In the energy equation, when Viscous heating is turned on, Pressure work must also be turned on.
Consider flow through a capillary ID 1 mm, Length 20 mm, adiabatic wall and inlet velocity 0.02 m/s, 300 K Fluid: m = 0.8 Pa.s, r = 600 kg/m3, k = 0.1 W/(mK), Cp = 2600 J/(kgK) Therefore, Re = 0.015 and Pr = 20800. In this case, when you turn on viscous heating only, the outlet temperature < Inlet temperature. The problem is rectified when turning on pressure work. when Viscous heating is turned on, Pressure work must also be turned on. Explanation:  We know h = u + P/rho. where h = specific enthalpy; u = specific internal energy; P = pressure; rho = density.Hence delta(h) = delta(u) + delta (p/rho) = delta (u) + P.delta(1/rho) + (1/rho).delta(P). For incompressible liquids with constant density , P.delta(1/rho) = 0 but we need to account for (1/rho).delta(P). For gases both incompressible and compressible, depending upon whether it is constant volume or constant pressure process, the appropriate pressure work remains. When viscous heating is turned on, the increase in enthalpy needs to be accounted for by the pressure work term. Otherwise, as in the sample problem case, you will see a fictitious drop in exit temperature to satisfy the enthalpy balance. How to turn on Pressure Work?  In the text interface Define>Models>Energy; FLUENT will prompt you for inclusion of viscous dissipation term, pressure work term, kinetic energy term. 

