Safety Performance Research Of Organic Rankine Cycle Using Flammable Mixtures For Engine Waste Heat Recovery

As the main fuel-consuming equipment,internal combustion engines lose a large amount of energy into the environment in the form of waste heat when it runs.Therefore,waste heat recovery is of great significance.Once such waste heat can be reused,the engine efficiency could be greatly improved,as well as the economic and environment benefit.Organic Rankine Cycle is considered as a promising approach for engine waste heat recovery.Working fluid selection is important because it determines the recovery efficiency to a great extent.Due to the high temperature of engine waste heat,mixture based on small molecule hydrocarbons and CO₂ can match well with the heat resource,as well as high efficient,environmental friendly and safe.However,the leakage of working fluid is unavoidable in practical application.It is dangerous for the reason that it may cause explosion once the concentration of flammable hydrocarbon exceeds a certain concentration.Thus,it is a threat for safety due to its flammability.Nowadays,there is a lack of investigation of the mixture working fluid leakage based on the system operation.Therefore,this study aims at establishing a comprehensive quantitative risk assessment system evaluating operation safety of Organic Rankine Cycle using flammable mixture working fluid.Besides,put forward specific risk mitigation measures to improve operation safety.The paper proposed a comprehensive risk assessment system to evaluate the operation safety of Organic Rankine Cycle using flammable mixture working fluid,which is comprised by concentration distribution analysis,explosive risk assessment and risk mitigation measures.Firstly,CFD numerical simulation of the concentration distribution of hazardous gas is carried out which is significant for investigating the diffusion patterns of leakage gas.The next part focuses on the evaluation of explosion consequences based on the TNT equivalent method.The final part investigates the effectiveness of specific risk mitigation measures through further simulations.It will provide scientific guidance to prevent gas explosion,which is of great significance to improve the security of the ORC system and reduce potential hazard caused by leakage.Safety assessment of ORC system for diesel engine waste heat recovery in the laboratory caused by propane/CO₂ leakage is carried out.What’s more,risk assessment of different hydrocarbon/CO₂ is investigated through the three level safety assessment model.Results show that the concentration distribution was governed by the leakage initial momentum,ventilation conditions,gravity and obstacle in the room.With the increment of the leakage time,the radius of all dangerous zones increase gradually.The overpressure decrease with the increment of distance.When the ratio of hydrocarbon is0.4,explosive zone only exist on the top of the leakage hole.In view of the security of leakage,the maximum ratio of propylene and pentane in the mixtures should be 0.4when blending with carbon dioxide.To improve the safe performance of hydrocarbon/CO₂ mixture in practical application,gas-detecting device is preferably installed in the middle of the ceiling,which can detect gas leakage in less than one second.The damage degree to buildings and individuals caused by explosion increase from propylene/CO₂to pentane/CO₂.The risk prediction and treatment measures of working fluid leakage can provide valuable guidance for ORC application with safe operation,as well as avoid the damage to buildings and individuals.Safety assessment of different hydrocarbon/CO₂,could promote the research of efficient,environmental protection and safe high temperature working fluid.Although the study has focused on a mixture of hydrocarbon and carbon dioxide,the model is generic enough to deal with cases of any kind of flammable gas.The model in this paper can also be widely applied to refrigeration and pump systems.