[~Akshay350~]

Hello CFD members,
I am currently project on vapor condensation in a condenser in an AC system.
For starting from basics I have made a simulations with various pipe lengths(one shorter and other longer) where pure water vapor enters and gets converted into liquid phase in the end due to heat transfer from surrounding water..
I used Pressure based type solver , absolute velocity formation, multiphase model as VOF, Viscous model-k-epsilon, standard and enhanced wall treatment.
Boundary Conditions 1. Inlet=velocity inlet- 0.1 m/s, 400 K, phase-vapor fraction-1.0
2. Outlet=Pressure outlet, phase- mixture.
3.Wall tube= Convection, Heat transfer coeff-400, Free stream temp-300 K, Material -Copper

I have made heat transfer from colder water to hot water vapor inside tube via a coupled wall; from mesh interface.

I have attached the pictures of the results of my both simulations.
This image shows the volume rendering of volume fraction.

This one show the temperature across the pipe on a contour.

In both shorter and longer pipe I observed same phenomenon.
My doubts are as follows:
1. Why is the volume fraction of water liquid at the end of pipe is suddenly rising towards 1 rather than gradual increase from the inlet side? Cause heat transfer through walls across the pipes is constant.Here is side view of the pipe for clarity.

2. What should be mesh type at the phase simulations to make a quicker simulations(this simulation took 2 hrs approx for steady state)
3. Does the pressure value at the outlet makes any changes to the volume fraction? As the value at pressure outlet was 0, so is that the reason behind sudden rise of volume fraction?
4.Whats the difference and significance of First order upwind and second order upwind? Which scheme is better suited for the condensation?
5.I have used water as the medium; while in my actual project I am going to use R-134a refrigerant; so what are steps to define the material?Should I make two different phases named liq and gaseous state? Should I keep the latent heat of vaporization 0?
Thanks in advance!