I investigated the Problem further and think found the cause for the problem. Within the NTU-Method there is a special case when R_1 = 1 (for my case this means R_1 = R_2 = 1, because i have water on both side which means the cp is equal on both sides), in this case phi_i is calculated through a different formula than normal.
I caculated phi 1 and 2 for m_dot,2 = const. = 5 l/min and m_dot,1 = 0.1...10 in 0.1 increments using an excel sheet. Furthermore i caculated phi_1 = phi_2 for m_dot,2 = m_dot,1 = 5 l/min. I put all of this into a graph to showcase the Problem.
I think the problem originates because of the jump from the normaly caculated phi to the phi caculated with the formula for the special case.
Could i just interpolate the values in between? What would be the best way to do this?
I really hope somebody can help me to solve this issue!