Hi!

I tried to simulate a heat exchanger. Water is flowing in counter flow and is separated by a solid wall out of copper. One side has an inlet velocity of 1 m/s and an inlet temperature of 283K the other has a velocity of 10 m/s and an inlet temperature of 363K. After 140 iterations and a residual of 10e-5 CFX-Post shows a temperature from over 370K at the solid domain. That is much warmer than the inlet!!! Below you can find the command file. Another person than me had also a look at the boundary conditions and could not find any error. Although I modelled the heat exchanger very short (100mm), I do not believe that entrance effects are the reason for this!? Because of creating not to much cells, I modelled this problem as "2 dimensional" with only one cell in one coordinate direction and symmetry boundary conditions. Is there anybody with experiences in modelling convective heat transfer in CFX or knows where I can find some information or examples.

Thanks for any help,

Mark

FLOW: DOMAIN INTERFACE: Default 1

Boundary List1 = Default 1 Side Fluid1 Part 1

Boundary List2 = Default 1 Side Solid Part 2

Connection Type = Automatic

Interface Region List1 = FLUID1 External

Interface Region List2 = SOLID External B

Interface Type = Fluid Solid END DOMAIN INTERFACE: Default 2

Boundary List1 = Default 2 Side Fluid2 Part 1

Boundary List2 = Default 2 Side Solid Part 2

Connection Type = Automatic

Interface Region List1 = FLUID2 External

Interface Region List2 = SOLID External A

Interface Type = Fluid Solid END DOMAIN: Fluid1

Coord Frame = Coord 0

Domain Type = Fluid

Fluids List = Water

Location = FLUID1

BOUNDARY: Default 1 Side Fluid1 Part 1

Boundary Type = INTERFACE

Interface Boundary = On

Location = FLUID1 External

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Conservative Interface Flux

END

WALL INFLUENCE ON FLOW:

Option = No Slip

END

END

END

BOUNDARY: Fluid1 Default

Boundary Type = WALL

Create Other Side = Off

Interface Boundary = Off

Location = WALL1

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Adiabatic

END

WALL INFLUENCE ON FLOW:

Option = Free Slip

END

END

END

BOUNDARY: In1

Boundary Type = INLET

Interface Boundary = Off

Location = BOTTOM1

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

HEAT TRANSFER:

Option = Static Temperature

Static Temperature = 10 [C]

END

MASS AND MOMENTUM:

Normal Speed = 1 [m s^-1]

Option = Normal Speed

END

END

END

BOUNDARY: Out1

Boundary Type = OUTLET

Interface Boundary = Off

Location = TOP1

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Average Static Pressure

Relative Pressure = 0 [Pa]

END

END

END

BOUNDARY: Symmetry

Boundary Type = SYMMETRY

Interface Boundary = Off

Location = LEFT1,RIGHT1

END

DOMAIN MODELS:

BUOYANCY MODEL:

Option = Non Buoyant

END

DOMAIN MOTION:

Option = Stationary

END

REFERENCE PRESSURE:

Reference Pressure = 1 [atm]

END

END

FLUID MODELS:

COMBUSTION MODEL:

Option = None

END

HEAT TRANSFER MODEL:

Option = Thermal Energy

END

THERMAL RADIATION MODEL:

Option = None

END

TURBULENCE MODEL:

Option = Laminar

END

END END DOMAIN: Fluid2

Coord Frame = Coord 0

Domain Type = Fluid

Fluids List = Water

Location = FLUID2

BOUNDARY: Default 2 Side Fluid2 Part 1

Boundary Type = INTERFACE

Interface Boundary = On

Location = FLUID2 External

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Conservative Interface Flux

END

WALL INFLUENCE ON FLOW:

Option = No Slip

END

END

END

BOUNDARY: Fluid2 Default

Boundary Type = WALL

Create Other Side = Off

Interface Boundary = Off

Location = WALL2

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Adiabatic

END

WALL INFLUENCE ON FLOW:

Option = Free Slip

END

END

END

BOUNDARY: In2

Boundary Type = INLET

Interface Boundary = Off

Location = TOP2

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

HEAT TRANSFER:

Option = Static Temperature

Static Temperature = 90 [C]

END

MASS AND MOMENTUM:

Normal Speed = 10 [m s^-1]

Option = Normal Speed

END

END

END

BOUNDARY: Out2

Boundary Type = OUTLET

Interface Boundary = Off

Location = BOTTOM2

BOUNDARY CONDITIONS:

FLOW REGIME:

Option = Subsonic

END

MASS AND MOMENTUM:

Option = Average Static Pressure

Relative Pressure = 0 [Pa]

END

END

END

BOUNDARY: Sym2

Boundary Type = SYMMETRY

Interface Boundary = Off

Location = LEFT2,RIGHT2

END

DOMAIN MODELS:

BUOYANCY MODEL:

Option = Non Buoyant

END

DOMAIN MOTION:

Option = Stationary

END

REFERENCE PRESSURE:

Reference Pressure = 1 [atm]

END

END

FLUID MODELS:

COMBUSTION MODEL:

Option = None

END

HEAT TRANSFER MODEL:

Option = Thermal Energy

END

THERMAL RADIATION MODEL:

Option = None

END

TURBULENCE MODEL:

Option = Laminar

END

END END DOMAIN: Solid

Domain Type = Solid

Location = SOLID

Solids List = Copper

BOUNDARY: Default 1 Side Solid Part 2

Boundary Type = INTERFACE

Interface Boundary = On

Location = SOLID External B

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Conservative Interface Flux

END

END

END

BOUNDARY: Default 2 Side Solid Part 2

Boundary Type = INTERFACE

Interface Boundary = On

Location = SOLID External A

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Conservative Interface Flux

END

END

END

BOUNDARY: Solid Default

Boundary Type = WALL

Create Other Side = Off

Interface Boundary = Off

Location = SOLIDBOTTOM,SOLIDTOP

BOUNDARY CONDITIONS:

HEAT TRANSFER:

Option = Adiabatic

END

END

END

BOUNDARY: SymSolid

Boundary Type = SYMMETRY

Interface Boundary = Off

Location = SOLIDLEFT,SOLIDRIGHT

END

DOMAIN MODELS:

DOMAIN MOTION:

Option = Stationary

END

END

INITIALISATION:

Option = Automatic

INITIAL CONDITIONS:

TEMPERATURE:

Option = Automatic with Value

Temperature = 10 [C]

END

END

END

SOLID MODELS:

HEAT TRANSFER MODEL:

Option = Thermal Energy

END

THERMAL RADIATION MODEL:

Option = None

END

END END OUTPUT CONTROL:

RESULTS:

File Compression Level = Default

Option = Full

END END SIMULATION TYPE:

Option = Steady State END SOLUTION UNITS:

Angle Units = [rad]

Length Units = [m]

Mass Units = [kg]

Solid Angle Units = [sr]

Temperature Units = [K]

Time Units = [s] END SOLVER CONTROL:

ADVECTION SCHEME:

Option = High Resolution

END

CONVERGENCE CONTROL:

Length Scale Option = Conservative

Maximum Number of Iterations = 200

Solid Timescale Control = Auto Timescale

Timescale Control = Auto Timescale

END

CONVERGENCE CRITERIA:

Residual Target = 0.00001

Residual Type = RMS

END

DYNAMIC MODEL CONTROL:

Global Dynamic Model Control = On

END END END

Hi,

I doubt your simulation is converged. Some pointers for CHT modelling:

1) Usually the solid timescales are much slower than the fluid timescales. Hence use a solid timescale factor, I often use about 1000. 2) Even though you have 1 to 1 node matching across your solid/fluid interfaces, I recommend specifiying GGI interfaces. I understand they work better than the default 1 to 1 interface. 3) Include balances in your convergence parameters. This is very important in CHT models. 4) Run it for longer! I bet if you look at the balances of this simulation they will not have converged yet.

Glenn Horrocks

Thanks again!! I will try the points you have posted. The convergence reaches the residual of 10e-5!! But it does look like that there is something wrong. First the convergence is getting "slower" but suddenly it starts going down.

Regards,

Mark

Hi Mark,

It is common for the residuals to slow convergence as the simulation proceeds. As I said in my previous posting, it is very important to converge on balances or conservation as well as residuals in CHT simulations.

Glenn Horrocks

I'm trying to find the the over all transfer heat coeficien in transicion between laminar flow and turbulent flow in a heat exchanger, and i'd like to know if you could mail me some information, thanks

Hi Marco!

The heat transfer coefficient depends on its definition. If you do not know the heat flow you have to calculate it by an energy balance on one side of the heat exchanger (mass flow * heat capacity * (T_in - T_out)). An average heat transfer coefficient can now be derived by using the logarithmic temperature difference (average heat transfer coefficient = heat flow / (area * logarithmic temperature difference))

I hope this is what your question was about?!

A good book is Heat and Mass Transfer by H.D. Baehr and K. Stephan