ABSTRACT
In fluid systems engineering, the solution of a great number of heat transfer and flow problems is obtained using empirical relationships whose validity, even justified by calculation rapidity needs, corresponding to the situations in which they have been experimentally obtained. Some semiempirical correlations are themselves special forms of the conservation equations, finalized to specific phenomenological situations. The status of knowledge is clearly lower in the case of two-phase phenomena with respect to single-phase phenomena. In the analysis of conservation equations, a distinction is made among single-phase laminar flows, single-phase turbulent flows, and two-phase flows. The continuity equation expresses the instantaneous balance among mass entering the system, mass leaving the system, and mass remaining in the system. The momentum equations are obtained as an extension of the second law of dynamics, with the consideration that, in flowing fluids, the following forces may be present: superficial forces, volume forces, and viscous forces.
