C# Infix to Postfix Conversion of Multi Digits?

Question

I'm trying to convert infix to postfix like that (22 + 3) = 22 3 + but this function gives me the output like that 2 2 3 + which is hard to read for my evaluation function. I want to read a multi-digit number so it's easy to evaluate. I am not really sure where the problem is though. Below I have included all of the code needed. Any tips would be greatly appreciated!

Note: I'm doing infix to Postfix Conversion, not Evaluation

 

When I'm given the Infix Equation and Output comes like that

 

 

postfix_evaluation Code need the Equation like that