Gas heat transfer error

Hello researchers. Excuse me. I am conducting research on biomass pyrolysis, and the product is tar. However, when I was calculating, this error occurred. I checked the wrong location and believed that there was a problem with gas heat transfer. However, I do not know which value to modify on the MFix operation interface to avoid this error. Or is there any other solution?
Best wishes!

type or paste code here

It looks like the value inside the SQRT is negative, I think you understand this since you have underlined it… the question is how did that happen? Hard to say since you did not attach your files.

You could try preventing this by putting in a check before line 177, something like this (untested!):

               IF( (DES_T_S(I)+DES_T_S(J)) .LT. ZERO) THEN
                  WRITE(*,*) "XXX DES_T_S(",I,")=", DES_T_S(I), "DES_T_S(",J,")=",DES_T_S(J)
                  K_Gas = 6.02D-5
                  K_Gas = 6.02D-5*SQRT(0.5d0*(DES_T_S(I)+DES_T_S(J))/300.D0) ! cal/(