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October 19, 2004, 00:36 
Convective Heat Transfer  Heat Exchanger

#1 
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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 10e5 CFXPost 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 

October 19, 2004, 18:55 
Re: Convective Heat Transfer  Heat Exchanger

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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 

October 20, 2004, 00:17 
Re: Convective Heat Transfer  Heat Exchanger

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Thanks again!! I will try the points you have posted. The convergence reaches the residual of 10e5!! But it does look like that there is something wrong. First the convergence is getting "slower" but suddenly it starts going down.
Regards, Mark 

October 20, 2004, 18:51 
Re: Convective Heat Transfer  Heat Exchanger

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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 

November 1, 2004, 07:57 
Re: Convective Heat Transfer  Heat Exchanger

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November 12, 2004, 22:55 
Re: Convective Heat Transfer  Heat Exchanger

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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


November 15, 2004, 16:55 
Re: Convective Heat Transfer  Heat Exchanger

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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 

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