I want to know if dell Inspiron 3521 support second hdd drive in place of CD drive.
Officially no - it doesn't.
Unofficially, if you can find a caddy to fit the space, one likely will work - but only for storage. A drive mounted in a caddy cannot be used as a boot device.
@ejn63 , I have tried with caddy but it's not showing in disk management but in boot setup it's visible.
You'll have to ask the manufacturer of the caddy you purchased - these aren't universally compatible devices.