Explosive Properties Research Of Flammable Mixtures Used In Organic Rankine Cycle

Internal combustion engine,as the main oil consumption equipment,has a thermal efficiency of about 40%,which means that most of energies came from oil scatter and disappear in environment in the form of heat.If the waste heat were retrieved,then the thermal efficiency of internal combustion engine could be improved highly.The temperature of exhaust gas is high and changes with the load of engine.The decomposition temperature of the traditional refrigerants such as R123and R134 are less than the temperature of exhaust gas.And the matching between the traditional refrigerant and waste heat from engine is bad.Investigation reveals that the alkane has a high decomposition temperature and excellent thermodynamics in ORC,however the flammability limit the use of alkane.CO₂ working medium with the fast response characteristics for variable working condition has some outstanding advantages on miniaturization for the ORC equipment.But the thermal efficiency of CO₂ in waste heat recovery is lower than alkane under same condition.Based on the investigations on alkane and CO₂,a new working medium,alkane-CO₂ mixture,is put forward to integrate the advantages of CO₂ and alkane.However,aim at the new working fluid,the flammable research is lack so far.From the viewpoint of safety,it is necessary to carry out study the flammability limits of alkane-CO₂ mixture from both experiment and theory.An experimental system based on ASTM E681 is designed to measure the flammability limits of mixture.Aim at the extinguishing problems occurred in glass tube vessel,the 12 L glass flask is used to avoid the problems.The experimental setup is divided into two big systems,gas distribution system and explosion system,which consist of assembly units and subsystems such as valves and temperature control system.In the design of the experiment,an innovative tactic about gas distribution is proposed to improve the precision.The flammable zones of five mixtures at 30?and the flammability limits of five alkanes under different temperature are measured carefully in this paper.Theoretical models to evaluate the flammable zones of mixtures made up of hydrocarbon,carbon dioxide and air have been proposed in present study.This high accuracy model mainly deal with the calculation of heat absorption at UFL.The calculated results indicate that there is a linear relationship at upper flammability limits between molar fraction of diluent in alkane-CO₂ mixture and calculated adiabatic flame temperature.However,the existing models to predicted flammability limits of alkane-CO₂ mixtures always use the FAFT.This may be the cause that leads to large error at UFL.The prediction ability of the VAFT model that incorporated the linear relationship above is greatly better than the models that adopted the fixed calculated adiabatic flame temperature at upper flammability limit.The average relative differences between results predicted by the new model and observed values are less than 2.7%for upper flammability limit evaluation.The prediction accuracy has been increased by four times.In the actual operating environment,the temperature of alkane-CO₂ in ORC is higher than the room temperature.Thus,the leaking gas is high.It is necessary to investigate the effect of temperature on the flammability limits of alkane because the temperature increase leads to the flammable zone extend.Theoretical equation to evaluate the effect of temperature on the flammability limits of pure hydrocarbons was proposed in present study and compared with other methods available in the literatures.Theoretical equation was based on the thermal theory.The equation possesses better prediction accuracy than other available methods at UFL.Finally,the temperature range of application of theoretical equation was discussed in detail.Combining the temperature model for alkane and model for predicting flammability limits for mixture,the flammable zone of mixture at different temperature is available.