Hello LAMMPS-Users
I am performing evaporation simulation on a flat copper surface. I
constructed seven layers of copper atoms to form bottom solid wall,
and different layers match with different functions. From outside to
inside, the first layer of copper atoms stayed still as a boundary
wall to keep the volume of the system constant (fix wall); the
following inside three layers were set as heat source from which heat
flux was generated (phantom atoms), the last three layers (wall atoms)
were set as solid walls through which heat conducted to argon fluids.
Above the solid surfece, there is a thin film of liquid and above the
liquid region there is vapor region with approximately 250 atoms. The
procedure that I used in my problem is
1- Carried out an EMD simulation at T=90K for whole system to achieve
a system with constant temperature.
2- Change the NVT ensemble to NVE ensemble for fluid domain while the
temperature of the solid wall was still fixed at 90 K by the
thermostat.
3- Increased the solid wall temperature to T=140 K for evaporation.
I have not any problem with step 1 and 2. My problems and questions
are related to step 3:
1- In a short interval after equilibration period the temperature of
argon get higher than temperature of solid substrate which is not
realistic. I played with different parameters such as time step and
height of computational domain but I couldn't fix the problem.
2- The result of pressure is not realistic. Due to fix volume of
system, the pressure should increase during evaporation period but it
just fluctuates. Even in equilibration period the average value of
pressure doesn't have good agreement with saturation pressure of Argon
at T=90K which is nearly 1.35 bar. Is the method that I used for
calculating pressure correct?
I attached the plots of temperature and pressure with respect to time
to this email.
1.bmp (896 KB)
pressure.bmp (1.24 MB)
Couple of comments below.
Steve
Hello LAMMPS-Users
I am performing evaporation simulation on a flat copper surface. I
constructed seven layers of copper atoms to form bottom solid wall,
and different layers match with different functions. From outside to
inside, the first layer of copper atoms stayed still as a boundary
wall to keep the volume of the system constant (fix wall); the
following inside three layers were set as heat source from which heat
flux was generated (phantom atoms), the last three layers (wall atoms)
were set as solid walls through which heat conducted to argon fluids.
Above the solid surfece, there is a thin film of liquid and above the
liquid region there is vapor region with approximately 250 atoms. The
procedure that I used in my problem is
1- Carried out an EMD simulation at T=90K for whole system to achieve
a system with constant temperature.
Is the pressure of the vapor in this system correct, before you start heating?
2- Change the NVT ensemble to NVE ensemble for fluid domain while the
temperature of the solid wall was still fixed at 90 K by the
thermostat.
Ditto for this step?
3- Increased the solid wall temperature to T=140 K for evaporation.
I have not any problem with step 1 and 2. My problems and questions
are related to step 3:
1- In a short interval after equilibration period the temperature of
argon get higher than temperature of solid substrate which is not
realistic. I played with different parameters such as time step and
height of computational domain but I couldn't fix the problem.
When you spike the substrate temperature, the vapor may take
a while to re-equilibrate. That's what your T plot seems to
be doing - why is this bad?
2- The result of pressure is not realistic. Due to fix volume of
system, the pressure should increase during evaporation period but it
just fluctuates. Even in equilibration period the average value of
pressure doesn't have good agreement with saturation pressure of Argon
at T=90K which is nearly 1.35 bar. Is the method that I used for
calculating pressure correct?
I attached the plots of temperature and pressure with respect to time
to this email.
Your plot is 100 bar, not 1 bar. Hence the questions as to whether
P is correct in steps 1 and 2, before heating. You can use
the compute pressure command or compute stress/atom to
look at the virial vs kinetic component. So you can see which
component you think is off. I presume you are calculating the
pressure of the gas with respect to its volume. 250 atoms is
not a lot to compute a pressure on. Do you have the density
of the gas correct?
