I am doing my PG, need to analise combuster in Fluent

I have generated solid model in PROE with 3 mm thickness, & exported in GAMBIT. In Gambit I have defined two volumes for Mesh (one for solid material ie shell & other one is for internal fluid), while giving grid to shell(solid) I am getting errors due to very small gap between inner surface & outer surface, what I have to do??

Can we have such kind of thickness?? or we can use "shell thickness" property in FLUENT->BC for wall??

Please helpme out

I think it would help the other users if you mentioned what the error message said. I don't think that you can define a wall boundary condition with a thickness. Why is that you want to build a wall with a thickness? If you are planning to get fluent to calculate a temperature distribution through the 3mm thickness then you will need to grid it. If however the only purpose of the "shell" is to bound or encapsulate the fluid then you do not need to specify the shell thickness, just create one volume that contains the fluid and define the shell as a wall boundary condition, a thickness will not be needed.