the calculation of convective flux

Hai-Wen Ge

I'm a graduate student.Now I work on the topic of numerical combustion.I should solve the N-S equations on unstructured mesh.I have a question about convective flux and need your help. I compute the convective flux of N-S equations by using Roe's approximate Rieman solver.With the evolution,it will give a density which is less than zero.(the value \rho continue to decrease) What's the reason of diverge?Is Roe's approximate Rieman solver not suitable to solve the field of reactive flows(\gama=Cp/Cv changes greatly in the computation domain,1.25~1.34) I need your suggestion!Thanks!