Flammability And Thermal Performances Research Of Mixtures Used In Organic Rankine Cycle For Engine Waste Heat Recovery

Internal combustion engines,which are the main fuel-consuming equipments,when it runs,a large amount of energy is emitted into the environment in the form of waste heat.Therefore,it is of great significance to recycle its waste heat.Exhaust gas waste heat is the highest grade and the most part.Accordingly,engine efficiency could be greatly improved if such waste heat could be reused,as well as the economic and environmental benefit.Organic Rankine Cycle is an efficient approach for engine waste heat recovery.But,the recovery effect is highly dependent on the working fluid.Besides,it is very hard to find out a kind of pure working medium,efficient,environmental pretection,safe and suitable to exchange heat with engine exhaust.Therefore,this study intends to put forward some kinds of high temperature mixtures based on CO₂ and small molecule hydrocarbons,efficient,environmental protection and safe,through conducting flammability and thermal performances researches on them.This paper established flammability limit prediction models against hydrocarbon/CO₂ mixtures on the basis of threshold flame temperature theory to analyze the flammability of these kinds of mixtures with different volume fraction of CO₂.Validation of these models has been conducted on existing experiment data.The mixture ratio was determined according to related safety standards.A certain type of Yuchai diesel engine was chose as the object,based on the first and second law of the thermodynamic analysis,a transcritical organic Rankine cycle waste heat recovery system was built to analyze their thermal performances.The model was validated by previous work.Combined with energy and exergy evaluating indicator,the influences of mass fraction of CO₂ and the system operation parameters on mixtures were investigated.The optimal mixture ratios of different mixtures under different system operation parameters were explored as well.Results show that,for small molecule hydrocarbon/CO₂ mixtures,the relative deviations between predicted upper flammability limit results and experimental data are less than 5.6% and those for lower flammability limit are less than 2.4%.The model developed in this paper turned out to be accurate and simple which could be used for practical applications.Four series of mixtures,propane/CO₂,n-butane/CO₂,isobutane/CO₂ and pentane/CO₂,were proposed according to the results of flammability analysis and relative safety standards.Mixtures possess higher safety while the mass fraction of CO₂ in each mixture is no less than 65%.It was also found that thermal performances of mixtures decreased with the increase of CO₂ mass fraction under low evaporation pressures.The net power output reached its maximum value for each mixture while the turbine inlet temperature is around 550 K.And the maximum net power outputs are more than 13.5kW for each mixture when the mass fraction of CO₂ is 65%.In the meantime,the thermal efficiency of each mixture is higher than 16%.It proved that the thermal performances of these kinds of mixtures are excellent.Furthermore,for obtaining equal net power output,transcritical cycle using these mixtures as working fluid could decrease the evaporation and condensation pressures 2MPa or over compared with using CO₂.Especially,the utilization effect of mixtures to low-middle temperature heat source is comparable to CO₂ to middle-high temperature source.Moreover,it is a potential method to solve the condensation problem of CO₂ by using mixtures.In general,conducting flammability and thermal performances research on these kinds of mixtures could promote the research of efficient,environmental protection and safe high temperature working fluid.