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 pratikmehta September 28, 2010 16:22

Ice melting Simulation

Hello Friends,

I am trying to perform a simulation for ice melting

Here is the Problem definition:

I have a aluminium hollow pipe ( 1mm thickness ) . Through this pipe water at 80 C is flowing at constant flow rate . Re number is about 20K
The ice deposition over this pipe is about 2mm thick .

My quest is to know how much time does it take for the ice at -5 C gets converted to water at 5 C . I just need to know when does water reach 5C.

What I have done so far:

I have got geometry and meshed it :).I have the fluid domain ( water )and solid domain ( Ice ) .I am using CFX 12.1v. Material in fluid domain is water & material in Ice domain is user created material as CHT solid & thermodynamic state as solid.

My doubts,,,
1) How should I treat the interface ??. Should I keep it as conservative interface flux as the boundary condition and should I create a thin wall of 1 mm aluminium ,so as to save my meshes to aviod the extra physical 1mm thickness of the pipe.

2) Variables like Density, Thermal conductivty & Sp heat capacity as all function of temperature. Regarding the thermal conductivity & density I have used step function CEL expression that will suitably feed in the right number at the existence of ice or water. My biggest worry is to specify Cp variation as the ice start to heat up from -5C .

I need to know how should I specify Cp that will consider the Cp of Ice from -5C to -1C approx 2kJ/KgK , then it must consider Cp as 166kJ/KgK frmo -1C to +1 C and later from 1C onwards Cp as 4kJ/KgK .

I tried to use the step function in order to get this variation but could not get it correct.

So how would you suggest me to go forward .

Best regards
Pratik

 ghorrocks September 28, 2010 18:55

Can't your problem be done as an analytical solution, heat input, assumed HTC, latent heat etc to get how much ice melts? What will CFD add to it?

 ghorrocks September 28, 2010 18:57

Actually, it probably won't be an analytical solution but sounds like a simple ODE so can be easily done in matlab or a hand written ODE solver.

 pratikmehta September 29, 2010 03:13

Hi Glenn,

Ya actually this is what could be the solution but currently i am just tryin to get the melting process to work. Actually problem definition is that the water flows through the pipe at varied temperature over period of 20 mins ok , ,, so over the time of 20 mins of constant flow rate with varied temperature of water flowing through the pipe I need to melt certain substance called Ad blue for SCR ( having its material properties ).

So for a start I thought of setting up a simple case to understand the complications before i taken the above mentioned problem addition.

Well all I would think of that this simulation is pretty the same as the Deicing / Defrosting on a windshield which is a regular CFD problem.

Also my question is ,, to use either a step function for Cp or use a user rountine fortan but I have heard before that fortan sub routine cannot be run properly on a windows platform ???, well i am just speculating i am not sure.

thanks

 ghorrocks September 29, 2010 08:26

There are many complexities in modelling freezing or melting. You still have not explained why CFD is needed here, why won't a simple ODE solution be good enough? You would have the answer after a few hours work rather than a few months by CFD.

If you insist on doing it by CFD then I would talk to CFX support and ask for some demo files on freezing or melting. I am pretty sure some example files are around.

Fortran runs fine on Windows, you just need to have the correct fortran compiler. Look in the documentation for details.

 pratikmehta September 29, 2010 11:39

Hi Glenn,

Melting the substance under consideration while supplying the water which is responsible for heat transfer whose temperature at the inlet is sent in at different temperatures ( starting with 20 C upto 70 C within the time duration of 20 mins ) .Hence I thought doing this in CFD would be helpful as also we have no details abt the local heat transfer cofficient and assuming a constant heat transfer coefficient or by empherical value would cause errors. Over all geometry is also complex in reality. Also we are interested to know whether how much melting in liquid fraction has been obtained.

PS: Dont you think that deicing is very similar issue and people in industries do go for such CHT CFD simulation in order to know the fraction of liquid to solid and get more info.

let me know if you still think if this can be done using ODEs.

Well the CFX support have not given me any documentation as of yet.

Best regards
Pratik

 ghorrocks September 29, 2010 18:57

Quote:
 we have no details abt the local heat transfer cofficient
Then do a simple single phase model to just get the HTC and use that in an ODE approach.

Quote:
 Over all geometry is also complex in reality.
That may mean the ODE approach is intractable depending on the geometry and how the stuff melts. Can't say one way or the other without further details.

Quote:
 PS: Dont you think that deicing is very similar issue and people in industries do go for such CHT CFD simulation in order to know the fraction of liquid to solid and get more info.
It is similar to deicing but many deicing simulations assume the ice is a thin layer on the surface and when it turns to liquid it disappears. This may not be appropriate for what you are doing. Can't be sure based on the sketchy details you have provided so far.

 pratikmehta September 30, 2010 02:35

Hi ,

Ya may be in this case , I can assume all the liquid disappears as well as long as that is indicating the formation of liquid state. But never the less my query is to implement the correct Cp so as to observe this process in CFX. Assuming an average Cp would insert errors in overall melting process so I was looking for help for the same.

PS: I would be glad if anyone has done even the deicing simulation to direct me in how have they managed to put in Cp or inserted the latent heat in the data as I see that CFX does not allow to put this information directly. With conductivity and densitx changed can be managed with Step function.

Thank again for your kind help

Best Regards
Pratik

 ghorrocks September 30, 2010 05:09

I do not understand what you are trying to do. Can you post some drawings and a description of how it works?

 10mmet21 February 29, 2012 04:39

Freezing of an ice cream

Dear all,

I am Working On an interesting M. Tech Project. Description is discussed bellow.

It is a Two phase problem in which convection takes place through air flowing outside and conduction in the ice cream.

1 litter rectangular Ice cream packet is placed in air blast freezer.
Air is flowing at the velocity of 2.5 m/s. from one side of rectangular duct and leave at the other end of the freezer. It flows over the Rectangular ice cream box. The Temperature of an air is (-30 C)

Initial Temperature of an ice cream is (-4 C)
initial freezing temperature is (-5.6 C)
specific heat above freezing point is 3.22 kJ/Kg K and bellow freezing Point is 2.74Kj/kg K
latent heat of fusion is 204 kj/kg
density 550 kg/cubic meter
conductivity of unfrozen ice cream is 0.518 w/m K and frozen ice cream is 1.8203 w/m k

I have to find that what time will it take to reach the center temperature of an ice cream to (-18 C)

Air properties are available. heat transfer coefficient h=15 w/m2*K

conductivity of an ice cream box which is made up of paper can be taken as 0.05 w/m k. Thickness of wraper is 0.5 mm.

If any one has related tutorial of freezing then please send it to me or upload on the forum.

I wanted to plot graph of Temperature vs time as an Output.

 ghorrocks February 29, 2012 06:02

Does the liquid ice cream move at all?

Are there any moulds, pipes, formers or anything else to hold things in place? I assume they are made of metal or plastic or something. Or is the liquid ice cream in a paper box?

 10mmet21 February 29, 2012 07:25

Thank you very much for taking an interest in my problem.

The ice cream is stationary in 1 liter rectangular family pack. (It is Kept in Rectangular Box made of paper card board... You have Already seen it)

This Box is placed in air blast freezer.

One more thing is Ice cream is in paste form (Not a liquid Not a solid) so i am confused that what should i consider it initially. finally it will became solid after freezing.

Another thing is Ice cream properties like specific heat, density and conductivity are Changing from paste form to solid form.

if i use solidification and melting model software ask me about solidus temperature and liquidious temperature. what value should i take. the freezing temperature is (-5.6 degree centigrade)

so cold air of (-30 C) at velocity 2.5 m/s (turbulent flow) is blown on (-4 C) temperature ice cream and air continuous to flow till the ice cream center temperature reaches to (-18 C)

 ghorrocks February 29, 2012 17:14

Sounds like the simplest approach is to model the ice cream as a solid with a variable specific heat as a function of temperature, and include in it a jump which represents the latent heat of freezing. Likewise the thermal conductivity is simply a function of temperature.

The paper is a thermal resistance on the outside of the ice cream solid.

Then you can do a CHT simulation to couple it to the fluid flow. But I suspect the coupling will be weak and you can probably do a fluid simulation to get heat transfer coefficients over the ice cream and then just do an isolated fluid flow model and isolated solid temperature model.

 evcelica February 29, 2012 21:14

Looks like you were given the convection coefficient of 15W/m^2*K?
If so forget CFD and just do a transient solid model and apply the convective boundary condition and temperature to the outside. Just increase the Cp over a small range like you were already doing to represent the latent heat.

A solid model like this shouldn't take more than 30 minutes to setup, mesh, and solve.

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