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• 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 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]

Hydrogen enriched with compressed natural gas is an efficient and environment-friendly gaseous fuel. However, the safety issues of mixture and the method to control or weaken their combustion are highly concerned. To explore the inhibition effect of halogenated fire suppressants on the mixture, the effect of HFC-227ea on the laminar premixed methane/air flames, with different fractions of H 2 , have been studied. Burning velocities have been measured with constant-volume combustion chamber and kinetically modelled a recently assembled kinetic mechanism. The fractions of H 2 influence the enhancement and inhibition effect of HFC-227ea, and it is less effective with the lean mixture. In stoichiometric condition, HFC-227ea showed good inhibition effect on the mixture flames. The HFC-227ea increased the burning velocities of CH 4 -0% H 2 -air and CH 4 -10% H 2 -air flames at leanest condition, whereas the increased burning velocity arising from HFC-227ea not occurred as the addition of H 2 above 20%. Experimental results coincided well with numerical results, however the agreement was poor for the leanest flames at low agent loading. Lastly, kinetic mechanism analysis was used to interpret the combustion enhancement and inhibition effect of hydrogen-doped methane flame by HFC-227ea. • Effect of HFC-227ea on methane flames with different fractions of H 2 were studied. • The burning velocity and kinetic mechanism were obtained and analyzed. • The fractions of H2 influence the enhancement and inhibition effect of HFC-227ea. [ABSTRACT FROM AUTHOR]

The burning velocity plays an important role in determining the safety level of new refrigerants. Based on the self-designed test system for burning velocity characteristics of flammable refrigerants, the burning velocities of R152a with different concentration were studied by vertical glass tube method. And the maximum burning velocity of R152a was analyzed, which was close to the standard recommended value. R1234yf with environmental advantages and lower flammability may form a new alternative refrigerant mixture with R152a. Therefore, the flame propagation and burning velocity characteristics of R152a+R1234yf were studied under the conditions of different component ratio and different concentration. Based on the theoretical analysis, three calculation methods for predicting the burning velocity of binary mixed flammable refrigerant were proposed for the first time. Then, three groups of predicted burning velocity of R152a+R1234yf under different composition or concentrations were calculated. Compared with the experimental data, it was found that the prediction accuracy of the component equivalent simplified method is the highest among the three methods. It was of great significance for the prediction of burning velocity and the assessment of safety level of new mixed refrigerant or flammable gas. [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]

• 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 oxidation process of R152a/R1234ze(E) mixture had been first studied by the synchrotron vacuum ultraviolet photoionization mass spectrometry. • The laminar burning velocities and flammability limits of R152a/R1234ze(E) were measured by a constant-volume combustion bomb. • Species of CH 2 :CHF, CH 3 -CHF 2 , CHFCHCF 3 , CHF:CHF, CH 2 :CF, CF 2 :O, C 2 H 2 , and CH 4 were identified in R152a/R1234ze(E) oxidation. • An improved flame retarding model for R1234ze(E) was developed. • The flame retarding mechanism of R1234ze(E) on R152a was revealed in terms of oxidation products, reaction pathways and flame characteristic parameters. R1234ze(E) (1,3,3,3-tetrafluoro-1-propene), with low global warming potential (GWP), is considered as environmentally friendly refrigerant for refrigeration and heat pump systems. Although R1234ze(E) is classified as weakly flammable refrigerant, it is non-flammable under low humidity conditions. Therefore, R1234ze(E) can be a flame retardant in blended refrigerants. This study analyzed the oxidation process of the mixture of R1234ze(E) and flammable R152a via the Synchrotron Vacuum Ultraviolet Photoionization Mass Spectrometry (SVUV-PIMS). The oxidation species of CH 2 :CHF, CH 3 -CHF 2 , CHFCHCF 3 , CHF:CHF, CH 2 :CF, CF 2 :O, C 2 H 2 , and CH 4 were identified by experiments. Besides, the laminar burning velocities and flammability limits of R152a/R1234ze(E) were measured on a constant volume combustion bomb. Based on these experimental results, a new kinetic model was developed to describe the inhibition of R1234ze(E) on the combustion process of R152a. In order to analyze the flame retarding effect of R1234ze(E), sensitivity analysis of key groups, generation and consumption paths of major components, distribution of important products and free radicals, and the adiabatic flame temperature were revealed. It was found that the addition of R1234ze(E) introduced new inhibitory pathways and suppressed the reactions between R152a and H, O and OH radicals. Furthermore, R1234ze(E) reduced the concentrations of flammable radicals H, O, and OH in freely propagating flames, while it increased the number of flame retarding radicals CF 2 and CF 3. The adiabatic temperature of R1234ze(E)/R152a decreased with the increasing proportion of R1234ze(E) as well. This study can provide valuable reference for the application of R1234ze(E) in the field of new flame retardants. [ABSTRACT FROM AUTHOR]