I got the following answer on the following link. You can check with them for more detail.

http://www.linkedin.com/groups/Reverse-flow-in-Ansys-Fluent-134907.S.224367061

http://www.cfd-online.com/Forums/fluent/29766-reverse-flow-error-message.html

1.You haven't run your simulation long enough to build up a stable flow. Usually, this message only appears at the initial stage of your simulation. 2) The quality of your computational grids is not good enough. You should check your grids before you start your calculation.

Thanks for ur input.

See link

http://www.cfd-online.com/Forums/fluent/94824-reverse-flow-pressure-outlet.html

Reversed flow can occur when pressure gradients due to geometry mean that the flow wants to re-enter the domain at the location where you have placed an outlet. Some flow reversal can be tolerated, but will have an effect on mass-conservation and thus convergence