Experimental And Theoretical Study On Explosive Properties Of HC/CO₂ Mixtures Under High Temperature

The waste heat energy represented by industrial production waste heat and internal combustion engine waste heat has a large quantity and high quality,which has great recovery potential.ORC can utilize the waste heat effectively,and the working fluid influence deeply on the recovery efficiency of waste heat.The traditional working fluid has the problem of high-temperature decomposition and environmental damage.Hydrocarbons owns high decomposition temperature,good cycling performance but flammable.CO₂ matches well with heat sources,which is conducive to the miniaturization of the system.However,its cycle performance is not as good as that of hydrocarbons.The mixing of HC/CO₂ can combine their advantages to achieve a balance of recycling performance,safety and environmental protection.The presence of CO₂ weakens the combustibility of hydrocarbons,but it cannot guarantee the complete non-combustibility.Therefore,it is necessary to study the flammability of the HC/CO₂ mixtures to guide the safe operation of the system.Existing studies lack the flammability limit data of HC/CO₂mixtures under high temperature.Therefore,this paper enriched the flammability limit data of HC/CO₂ mixtures in the high temperature condition through the flammability experiment and the establishment of theoretical prediction models.The flammability limit of C₃H₈,n C₄H₁₀,i C₄H₁₀ and n C₅H₁₂ mixed with CO₂ in different proportions at 90℃was tested through the experiment.when the temperature increased,the flame at the upper flammability limits was brighter than that at the lower limit.When the number of carbon atoms increased,there was no significant difference in the flame at both upper and lower flammability limits.The upper and lower flammability limit increase and decrease with the increase of temperature,respectively.When CO₂ dilution is constant,it is proved by theoretical analysis that the flammability limit changes linearly with the temperature.Through the analysis and of the parameters of the flammable region,it is shown that the expansion of the flammable region caused by the temperature rise and the inhibition of the flammability caused by the addition of carbon dioxide quantificational.When the dilution reached 90%,the inerting effect was isobutane>n-pentane>n-butane>propane.The theoretical analysis was carried out by CHEMKIN.When the temperature increased from 30℃to 90℃,the product species and quantity did not change significantly.When the number of carbon atoms increased,the number of CO and H₂of n-pentane was more than that of n-butane,while CO₂ and H₂O were less.With the increase of CO₂dilution,the species and quantity of products at the upper and lower flammability limits were obviously different,and the flame phenomenon was well explained by kinetic analysis.Based on the advantages and rules of existing theoretical thermal models,a model was established to predict the flammability limit at high temperature and verified by experimental data at 90℃.The average relative deviations of the upper and lower flammability limits were 3.78%and 1.95%,respectively.A CFD model was established to simulate the combustion process of flammability test.CFD and chemical dynamics skeleton mechanism were combined to simulate the flame propagation in the combustion process.The rules of simulated flame and real flame are compared and analyzed,the temperature basis used for simulating flame is explained,and the 90°visual criterion is applied to the judgment of flammability limit in the model.The model is used to predict the flammability limit of propane-carbon dioxide mixture under high temperature.Compared with the existing model and experimental values,the accuracy of the model is verified.The application of chemical kinetics mechanism makes the prediction effect of this model outstanding when the dilution is 90%.This conclusion provides guidance for the study of the upper flammability limit.