Error in burner model.

I am trying to simulate a version of the "syngas combustion in a round-jet burner" where heat transfer in fluids and reactive flow are coupled. The model works fine in the first step where the reactive flow is simulated. But when i'm trying to solve the coupled heat transfer and reactive flow i get the following error after about 10 solver steps:

Segregated group 4
Attempt to evaluate non-integral power of negative number.
- Function: ^
Failed to evaluate operator.
- Operator: nif
- Geometry: geom2
- Boundaries: 5, 6, 7, 8, 9
Failed to evaluate expression.
- Expression: nif((comp2.rspf.d_w_plus<(10/(comp2.rspf.Sc_wCO^0.33333333333333331))),nojac((comp2.rspf.d_w_plus*comp2.rspf.Sc_wCO)),(comp2.rspf.d_w_plus<sqrt(((comp2.rspf.kappav*1000)/comp2.rspf.ScT_wCO))),nojac((((comp2.rspf.Sc_wCO^0.66666666666666663)*15)-(500/(comp2.rspf.d_w_plus^2)))),nojac(((((comp2.rspf.Sc_wCO^0.66666666666666663)*15)+((log(comp2.rspf.d_w_plus)*comp2.rspf.ScT_wCO)/comp2.rspf.kappav))-(((0.5*comp2.rspf.ScT_wCO)*(1+log(((comp2.rspf.kappav*1000)/comp2.rspf.ScT_wCO))))/comp2.rspf.kappav))))
Failed to evaluate variable.
- Variable: comp2.rspf.wplus_wCO
- Defined as: nif((comp2.rspf.d_w_plus<(10/(comp2.rspf.Sc_wCO^0.33333333333333331))),nojac((comp2.rspf.d_w_plus*comp2.rspf.Sc_wCO)),(comp2.rspf.d_w_plus<sqrt(((comp2.rspf.kappav*1000)/comp2.rspf.ScT_wCO))),nojac((((comp2.rspf.Sc_wCO^0.66666666666666663)*15)-(500/(comp2.rspf.d_w_plus^2)))),nojac(((((comp2.rspf.Sc_wCO^0.66666666666666663)*15)+((log(comp2.rspf.d_w_plus)*comp2.rspf.ScT_wCO)/comp2.rspf.kappav))-(((0.5*comp2.rspf.ScT_wCO)*(1+log(((comp2.rspf.kappav*1000)/comp2.rspf.ScT_wCO))))/comp2.rspf.kappav))))
Failed to evaluate expression.
- Expression: d((2*down(comp2.rspf.rho)*comp2.rspf.beta0s^0.25*sqrt(max(comp2.k2,eps))*(comp2.rspf.wCOWall-comp2.wCO)*test(comp2.wCO)*pi*r/comp2.rspf.wplus_wCO)*(dvol),{test@24})
- Feature: Stationary Solver 2 (sol2/s2)

The model simulates a recuperative burner which uses radiation through an outer surface to transfer heat to its surroundings. I have attached the model to this post.

Thank you for your help.