It is not essential to tell the person who is entitled to zakah that the money is zakah
Question

I sent a sum of money to my maternal uncle , and I did not tell him that it is zakah, because if he knew that it was zakah he would not take it. So I left this matter between me and Allah. Is my zakah valid?.
Praise be to Allah.If you gave your zakah to someone whom you know is entitled to it, with the intention of giving zakah, then your zakah is valid and we hope that Allah will accept it from you. You do not have to tell the recipient that it is zakah. 
And Allah is the Source of strength.