The facility's objective is to characterize the effect of different fuel grid spacers on the onset of the departure from nucleate boiling ( DNB ) phenomenon and to quantify the critical heat flux ( CHF ) as the DNB event occurs.
22.
The increased coolant flow rate through the reactor produced an increase in the inlet coolant temperature of the reactor core ( the coolant no longer having sufficient time to release its heat in the turbine and cooling towers ), which now more closely approached the nucleate boiling temperature of water, reducing the safety margin.
23.
DNB may be avoided in practice by increasing the pressure of the fluid, increasing its flow rate, or by utilizing a lower temperature bulk fluid which has a higher CHF . If the bulk fluid temperature is too low or the pressure of the fluid is too high, nucleate boiling is however not possible.
24.
The effect of the Marangoni effect on heat transfer in the presence of gas bubbles on the heating surface ( e . g ., in subcooled nucleate boiling ) has long been ignored, but it is currently a topic of ongoing research interest because of its potential fundamental importance to the understanding of heat transfer in boiling.
25.
A number of different terms are used to denote the CHF condition : departure from nucleate boiling ( DNB ), liquid film dryout ( LFD ), annular film dryout ( AFD ), dryout ( DO ), burnout ( BO ), boiling crisis ( BC ), boiling transition ( BT ), etc . DNB, LFD and AFD represent specific mechanisms which will be introduced later.
