Hi Shashi,
That directory is where Apex stores application information, like application jar files, checkpoints, container information, etc.
Please run this command to see which directory is taking the most space.
$ hdfs dfs -du /user/dtadmin/datatorrent/apps
Then open dtcli and use the get-app-info command look at the information of that application. For example:
dt> get-app-info application_1439598948299_0557
The field "state" will tell you whether the application is running or not.
If you don't care about the application, you can safely kill it if it's running and delete the HDFS directory by doing hdfs dfs -rm -r /user/dtadmin/datatorrent/apps/application_xxx_yyy (replace xxx and yyy with appropriate values). Note that doing so will wipe all stored information about that application.
David