CFX Expression Language help

JuPa · April 20, 2013, 09:43

Hi,

There is a well known condensation heat transfer coefficient formulated by Nusselt (you might be familiar with it):

$\alpha = 0.5 \frac{\lambda_{L}}{D} \left[ \frac{g \rho_{L} \left(\rho_{L} - \rho_{G} \right) h_{FG} D^{3}}{\mu_{L}\lambda_{L}\left(T_{Steam}-T_{Wall}\right)}\right]^{0.25}$

The subscripts

and

denote the liquid and gas phases, and the following symbols have their meanings:

$\alpha$ is the heat transfer coefficient.
$\lambda$ is the thermal conductivity.

is the gravitational constant.
$\rho$ is the density.
$h_{FG}$ is the enthalpy of vaporisation from liquid to gas phases.

is the characteristic dimension.
$\mu$ is the viscosity.
$T_{Steam}$ is the steam temperature.
$T_{Wall}$ is the wall temperature.

Now inputting this into CFX at a boundary is dead easy in principle. You just enter it in at a boundary as a heat transfer coefficient (as a CEL expression) and select the steam temperature as the outside temperature.

However the problem I am having is the symbols are functions of the steam temperature.

Let's say for one simulation my steam temperature $T_{Steam}$ is 400K. Then my density at the liquid phase is 937 kg/m^3 and at the gas phase is 1.37 kg/m^3.

Now I can enter each value for each property myself (density, viscosity, thermal conductivity etc) for my steam temperature of 400K. However I don't want to do this if I change my steam value from 400K to 500K 600K etc.

In CFX Expression Language, is there a way of telling CFX that I want viscosity, density etc as a function of some temperature that I specify?

I've read the CFX Reference Guide on CEL (CFX Expression Language) and it isn't very clear how this can be achieved.

Thank you for your help!

oj.bulmer · April 20, 2013, 10:45

Unless am missing something, it's pretty easy.

1) Create expression for temperature, say FluidTemp.
2) Create expressions for density and a viscosity as a function of temperature, say FluidDensity=f(FluidTemp)
3) In the definition of your materials, use "expressions" to specify density and viscosity instead of the actual value with units.
4) Create Workbench parameter for FluidTemp

Once you change the value from say 400K to 800K, the density and viscosity values, since they are parameterised, should change according to your function.

OJ

JuPa · April 20, 2013, 13:07

OJ, I am using IAPWS water so the thermophysical properties are a function of temperature and pressure.

What I am referring to in my original post is making the thermophysical properties in the heat transfer coefficient a function of some known temperature.

ghorrocks · April 21, 2013, 08:46

You will have to define a function for this. It could be a CEL expression or 1D interpolation function or user fortran.

For the CEL expression you could put the IAPWS equations (simplified a bit as they are the full equations aer iterative if I remember correctly) or a simplified approximation.

For a 1D interpolation function you could just map out the property against temperature and use that as a 1D interpolation function.

For a user fortran routine you could use the full IAPWS equations in their full iterative glory, but of course you will have to code it.

April 20, 2013, 09:43	CFX Expression Language help	#1
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Hi, There is a well known condensation heat transfer coefficient formulated by Nusselt (you might be familiar with it): $\alpha = 0.5 \frac{\lambda_{L}}{D} \left[ \frac{g \rho_{L} \left(\rho_{L} - \rho_{G} \right) h_{FG} D^{3}}{\mu_{L}\lambda_{L}\left(T_{Steam}-T_{Wall}\right)}\right]^{0.25}$ The subscripts and denote the liquid and gas phases, and the following symbols have their meanings: $\alpha$ is the heat transfer coefficient. $\lambda$ is the thermal conductivity. is the gravitational constant. $\rho$ is the density. $h_{FG}$ is the enthalpy of vaporisation from liquid to gas phases. is the characteristic dimension. $\mu$ is the viscosity. $T_{Steam}$ is the steam temperature. $T_{Wall}$ is the wall temperature. Now inputting this into CFX at a boundary is dead easy in principle. You just enter it in at a boundary as a heat transfer coefficient (as a CEL expression) and select the steam temperature as the outside temperature. However the problem I am having is the symbols are functions of the steam temperature. Let's say for one simulation my steam temperature $T_{Steam}$ is 400K. Then my density at the liquid phase is 937 kg/m^3 and at the gas phase is 1.37 kg/m^3. Now I can enter each value for each property myself (density, viscosity, thermal conductivity etc) for my steam temperature of 400K. However I don't want to do this if I change my steam value from 400K to 500K 600K etc. In CFX Expression Language, is there a way of telling CFX that I want viscosity, density etc as a function of some temperature that I specify? I've read the CFX Reference Guide on CEL (CFX Expression Language) and it isn't very clear how this can be achieved. Thank you for your help!

April 21, 2013, 08:46		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,852 Rep Power: 144	You will have to define a function for this. It could be a CEL expression or 1D interpolation function or user fortran. For the CEL expression you could put the IAPWS equations (simplified a bit as they are the full equations aer iterative if I remember correctly) or a simplified approximation. For a 1D interpolation function you could just map out the property against temperature and use that as a 1D interpolation function. For a user fortran routine you could use the full IAPWS equations in their full iterative glory, but of course you will have to code it. wwjshirley likes this.

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April 20, 2013, 10:45		#2
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	Unless am missing something, it's pretty easy. 1) Create expression for temperature, say FluidTemp. 2) Create expressions for density and a viscosity as a function of temperature, say FluidDensity=f(FluidTemp) 3) In the definition of your materials, use "expressions" to specify density and viscosity instead of the actual value with units. 4) Create Workbench parameter for FluidTemp Once you change the value from say 400K to 800K, the density and viscosity values, since they are parameterised, should change according to your function. OJ

April 20, 2013, 13:07		#3
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	OJ, I am using IAPWS water so the thermophysical properties are a function of temperature and pressure. What I am referring to in my original post is making the thermophysical properties in the heat transfer coefficient a function of some known temperature.