Rivista La Termotecnica - Accesso riservato soci CTI

Autore: M. Napolitano

Collana: 2017, numero 07

Note:
This paper addresses a specific peculiarity of the quasi-one-dimensional conservation laws for a compressible fluid. The general three-dimensional equations for a finite control volume are provided and the corresponding differential equations are derived from them. It is shown that the viscous forces produce both a mechanical work – which can be either positive or negative, i. e., can either accelerate or decelerate a given fluid particle, depending on the surrounding flow-field – and a non-negative thermodynamic dissipation work. The quasi-one-dimensional equations are then derived: for them, no mechanical work is produced by the wall-friction, since the physical velocity is zero at the wall, and the thermodynamic dissipation work is shown to be equal to minus the product of the wall shear times the average (one-dimensional) velocity of the flow inside the duct.