At low pressure and low flow conditions, the prediction of two-phase flow transients is much more difficult than at relatively higher pressure or at high flow due to the large density ratio and thermal and mechanical non-equilibrium between the phases. A mechanical non-equilibrium effect was studied with air-water two-phase critical flow in pipes at low pressure (< 1 MPa). Critical flow test were conducted in a well-scaled test facility with several on-line instruments. The slip ratio, which is the key factor in the mechanical non-equilibrium, is directly measured at the upstream of the critical section. The break geometry effect was investigated using the nozzle and orifice as critical flow sections. The experimental results showed that the slip ratio increased as the quality increased. The slip ratio value at low pressure was relatively higher than the slip ratio at the high pressure for the same flow quality. The measured critical mass flux for the nozzle was higher than the orifice at the low flow quality. Thus there is a geometry effect on critical mass flux at the low quality region, even though there is no difference in the slip ratio at the upstream of choking plane. Thus, it is concluded that there is a strong mechanical non-equilibrium at the choking plane.

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