Final answer:
During boiling, burnout can occur when the phase transition goes from nucleate boiling to film boiling due to excessive heat flux. This results in a reduced heat transfer efficiency and potential damage to the heating element.
Step-by-step explanation:
When boiling a saturated liquid and increasing the heat flux, one must be careful to avoid burnout. Burnout occurs when boiling transitions from nucleate boiling to film boiling. The answer to the question is D. Nucleate to Film. During nucleate boiling, bubbles form on heated surfaces and contribute to heat transfer by their subsequent rise through the liquid. As the heat flux increases, these bubbles coalesce into a continuous layer of vapor at the surface in what is called film boiling, which greatly reduces the heat transfer efficiency because the vapor layer acts as an insulator. This can lead to a rapid rise in surface temperature, known as burnout, which can damage the heating element.
In the context of phase changes, heat added during boiling does not change the temperature but goes into breaking the intermolecular forces to transition from the liquid to the gas phase. So, the process of boiling is isothermal. This continues until the liquid is fully vaporized.