Hi,there. 
I want to simulate the heat exchange process between hot particles and cold gas in a two- dimensional bubble bed.But，when the program starts running, the gas flows in from the inlet set at the bottom, and all particles in the bed will rise rapidly, as shown in the figure. After about 0.8S, all the particles fell, some of which overflowed from the top, because the accumulation of particles after 0.8S was much less than the initial time. At present, I think this process should not exist. I want to know how to solve such an overflow problem, my program is attached.Thanks! 
This occurs due to the step input of gas into the system with an unsettled particle bed. To avoid this, you can ramp up the inlet BC over a short period to reach your desired inlet velocity.
Thank you.I will try. 
Hi shaochong,
You have 5 m/s velocity in the vessel, which is way higher than the fluidization velocity of 1.5 mm particles. You can either increase the diameter of particles or reduce the superficial velocity through the vessel.
Sefa
Hi Shaochong
It maybe the problem of the lastest version of MFIX(v-20.3.1). I encountered with the same issue when fluidize particles. You can try to the previous versions, e.g. MFIX-19.2.
Shuai
Thank you for your reply. In fact, the particles fluidized normally after 0.8S. In this case, particles with a velocity of 5m/s could not be fluidized. I used a velocity of 6m/s and the particles were fluidized normally afterwards. Therefore, I think the abnormal behavior of particles before 0.8S should be the wrong setting in other places, not the speed.
Thanks, I will try the previous version.
Recently, I made some attempts. Currently increasing the intake boundary and trying the previous version cannot solve this problem. But I found that when I changed the gas temperature in the Backgrand IC and bed in the initial condition interface to 1200K (as shown in the figure), this situation would not happen again. It makes me a little confused.
I’m sorry to trouble you again. I find that I am not quite sure what ramp up the inlet BC means. Should I increase the inlet boundary or increase the inlet speed or other?
I was referring to increasing the gas inlet velocity gradually to your desired value.