Nu_oil (nipfl.f/8)*Pr_oil*nipfl.Re/(1.07+12.7*(nipfl.f/8)^0.5*(Pr_oil^(2/3)-1)) Nusselt number
h_inner Nu_oil*k_oil/nipfl.dh Film heat transfer coefficient, oil side
Re_air rho_air*v_air*nipfl.dh/mu_air Reynolds number
Nu_air 0.027*Re_air^0.805*Pr_air^(1/3) Nusselt number
h_outer Nu_air*k_air/nipfl.dh Film heat transfer coefficient, air side
U 1/(1/h_inner+d_wall/k_wall+d_ins/k_ins+1/h_outer) Overall heat transfer coefficient