Study on analytical prediction of forced convective CHF based on multi-fluid model

Abstract

In relation to the design and safety analysis of various industrial equipments using boiling two-phase flow, an analytical study was carried out on the prediction of critical heat flux (CHF) forced convective boiling in straight pipe. Using multi-fluid model (two-fluid model and three fluid model) of two-phase flow and appropriate CHF model, CHF for wide range of flow conditions was successfully predicted. For annular dispersed flow regime where CHF mechanism is film dryout, the constitutive equation of droplet entrainment and deposition were found to be quite important. Then the sensitivity of these correlations on the prediction of CHF was evaluated and recommendation of appropriate constitutive correlations was made. For bubbly, churn and slug flow regime, where CHF mechanism is DNB (transition from nucleate boiling to film boiling) two types of DNB models were evaluated coupled with accurate calculation of two-phase flow parameters based on two-fluid models. A wide range of DNB data were predicted successfully with modified DNB models. In predicting CHF, particularly for subcooled boiling, accurate prediction of net vapor generation point was found to be quite important. A modified correlation for net vapor generation point was proposed which predicts a wide range of CHF data both for dryout and DNB.