Search

• The scientific law of burning velocities of working fluids was found. • The effect of flame retardants on combustion characteristics was acquired. • The relationship between LFL/UFL and burning velocities was researched. • The inhibition mechanism of retardants on combustion characteristics was obtained. Despite its good environmental properties, the application of 1,1-Difluoroethane (R152a) is hindered by its flammability. So, an accurate instrument is required to solve the safety issues of R152a. In this study, the combustion characteristics of R152a mixtures with non-flammable CF 3 I/C 3 F 6 were measured using the vertical tube method by ISO-817. It could be found that the maximum burning velocity (S u, max) of R152a decreased from 21.163 cm/s to 7.805 cm/s (with R13I1) or 7.675 cm/s (with R1216) at concentration of 15.403%(with R13I1) and 11.508%(with R1216) at an ambient temperature. By adding non-flammable retardants to R152a from volumetric concentration of 10% – 40%, the flammability of the mixtures was lowered. And S u, max of both were reduced by more than 60%, leading the mixture to reach A2L. According to the S u, max data, the dilution effect of the three refrigerants is: R1216 > R13I1 > R227ea. Meanwhile, the density functional analysis theory was used to study the relationship between the molecular bonds of these refrigerants. The results showed that the refrigerants containing higher F/H ratio and the molecules with -CF 3 and -I groups were more prone to inhibit combustion. [ABSTRACT FROM AUTHOR]

The inerting mechanism of the new flame retardants to flammable gases was investigated theoretically. All calculations were performed at the level of M06-2X/6-311 + G(d,p) using the density function method. Five different reaction pathways of traditional flame retardant group CF3 and 2,3,3,3-tetrafluoropropene (R1234yf) were calculated. Then, three novel flame retardant groups (BF2, BCl2 and BI2) for R1234yf were compared under the same reaction path. The results showed that the reaction energy barrier of the novel flame retardant groups was smaller than that of CF3, and the heat release was larger. It revealed that the flame retardant effect of the novel flame retardant groups is better than that of the traditional flame retardant group CF3. Furthermore, the deep discussions were made to the reconstruction of flame retardant molecules, which drew a material system map for the new flame retardant groups. Based on the analysis of environmental protection, flammability and thermophysical properties, new environment-friendly flame retardants suitable for different application fields can be obtained. [ABSTRACT FROM AUTHOR]

The flammability of refrigerants seriously affects the safe use of new generation environment-friendly refrigerants. Mixing inert gas or mildly flammable refrigerants with other flammable refrigerants has a specific inhibition effect, but the mechanism is still unclear. The burning velocity inhibition effect was theoretically studied by analyzing the influence of working fluids such as R1234yf and CO 2 on the burning velocity of R32 with and without water and analyzing the reaction pathways. Results have shown that a small amount (5% molar concentration) of weakly flammable R1234yf has a more obvious inhibition effect on the burning velocity of R32 than the same amount (5% molar concentration) of inert gas CO 2 ; when the mole fraction of the additives is large (50% mole concentration), the result is the opposite. The presence of water slightly increases the burning velocity, and the increase of the burning velocity is affected by the ratio of fluorine. Through the reaction path analysis, the reactive group CHF3 after thermal decomposition of R1234yf reacts with R32, which changes the reaction path of R32 directly, reduces the rapid reaction of R32, and reduces the initial burning velocity of combustion. [ABSTRACT FROM AUTHOR]

Current alternatives to R404A are either refrigerants with higher global warming potentials (GWP) from 1200 to 2000 or mildly flammable refrigerants with a charge limit of 1.2 kg. Therefore, nonflammable alternatives to R404A with lower GWP values need to be developed. In this study, a new non-flammable refrigerant blend, named RTJU-Z (GWP of 794), has been proposed to directly replace R404A in a cold storage system with no modification. The flammability and thermodynamic performance of RTJU-Z were studied experimentally. The results show that RTJU-Z couldn't be ignited successfully and had a higher coefficient of performance (COP) value than that of R404A by 2.15%-5.71% at different cold storage temperatures. The optimal charge of RTJU-Z for the experimental system was 11.76% lower than that of R404A. The refrigeration capacity of RTJU-Z was about the same as that of R404A at the cold storage temperature from -20 °C to 10 °C. It is concluded that RTJU-Z can be directly used as a non-flammable alternative to R404A in a cold storage system with no modification. [ABSTRACT FROM AUTHOR]

As the key part of the refrigeration systems, heat pump systems, and Organic Rankine Cycle (ORC) systems, most working fluids with low global warming potential (GWP) are flammable. This paper proposed a novel method called indirect inhibition to mitigate their flammability to enhance the applicability of low-GWP working fluids. Firstly, the effect of lubricants oil, indirect flame retardants (IFR), and mixtures of the two on the flammability of propane were investigated. The experimental results showed that lubricating oil promoted the combustion of propane to a certain extent. The introduction of indirect flame retardant (CP-52) weakened the combustion intensity of propane. The lubricating oil, in conjunction with a flame retardant, could inhibit the combustion of propane. Secondly, the effects of the indirect flame retardant and mineral oil on the flammability of propane were theoretically analyzed using density functional theory (DFT). The theoretical calculation results showed that the ability of IFR to capture reactive free radicals was comparable to that of traditional flame retardants. The research results are of great significance for improving the safety of the cycle system and reducing the risk of accidents. [ABSTRACT FROM AUTHOR]

[Display omitted] • The change laws of flammability limits with ignition energy for flammable working fluids are studied. • The influence laws of ignition energy on flame propagation are acquired. • The burning velocity in stable propagation stage is independent of ignition energy. • The influence mechanism of ignition energy on combustion characteristics is obtained. Working fluids with low global warming potential (GWP) will play an important role in the research and development of alternative working fluids for organic Rankine cycles and refrigeration cycles. However, the flammability of low GWP working fluid limits their development, and the energy of typical fire sources has a significant impact on their safe application. At present, there is a lack of relevant research on the effect of ignition energy on their flammability. In this paper, the effect of ignition energy on the combustion characteristics for propane (R290), fluoroethane (R161), 1,1-Difluoroethane (R152a), and 3,3,3-Trifluoropropene (R1243zf) was studied using a variable ignition energy system. The initial flame propagation process, flammability limit, and burning velocity were tested and analyzed in the range of 50 mJ to 20 J. The results showed that with the increase of ignition energy, the lower flammability limit (LFL) decreased and the upper flammability limit (UFL) increased, but their rate of change decreased with the increasing ignition energy. The effect of ignition energy on the flammability limit of R1243zf was significantly greater than that of the other three working fluids. In addition, the effect of ignition energy on flame propagation was mainly limited to the initial stage, and the burning velocity was independent of ignition energy. The mechanism of ignition energy on combustion characteristics was obtained. The research results have guiding significance for the safe application of flammable working fluids. [ABSTRACT FROM AUTHOR]

• Flammability characteristics of R1216 mixed with R32 and R152a were investigated. • The suppression effect of R1216, R134a, R1233zd(E), and R13I1 was compared. • The flame inhibition mechanism of R1216 was analyzed. R1216 (1,1,2,3,3,3-hexafluoro-1-propene), which has a low global warming potential, is regarded as the new generation of environmentally friendly working fluid in ORCs (organic Rankine cycle) and refrigeration systems. Besides, it also has the advantage of nonflammable property. Thus, R1216 can be mixed with some flammable working fluids to develop new mixed alternatives, which can possess a combination of chemical, environmental, thermodynamic, and safety properties. In this work, the flammability limits of R1216/R32 and R1216/R152a were measured in a 12L spherical glass vessel according to the method of ASTM E681. And the critical flammable volumetric ratios of R1216 on R32 and R152a were about 0.15 and 1.1, respectively. This study also found that the ratio of F atoms to H atoms for the two mixtures had a close relationship with the flammability limit ranges. Finally, the flame inhibition performances of R1216 were compared with other commonly used flame retardants, such as R134a, R13I1 (trifluoroiodomethane), and R1233zd(E) (trans -1-Chloro-3,3,3-trifluoro-propene). It can be concluded that the rank of the inhibition effects was R1216 > R13I1 > R134a > R1233zd(E). And a flame retardant, which contained a higher F/H ratio and much more Cl, I, and CF 3 radicals, can possess a higher flammability inhibition efficiency. [ABSTRACT FROM AUTHOR]

• Flammability limits of azeotropic mixtures consist of R1243zf are tested for the first time. • A brief mechanism for flame-retardant effect of R13I1, R227ea and R134a is analyzed. • CSCs of R1243zf mixtures are tested and compared with calculation results by 3 methods. 3,3,3-trifluoropropene (R1243zf) is a low climate change effect refrigerant that has great development potential. However, R1243zf is flammable and its safety classification is A2 as per ISO standard 817. To reduce its risk of flammability, the method of using mixed refrigerants in practical applications is an effective approach. In this paper, an experimental system was established according to the ASHRAE standard 34 to measure the flammability limits of four azeotropic mixtures containing R1243zf. First, the critical suppression ratio (CSR) of R13I1/R1243zf was tested out that is 0.353. Further measurements were made to determine the lower flammability limits (LFLs) and critical suppression concentrations (CSCs) of three ternary mixtures consisted of R1243zf, thermodynamic performance enhancer (RE170) and flame retardants (R13I1, R134a or R227ea), at various compositions. The experimental non-flammable proportions of R1243zf/RE170/R13I1, R1243zf/RE170/R134a, and R1243zf/RE170/R227ea were 31.81 %, 5.49 %, and 22.42 %, respectively. Besides, a comparison was made between the experimental data and the predicted results by means of three different theoretical methods (the equal effect method, thermal balance method, and group contribution method). The fresh experimental results will provide practical guidance for the applications that utilize the R1243zf blends in low flammability risk. [ABSTRACT FROM AUTHOR]

[Display omitted] • Flammability limits of the new binary mixture R1234ze(E)/R32 was measured. • Oxidative decomposition and interaction mechanisms of R234ze(E)/R32 were studied. • R32 decomposes preferentially and R1234ze(E) is more likely to react with radicals. • CF 3 plays an important role in R1234ze(E) inhibiting flammability of R32. At present, mixed working fluids have received increasing attention due to their excellent comprehensive performance. Among them, R1234ze(E) (trans -1,3,3,3-tetrafluoropropene) and the R32 (difluoromethane) with excellent thermal properties can form a safe and environmentally friendly new mixture through advantageous complementarity. In this paper, the flammability of the binary mixture R1234ze(E)/R32 mentioned above was studied. Firstly, the flammability limits and critical concentration inhibition ratio of R1234ze(E)/R32 were tested at different ratios. Then, based on density functional theory (DFT), the interaction mechanism between the two components in the mixture was analyzed. The results show that with the increase of R1234ze(E), the flammable limit range becomes narrower. In the early stage of the reaction between R1234ze(E)/R32, R32 preferentially breaks the bond to generate H and F radicals, but the reactions between R1234ze(E) and free radicals are dominant. CF 3 radical is an important free radical for R1234ze(E) to inhibit the flammability of R32. The research results will have guiding significance for the internal interaction mechanism of the flammability of new binary mixtures and their safe applications. [ABSTRACT FROM AUTHOR]