Ansys Employee
Hello Muzzammil,
Classic textbook derivations of the energy balance do not include the effects of viscous dissipation, and thus comparing m*Cp*(T-Tref) at the inlet and outlet becomes only approximate when the viscous dissipation is included. For flows with Brinkman numbers, Br << 1, this effect is negligible and the theoretical estimate should be accurate. For flows with Br greater that 1 - not so much so. A theoretical approach to estimating viscous dissipation is pretty much the same you would take to estimate effects of viscosity in the momentum equations, which I am sure you know from your fluid dynamics is possible only for simple flows. True theory can be derived only for laminar, and for turbulent - correlations exist for flat plate boundary layers, channel and pipe flows (e. g. refer to Incropera's book). In Fluent 2021 you can separately report viscous work by activating beta features. After that, Viscous Work option will be available in the Flux Report Panel.

Hope this helps.