Boiling incipience in liquid nitrogen induced by a step change in heat flux

In the present paper are reported boiling experiments performed on a 25 μm brass wire immersed in saturated liquid nitrogen at 1 bar. A boiling curve performed under quasi-stationary heating conditions is presented. The boiling performance of the system induced by a step change in heat flux is next investigated. It is shown that a premature transition to film boiling is systematically observed when the applied heat flux is higher than 80% of the steady critical heat flux. This transition is however likely to happen as soon as the applied heat flux exceeds the steady minimum heat flux value. Boiling incipience results from the activation of pre-existing vapour embryos entrapped in the cavities of the wall. A gradual activation of the nucleation sites is observed for low values of the applied heat flux. Values higher than qFDNB″ (minimum heat flux for fully developed nucleate boiling measured under stationary heating conditions) lead to explosive boiling incipience in which bubble dynamics and interfacial motions are intense enough to produce an acoustic emission.