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[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]

• The flammability limits and burning velocities of R1234yf mixtures were measured. • The relationship between flammability limit ranges and the n F /n H was analyzed. • The factors affecting the burning velocity were explored in the molecular structure. In response to the pressure of environmental protection, a series of environment-hazardous refrigerants will be phased out. And the selection of the substitutions for the future will need to balance thermodynamic property, safety and sustainability. Thus, the mixed refrigerants are receiving increasing attentions for their special performances. In this paper, the flammability characteristics of the commonly used mixtures R32/R1234yf and R152a/R1234yf were experimentally researched. The flammability limits of the two binary mixtures at different proportions of R1234yf were measured in a 12L spherical glass vessel. And the burning velocities of the two mixtures at different equivalence ratios were measured in a cylindrical quartz vertical tube. It can be found that the flammability limit curves of R32/R1234yf were different from that of R152a/R1234yf. The ratio of F atoms to H atoms (n F /n H) for the two mixtures had a close relationship with the flammability limit ranges. The maximum burning velocity for R32/R1234yf and R152a/R1234yf tended to decrease with the increase of the R1234yf proportion, and also decreased with the increasing of the n F /n H. But the n F /n H was not the only impact on the burning velocity. The number of C–H bonds and the molecular structure reflected in the heat of combustion would be correlated with the burning velocities of the mixtures. The results will provide a guidance for the development and safe application of R1234yf mixtures. [ABSTRACT FROM AUTHOR]

• The flammability limits of R290 and R600a were measured and analyzed in different temperature and humidity range. • The flammability limits of R290 and R600a showed linear temperature dependences. • A special case for the influence of humidity on the flammability limits of hydrocarbons was found, especially for UFLs. • Two effects with maximum and minimum were found for the UFLs of R290 and R600a. In this study, the flammability limits of R290 and R600a were measured in different environmental conditions. All the experiments were performed on a self-made experimental rig at temperature ranging from 5 to 55 °C and relative humidity of air ranging from 0 to 100%. The results showed that the temperature dependences for the flammability limits of both R290 and R600a can be predicted by linear model precisely. The humidity dependences for the flammability limits of R290 and R600a were distinctive, which was increased slowly for lower flammability limits (LFLs) but changeable for the upper flammability limits (UFLs). Meanwhile, it was found that the flammability limits of both R290 and R600a became sensitive with the relative humidity changing. A special case for the water vapor influencing on the flammability limits of hydrocarbons was found, especially for UFLs. It has maximum and minimum values in the experimental curve of UFLs when the relative humidity increased from 0 to 100%, which revealed that water vapor does not inhibit the combustion of hydrocarbons simply. [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]