POLYFLOW - On the importance/relevance of viscous heating and its influence/effect on a flow?
How canone quickly estimate the importance or relevance of viscous heating and its influence/effect on a flow, without performing too much calculations? This would be useful to know whether viscous heating has to be incorporated into the calculation or not.
The post-calculation of total dissipated power already gives an indication, but the resulting effect depends on the amount of material involved and on physical parameters. Instead, one can suggest two relatively simple approaches.
- On the basis of a similar calculation where temperature dependence of parameters (e.g. viscosity) is omitted, one obtains a decoupled system. If viscous heating is incorporated, this will lead to a temperature increase, which can be viewed as an upper-limit of the expected temperature increase that would actually be observed under normal conditions.
- If the flow domain is surrounded by thermally insulated walls except at the entry and exit, energy balance can be used for estimating the temperature increase. We have indeed rho.Cp.Q.deltaT = total dissipated power; this allows evaluating the average temperature increase.
A general comment. For flows of highly viscous materials, temperature increase can be usually expected. In a way, viscous heating can therefore always be asked, unless one definitely wants to run an isothermal case. Its first effect is of course the introduction of an additional non-linear term, while the corresponding increase of calculation time would often remain marginal. A posteriori, if no temperature increase is reported, the calculation has not become more difficult, while, if a temperature increase is indeed reported, it was then a good choice to include the viscous heating into the calculation.