boundary conditions for heat exchanger
 

hi to everyone, i have a problem with boundary conditions for heat exchanger with coaxial pipes, in the inner pipe there is air, in the external there is water liquid. for two sides of inner pipe boundary conditions are: "wall" with temperature = 1073 k and other side "pressure outlet". for external pipe there are "mass flow inlet" =0.01 kg/s and "outflow". the problem is that outflow and pressure outlet can't subsist in the same domain. someone has any idea for avoid this problem?
i changed outflow with pressure outlet and flag "target mass flow rate" but in this way mass flow inlet is different from outlet mass flow and the temperature domain is not real.thanks for helping. the scheme of heat exchanger is in the picture

There should be no problem in your case. Do you specify the same mass flow rate of inlet to pressure outlet B.C. ?

yes, specified the same mass flow at pressure outlet, but after 250 iterations inlet mass flow is 0.01 kg/s, outlet is 0.015 kg/s. i know that with more iterations the mass flow became the same, but the temperature of the water is very low. inlet temperature is 293 k, outlet temperature is 300 k with inner pipe temperature of 350 k.

in this case have i to use vof model? or is it useless?

There is no need to use VOF. How much is the length?

one metre

how much the temperature difference you think should be? Do you have any experimental results?

i have not experimental results but i dont' think that the field of temperature is real becouse the residuals don't go down. after 10000 iterations they are in the range between 10e-2 and 10e-3 i want they are between 10e-5 and 10e-6. what can i change for having convergence of iterations?

Well, it depends on several factors, but let's try these at first.
1-how fine is your mesh?
2-what is flow type(laminar or turbulent)?
3-what residual does not converge(mom, energy or turb.)?

the flow is turbolent k and epsilon standard model, the mesh i thik is well done, about residual they are continuity, x, y, z velocity, k and epsilon. energy and DO are under 10e-6

i have to change BC. for wather BC are mass flow inlet and outflow, for air i have a problem i can use wall T=1073 k but can't use pressoure outlet.Which BC can i use?

after many changes the proble is the same, if in fluent i consider both volumes , air and water liquid, i have reverse flow at mass flow inlet.if i consider only the volume of water with mass flow inlet, pressure outlet and heat generation (w/m^3) there isn't reverse flow for the water.i don't know why...when i use gas and liquid in fluent have i to define some in particular?

Mike2 can you plzz specify the inlet boundary condition for inner pipe