**Jiaming Jin, Renming Pan \*, Ruiyu Chen, Xiaokang Xu and Quanwei Li \***

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; jmjin\_njust@163.com (J.J.); crynjust@njust.edu.cn (R.C.); xiaokangxu@njust.edu.cn (X.X.) **\*** Correspondence: panrenming@njust.edu.cn (R.P.); liqw83@163.com (Q.L.); Tel.: +86-025-8431-5511 (R.P.)

Received: 3 December 2020; Accepted: 16 December 2020; Published: 21 December 2020

**Abstract:** To provide guidance towards reducing the weight of the HFC-125 storage vessel by reducing the release pressure and to reveal the effects of release pressure on the extinguishing efficiency of HFC-125, we investigated the flow and diffusion characteristics of HFC-125 under six release pressures in the present study. The influence of release pressure on the degree of superheat, injection duration, pressure loss, jet angle, and concentration distribution were analyzed. Results show that the degree of superheat and the injection duration both decreased with the release pressure. The bubble expansion in the HFC-125 could slow down the pressure decrease in the storage vessel. The flow process in the pipeline can be divided into three phases: pipeline filling, stable flow, and mixed gases release. Both of the maximum and mean values of the pipeline pressure loss increased with the release pressure. The maximum concentration value decreased with the increase of the distance from the nozzle. The maximum concentration value in the near field from the nozzle increased with the release pressure. The concentration and holding time (duration above 17.6% volume concentration) of HFC-125 in the near field from the nozzle met the requirements of minimum performance standards (MPS) for HFC-125.

**Keywords:** Halon candidate substitute; aircraft weight reduction; HFC-125; release pressure; flow; diffusion
