Effect Of Heat Transfer And Cycle Characteristics On Carbon Deposition In Stirling Engine Heating Pipes

Stirling engine,whose energy comes from external combustion,has outstanding advantages and good energy adaptability.The heat sources include fossil energy,such as coal,gasoline,diesel and natural gas,biomass energy(i.e.wood chips,straw,alcohol and biogas)and low-grade energy of waste heat and solar energy.To seal and reduce the abrasion of the moving parts,some engines need lubricating oil.Meanwhile,a part of the lubricating oil flees into the chambers of the engine.For long-run closed-loop Stirling engine,the lubricating oil will be decompose into carbon deposition,oil sludge and other byproducts,resulting in the reduction of heat exchanger capacity.This paper focuses on the carbon deposition in the heating pipes of Stirling engine.The main research contents and results are shown as follows:The heating pipe is a key component for optimal design of Stirling engine.The leakage of lubricating oil into the heat pipe will crack carbon deposition at high temperature,leading to reduce heat transfer.The losses of synthetic lubricants in mass are compared under nitrogen and carbon dioxide atmosphere.When the temperature is 180 0C,the losses are almost the same.However,when the temperature is 450 ℃,the losses become exhausted.At the temperature of 250℃ and 350℃,the mass loss in carbon dioxide is less than that in nitrogen.On the other hand,the mass decreases with increasing the heating time.At 250℃ and 350℃,the linear paraffin volatilize and crack easily,and more easily reduced to PAO lubricants under nitrogen atmosphere.At 250℃,C-H attached to the vinyl group reduces.But at 350℃,C-H attached to the vinyl group was broken and recombined by substituent.For the diester lubricating oils at 250℃ and 350℃,all the vibration absorption peaks of-CH3 and-CH2,the vibration absorption peaks of C=O-O-,the characteristic absorption peaks of linear alkanes increase respectively.At 250℃,The stretching vibration bonds of-CH3 and-CH2 do not change significantly,whereas decreases at 350℃.Both at the temperature of 180℃and 250℃,the PAO lubricants are decomposed into short-chain hydrocarbons.It is obtained that more short-chain hydrocarbons are formed at 250℃than that at 180℃.PAO lubricants are pyrolysed into high boiling point substances at 350℃ and form carbon deposition at 450℃ which is easy to absorb moisture in the air.The diester lubricants are broken into short-chain hydrocarbons and have the same substance at 180℃ and 250℃.The diester lubricants are pyrolysed into high boiling point substances and forms carbon deposition at 450 ℃.The PAO lubricants form by the relatively smooth solid residue.The diester lubricants form by solid residue which is smoother at carbon dioxide than ones at nitrogen.Comparing above all,PAO lubricants are smoother,effectively inhibiting the formation of solid residue.It is found that the amount of carbon deposition is largest when Reynolds number is 10434 among for 5 different Reynolds numbers,which is 1.36～1.75 times of other Reynolds numbers.The temperature of mainstream fluid and the type of fluid have the obvious impact on carbon deposition.At the same Reynolds number,the amount of carbon deposition at 110℃ is 10 times higher than that at 190℃.The distribution of carbon deposition along the tube is uniform and the uniform increase alone with longer time.For the same fluid,high temperature conditions can effectively alleviate the formation of carbon deposition in Stirling engine.While for different types of fluids,oxidizing atmosphere(such as air)can tremendously inhibit the formation of carbon deposition.The heat transfer experiment of the heating pipes was carried out.According to the experimental results,the non-dimensional heat transfer,relations of the clean state and the carbon deposition 180h were fitted:Nu = 1.5136*10-7*Re00.6627*Pr0-39.5630(clean state),where R2=0.9958,3567 ≤Re≤33524;Nu = 2.8171*10-7*Ref0.6627*Prf-39.1176(180h),where R2=0.9925,3567 ≤Re≤33524.During the process of carbon deposition in the heating pipes,a spot of carbon deposition can strengthen the heat transfer performance and then the heat transfer performance approximately reduces linearly.At the same Reynolds number,the wall surface temperature and the temperature difference of the main fluid decrease and then increase with the increase of carbon deposition.The thermal cycle efficiency and the circulating power increase or decrease simultaneously with the amount of carbon deposition,where a spot of carbon deposition can enhance performance output.This distinguishing feature can provide guidance and tips for overhauling carbon deposition.A suggestion can be proposed that the carbon deposition in the Stirling engine should be cleaned when the cycle power is less than the initial state power(without carbon deposition),and the carbon deposition per unit area is about 5.11mg/cm2～6.88mg/cm2 with the Reynolds number 3688～33523.Combined with experimental design,a simple device for removing carbon deposition of Stirling engine is designed.