This numerical paper presents the effects of viscous dissipation on both hydrodynamic flow behavior and thermal flow characteristics of fluid included in rheological polymer flow analysis. The shear rate dependence of the viscosity is modeled using a modified form of the Cross constitutive equation, while the density changes are modeled using the modified Tait state of equation. The Navier-Stokes equations are solved in a sequential, decoupled manner with energy conservation equations using a finite volume method based fluid flow solver. Hydrodynamic and thermal boundary layer developments in an asymmetric sudden expansion for different velocity and melt flow injection temperature boundary and geometry conditions are determined under the influence of viscous dissipation effects and the results are compared with each other to measure the relative effects of viscous dissipation on the interactions of these layers for a commercial polymer melt flow, namely polypropylene (PP). The numerical results demonstrate that proposed mathematical and numerical formulations for viscosity and density variations including viscous heating terms lead to more accurate representation of the polymer melt flow and heat transfer phenomena in plane channels or mold cavity associated with a sudden expansion.