UNDERSTANDING AND MODELLING DNB IN FORCED CONVECTIVE BOILING: MODELLING OF A MECHANISM BASED ON NUCLEATION SITE DRYOUT

Abstract

Two existing mechanisms for DNB in forced convective boiling are unified into one which, based on nucleation site dryout, a priori covers the whole range of DNB, from bubbly to slug flow Two modelling approaches are explored. A first, simplified model, based on a global heat balance in the wall under a dry patch, enables the CHF to be calculated, distinguishing the case of thin and thick heating walls. From a comparison with CHF data, good results are obtained in a wide enough range. A second, detailed model, based on the two-dimensional unsteady thermal analysis of the wall in response to a nucleation cycle, is then developed in order to support the likeliness of the postulated mechanism in bubbly flow. A parametric study around a well-documented CHF data point, indicates some requirements for DNB to be reached.