Explosive Properties Research Of Flammable Mixtures Used In Organic Rankine Cycle

Organic Rankine Cycle is one of the effective ways of waste heat recovery,moreover,its efficiency is highly depends on the physical properties of working fluid.Alkanes are natural refrigerants with good cycling performance,but their flammable properties limit their practical application.Carbon dioxide is non-flammable,and matches well with heat sources.However,the cycle thermal efficiency of carbon dioxide is lower than that of alkanes,and the operating pressure is relatively higher.Therefore,mixtures of alkanes and carbon dioxide could be selected for a promising performance.However,the mixtures are still flammable.Once the mixtures leak,there would be a risk of combustion and explosion.It is necessary to research the flammability limit of alkane-carbon dioxide mixtures to provide safety guidance.As the working temperature range in Organic Rankine Cycle is 25 °C-222.5 °C,it is necessary to obtain the flammability limitation at high temperatures through experiments and prediction methods.Meanwhile,theoretical prediction models for mixtures of carbon dioxide and alkane with complex molecular structure are necessary to enrich the flammability limit data.The flammability limitations were measured on the test bench based on ASTM E681-09 standard.It was observed that the flame phenomena did not change lot with the initial reaction temperature,however,the flame phenomena of the upper and lower flammable limits were obviously different.The experimental data show that the lower flammable limit decreases and the upper flammable limit increases with the increase of initial temperature,and the trend of the upper flammable limit is obviously larger than that of lower flammable limit.When the dilute concentration is constant,the flammability limit of the mixture varies linearly with temperature.Finally,the safety of paraffin-carbon dioxide mixtures is classified.When the dilution is not greater than FIP point dilution,they belong to A3,and when the dilution is greater than FIP point concentration,they belong to A1.Based on the theory of critical flame temperature,a prediction model of alkane-carbon dioxide mixture at variable temperature was established.Firstly,Chemkin-based analysis shows that the composition of combustion products has few changes in the temperature range of 30 °C to 70 °C.Then,based on the mass and energy conservation equations,the relationship between flammability limit and adiabatic flame temperature is established.It is assumed that the calculated adiabatic flame temperature does not vary with the initial reaction temperature,and the linear relationship between the calculated adiabatic flame temperature and the dilution is introduced to predict the flammability limit of the mixture of alkane and carbon dioxide.Finally,the theoretical analysis model has good accuracy in the temperature range of 25 °C-222.5 °C,which verifies the reliability of the prediction model.Based on group contribution method,a prediction model of hydrocarbon-inert gas mixture was established.The MG group contribution method was used to establish the prediction model of the flammability limit of pure hydrocarbons.The dilution and contribution factors of inert gases were introduced to modify the model to predict the flammability limit of mixtures.At the same time,the uncertainty of group contribution prediction model was analyzed and 95 % confidence interval was estimated.In practical application,the lower limit value of the lower flammability limit confidence interval and the upper limit value of the upper flammability limit confidence interval could be used to make a conservative prediction.The results indicated that the study of flammability limits of alkane-carbon dioxide mixtures could provide a safe guidance for the actual cycles.