I have a problem with the sign of heat flux. I set the boundary condition of the wall having negative heat flux. However, it seems that FLUENT recognize both nagative and positive heat flux as positive heat flux. I tried both negative and positive and checked the temperture contour, but the results were the same (positive heat flux). Is there any other setup to make the negative heat flux? Please help me out.

Try increaisng the value of the heat flux substantially. It is possible that the effect of heat flux is being overshadowed by some other phenomena. In this way you may know what sign to put.