Two-phase flow instabilities in horizontal straight tube evaporator

It is essential to ensure the stability of a refrigeration system if the oscillation in evaporation process is the primary cause for the whole system instability. This paper is concerned with an experimental investigation of two-phase flow instabilities in a horizontal straight tube evaporator of a refrigeration system. The relationship between pressure drop and mass flow with constant heat flux and evaporation pressure is measured and determined. It is found that there is a negative slope section in the middle of positive slope pressure drop versus mass flow velocity thus making the velocity a multi-valued function of pressure drop. Three types of dynamic instabilities including the density-wave instability, pressure drop instability and thermal instability are found under the conditions of heat flux from 5 to 17 kW/m2, mass velocity from 150 to 1500 kg/(m2 s), and evaporating pressure from 0.5 to 0.8 MPa. The density-wave oscillation occurs at almost all mass velocities; its period is about 1–3 s and its amplitude is the lowest. The pressure drop oscillation takes place in a negative slope section; its period is about 10 s. The thermal oscillation can be induced at a high mass flow velocity, and its period is of 60 s and the amplitude is the highest among the three oscillations. The boundary as the onset of pressure drop oscillation and that of thermal oscillation are obtained experimentally. Finally, the empirical equations for the boundaries between three types of oscillations are obtained according to the mathematical model and test data.