Learning Forum

[Muzzammil]

I am simulating a case of an electric motor which is actively cooled using a coolant. I am using Ansys fluent, steady state, pressure based solver. I have activated the viscous heating option from viscous models to include the power loss occurring due to viscous dissipation effect of the coolant. I have run the simulation and have achieved convergence. I am interested to estimate the heat absorbed by the coolant, for that i need to estimate the enthalpy of the coolant at the inlet and outlet.

From Surface integrals - heat fluxes- I got the values of enthalpies at the inlet and outlet, but these values does not match properly with a theoretical expression of Hi = m*Cp*(T-Tref) where Tref is 298.15 K.

I would like to know, is there any other way to estimate the enthalpy or any other theoretical expression exclusively used when using viscous heating.

Here i would like to let you know that the values of enthalpy at the inlet and outlet goes hand in hand with the theoretical expression only when there is No Viscous Heating happening in the fluid.

[Kalyan Goparaju]

Hello,
An estimate can be made for only simple flows, e. g. flat plate boundary layers. For a general flow, a numerical solution is perhaps the only possible option.
Thanks Kalyan

[Konstantin]

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.

[Amine Ben Hadj Ali]

:)