**1. Introduction**

Halon fire extinguishing agents, especially for Halon 1301, negatively impact the ozone layer and have a high global warming potential (GWP) index, which goes against the environment protection and the sustainable development of the world [1]. The search for appropriate alternatives for the halon fire extinguishing agents has received increasing attention in the past decades [2,3]. Pentafluoroethane (HFC-125) does not destroy the ozone layer and its GWP index is only half of that of Halon 1301. It is regarded as a candidate substitute for Halon 1301 in the engine nacelle and auxiliary power unit of commercial aircrafts at low temperatures [4,5]. However, when HFC-125 is applied in aircrafts, owing to the lower extinguishing efficiency of HFC-125 compared with Halon 1301, more HFC-125 agents (an increase of approximately 80% in weight) and larger fire extinguishing agent storage vessels (approximately 2.3 to 4.3 times in volume larger than that of Halon 1301) are needed [6]. Such an increase in weight and volume will pose a great challenge to the aircraft design in the aspect of cost control, fuel consumption, and safety [7], which goes against the sustainable development of the aviation industry. It is noted that if the release pressure is appropriately reduced on the condition of meeting the requirements of the airworthiness provisions for the volume concentration and holding time of the fire extinguishing agent, the weight of the agent storage vessel can be reduced, which is considerably beneficial to the weight reduction of the aircraft. The weight reduction can greatly reduce

the greenhouse gas emissions generated by the fuel consumption. In order to evaluate whether the requirements of the minimum performance standards (MPS) [8] for HFC-125 can be satisfied or not on the condition of reducing the release pressure, it is of great necessity to study the flow characteristics in the pipeline and the diffusion behaviors in the power nacelle of HFC-125 under different pressures.

Some attention was devoted to the flow behaviors of fire extinguishing agents in pipelines. However, few studies have been reported on HFC-125. Williamson studied the flow behaviors of Halon 1301 in pipelines under 2.48 MPa [9]. He found that the pressure decreased in a nonlinear way when the Halon 1301 agent flowed in the pipeline, and the agent boiling would slow down the pressure decrease. Moreover, the release rate of the Halon 1301 agent increased with the bottle volume. In the case of 5.2 MPa, Elliott et al. [10] proposed a homogeneous and equilibrium model of two-phase (gas and liquid phase) flow to estimate the flow behaviors of Halon 1301 in the pipeline. It was found that the predicted data based on the model were in accordance with the experimental data. Yang et al. [11] presented a two-phase (gas and liquid phase) equilibrium model to calculate the thermodynamic properties and filling conditions of five selected agents: HFC-227ea, CF3I, FC-218, HFC-125, and CF3Br. The accuracy of the presented model was verified under the release pressures from 2.8 to 4.1 MPa. The predicted values based on the two-phase quilibrium model were found to be in good agreement with the measured values. Tuzla et al. [12] developed a computer code to predict the single-phase and two-phase flow behaviors of the fire extinguishing agents in the pipeline under 5.71 MPa on the basis of the multi-phase flow algorithms generally used in the nuclear power plant. Kim et al. [13] employed FLUENT software to simulate the flow behaviors of Halon 1301 in the fire extinguishing system under 4.1 MPa. The volume percentage of the Halon 1301 agent in the pipeline and the outlet was obtained. In addition, the release behaviors of the Halon 1301 agent in the case of different surface areas of the rupture disk were analyzed. The results indicated that the release rate of the Halon 1301 agent increased with the surface area of the rupture disk. Moreover, it was found that little influence of the pipeline diameter was exerted on the release process of the Halon 1301 agent in the storage vessel. However, the release rate of the Halon 1301 agent at the pipeline outlet increased with the pipeline diameter. Some studies were reported on the diffusion behaviors of fire extinguishing agents in enclosure spaces, with the majority being numerical simulation studies. Among them, the study concerning the diffusion of HFC-125 in enclosure spaces was rare. Sarkos [14] studied the diffusion behaviors of Halon 1301 in a full-scale aircraft cabin under 2.48 MPa. The profiles of the agent concentration, visibility, pressure, temperature, noise, etc. were measured. The results indicated that the influence of the ultralow-pressure, over-temperature, and the agent concentration overshoot on passengers can be alleviated by the air disturbance. In addition, the air disturbance was helpful in the diffusion of the agent. The fire extinguishing agent released from the ceil can finally enter the lavatory and other complex areas through diffusion. Niu et al. [15] employed fire dynamics simulator (FDS) to study the diffusion behaviors of the Halon 1301 agent in the helicopter engine nacelle in the case of no-ventilation. The concentration distribution of the Halon 1301 agent in 6–10 s in the case of different mass flow rates and injection time were measured. It was found that reducing the mass flow rate of the fire extinguishing agent was beneficial to improve the system reliability and reduce the amount of the fire extinguishing agent. Using the lumped parameter approach, Kurokawa et al. [16,17] proposed a one-dimensional model to predict the volume concentration of the Halon 1301 fire extinguishing agent with the assumption that the profile of the flow rate was as a ladder shape. The proposed model was found to acceptably predict the volume concentration of the fire extinguishing agent. Adopting Fluent software, Zaparoli [18] investigated the diffusion behaviors of the Halon 1301 agent in the cases of three air flow rates in the cargo hold of the aircraft. It was indicated that the agent concentration decreased continuously with time when the air flow rate was 0.08 kg/s. Using Hflowx and Fluent, Lee [19] simulated the diffusion of the fire extinguishing agent (HFC-125, CF3I, and Halon 1301) in the engine nacelle and auxiliary power unit (APU) nacelle on the basis of a one-dimensional two-phase flow algorithm. It was found that small differences occurred between the simulation results and the experimental results. In summary, the flow and diffusion

behaviors of the Halon fire extinguishing agent have been analyzed under single release pressure in the previous studies. However, scarce attention was focused on the flow and diffusion behaviors of HFC-125. Moreover, it is indicated in the previous studies that the flow and diffusion characteristics of the fire extinguishing agent are in close relationship with the volume of the storage vessel, the pipe diameter, the nozzle location and configuration, the outlet flow rate, the degree of phase transition, etc., whose design are greatly dependent on the initial release pressure. Therefore, considering the urgent need of the alternatives for the halon fire extinguishing agents and the deduction of the aircraft weight, it is of considerable necessity and importance to study the flow and diffusion behaviors of HFC-125 under different release pressures.

In the present study, the flow behaviors in the pipeline and the diffusion behaviors in the enclosure spaces of HFC-125 were studied under different release pressures using a full-scale airborne fire extinguishing system in the engine nacelle. Many parameters including the degree of superheat, the injection duration, the jet structure, and the concentration distribution were measured and discussed. The effects of release pressure on the above-mentioned parameters were then analyzed.

### **2. Experiment Apparatus**

Figure 1 shows the schematic diagram of the full-scale airborne fire extinguishing system. It was mainly composed of 3 parts: agents release system, enclosure space, and data acquisition system. The agents release system consisted of a high-pressure storage vessel with the working capacity of 1.4 L, a vessel head valve, a pipe, and a nozzle. The storage vessel was pressurized by nitrogen to drive the agents in the storage vessel. In the present study, the storage vessel was filled with HFC-125 of 0.95 kg. In order to saturate the nitrogen dissolved in the fire extinguishing agent, we increased the vessel pressure slowly by nitrogen to 2.41, 2.76, 3.1, 3.45, 3.79, and 4.14 MPa under 294.25 K. The vessel head valve was installed at the outlet of the vessel to control the opening and close of the fire extinguishing system. The downstream pipe was 2400 mm long and 15.6 mm in diameter and employed the same straight-through nozzle. The diffusion characteristics of the fire extinguishing agent jet was proceeded in an enclosure space (2200 mm (long) × 2300 mm (wide) × 2000 mm (high)) with a pressure relief port.

**Figure 1.** The full-scale airborne fire extinguishing system.

The data acquisition system consisted of two K-type thermocouples, three pressure transmitters, a fire extinguishing agent concentration tester, and a high-speed camera. The variations of pressure in the vessel and pipeline and the equilibrium temperature of the agents throughout the release process were monitored by the pressure transmitter and the thermocouple, respectively. The thermocouples were installed at the top and bottom of the vessel. Three pressure transmitters with the range of 0–5 MPa were installed to record the vessel pressure at the top of the vessel (*P*) and the pressure loss of the HFC-125 agent at the inlet of the pipe (*P*in) and the inlet of the nozzle (*P*out) in the pipeline. The length between *P*in and *P*out in the downstream pipeline was 2 m. There were 12 channels in the fire extinguishing agent concentration tester, and the measurement range was 0–80%. It was noted that the concentration mentioned here refers to the volume concentration of the gaseous fire extinguishing agent. The concentration distribution of the HFC-125 at 3 imaginary isometric sections (shown in Figure 2) was measured and analyzed in the present study. Therein, Section II was the central section of the enclosure space, and Section I and III were 550 mm away from the left and right of Section II, as shown in Figure 2. Point 2, point 4, and point 10 were located on the circle with a diameter of 109.7 mm, while point 3, point 5, and point 11 were located on the circle with a diameter of 219.4 mm. Point 12 was located on the circle with a diameter of 329.1 mm. The orifices of the sampling pipe were fixed at the 12 points located in the enclosure space to measure the concentration of the fire extinguishing agent. A Photron FASTCAM UX50 high-speed camera with the frame rate of 1000 fps was used to study the diffusion behaviors of the fire extinguishing agent in the ejection process in the enclosure space.

**Figure 2.** The location of sampling points.

### **3. Fire Extinguishing Agent Properties and Experimental Conditions**

HFC-125 is named pentafluoroethane, whose molecular formula is C2HF5. Its molecular weight is 120.02 and its boiling point at standard atmospheric pressure is 224.7 K. It is in a gas state under normal temperature and pressure, while it can be liquefied when it is pressured. The value of ozone depletion potential (ODP) of HFC-125 is far below 0.001, and it has been recognized as the candidate substitute for the halon fire extinguishing agents by the EPA (United States Environmental Protection Agency)'s Significant New Alternatives Policy (SNAP).

In the present study, the structure of the storage vessel and filling conditions were consistent with those of the airborne APU fire extinguishing system. The flow and diffusion characteristics of HFC-125 were studied under six release pressures of 2.41, 2.76, 3.1, 3.45, 3.79, and 4.14 MPa.
