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

ABSTRACT

By 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 Ų, 4.03 × 105 Ų, 2.49 × 105 Ų, 4.07 × 105 Ų, 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 dynamics
• supercritical
• gas–liquid interface
• diffusion coefficient
• supercritical phase transition mode

Nomenclature

D100	=	Pure diesel oil
RDF	=	Radial distribution function
ADI	=	the mean displacement increment
MD	=	Molecular dynamics
CFD	=	Computational Fluid Dynamics
LED	=	Light-emitting diode
N2	=	Nitrogen gas
N-N	=	The covalent bond formed between nitrogen atoms
Etotal	=	The total energy of the bond
Ebond	=	the bond’s tensile energy
Eangle	=	the bond’s bending energy
Etorsion	=	the bond’s dihedral twisting energy
Eoop	=	the bond’s off-plane vibration energy
Ecross	=	the bond’s cross energy term
Eelec	=	the bond’s coulomb electrostatic force
ELJ	=	the bond’s coulomb electrostatic force
ρ	=	the density of the system
$\overrightarrow{F_{ij}}$	=	the force between atoms i and j
$\overrightarrow{r_{ij}}$	=	the distance vector
kB	=	Boltzmann’s constant
Lz	=	the height of the frame on the z axis
Pxx,Pyy and Pzz	=	the pressures in the x, y, and z directions respectively
ri(t)	=	the position of atom i at time t
n	=	the total number of atoms in the system
D	=	the diffusion coefficient
Nα	=	the number of diffused atoms in the system
t	=	time
N	=	the total number of molecules
T	=	the total calculated time (number of steps)
δr	=	the set distance difference
$N$	=	the number of molecules between $\left(r\to r+\delta r\right)$
AC	=	Amorphous Cell-construction
FCC	=	face-centered cubic
Lx, Ly, Lz	=	the size of the simulation box in the x, y, and z directions respectively
NVT	=	canonical ensemble
NVE	=	microcanonical ensemble
C7H16	=	n-heptane
MSD	=	mean square displacement
C8H18	=	n-octane
C12H26	=	Dodecane

Acknowledgements

This 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 statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The 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 contributors

Ruina Li

Dr. Ruina Li is an associate professor at the School of Automotive and Transportation Engineering at Jiangsu University.

Liang Zhang

Mr. Liang Zhang is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.

Yang Song

Mr. Yang Song is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.

Chunyi Tang

Mr. Chunyi Tang is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.

Quan Hu

Mr. Quan Hu is a master's student at the School of Automotive and Transportation Engineering, Jiangsu University.