Model for a profile temperature inside a digester

enzome · November 7, 2011, 12:12

Hi to everyone. I am not able to solve this problem.
I'm working on a digester which has 2 velocity inlets at the top and 2 pressure outlet at the bottom. The sludge is entering from the top, it is moving in the digester and it is going out from the pipes at the bottom. I already have the solution that shows the movement of the fluid, streamlines, velocity vectors etc. Now I have to look for the correct model (always in monophase) that let me find the profile of temperature when the sludge enter with a temperature inside the digester and goes out with another a bit lower. That is because the microorganisms which are inside could die if there is a change of + or - 1 C.
Questions:
Should I create 2 regions, solid (Concrete) and liquid (sludge)?
Should I create 2 Physics continuum? (solid and liquid)?
Does anyone have a tutorial whit a similar problem?!
Please help me, and don't hesitate to ask for additional data. Thank you

willimanili · November 7, 2011, 17:52

Yes, you need two regions and two physics continua. Set the concrete region to solid with constant density and segregated temperature, the sludge should be modeled as a liqiud region also with segregated temperature. Important is that you have the right material values for the sludge as dyn. viscosity, heat capacity, density and heat conductivity. Because you said that the bacteria will die if there is a change of temperature more than +/- 1 degree is think you dont need to take temperature depended material values so you can set them all to constant. You can change the values under the physics continuum child.
Also very important is how the digester get in contact to the environment?! You need to specify the thermal specification of the outer boundaries of your digester as close to reality as possible because they will have a main influence to the solution.

ping · November 8, 2011, 03:41

this is good advice but can i add that a quicker and possibly less accurate method is to use your current model and apply a convection thermal specification on the outer wall, then you also need the the heat transfer coef which includes the conduction through the contrete and the external transfer to the air, and an external air temp.. a good first step method which is much easier to get going and if the tank is pretty constant in thickness then probably just as accurate. and if even greater accuracy is required then you might need to model the outside air flow too to get more accurate tank external heat transfer coefs.

enzome · November 9, 2011, 05:28

Quote:

Originally Posted by ping

this is good advice but can i add that a quicker and possibly less accurate method is to use your current model and apply a convection thermal specification on the outer wall, then you also need the the heat transfer coef which includes the conduction through the contrete and the external transfer to the air, and an external air temp.. a good first step method which is much easier to get going and if the tank is pretty constant in thickness then probably just as accurate. and if even greater accuracy is required then you might need to model the outside air flow too to get more accurate tank external heat transfer coefs.

Thank you to both of them. Without reading you I was trying to work with the advice of Ping cos the thickness is constant. I've calculated the heat transfer coef of the conduction through the concrete ( i supposed a plane surfaces cos the diameter is quite big), K=thermal conductivity concrete/ thickness= 7,5 W/m^2K. how can i evaluate the conduction through the external transfer to the air? I can suppose an air temperature in the worst conditions in summer and winter (35 Celsius and 5 Celsius, we are in Spain

)And after it, should I sum to the one calculated above? Thank you for the useful suggestions

abdul099 · November 11, 2011, 14:44

You could calculate a heat transfer resistance from a heat conductivity through the wall and the heat transfer to the outer air. BUT:

Heat transfer to the outside air depens on:
- wall temperature
- air temperature
- heat transfer coefficient, which depens on the geometry, wind speed, surrounding objects etc.

So it's too complicated to invent a single value. I advice to follow ping's suggestions: Either live with the inaccuracy or model the outer air flow.

November 7, 2011, 12:12	Model for a profile temperature inside a digester	#1
enzome New Member vincenzo Join Date: Nov 2011 Posts: 3 Rep Power: 14	Hi to everyone. I am not able to solve this problem. I'm working on a digester which has 2 velocity inlets at the top and 2 pressure outlet at the bottom. The sludge is entering from the top, it is moving in the digester and it is going out from the pipes at the bottom. I already have the solution that shows the movement of the fluid, streamlines, velocity vectors etc. Now I have to look for the correct model (always in monophase) that let me find the profile of temperature when the sludge enter with a temperature inside the digester and goes out with another a bit lower. That is because the microorganisms which are inside could die if there is a change of + or - 1 C. Questions: Should I create 2 regions, solid (Concrete) and liquid (sludge)? Should I create 2 Physics continuum? (solid and liquid)? Does anyone have a tutorial whit a similar problem?! Please help me, and don't hesitate to ask for additional data. Thank you

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November 7, 2011, 17:52		#2
willimanili Member Join Date: Feb 2011 Location: DE-PB Posts: 56 Rep Power: 15	Yes, you need two regions and two physics continua. Set the concrete region to solid with constant density and segregated temperature, the sludge should be modeled as a liqiud region also with segregated temperature. Important is that you have the right material values for the sludge as dyn. viscosity, heat capacity, density and heat conductivity. Because you said that the bacteria will die if there is a change of temperature more than +/- 1 degree is think you dont need to take temperature depended material values so you can set them all to constant. You can change the values under the physics continuum child. Also very important is how the digester get in contact to the environment?! You need to specify the thermal specification of the outer boundaries of your digester as close to reality as possible because they will have a main influence to the solution.

November 8, 2011, 03:41		#3
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	this is good advice but can i add that a quicker and possibly less accurate method is to use your current model and apply a convection thermal specification on the outer wall, then you also need the the heat transfer coef which includes the conduction through the contrete and the external transfer to the air, and an external air temp.. a good first step method which is much easier to get going and if the tank is pretty constant in thickness then probably just as accurate. and if even greater accuracy is required then you might need to model the outside air flow too to get more accurate tank external heat transfer coefs.

November 11, 2011, 14:44		#5
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	You could calculate a heat transfer resistance from a heat conductivity through the wall and the heat transfer to the outer air. BUT: Heat transfer to the outside air depens on: - wall temperature - air temperature - heat transfer coefficient, which depens on the geometry, wind speed, surrounding objects etc. So it's too complicated to invent a single value. I advice to follow ping's suggestions: Either live with the inaccuracy or model the outer air flow.