Experimental study of natural circulation flow instability induced by flow boiling and loop circulation

Highlights: • Natural circulation flow instabilitiy induced by flow boiling and loop circulation are identified. • Flow boiling instabilities are found at three exit vapor qualities: x_e < 0 (sub-cooled boiling), 0 ≤ x_e < 1 (saturated boiling) and x_e = 1 (periodic dryout). • Flow instability induced by loop circulation is classified as Natural Circulation Oscillations. • Natural Circulation Oscillations lie in the transition zone from saturated boiling instability to periodic dryout. An experimental study of natural circulation flow instability has been carried out to investigate instability modes at low pressure. Water in the primary loop is heated in a single macro-channel and condensed in a tube-shell shape condenser by circulating coolant in the secondary loop. Two kinds of flow instability modes are identified: flow instability induced by flow boiling in the heated channel and flow instability induced by loop circulation. Flow boiling instabilities are found at three exit vapor qualities: x_e < 0 (sub-cooled boiling), 0 ≤ x_e < 1 (saturated boiling) and x_e = 1 (periodic dryout). Sub-cooled boiling instability is caused by bubble growth and condensation in the long sub-cooled flow boiling region. Saturated boiling instability is classified as Pressure Drop Oscillations because it operates in the negative slope region of internal characteristic curve. Combined with the oscillations of heating surface temperature, periodic dryout is confirmed by predicting CHF from empirical correlations. Flow instability induced by loop circulation is classified as Natural Circulation Oscillations, which is governed by the imbalance between heat transfer in the heated channel and condenser. The frequency and amplitude of flow boiling instability are given. The evolution of flow instability is indicated in the stability map.