Investigation on sub-to-supercritical transition of diesel: Gas–liquid interface properties

ABSTRACTABSTRACTBy establishing a high-temperature droplet evaporation test device, the influence of ambient temperature on droplet morphology and droplet diameter is studied using diesel oil (D100). The variation of droplet evaporation characteristics is studied. The phase transition model of n-heptane in a supercritical environment is established and verified by the molecular dynamics simulation method. The diffusion coefficient, radial distribution function (RDF), surface tension, interfacial thickness, and liquid film temperature of n-heptane liquid film are analyzed. The results show that when the environmental conditions change from subcritical to supercritical, the diffusion coefficient decreases first and then increases from 8.55 × 10−4 cm/s−1 to 71.75 × 10−4 cm/s−1. The peak value of RDF decreases from 1627 to 438, and the smaller peaks after the main peaks tend to be smooth, indicating that n-heptane appears as a gas under supercritical conditions, and the phase transition changes from evaporation to diffusion. Cases 5(T = 973 K P = 5 MPa), 6(T = 573 K P = 7 MPa), 8(T = 773 K P = 7 MPa), and 9(T = 973 K P = 7 MPa) are high-supercritical calculation cases, with MSD of 1.71 × 105 Å2, 4.03 × 105 Å2, 2.49 × 105 Å2, 4.07 × 105 Å2, compare with case 1(T = 573 K P = 3 MPa), MSD increased by 11 times. When T/Tc ≥1.5 and P/Pc ≥1.5, n-heptane undergo three stages of transition from subcritical state to supercritical state. The first stage is the subcritical evaporation stage. With the evaporation of n-heptane, the fuel surface tension gradually disappears, the subcritical evaporation stage ends, and the transition stage begins. When the liquid film temperature exceeds the critical value, the transition phase ends, and the supercritical diffusion phase begins. In addition, when the ambient pressure is 5 MPa and 7MPa, the ambient temperature rises from 773 K to 973 K. The proportion of the transition process in the evaporation process increases by 19% and decreases by 3%, respectively.KEYWORDS: Molecular dynamicssupercriticalgas–liquid interfacediffusion coefficientsupercritical phase transition mode AcknowledgementsThis study was financially supported by the project of natural science foundation of Jiangsu province (BK20200910), open project of state key laboratory of engines (Tianjin University) (K2020-12), provincial engineering research center for new energy vehicle intelligent control and simulation test technology of Sichuan (XNYQ2021-003), Nantong science and technology plan project (JC2021166), and state key laboratory of automotive safety and energy (KFY2227).Disclosure statementNo potential conflict of interest was reported by the author(s).Nomenclature D100Pure diesel oilRDFRadial distribution functionADIthe mean displacement incrementMDMolecular dynamicsCFDComputational Fluid DynamicsLEDLight-emitting diodeN2Nitrogen gasN-NThe covalent bond formed between nitrogen atomsEtotalThe total energy of the bondEbondthe bond’s tensile energyEanglethe bond’s bending energyEtorsionthe bond’s dihedral twisting energyEoopthe bond’s off-plane vibration energyEcrossthe bond’s cross energy termEelecthe bond’s coulomb electrostatic forceELJthe bond’s coulomb electrostatic forceρthe density of the systemFij→the force between atoms i and jrij→the distance vectorkBBoltzmann’s constantLzthe height of the frame on the z axisPxx,Pyy and Pzzthe pressures in the x, y, and z directions respectivelyri(t)the position of atom i at time tnthe total number of atoms in the systemDthe diffusion coefficientNαthe number of diffused atoms in the systemttimeNthe total number of moleculesTthe total calculated time (number of steps)δrthe set distance differenceNthe number of molecules between r→r+δrACAmorphous Cell-constructionFCCface-centered cubicLx, Ly, Lzthe size of the simulation box in the x, y, and z directions respectivelyNVTcanonical ensembleNVEmicrocanonical ensembleC7H16n-heptaneMSDmean square displacementC8H18n-octaneC12H26DodecaneAdditional informationFundingThe work was supported by the Open project of State Key Laboratory of Internal Combustion Engine Combustion, Tianjin University [K2020-12]; Project of Natural Science Foundation of Jiangsu Province [BK20200910]; provincial engineering research center for new energy vehicle intelligent control and simulation test technology of Sichuan [XNYQ2021-003]; State Key Laboratory of Automotive Safety and Energy [KFY2227].Notes on contributorsRuina LiDr. Ruina Li is an associate professor at the School of Automotive and Transportation Engineering at Jiangsu University.Liang ZhangMr. Liang Zhang is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.Yang SongMr. Yang Song is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.Chunyi TangMr. Chunyi Tang is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.Quan HuMr. Quan Hu is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.