what should be the backflow temperature specified at the outlet?
 

Hi,

I am simulating steady-state heat transfer inside the packed bed system. For that, I have specified mass flow inlet boundary condition at the inlet. At the inlet, mass flow rate and inlet temperature of 800 K has been specified. At the wall, heat flux is applied. At the outlet, pressure outlet boundary condition has been considered, where zero pressure is specified.

My query is about the backflow temperature at the outlet (by default 300 K is given). What shout be the backflow temperature specified. Is there any way to choose this temperature? or it should be by default 300 K?

Ideally, the computational domain should be designed in a way that backflow at an outlet is minimized. I.e. the specified back flow does not matter.
If you somehow can't avoid that, you need an informed guess. Since in your case the temperature at the inlet is already 800K, and you have another heat source(?) in your domain, 800K seems like the minimum sensible backflow temperature as a starting point. Look at the results you are getting. Look at the average temperature leaving the domain at the outlet. Maybe you can then derive a better guess from the initial results.

Thank You, Alex for your reply.

I have one case in which an artificial heat sink is applied inside the solid particles, which is 5 times higher (in W) than the heat flux applied on the wall. In this case, due to the high heat sink value temperature will be dropped inside the bed. Therefore, the outlet temperature will be lower than the inlet temperature.

Since, in my case, there is some backflow. therefore, I am unable to guess the backflow temperature at the outlet.

I have simulated this case in which the backflow temperature is specified as the same as inlet temperature. However, I am not sure whether the temperature which I am getting at the outlet is correct or not?

If you really can't avoid backflow, and you know the heat sources/sinks, you could estimate the average temperature of the fluid leaving the computational domain. Based on mass flow rate, and the sum of all heat sources. That can be a better guess for the backflow temperature.

But just to drive home this point once more: If the simulation results you are interested in strongly depend on the specified backflow temperature, you HAVE to change your simulation setup. Typically, this involves moving the outlet boundary in the computational model further downstream in the physical model.

Thank you, Alex for your suggestions. I will estimate the temperature at the outlet as per your suggestion.

The advice to move the outlet section in such a way to avoid backflow is the best way to solve your issue.
If that is not possible, you could use Neumann type outlet condition.