DNBR is the ratio of which two heat flux quantities at a location?

• A. Critical heat flux to actual heat flux
• B. Actual heat flux to critical heat flux
• C. Heat flux to surface temperature difference
• D. Maximum heat flux to average heat flux

Answer: (A)

DNBR measures how close a fuel location is to the boiling crisis by comparing the critical heat flux to the actual local heat flux. The critical heat flux is the threshold where nucleate boiling becomes unstable and film boiling can form, reducing heat transfer efficiency. The actual heat flux is the heat being deposited at that location. A DNBR greater than one means there is a safety margin; as the local heat flux nears CHF, DNBR approaches one, and if the actual heat flux exceeds CHF, DNBR falls below one, indicating potential departure from nucleate boiling.