CFX, temperature gradient inside particles

hmdl · January 29, 2016, 07:11

Hi everyone,

I'm simulating particle-laden flow in a hot jet flow in CFX, steady state, particle morphology-Particle Transport Solid. As a result I get particle trajectories, their velocity and temperature at eat point on these trajectories. Now my problem is that it is assumed in CFX that particles are heated uniformly and the temperature value applies to the whole particle volume. The result of my simulations should be the temperature gradients inside the particles. Usually, because of very small size of the particles heat conduction inside a particle is neglected and it is safe to assume that the particle is heated uniformly. But in my case jet velocity and temperature are so high that a particle can be melted on the outside and still be cold on the inside.

To tackle this problem I can think of two approaches. One approach would be to implement in CFL the solution of a heating of a sphere depending on the local temperature of the ambient gas, relative velocity of gas to particle and material properties and let CFX compute the gradients for each time step for each particle.
Another way would be creating a separate simulation in workbench for transient heating of a sphere problem (strongly reduced problem to save time). This should be solved for each particle with different size, ambient temperature and velocity. I would be glad if anyone could advice on which method is better to get the temperature gradients inside the particles.

Best Regards,
hmdl

ghorrocks · January 29, 2016, 08:23

If the heating effects can be reduced to a algebraic equation then this can be implemented in CFX as a CEL function. For this to be applicable the particle coupling to the flow must simple and the particle condition is only a function of the local flow condition. Your first suggestion (heating of sphere relative to local temperature) is an example of this and if this is possible is by far the easiest way of doing it. The main assumption here is that the particle history is not significant - this means the particle cannot progressively melt, the melt is just determined on the local temperature.

Your second suggestion just seems to be the same as the first. I see no difference in it.

If not and the coupling between the particle temperature profile and the flow is strong then you are forced to model the temperature profile for each particle and integrate it over time. This is a far trickier simulation and I can only see this being done as a user particle routine in user fortran in CFX.

hmdl · January 29, 2016, 09:39

Hi Glenn,

the coupling between the particle temperature profile and the flow is indeed very strong, because of enormous temperature gradients inside the jet. The jet temperature is over 10000 K upstream and cools down to few hundreds of K downstream. Therefore, I have to take into account the particle histories. Do you mean there is no way to implement it in CEL? Transient heating of a sphere problem can't be easily reduced to algebraic equations and some parts should be solved numerically, that could be another reason why a user particle routine in fortran should be the way to go.

About the second method with a separate transient heat transfer simulation, I forgot to mention that its boundary conditions are also time depentend and should be updated from the particle tracking in the jet simulation. So, the ambient gas temperature and relative velocity for each particle at each time step is taken from the jet simulation and read into the sphere heating simulation as boundary conditions. This could be done in parallel or after the jet simulation is complete, because I can assume that the temperature gradients inside the particles doesn't affect the jet tempeture. Nevertheless, the particles are two-way coupled with the jet, so the jet gets colder while warming the particles up. This is already the case in the current set-up. Of course, in reality the temperature profile inside a particle has a direct impact on how much heat it absorbs from the gas, but this is level of abstraction I can live with at the moment.

I figured, as long as differential equations has to be solved for the heat transfer problem, why not use a seperate simulation for that? CFX is a differential equation calculator after all. Maybe the most ideal way to go would be to simulate heat transfer separately and use particle fortran routine in order to read the time dependent boundary conditions from the results file of the jet simulation ?

My Best Regards,
hmdl

ghorrocks · January 30, 2016, 05:40

I cannot see how to implement your second option. If you know how to do it then please let me know. The only way I can see to implement your particle thermal profile is via a user particle routine in user fortran.

January 29, 2016, 07:11	CFX, temperature gradient inside particles	#1
hmdl New Member Join Date: Jan 2016 Posts: 12 Rep Power: 10	Hi everyone, I'm simulating particle-laden flow in a hot jet flow in CFX, steady state, particle morphology-Particle Transport Solid. As a result I get particle trajectories, their velocity and temperature at eat point on these trajectories. Now my problem is that it is assumed in CFX that particles are heated uniformly and the temperature value applies to the whole particle volume. The result of my simulations should be the temperature gradients inside the particles. Usually, because of very small size of the particles heat conduction inside a particle is neglected and it is safe to assume that the particle is heated uniformly. But in my case jet velocity and temperature are so high that a particle can be melted on the outside and still be cold on the inside. To tackle this problem I can think of two approaches. One approach would be to implement in CFL the solution of a heating of a sphere depending on the local temperature of the ambient gas, relative velocity of gas to particle and material properties and let CFX compute the gradients for each time step for each particle. Another way would be creating a separate simulation in workbench for transient heating of a sphere problem (strongly reduced problem to save time). This should be solved for each particle with different size, ambient temperature and velocity. I would be glad if anyone could advice on which method is better to get the temperature gradients inside the particles. Best Regards, hmdl

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January 29, 2016, 08:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	If the heating effects can be reduced to a algebraic equation then this can be implemented in CFX as a CEL function. For this to be applicable the particle coupling to the flow must simple and the particle condition is only a function of the local flow condition. Your first suggestion (heating of sphere relative to local temperature) is an example of this and if this is possible is by far the easiest way of doing it. The main assumption here is that the particle history is not significant - this means the particle cannot progressively melt, the melt is just determined on the local temperature. Your second suggestion just seems to be the same as the first. I see no difference in it. If not and the coupling between the particle temperature profile and the flow is strong then you are forced to model the temperature profile for each particle and integrate it over time. This is a far trickier simulation and I can only see this being done as a user particle routine in user fortran in CFX.

January 29, 2016, 09:39		#3
hmdl New Member Join Date: Jan 2016 Posts: 12 Rep Power: 10	Hi Glenn, the coupling between the particle temperature profile and the flow is indeed very strong, because of enormous temperature gradients inside the jet. The jet temperature is over 10000 K upstream and cools down to few hundreds of K downstream. Therefore, I have to take into account the particle histories. Do you mean there is no way to implement it in CEL? Transient heating of a sphere problem can't be easily reduced to algebraic equations and some parts should be solved numerically, that could be another reason why a user particle routine in fortran should be the way to go. About the second method with a separate transient heat transfer simulation, I forgot to mention that its boundary conditions are also time depentend and should be updated from the particle tracking in the jet simulation. So, the ambient gas temperature and relative velocity for each particle at each time step is taken from the jet simulation and read into the sphere heating simulation as boundary conditions. This could be done in parallel or after the jet simulation is complete, because I can assume that the temperature gradients inside the particles doesn't affect the jet tempeture. Nevertheless, the particles are two-way coupled with the jet, so the jet gets colder while warming the particles up. This is already the case in the current set-up. Of course, in reality the temperature profile inside a particle has a direct impact on how much heat it absorbs from the gas, but this is level of abstraction I can live with at the moment. I figured, as long as differential equations has to be solved for the heat transfer problem, why not use a seperate simulation for that? CFX is a differential equation calculator after all. Maybe the most ideal way to go would be to simulate heat transfer separately and use particle fortran routine in order to read the time dependent boundary conditions from the results file of the jet simulation ? My Best Regards, hmdl

January 30, 2016, 05:40		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	I cannot see how to implement your second option. If you know how to do it then please let me know. The only way I can see to implement your particle thermal profile is via a user particle routine in user fortran.