Two-Phase Flow Instabilities
This chapter examines the various kinds of instabilities, according to a classification based on the related physical mechanisms. Thermodynamic instabilities are undesirable and potentially harmful in boiling and condensation processes and generally in the components with a two-phase flow. The flow excursion instability involves a sudden change of the flow to a considerably lower value. In a heated channel with a forced convection liquid flow, if the flow rate decreases while the other parameters remain constant and the subcooled boiling conditions are reached, the pressure losses may grow considerably. Thermal crisis is caused by a change in the heat transfer mechanism and is characterized by a sudden increase in the wall temperature. Flow-regime transition instabilities have been hypothesized with flow conditions close to the transition between the bubbly and annular flow. When nonequilibrium states occur, as in the interior of a moving shock wave, disturbance waves propagate along the system, belonging to two main types: recoverable-work disturbances and heating disturbances.