Numerical Study on Effects of Geometric Parameters on the Release Characteristics of Straight Sudden Expansion Gas Extinguishing Nozzles

In order to guide the optimization design of the nozzle of the aircraft-fixed gas fire extinguishing system, we studied the influence of nozzle geometric parameters including outlet–inlet area ratio, length–diameter aspect ratio, and wall roughness on the distribution of pressure and velocity in the nozzle on the basis of CFD simulations. Although the structure of the nozzle is axisymmetric, the spatial distribution of the pressure and velocity during the flow and release of gas extinguishing agent is not completely symmetric. It was found that both of the outlet–inlet area ratio ($\mathsf{\delta }$) and the length–diameter aspect ratio ($\xi$) had a significant impact on the distribution characteristics of the pressure and axial velocity in the nozzle. With the increase of $\mathsf{\delta }$, the average pressure at the outlet cross-section of the nozzle decreased monotonically, while the average axial velocity at the outlet increased approximately linearly. When $\xi \ge 2$, the uniformity of the pressure and velocity distribution at the nozzle outlet was significantly improved. Moreover, with the increase of $\xi$, the average pressure and the average axial velocity of the outlet both showed a non-monotonic change trend, and the optimal value of $\xi$ should be about 3.0. Compared with $\mathsf{\delta }$ and $\xi$, the influence of the nozzle wall roughness (${\epsilon }_N)$ on the flow and release characteristics of the extinguishing agent was weak. With the increase of ${\epsilon }_N$, the average pressure of the nozzle outlet increased slightly, while the average axial velocity at the nozzle outlet decreased slightly.