The Prediction Theory,Numerical Method and Experiment of Flammability Limits for Gases at High Temperatures and Pressures

Author:Wan Zuo

Supervisor:zhang qi

Database:Doctor

Degree Year:2017

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Pages:149

Size:1744K

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Flammable gas fires and explosion accidents occur frequently.The flammability limit is an important parameter reflecting the risk of gas explosion,with the value affected by temperature,pressure,inert gas,as well as other factors.In the industrial production process,the unit operation is often in environments with high temperature and pressure;therefore,it is difficult to determine the flammability limits with poor experimental security and fewer results.In particular,there has been no significant progress in the theoretical prediction for a long time,and a perfect theory and data accumulation of the system are lacking.According to the theoretical prediction,numerical method and experiment of high temperature and high pressure gas flammability limit,the main innovative achievements are as follows:Based on the adiabatic flame temperature,a prediction model of combustible gas flammability limit under high temperature and high pressure was established.The numerical prediction of the flammability limit of combustible gas under high temperature and high pressure was realized by determining the temperature of the limit adiabatic flame,the thermodynamic conservation of the combustion reaction and the calculation of the minimum free energy.Simultaneously,we obtained the flammability limits of combustible gases in different atmospheres under different temperaures and pressures,such as air,oxygen,multicomponent,inert gas,etc.The numerical prediction agreed well with the experimental results,proving that the prediction theory and numerical method are both correct and reliable.The calculation model of thermodynamic properties in the prediction model of high temperature and high pressure explosion was established.The calculated thermodynamic parameters such as absolute enthalpy,absolute entropy,enthalpy of formation and constant heat capacity were consistent with the experimental data,proving that the model was reliable.The numerical results of gas thermodynamic parameters at high temperature and high pressure show that the lower the initial temperature,the greater the effect of the pressure on the thermodynamic parameters of the gas;furthermore,as the temperature increases,the difference in thermodynamic parameters between the varying pressures becomes smaller.It is primarily owing to dissimilarities in the forces and volumetric effects of molecules at different temperatures and pressures.Taking hydrocarbons and oxygen-containing fuels as the object in this study,the variation of the lower limit of combustible gas explosion with temperature was predicted numerically based on the independence of adiabatic flame temperature and temperature.The reliability of the lower flammability limit(LFL)prediction theory at high temperature and numerical results were verified through the experiment and comparison.Though the decreasing trend is closely related to the fuel type,the results illustrate that the LFL of single-component combustible gas decreased with the increase of temperature.With the number of carbon atoms increasing,the decreasing trend of LFL was slower.When the number of carbon atoms was the same,the decreasing rate of LFL was slower when the content of hydrogen and oxygen in the oxygen-containing fuel was lower.For the high temperature multicomponent system,the LFL showed a linear decrease with the increase of temperature,and when the proportion of substances with relatively large LFL was high,the LFL decreased more quickly.For fuel-oxygen systems,the increase of temperature had a greater effect on combustible gases in oxygen than in air,and the LFL decreased faster,indicating a higher risk of explosion.For the system containing the inert component N2,the change in the composition of the N2at high temperatures had little effect on the LFL of the mixture.It is pointed out that the adiabatic flame temperature in the upper flammability limit(UFL)at high temperatures should consider the important point of concentration.Based on the concept of adiabatic flame temperature at stoichiometric ratio,a numerical calculating method about the UFL adiabatic flame temperature at various temperatures was established and the upper limit adiabatic flame temperatures of C1-C4 alkanes,C2-C3 olefins,methanol and ethanol in the temperature range of 298K-473K were obtained.The result revealed the variation law of UFL adiabatic flame temperature with temperature,and it is an important foundation for the prediction of the UFL of gas at high temperatures.The changes of UFL of typical alkanes,olefins and alcohols with temperature were studied based on the adiabatic flame temperature and the explosion limit prediction model.The results show that the relative error between the predicted value of the UFL and the experimental value was only 0.84%,and an accurate prediction of the UFL of the high temperature gas was realized.For the high temperature multicomponent system,the UFL linearly increases with temperature.When the proportion of substances with relatively large UFL was higher,the trend of the increasing UFL was more pronounced.For the system containing the inert component N2,UFL decreased faster with the increase of N2content and a higher temperature.When N2content was increased,the UFL slowly increased with the increase of temperature,indicating that the influence of temperature was insignificant.Based on the discussion of the correlation between the UFL adiabatic flame temperature and pressure,the numerical models of the UFL adiabatic flame temperature of the alkane and olefins with pressure were established when the pressure was less than 20bar.Furthermore,the UFL adiabatic flame temperatures of C1-C4 alkanes and C2-C3olefins at varied pressures were obtained,which became the key link in the prediction of the UFL under high pressure.Based on the adiabatic flame temperature,we explored the change rule of typical alkane and olefin UFL with pressure using the explosion limit numerical model.The results show that the average relative error between the predicted value and the experimental value of the UFL was 3.64%,which accomplished the accurate prediction of the high pressure gas UFL.For the high pressure multicomponent system,the UFL increased with the increase of pressure.When the proportion of substances with relatively large UFL was smaller,the trend of UFL increasing was more pronounced.For the system containing the inert component N2,with the increase of N2content,the higher the pressure,the faster the decrease of UFL.When N2content was increased,the UFL slowly increased with the increase of pressure,indicating that the influence of pressure was insignificant.Taking hydrocarbons and oxygen-containing fuels as the object,the method of determining the LFL adiabatic flame temperature under high temperature and high pressure was established.Based on the flammability limit theory model,the LFL of gas under high temperature and high pressure coupling conditions was predicted,and the changing law of the LFL with temperature and pressure in a variety of environmental atmospheres was explored.The results show that the prediction of the fuel LFL in the air and oxygen was consistent with the experimental results in this paper,demonstrating that the numerical model is reliable.At high temperatures,with the increase of pressure,the descending rate of gas LFL was evidently higher than juxtaposed to that of the low temperature,but the decreasing trend is closely related to the fuel type.With the number of carbon atoms increasing,the decreasing trend of LFL was slower.When the number of carbon atoms was same,the higher the content of oxygen in the oxygen-containing fuel,the faster the decreasing rate of LFL.For the multicomponent mixing system,the LFL decreased with increasing pressure,and the proportion of the large LFL gas was higher;the LFL of the mixture increased regardless of the pressure at high temperatures.For fuel-oxygen systems,the increase of temperature and pressure had a greater effect on combustible gases in oxygen than in air,and the LFL decreased faster,indicating a higher risk of explosion.For systems containing the inert components N2,the addition of N2had little effect on the LFL of the mixture within the range of N2proportion at high temperatures,regardless of the pressure.