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The issue of global warming prompts us to consider some questions related to energy supply and use. Organic Rankine Cycles (ORC) is a suitable technology for converting low-grade thermal energy into electrical energy, vital for conserving energy and mitigating emissions. Currently, most environmentally friendly working fluids are characterized by their flammability. Analyzing the minimum ignition energy (MIE) of these working fluids is crucial in guaranteeing secure utilization. This paper introduced a novel approach to predicting the MIE of flammable working fluids using microscopic molecular structure analysis. Firstly, the bond energy of molecules was derived by optimizing the microscopic molecular structure of prevalent combustible agents based on the density functional theory (DFT), which was employed to calculate the heat of combustion. Secondly, a mathematical model for predicting the MIE of combustible working fluids was constructed, drawing inspiration from the energy relationship during combustion. Then, the newly formulated model was used to predict the MIE of propane and nine other combustible working fluids, and the results were compared with the existing data. The results show that the predicted values of the MIE exhibit an impressive accuracy rate exceeding 95%. Finally, the MIE prediction model has been modified to provide higher prediction accuracy. This study is of great significance to ensure the safe application of combustible working fluids and to explore environmentally friendly alternatives. [Display omitted] • A new method for predicting the MIE based on molecular structure was proposed. • The molecular structure was optimized based on density functional theory. • The mathematical model has a prediction accuracy of more than 95 %. • The model can predict the ignition energy of substances outside the database. [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]

Organic Rankine cycle (ORC) is an effective way to utilize low-grade energy. 2,3,3,3-tetrafluoropropene (R1234yf) has attracted extensive attention due to its good environmental protection performance and is considered as an alternative working fluid in ORC. However, its thermodynamic, environmental and safety characteristics need to be fully considered. In this paper, the combustion mechanism and product characteristics of R1234yf were investigated by ReaxFF molecular dynamics simulations. The reaction temperature and equivalence ratio have important effects on the combustion of R1234yf. The early stage of the combustion process is mainly the unimolecular decomposition reaction of R1234yf, and then O 2 participates in the reaction, the consumption rate of R1234yf increases rapidly, the radicals O, H, OH, CF 3 , CF 2 , and CFO play important roles in the combustion reaction. The main combustion products include CO, CO 2 , HF, and COF 2. In addition, the combustion product concentration and yield of R1234yf were tested by an improved method. The results showed that the change law of combustion product yield with equivalence ratio was in good agreement with the simulation results, and the risk of high CO and HF concentrations still exists. The research results have practical significance for the safe application of R1234yf in ORC system. • The combustion mechanism of R1234yf was studied by ReaxFF-MD simulations. • The reaction conditions have important effects on the combustion of R1234yf. • The concentration and yield of combustion products were measured and analyzed. • The variation law of combustion products of R1234yf was obtained. [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]

2-BTP (2-Bromo-3,3,3-trifluoropropene) is a promising environment-friendly flame retardant showing high effectiveness. Because of the good mutual solubility with polyol ester oil, 2-BTP was selected as a lubricant additive in this work. The flame inhibition performance of lubricating oil with 2-BTP on the combustion of R152a (1,1-difluoroethane) and R1234yf (2,3,3,3-tetrafluoroprop-1-ene) was investigated, which was expected to enhance the safety level of accidental leaks of flammable working fluids. To assess the suppression capability, experiments were conducted using a 12 L spherical flask according to the test method of ASTM E681. The results indicated that the lower flammable limit of R152a was increased from 4.8% to 5.0% in volumetric concentration as the mass fraction of 2-BTP contained in the POE oil was 20%. As to R1234yf, the lower flammable limit was increased from 7.2% to 10.3%, and the upper flammable limit was decreased from 13.0% to 10.3%. Besides, with the addition of 2-BTP, the combustion intensity of R152a and R1234yf was weakened. Both of the flame diameter and duration were reduced. And the maximum flame transient temperature of R152a also had a decrease. The inhibition effect of lubricating oil with 2-BTP on the mildly flammable working fluid R1234yf was much better. [ABSTRACT FROM AUTHOR]

• Initial oxidative decomposition path and energy change of fluoroethane are obtained. • Energy changes of reactions of six active radicals with fluoroethane are compared. • Fluoroethane generates more than 20 products through two main radicals ·CH3 and ·CH2F. • Intermediates are easy to combine to form products rather than continue to decompose. Fluoroethane is a potential fluid or component of new environmentally friendly working fluids, but its flammability affects its direct application. The oxidative decomposition mechanism of the working fluid is the key to further study its flammability or stability. Therefore, the oxidative decomposition mechanism of fluoroethane was researched by density functional theory. Firstly, seven initial thermal decomposition paths of fluoroethane and four oxidative decomposition paths involving oxygen were obtained, among which the main initial decomposition products were CH 2 =CH 2 and HF, and the free radicals were ·CH 3 , ·CH 2 F, H·, HOO·, etc.; Secondly, the main reaction process of six typical active radicals generated by the chain reaction and fluoroethane was also studied, and it was found that the energy barrier of the hydrogen-extraction reaction was lower than that of the fluorine-extraction reaction, and most of the reactions were exothermic reactions; Finally, the subsequent reaction processes of the main radicals ·CH 3 and ·CH 2 F in the decomposition of fluoroethane were analyzed, and more than 20 possible products and their energy changes were obtained. The relevant results have guiding significance for weakening the flammability of fluoroethane, improving its thermal stability and evaluating its oxidative decomposition products from the deep mechanism. [ABSTRACT FROM AUTHOR]

CO 2 /C 3 H 6 mixture has a wide range of applications in refrigeration systems, heat pumps and Organic Rankine Cycle (ORC) systems, but its flammability safety needs to be considered. An experimental and computational investigation has been conducted on the determination of the flammability limits of CO 2 /C 3 H 6 and on the understanding of associated limit phenomena in general. We tested the flammable zone of CO 2 /C 3 H 6 at variable initial temperature. The accuracy of the prediction model of calculation adiabatic flame temperature were verified based on the experimental data. Meanwhile, a high-speed camera was used to record the process of CO 2 suppression of flame propagation near flammability limit. Furthermore, based on kinetic numerical calculation, the effect of CO 2 chemical and thermal on the flammability limit of C 3 H 6 were analyzed. The results show that the thermal effect of CO 2 is greater than that of chemical effect near the flammability limit. The theory of chemical reactivity inhibition reduces the yield and molar ratio of key radicals. The addition of CO 2 can inhibit the activity of the dominant chain and increase the activity of the dominant chain termination reaction in the global combustion kinetics. Results of this study provide theoretical guidance for the safe application of CO 2 /C 3 H 6 mixture. • The higher the ambient temperature, the larger the CO 2 /C 3 H 6 flammable zone is. • Validation of the accuracy of calculated adiabatic flame models. • Introducing innovative computational strategies to clarify CO 2 suppression theory. • The thermal inhibition effect of CO 2 is more significant than the chemical effect. [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]