Define dpm bc
 

I want to achieve the aim that the droplets are escape from a specified interface in the flow zone, and store the temperature and flow rate of droplets in UDM. the continuous phase flows as normal.
Can I use DEFINE_DPM_BC to do this?
pulling~my~hair~~

User-Defined Memory (UDM) is for either the cells or nodes within the domain (and is therefore generally used from the perspective of the continuous phase). You could have a UDM for accumulating the mass flow rate of droplets in cells at the boundary. As for the temperature, you could store the last droplet's temperature in a UDM in a similar way.

What exactly is your goal? Because if you're after a list of droplet temperature and flow rates after they have escaped the domain at this particular boundary, then simply save this data to a text file.