The pipe diameter is an important technical parameter of pipelines, which often has significant effects on the flow characteristics of their internal medium. In order to study the influence of the pipeline diameter on the flow and release characteristics of the Halon 1301 agent, the diameters of the Pipe 1 and Pipe 2 shown in
Figure 3 were both set to 10
, 15
, 20
and 25
respectively, which is for the contrast test with the same filling status of the extinguishing bottle. The lengths of Pipe 1 and Pipe 2 were set to 220
and 427.7
, respectively. The initial filling pressure of the extinguishing bottle was 4.832
and the filling amount of the extinguishing agent was 1.195
. At the temperature of 294.25
, the volume percentages of the Halon 1301 agent in the gas phase and liquid phase were 47.21
and 94.90
, respectively, and the remaining proportions were occupied by the compressed nitrogen.
3.1.1. Effects of Pipeline Diameter on the Release Time
The release duration of the extinguishing agent is an important target to measure the performance of the gas extinguishing system. The length of the release time significantly affects the spatial and temporal distribution characteristics of the extinguishing agent in protected spaces, which not only affects the fire extinguishing rate but also affects the interaction time between the extinguishing agent and high-temperature flames, thereby affecting the amount of toxic and harmful products generated by the thermal decomposition of the extinguishing agent [
25,
26,
27].
Figure 5a,b show the variation curves of the vessel pressure and the pressure drop rate during the release under the conditions of different pipeline diameters, respectively. It can be seen from
Figure 5a that, under the conditions of the same filling amount of the extinguishing agent, the total release duration decreased significantly with the increase in the pipeline diameter. As shown in
Figure 5b, the overall pressure drop rate of the vessel increases with the increase in the pipeline diameter. In addition, at the initial stage of the release, there is an obvious peak occurring in the pressure drop rate curve, and the peak value increased significantly with the increase in the pipeline diameter. In this study, the values of the pressure drop peak corresponding to the four pipeline diameters from thin to thick were 14.72
, 39.06
, 69.73
and 103.01
, respectively. It can be found that the peak value has a good quadratic function relationship with the pipeline diameter, and the fitting result is shown in Equation (7).
where
represents the maximum pressure drop rate of the vessel (
);
denotes the pipeline diameter (
).
In order to reveal the release process of the extinguishing agent, the pressure and the pressure drop rate of the vessel and the mass flow rate of the extinguishing agent at the pipeline outlet were compared, as shown in
Figure 5c, it can be seen from the figure that the pressure of the vessel drops rapidly at the beginning of the release, and an obvious peak occurs in the pressure drop rate curve of the vessel. At this moment, the extinguishing agent was about to be sprayed out of the pipeline. Then the mass flow rate of the extinguishing agent at the pipeline outlet increases rapidly to the peak value, while the pressure drop rate of the vessel decreased rapidly. Therefore, it can be inferred that this stage was the process of rapid filling of the pipeline by the extinguishing agent. In this paper, this stage was named Phase I: the rapid pipeline filling by the extinguishing agent.
Then, the mass flow rate of the extinguishing agent at the pipeline outlet which is decreased slowly at first and then decreased rapidly shows a two-stage trend. While the pressure drop rate of the vessel decreased at first and then increased. In addition, by comparing the mass flow rate of the extinguishing agent with that of the nitrogen, it can be seen that the mass flow rate of the nitrogen started to increase slowly from the inflection point (as shown by the red arrow in
Figure 5c) of the above mentioned two-stage trend, indicating that the compressed nitrogen in the vessel begins to release gradually at this moment. Thus, it can be inferred that the first-half stage (Part 1) should be the continuous release of the liquid extinguishing agent in the vessel, and the remained half stage (Part 2) was the release of the residual liquid extinguishing agent in the pipeline after the complete release of liquid extinguishing agent in the vessel. In this paper, this phase was named Phase II: the concentrated release of the liquid extinguishing agent.
Finally, the mass flow rate of the extinguishing agent and the compressed nitrogen at the pipeline outlet decreased gradually until the release ended. Meanwhile, the pressure drop rate of the vessel decreased gradually to zero. It can be speculated that this stage should be the release of the residual gaseous extinguishing agent and the compressed nitrogen through the pipeline. In this paper, it was named Phase III: the release of the residual gases.
In order to analyze the difference in the release time of the extinguishing agent under different pipeline diameter conditions, as shown in
Figure 5d, the duration of the liquid extinguishing agent (
, the total release time of the extinguishing agent (
, and the percentage of the liquid release time to the total release time (
were plotted against the pipeline diameter. It can be seen from the figure that
and
both decreased significantly with the increase in the pipeline diameter, which can be fitted by the exponential functions. The fitting results are shown as Equations (8) and (9), respectively. However,
did not change significantly with the increase in the pipeline diameter, which means that under the same conditions of the extinguishing bottle and the pipeline length, increasing the pipeline diameter is conducive to the rapid release of the extinguishing agent. Especially when the pipeline diameter was relatively small (such as 10
and 15
), with the increase in the pipeline diameter, the release duration of the extinguishing agent decreased significantly, while the decreasing rate of the release duration slows down gradually. Combined with the aforementioned analysis of the release process, it can be inferred that the increase in the pipeline diameter not only increased the mass flow rate of the liquid extinguishing agent flowing into the pipeline but also reduced the resistance loss during the flow, which was conducive to the rapid release of the extinguishing agent. However, due to the limited driving capacity of the extinguishing bottle, the pressure drop rate of the vessel increased significantly with the increase in the pipeline diameter, resulting in the decrease in the driving force for the extinguishing agent, which in turn was not conducive to the rapid release of the extinguishing agent.
where
and
denote the liquid release time (
) and the total release time (
) of the extinguishing agent, respectively;
denotes the pipeline diameter (
).
3.1.2. Effects of Pipeline Diameter on the Mass Flow Rate
Mass flow rate is one of the important parameters to characterize the release rate of fire extinguishing systems, which directly reflects the amount of the extinguishing agent released into the protected space per unit time, and has a vital influence on the establishment of the extinguishing concentration in protected spaces. There is obvious vaporization during the release of the Halon 1301 agent in the pipeline, and the vaporization rate not only affects the release rate of the extinguishing agent but also affects its flow and diffusion characteristics in protected spaces. Therefore, it is necessary to study the characteristics of the mass flow rates of the liquid and gaseous extinguishing agents flowing in the pipeline, respectively.
Figure 6a,b show the mass flow rate curves of the liquid and gaseous extinguishing agents at the pipeline outlet under different pipeline diameter conditions, respectively. It can be seen from these two figures that with the increase in the pipeline diameter, the mass flow rates of the liquid extinguishing agent and the gaseous extinguishing agent both increased significantly, while the release durations of them both decreased significantly. In addition, as shown in
Figure 6a, when the pipeline diameter was relatively small (10
and 15
), the mass flow rate of the liquid extinguishing agent in Phase II decreased monotonically, while when the pipeline diameter was relatively large (20
and 25
), the mass flow rate of the liquid extinguishing agent in Phase II showed a non-monotonic trend, as shown by the ellipses in
Figure 6a, which indicates that when the pipeline diameter is large, the gas–liquid two-phase flow of the extinguishing agent in the pipeline becomes more complicated.
In order to further analyze the difference of the mass flow rate of the extinguishing agent under different pipeline diameter conditions, taking the maximum (25
) and the minimum (10
) pipeline diameters in this study as examples, the mass flow rates of the extinguishing agent and the compressed nitrogen, and the pressure drop rate of the vessel were plotted, as shown in
Figure 6c,d, respectively. It can be seen from
Figure 6c,d that the mass flow rate of the gaseous extinguishing agent is much lower than that of the liquid extinguishing agent in the whole release process under these two different pipeline diameter conditions and the release process presented the same stage characteristics described in
Section 3.1.1, namely: Phase I: rapid filling of the pipeline by the extinguishing agent; Phase II: concentrated release of the liquid extinguishing agent; Phase III: release of the residual gases. By comparing
Figure 6c with
Figure 6d, it can be found that there are significant differences between the release characteristics in Phase II under the two different pipeline diameter conditions. Firstly, in the Part 1 stage, when the pipeline diameter was 10
, the pressure drop rate of the vessel was relatively small (the average value was about 5.86
) and decreases slowly with time, and the mass flow rate of the liquid extinguishing agent decreased while that of the gaseous extinguishing agent increased, indicating that the gasification rate of the extinguishing agent increased continuously during this stage; While, when the pipeline diameter was 25
, the pressure drop rate of the vessel was significantly larger than that of 10
(the average value was about 35.02
) and decreased rapidly with time, and the mass flow rate of the liquid extinguishing agent showed a non-monotonic variation trend, and that of the gaseous extinguishing agent decreased rapidly at first and then increased significantly, which indicates that the gasification rate of the extinguishing agent decreased rapidly at first and then increased significantly during this stage. Secondly, in the Part 2 stage, when the pipeline diameter was 10
, the mass flow rate of the gaseous extinguishing agent decreased with time, and that of the nitrogen increased slowly at the beginning of this stage; While, when the pipeline diameter was 25
, the mass flow rate of the gaseous extinguishing agent decreased rapidly at first and then increased slowly, and that of the nitrogen increased significantly at the beginning of this stage, it can be inferred that the rapid outflow of the compressed nitrogen accelerated the gasification rate of the extinguishing agent to a certain extent.
In order to compare the differences between the liquid mass flow rate and the total mass flow rate of the extinguishing agent, taking the case with the pipeline diameter of 20
as an example, the variation curves of the two mass flow rates with time were shown in
Figure 7a. As can be seen from the figure, the liquid phase mass flow rate of the extinguishing agent was close to the total mass flow rate throughout the release process, and the trend of the two curves was consistent. The maximum difference between the two parameters was 0.69
(about
of the total mass flow rate at that moment), which showed that the release of the extinguishing agent was still dominated by the liquid agent under this relatively large pipeline diameter.
Considering that the extinguishing agent was mainly released in Phase II, in order to analyze the difference of the release rate of the extinguishing agent under different pipeline diameter conditions, the maximum mass flow rates of the liquid extinguishing agent and that of the total extinguishing agent (represented by
and
, respectively) and the average mass flow rates of the liquid extinguishing agent and that of the total extinguishing agent (represented by
and
, respectively) in Phase II were plotted against the pipeline diameter, as shown in
Figure 7b. It can be seen from the figure that the four parameters all showed a good quadratic function relationship with the pipeline diameter, and the fitting results are shown in Equations (10)–(13) respectively. In addition, both
and
increased gradually with the increase in the pipeline diameter, indicating that the gasification rate of the extinguishing agent during the release increased with the increase in the pipeline diameter. This may be because the gas–liquid two-phase flow of the extinguishing agent became more complex with the increase in the pipeline diameter.
where
and
denote the maximum mass flow rates of the liquid extinguishing agent and that of the total extinguishing agent (
), respectively;
and
denote the average mass flow rates of the liquid extinguishing agent and that of the total extinguishing agent in Phase II (
), respectively;
denotes the pipe diameter (
).
In order to further analyze the influence of the pipeline diameter on the gasification rate of the extinguishing agent during the release, the per unit area mass flow rates of the extinguishing agent were plotted against the pipeline diameter, as shown in
Figure 7c (the maximum mass flow rate per unit area of the liquid extinguishing agent and that of the total extinguishing agent were represented by
and
, respectively, and the average mass flow rate per unit area of the liquid extinguishing agent and that of the total extinguishing agent in Phase II were represented by
and
, respectively). It can be seen from the figure that with the increase in the pipeline diameter,
,
,
and
all increased at first and then decreased. Under the conditions of the four pipeline diameters adopted in this study,
and
varied slightly with the pipeline diameter (the maximum values of these two parameters are 1.06 times and 1.04 times of the minimum values, respectively), and their maximum values both appear at the pipeline diameter of 15
. While the variations of
and
with the pipeline diameter are larger than
and
(the maximum values of
and
are 1.18 times and 1.20 times of the minimum values, respectively), and the maximum values of
and
both appear at the pipeline diameter of 20
.
The reasons why the aforementioned maximum mass flow rate per unit area varied slightly with the pipeline diameter may be due to the fact that, on the one hand, the flow resistance of the extinguishing agent in the pipeline decreased with the increase in the pipeline diameter, which was conducive to the increase in the maximum velocity of the extinguishing agent; On the other hand, the pressure drop rate of the vessel increased with the increase in the pipeline diameter, resulting in the decrease in the driving force for the extinguishing agent, which in turn reduced the maximum velocity of the extinguishing agent. For the average mass flow rate per unit area, there were more influencing factors. In addition to the above reasons, it was also affected by two other aspects: on the one hand, the large pipeline diameter was conducive to significantly reduce the release time of the extinguishing agent (as shown in
Figure 5d), which reduced the heat absorption of the extinguishing agent from the environment and reduced its gasification rate; On the other hand, it may also be related to the flow state in the Part 2 stage. The large cross-sectional area of the pipeline made the liquid extinguishing agent and the compressed nitrogen mixed intensely in this stage, and the nitrogen flowed out of the pipe too quickly, which was not conducive to the full utilization of its driving capability. In addition, it can be seen from
Figure 7c that when the pipeline diameter was large,
and
were significantly greater than those when the pipeline diameter was small, and the values of these two parameters at the pipeline diameter of 25
were 2.1 times and 1.7 times of those at the pipeline diameter of 10
, respectively. This indicated that the large pipeline diameter will accelerate the gasification rate of the extinguishing agent during the pipe flow, which may be due to the pressure drop rate of the vessel increased with the increase in the pipeline diameter so that the pressure of the extinguishing agent during the flow decreased and the degree of the superheat increased.
3.1.3. Effects of Pipeline Diameter on the Gasification Ratio
The vaporization of the extinguishing agent occurs during the flow in the pipeline, which not only affects the transport efficiency of the extinguishing agent in the pipeline and then affects its mass flow rate and release time, but also affects its performance of heat absorption through gasification in the protected space. Therefore, it is necessary to study the gasification characteristics of the extinguishing agent under different pipeline diameters.
Figure 8 shows the comparison curves between the flow velocity at the pipeline outlet and the total mass flow rate of the extinguishing agent when the pipeline diameter was 20
. It can be seen from the figure that in Phase I, an obvious peak appeared in the flow velocity curve at the beginning of the release, at this moment the extinguishing agent had not yet flowed out of the pipeline. Therefore, it can be inferred that this peak velocity should be caused by the rapid discharge of the original gas in the pipeline.
In Phase II, the total mass flow rate and the flow velocity both showed obvious two-stage characteristics: In the Part 1 stage, the total mass flow rate of the extinguishing agent decreased gradually, while the flow velocity at the pipeline outlet decreased slowly; In the Part 2 stage, the total mass flow rate of the extinguishing agent decreased rapidly, while the flow velocity at the pipeline outlet increased rapidly. This indicated that there was a significant change in the state of the fluid in the pipeline between the Part 1 stage and the Part 2 stage, and it can well confirm the previous speculation about Part 1 and Part 2, that is: the Part 1 stage was mainly the concentrated release of the liquid extinguishing agents, and the Part 2 stage was the release of the residual liquid extinguishing agents in the pipeline carried by the compressed nitrogen after the release of the liquid extinguishing agent in the vessel was completed.
In Phase III, the outlet velocity decreased rapidly to zero, and the mass flow rate of the extinguishing agent gradually decreased from a small initial value to zero, which also confirms the inference in the previous section that this stage was a single-phase flow of the mixture of the compressed nitrogen and the gaseous extinguishing agent.
In addition, through the further analysis of Phase II shown in
Figure 8, it can be found that in the Part 1 stage, the flow velocity at the pipeline outlet decreased slowly, while the mass flow rate of the extinguishing agent decreased significantly, which indicated that the average nominal density of the extinguishing agent decreased continuously in this stage. However, according to the analysis in
Section 3.1.2, it can be seen that the compressed nitrogen in the vessel had not yet flowed out, which means that the decrease in average nominal density was mainly due to the continuous gasification of the extinguishing agent; In the Part 2 stage, since there was no liquid extinguishing agent flowed into the pipeline from the vessel, the residual liquid extinguishing agents in the pipeline carried by the compressed nitrogen were released in a gas–liquid two-phase mixed state. In this stage, due to the rapid decrease in the liquid extinguishing agent content in the pipeline, the flow velocity at the pipeline outlet increased rapidly.
Figure 9 shows the variation of the percentage of the gas mass to the total mass of the extinguishing agent released through the pipeline with the pipeline diameter. It can be seen from the figure that, with the increase in the pipeline diameter, the gasification ratio of the extinguishing agent released through the pipeline decreased at first and then increased. It can be fitted by a quadratic polynomial function, and the fitting result is shown in Equation (14). It can be concluded that the pipeline diameter had a significant impact on the gasification ratio of the extinguishing agent during the pipe flow. Therefore, in the design of the fire extinguishing system, the pipeline diameter should be reasonably matched according to the drive capability of the extinguishing bottle so as to reduce the gasification ratio of the extinguishing agent during the release.
where
represents the mass percentage of the vaporized extinguishing agent during the pipe flow (
); and
denotes the pipeline diameter (
).