*Article* **Effect of Wind Speed on the Natural Ventilation and Smoke Exhaust Performance of an Optimized Unpowered Ventilator**

**Mao Li <sup>1</sup> , Yukai Qiang 2,\*, Xiaofei Wang <sup>3</sup> , Weidong Shi <sup>2</sup> , Yang Zhou <sup>2</sup> and Liang Yi 2,\***

<sup>1</sup> Headquarters of Mazhao Highway Project, Kunming 650034, China; limao9025@hotmail.com

<sup>2</sup> School of Civil Engineering, Central South University, Changsha 410075, China;

shiweidong@csu.edu.cn (W.S.); zyzhou@csu.edu.cn (Y.Z.)

<sup>3</sup> Anhui Transport Consulting & Design Institute Co., Ltd., Hefei 230088, China; wangxiaofei@acdi.ah.cn

**\*** Correspondence: yukaiqiang@csu.edu.cn (Y.Q.); yiliang@csu.edu.cn (L.Y.)

**Abstract:** Natural ventilators can maintain the ventilation of buildings and tunnels, and can exhaust fire smoke without requiring energy. In this study, we optimized a natural ventilator by adding axial fan blades (equivalent to adding a fan system) to investigate the effect of wind speed on the ventilation and smoke exhaust performance of an optimized natural ventilator. The experimental results showed that the best configuration of the ventilator was five fan blades at an angle of 25◦ with set-forward curved fan blades. With this configuration, the ventilation volume of the optimized natural ventilator was increased by 11.1%, and the energy consumption was reduced by 2.952 J. The third experiment showed that, in the case of a fire, the optimized ventilator can reduce the temperature of the ventilator faster than the original ventilator, indicating better smoke exhaust performance. The reason for this effect is that, when the optimized natural ventilator rotates, the rotation of the blades creates a flow field with a more evenly distributed wind speed. The experiments proved that natural ventilators can be optimized by adding a fan system. The results of this study can be applied to effectively improve the ventilation performance of natural ventilators to quickly exhaust smoke in building and tunnel fires, and provide a reference for related research on natural ventilators.

**Keywords:** optimized unpowered ventilator; axial fan blade; wind speed; ventilation performance; energy consumption; smoke exhaust performance
