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Akash February 4, 2008 22:20

Radiation effects in a transient analysis
 
Hi,

I am using Ansys to solve a 2-D conduction/radiation problem comprising two concentric, hollow cylinders which are connected using a covering plate at one end. The arrangement is placed in vacuum. The outer surface of the outer cylinder is subjected to a specified heating rate and allowed to stabilize, provided as a load step. After a pre-determined period of time has elpased, the inner surface of the inner cylinder is subjected to a slightly lower temperature, provided as another load step. I am trying to include the radiation heat transfer between the outer cylinder and the inner cylinder. Could somebody provide me with any tips on how to go about with it? I have tried to select the appropriate lines and specify emissivities. However, I am unable to get around the space temperature option as the assembly is in vacuum. Any assistance will be greatly appreciated.

Thanks, Akash

harish kumar February 5, 2008 12:27

Re: Radiation effects in a transient analysis
 
Dear Akash,

are you using Radiosity solver method? What about enclosure number [ i mean you should specify same enclosure number for the all edges].

since the system is in vacuum specify space temperature -273 C.

spctemp,encl,value example spctemp,1,-273


harish kumar February 5, 2008 12:28

Re: Radiation effects in a transient analysis
 
Dear Akash,

are you using Radiosity solver method? What about enclosure number [ i mean you should specify same enclosure number for the all edges].

since the system is in vacuum specify space temperature -273 C.

spctemp,encl,value example spctemp,1,-273

with regards, harish


Akash February 5, 2008 12:48

Re: Radiation effects in a transient analysis
 
Hello Harish,

Thanks so much for your quick response. I am using the readiosity solver method and I have specified the same enclosure number for all the edges.

Regards, Akash

harish February 5, 2008 14:07

Re: Radiation effects in a transient analysis
 
Dear Akash,

so where is the problem ? you have mentioned about the space temperature it can be accessed with the help of command.

best regards harish

Akash February 5, 2008 20:23

Re: Radiation effects in a transient analysis
 
Hello Harish,

The problem is that when I assign the space temperature, I get temperatures lower than expected for the inner surface of the inner cylinder. If I neglect radiation effects, I can reduce the error significantly. Any assistance here would be appreciated.

Thanks, Akash

harish kumar February 6, 2008 05:11

Re: Radiation effects in a transient analysis
 
Dear Akash,

let me understand your problem so you have concentric cylinder which is called inner cylinder and this is inclosed in an another concentric cylinder which is called outer cylinder and at one end both are connected [i.e outer surface of the inner cylinder and inner surface of the outer cylinder].

atmosphere which is existing between the outer cylinder and inner cylinder is vaccuum.

what about the atmosphere existing at the outer surface of the outer cylinder?

what about the atmospher existing at the inner surface of the inner cylinder?

did you specify emissivity for the outer surface of the outer cylinder and inner surface of the inner cylinder?

what about your initial conditions?

with regards, harish


Akash February 6, 2008 15:00

Re: Radiation effects in a transient analysis
 
Dear Harish,

The representation is a simplified version of the actual setup.

The entire assembly is in vacuum. The first load step assumes a uniform temperature of 303 K for a short period. The outer cylinder is ramped at a constant rate using a heater/controller to a temperature of t1. This is the second load step in Ansys specified as a temperature function Temp. = 303 + (heating rate*time_at_current time step), 0<=time<=6000 and Temp. = t1 for 6000<time<=9000. The third load step involves setting the inner surface to t1-10 for another 2000 seconds. The outer surface of the inner cylinder and the inner surface of the outer cylinder and the inner surface of the cover connecting the end surfaces of the two cylinders were provided with emissivity values using the SFL,ALL,RDSF...option and the space temp. was set to 0.

If I exclude radiation, the inner surface reaches t1 during the stabilization period. However, with radiation, the inner surface reaches only t1-10 during the stabilization period. I was expecting the radiation case to reach t1 sooner than in the case where only conduction is pre-dominant. So I was wondering if there is anything else that I should have included or someplace, I am overlooking. I greatly appreciate your information in this regard.

Thanks, Akash

harish kumar February 9, 2008 11:21

Re: Radiation effects in a transient analysis
 
Dear Akash,

Yes you are absolutely correct radiation is an additional mode of heat transfer at high temperatures. In your case stabilisation period should be less when radiation mode is present.

what is the boundary condition specified at the inner surface of the inner cylinder[ i think in case it is insulated]?

In this case one boundary condition is outer surface temperature of the outer cylinder is specified as function of time [this is correct].

inner surface of the outer cylinder should have emissivity and enclosure number[no boundary conditions]

outer surface of the inner cylinder should have emissivity and same enclosure number [ no boundary conditions required].

But inner surface of the inner cylinder [boundary condition is required]. I think in you case it may be insulated or some temperature.

since it is concentric [closed] so no radiation heat loss out of the system.

with above boundary conditions and initial temperatures problem is correctly specified.

with this you should get correct result

with regards, harish


Akash February 9, 2008 16:58

Re: Radiation effects in a transient analysis
 
Hello Harish,

Thanks a bunch for your assistance. The inner surface has a specified temperature after a certain period of time has elpased. I will work on it further and let you know of my status.

Many thanks, Akash

harish kumar February 10, 2008 11:48

Re: Radiation effects in a transient analysis
 
Dear Akash,

That is incorrect inner surface temperature can't come into the picture after certain period of time. it should be present right from the time (t=0 onwards)

Inner surface of the inner cylinder temperature can be a function of temperature or a constant temperature. but that value should be available at all times.

You can solve like this for certain period of time you specify 300K (0 to 3sec, 300K)then (3sec to final time(depends on the transient analysis)) specify either temperature or flux which can be function of time or a constant value.

best of luck

with regards, harish


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