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[FAQ]

Consider flow through a capillary ID 1 mm, Length 20 mm, adiabatic wall and inlet velocity 0.02 m/s, 300 K. The fluid has a viscosity = 0.8 Pa-s, density = 600 kg/m3, thermal conductivity = 0.1 W/m-K, and specific heat = 2600 J/kg-K. Therefore, the Ryenolds number is very low, Re = 0.015, and the Prandtl number if very high, Pr = 20800. In such a case, when you turn on viscous heating only, the outlet temperature will be LESS than the inlet temperature! This problem is rectified by turning on pressure work. The procedure will be explained and described further below. 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 ANSYS Fluent? In the text interface Define->Models->Energy; Fluent will prompt you for inclusion of viscous dissipation term, pressure work term, kinetic energy term.