Question: i have added a splash screen to my application by adding Default.png.
the splash screen works fine in the simulator but not showing on real device

Answer: 
The file name must be Default.png (with a capital D). Also, do a clean/rebuild of the entire project to ensure the most recent version of the file is being included in the project.
Select your Default.png in the groups and files window and make sure in the file listing above the editor window that the checkbox on the far right is checked. Without this the file will not be copied to the application bundle after compilation is finished.