A limiting heat flux associated with a transition to a less effective boiling heat-transfer regime under specified conditions. Its value depends on fluid properties, pressure, flow, heating and geometry; a single number cannot be transferred indiscriminately between different flow passages.
A transition from effective nucleate boiling toward impaired surface wetting and substantially poorer heat removal in the relevant boiling regime. It is a concern when a heated fuel surface can become much hotter at an otherwise similar imposed heat load.
The ratio of the heat flux needed to reach departure from nucleate boiling to the actual local heat flux. Greater values indicate more margin under the model and correlation used.[ref]
Reactor glossary by the United States Nuclear Regulatory Commission. Departure from nucleate boiling ratio entry
The minimum local departure from nucleate boiling ratio over the specified core region or assessment. It depends on local heat loading and coolant conditions, not only on average core power.
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Critical Heat Flux (CHF) is a key concept in thermodynamics and heat transfer, particularly in the field of boiling heat transfer. It refers to the maximum heat flux (the rate of heat transfer per unit area) that can be achieved during the boiling of a liquid before a dramatic drop in heat transfer efficiency occurs. Beyond this point, the liquid begins to transition to a vapor phase, and the heat transfer mechanism changes significantly.