Eventos Anais de eventos
COBEM 2017
24th ABCM International Congress of Mechanical Engineering
Analysis of Unsteady Friction Models for One-Dimensional Pipe Flow at the Light of the Second Law of Thermodynamics
Submission Author:
Jaguarê Smith Gonzaga Filho , RJ
Co-Authors:
Jaguarê Smith Gonzaga Filho, Felipe Bastos de Freitas Rachid
Presenter: Jaguarê Smith Gonzaga Filho
doi://10.26678/ABCM.COBEM2017.COB17-0955
Abstract
For decades, unsteady friction models have been developed to predict more accurately the water hammer phenomenon. However, the authors usually do not analyze whether their models do satisfy the Second Law of Thermodynamics (SLT), i.e., whether the rate of energy dissipation caused by the friction is in fact non-negative. This analysis is crucial to, if not validate, to rule out a proposed model. Therefore, this work aims to performing such analysis for two well-known unsteady friction models proposed in the literature. These models describe through different approaches the wall shear stress in unsteady slightly compressible viscous flow in complainant pipes, under laminar or turbulent regimes. For some models in particular, it can be analytically proved that they do unconditionally satisfy the SLT. However, it is not trivial to do so for these two models under investigation due to the complex analytical nature of their equations. Thus, in the lack of analytical tools, numerical simulations seem to be a feasible shortcut. The two selected models were implemented in the context of the Method of Characteristics (MOC) so that the overall rate of energy dissipation produced by the friction could be assessed. Then numerical simulations were carried out for a typical sudden valve closure in a reservoir-pipe-valve system for an isothermal process. The computed results reveal that one tested model produces negative dissipations, indicating that it apparently violates the SLT.
Keywords
water hammer, unsteady friction model, energy dissipation, unsteady fluid flow.
