Boiling is a complicated heat and mass transfer process involving at least two phases. It is highly sensitive to a large number of experimental parameters such as the heater surface superheat, wettability, roughness, orientation, dissolved gas concentration, the level of subcooling in the boiling fluid, heating pattern, and system operating pressure, among many others. Boiling heat transfer is also highly vulnerable to practical considerations such as the fouling of the heater surface and the level of contamination in the boiling fluid. Accordingly, a clear mechanistic understanding of the boiling heat transfer, and in particular the triggering mechanism of CHF (critical heat flux), has remained elusive, despite extensive research over the past several decades
The first type of bubble that appears almost immediately is an air bubble. Air that is trapped in the water will start expanding as the temperature rises- nitrogen, oxygen and inert gases as well. These gases become less soluble in water as temperature increases, resulting in the large bubbles observed before the water actually begins to boil. What is known as "nucleation" will occur. Imperfections on the sides of the container or impurities in the water will act as "sites of nucleation" and bubbles will start forming there. Nucleate boiling is a type of boiling that takes place when the surface temperature is hotter than the saturated fluid temperature by a certain amount but where the heat flux is below the critical heat flux (point at which boiling stops being the most efficient method of transferring energy in fluid).