| Nowadays,global warming is becoming more and more serious,which leads to a new wave of alternative refrigerants.HCFCs and HFCs with high Global Warming Potential(GWP)will be eliminated.New generation refrigerants mainly include HFOs(such as R1234yf,R1234ze(E),etc.),natural refrigerants(R290,R600a,etc.)and mixed refrigerants with low GWP,etc.However,most of these alternative refrigerants are flammable or explosive,and some of the deep mechanisms of combustion processes were not clear.The scientific research foundation of flammable refrigerants in the field of combustion characteristics is seriously insufficient.The researches on the safety characteristics of low GWP flammable refrigerants have been a hot scientific issue in the field of refrigeration science.In this paper,hydrofluoroolefins(R1234yf and R1234ze(E)),hydrocarbons(R290 and R600a)and NH3 are taken as the research objects.Theoretical analysis,simulation calculation and experimental testing were carried out on the micro and macro characteristic parameters of the combustion processes and the action mechanism of the multi-factor environment,the following results are obtained:(1)According to the combustion theory and engineering thermodynamics,the characteristic laws of combustion process of flammable refrigerants were discussed,and the numerical calculation models of flammability limits were established based on group contribution.The basic theory of combustion,micro and macro parameters during combustion and flammability limit theory were analyzed.The theoretical research system of combustion of flammable refrigerants was completed.A new comprehensive evaluation method was proposed to evaluated the safe concentration of refrigerants,which took R290 as an example.(2)A high-precision experimental system was used to explore the combustion characteristic experimental system with variable environmental parameters.Two kinds of environmental friendly combustible refrigerants hydrofluoroolefins(R1234yf and R1234ze(E))and hydrocarbons(R290 and R600a)were taken as research objects.The flammability limits were measured at different temperatures(5-55℃)and relative humidity(0-100%RH).The results showed that the temperature is linearly related to the lower and upper flammability limits for the testing flammable refrigerants.The higher the temperature is,the more flammable the refrigerant is.The effects of humidity on flammability limits were particular which is related to the content of H atom in combustible molecules.For hydrofluoroolefins,humidity promotes combustion,the lower flammability limit decreases and the upper flammability limit increases with the increasing of humidity.For hydrocarbons,there are both promotion and inhibition effects.In different humidity ranges,the change rules of flammability limits are different.For nonflammble refrigerants,they could be stimulated to ignite under certain humidity conditions.(3)The combustion microscopic reaction pathways of hydrofluoroolefins(R1234yf and R1234ze(E))and NH3 were simulated by density functional theory at basis set of M06-2X/6-311+G(d,p).The possible pathways of reaction processes and the molecular configuration,energy and thermodynamic parameters of substances(reactants,intermediate,transition states and products)involved in the reaction were obtained.The micro mechanism of combustion of flammable refrigerants were revealed.(4)An experimental system for the characteristics of combustion products of combustible working medium was designed and built with ball burner and intermediate pressure relief under the modificatory ASTM E 681 and ASHRAE 34 standards.The concentration of combustion products of R1234ze(E)at different concentrations and R1234yf at different humidity were measured.The change rule of combustion products characteristics was obtained.Combined with the above the results of micro path analysis of combustion processes,the combustion models of R1234yf and R1234ze(E)were established respectively. |
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