilation systems in road tunnels.