question: When heating water to a boil, how does heat cause the water to bubble, and eventually come to a intense rolling boil?
comment: Little bits of the water turn to steam. Steam is a gas. The bubbles you see in the water are bits of steam (gas) surrounded by water (liquid).
passages: Section::::Types.:Transition.
  
  "Transition boiling" may be defined as the unstable boiling, which occurs at surface temperatures between the maximum attainable in nucleate and the minimum attainable in film boiling.
  
  The formation of bubbles in a heated liquid is a complex physical process which often involves cavitation and acoustic effects, such as the broad-spectrum hiss one hears in a kettle not yet heated to the point where bubbles boil to the surface.
  
  Section::::Types.:Film.
   At standard atmospheric pressure and low temperatures, no boiling occurs and the heat transfer rate is controlled by the usual single-phase mechanisms. As the surface temperature is increased, local boiling occurs and vapor bubbles nucleate, grow into the surrounding cooler fluid, and collapse. This is "sub-cooled nucleate boiling", and is a very efficient heat transfer mechanism. At high bubble generation rates, the bubbles begin to interfere and the heat flux no longer increases rapidly with surface temperature (this is the departure from nucleate boiling, or DNB).
   The process of forming steam bubbles within liquid in micro cavities adjacent to the wall if the wall temperature at the heat transfer surface rises above the saturation temperature while the bulk of the liquid (heat exchanger) is subcooled. The bubbles grow until they reach some critical size, at which point they separate from the wall and are carried into the main fluid stream. There the bubbles collapse because the temperature of bulk fluid is not as high as at the heat transfer surface, where the bubbles were created. This collapsing is also responsible for the sound a water kettle produces during heat up but before the temperature at which bulk boiling is reached.
   BULLET::::- Nucleation in boiling can occur in the bulk liquid if the pressure is reduced so that the liquid becomes superheated with respect to the pressure-dependent boiling point. More often, nucleation occurs on the heating surface, at "nucleation sites". Typically, nucleation sites are tiny crevices where free gas-liquid surface is maintained or spots on the heating surface with lower wetting properties. Substantial superheating of a liquid can be achieved after the liquid is de-gassed and if the heating surfaces are clean, smooth and made of materials well wetted by the liquid.
   Section::::Light water reactor examples.
  
  In a boiling water reactor (BWR), the emergency core cooling system (ECCS) injects liquid water into the reactor core from the top. Water vapor produced from boiling will flow in the opposite direction. Given a high enough flow rate of steam, reversal of the ECCS-injected liquid water occurs.
   "Nucleate boiling" is characterized by the growth of bubbles or pops on a heated surface, which rises from discrete points on a surface, whose temperature is only slightly above the liquids. In general, the number of nucleation sites are increased by an increasing surface temperature.
   Heat transfer and mass transfer during nucleate boiling has a significant effect on the heat transfer rate. This heat transfer process helps quickly and efficiently to carry away the energy created at the heat transfer surface and is therefore sometimes desirable—for example in nuclear power plants, where liquid is used as a coolant.
  
  The effects of nucleate boiling take place at two locations:
  
  BULLET::::- the liquid-wall interface
  
  BULLET::::- the bubble-liquid interface
   Many shell-type boilers carry a large bath of liquid water which is heated to a higher temperature and pressure (enthalpy) than boiling water would be at atmospheric pressure. During normal operation, the liquid water remains in the bottom of the boiler due to gravity, steam bubbles rise through the liquid water and collect at the top for use until saturation pressure is reached, then the boiling stops. If some pressure is released, boiling begins again, and so on.
   The nucleate boiling process has a complex nature. A limited number of experimental studies provided valuable insights into the boiling phenomena, however these studies provided often contradictory data due to internal recalculation (state of chaos in the fluid not applying to classical thermodynamic methods of calculation, therefore giving wrong return values) and have not provided conclusive findings yet to develop models and correlations. Nucleate boiling phenomenon still requires more understanding.
  
  Section::::Boiling heat transfer correlations.
   Nucleate boiling
  
  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. For water, as shown in the graph below, nucleate boiling occurs when the surface temperature is higher than the saturation temperature (T) by between to . The critical heat flux is the peak on the curve between nucleate boiling and transition boiling. The heat transfer from surface to liquid is greater than that in film boiling.
  
  Section::::Mechanism.
   Two different regimes may be distinguished in the nucleate boiling range. When the temperature difference is between approximately to above T, isolated bubbles form at nucleation sites and separate from the surface. This separation induces considerable fluid mixing near the surface, substantially increasing the convective heat transfer coefficient and the heat flux. In this regime, most of the heat transfer is through direct transfer from the surface to the liquid in motion at the surface and not through the vapor bubbles rising from the surface.
   "Boiling" is the method of cooking food in boiling water or other water-based liquids such as stock or milk. Simmering is gentle boiling, while in poaching the cooking liquid moves but scarcely bubbles.
   As water is poured onto the fuel, it quickly sinks to the bottom of the container due to the water's higher density, and has little effect on extinguishing the flames on the surface. Under certain conditions, after some time, the water on the bottom rapidly vaporises into steam, causing it to expand more than 1700 times in volume. The rapidly expanding steam (possibly superheated) expels the oil or fuel above upward and out of the tank, resulting in the discharging of burning oil onto a large and uncontrolled area outside of the container. Mathematical modelling of the phenomenon is difficult and complicated, making boilover prediction unreliable.
   In a pressurized water reactor (PWR), the ECCS injects liquid into the hot and/or cold leg of the reactor. The cold leg flows through a downcomer on the outside of the core, before flowing up through the core. The core barrel and the reactor vessel wall form a cylindrical shell that is referred to as the downcomer. In the cold leg, boiling in the downcomer creates an upward flow of steam that can reverse the flow of liquid water coming in through the cold leg. The flooding rate in a PWR is pressure dependent.
  
  Section::::Similar terminology.
   Pool boiling is boiling at a stagnant fluid. Its behavior is well characterized by Nukiyama boiling curve, which shows the relation between the amount of surface superheat and applied heat flux on the surface. With the varying degrees of the superheat, the curve is composed of natural convection, onset of nucleate boiling, nucleate boiling, critical heat flux, transition boiling, and film boiling. Each regime has a different mechanism of heat transfer and has different correlation for heat transfer coefficient.
  
  Section::::Multi-phase heat transfer.:Flow boiling.
   Typically, a BLEVE starts with a container of liquid which is held above its normal, atmospheric-pressure boiling temperature. Many substances normally stored as liquids, such as CO, propane, and other similar industrial gases have boiling temperatures, at atmospheric pressure, far below room temperature. In the case of water, a BLEVE could occur if a pressurized chamber of water is heated far beyond the standard . That container, because the boiling water pressurizes it, is capable of holding "liquid" water at very high temperatures.
   At "T"+40, core temperature is at and rising steadily; CS and LPCI kick in and begins deluging the steam above the core, and then the core itself. First, a large amount of steam still trapped above and within the core has to be knocked down first, or the water will be flashed to steam prior to it hitting the rods. This happens after a few seconds, as the approximately 200,000 L/min (3,300 L/s, 52,500 US gal/min, 875 US gal/s) of water these systems release begin to cool first the top of the core, with LPCI deluging the fuel rods, and CS suppressing the generated steam until at approximately "T"+100 seconds, all of the fuel is now subject to deluge and the last remaining hot-spots at the bottom of the core are now being cooled. The peak temperature that was attained was (well below the maximum of established by the NRC) at the bottom of the core, which was the last hot spot to be affected by the water deluge.
   At atmospheric pressure the boiling point of water is - liquid water at atmospheric pressure does not exist at temperatures higher than . At that moment, the water would boil and turn to vapor explosively, and the liquid water turned to gas would take up significantly more volume (~1,600-fold) than it did as liquid, causing a vapor explosion. Such explosions can happen when the superheated water of a Boiler escapes through a crack in a boiler, causing a boiler explosion.
  
  Section::::Mechanism.:BLEVEs without chemical reactions.
   BULLET::::- Geysers: Geysers such as Old Faithful are a type of geothermal feature that periodically erupt scalding hot water. Increased pressure exerted by the enormous weight of the overlying rock and water prevents deeper water from boiling. As the hot water rises it is under less pressure and steam bubbles form. They, in turn, expand on their ascent until the bubbles are too big and numerous to pass freely through constrictions. At a critical point the confined bubbles actually lift the water above, causing the geyser to splash or overflow. This decreases the pressure of the system and violent boiling results. Large quantities of water flash into tremendous amounts of steam that force a jet of water out of the vent: an eruption begins. Water (and heat) is expelled faster than the geyser's recharge rate, gradually decreasing the system's pressure and eventually ending the eruption.
   As the geyser fills, the water at the top of the column cools off, but because of the narrowness of the channel, convective cooling of the water in the reservoir is impossible. The cooler water above presses down on the hotter water beneath, not unlike the lid of a pressure cooker, allowing the water in the reservoir to become superheated, i.e. to remain liquid at temperatures well above the standard-pressure boiling point.
   A cooking technique called flash boiling uses a small amount of water to quicken the process of boiling. For example, this technique can be used to melt a slice of cheese onto a hamburger patty. The cheese slice is placed on top of the meat on a hot surface such as a frying pan, and a small quantity of cold water is thrown onto the surface near the patty. A vessel (such as a pot or frying-pan cover) is then used to quickly seal the steam-flash reaction, dispersing much of the steamed water on the cheese and patty. This results in a large release of heat, transferred via vaporized water condensing back into a liquid (a principle also used in refrigerator and freezer production).
   Periodically, there have been fluctuations in the level and activity of the lake. In the 1870s it was deep; after a phreatic eruption nearby in 1880, the lake disappeared and formed a fountain of hot water and steam. Another phreatic eruption lowered the lake level by some from December 2004 to April 2005; later the lake level rose again, refilling the lake in just one day. The rapid draining and refilling of the lake implies that it is suspended well above the local water table and that a continuous flux of steam or gas generated by an underlying magmatic intrusion drives water up into the lake. A disturbance to the supply of gas can cause the lake to drain through the porous connection that normally allows steam to rise and heat the lake.
   A nova results from runaway hydrogen fusion (via the CNO cycle) in the outer layer of a carbon-oxygen white dwarf star. If a white dwarf has a companion star from which it can accrete gas, the material will accumulate in a surface layer made degenerate by the dwarf's intense gravity. Under the right conditions, a sufficiently thick layer of hydrogen is eventually heated to a temperature of 20 million K, igniting runaway fusion. The surface layer is blasted off the white dwarf, increasing luminosity by a factor on the order of 50,000. The white dwarf and companion remain intact, however, so the process can repeat. A much rarer type of nova may occur when the outer layer that ignites is composed of helium.
   Interference between the densely populated bubbles inhibits the motion of liquid near the surface. This is observed on the graph as a change in the direction of the gradient of the curve or an inflection in the boiling curve. After this point, the heat transfer coefficient starts to reduce as the surface temperature is further increased although the product of the heat transfer coefficient and the temperature difference (the heat flux) is still increasing.
   As the reactor heats the water flowing through coolant channels, subcooled nucleate boiling takes place, in which some of the water becomes small bubbles of steam on the cladding of the fuel rods. These are then stripped from the fuel cladding and into the coolant channel by the flow of water. Normally, these bubbles collapse in the channel, transferring enthalpy to the surrounding coolant. When the pressure is below the saturation pressure for the given temperature, the bubbles will not collapse. As more bubbles accumulate in the channel and combine, the steam space within the channel becomes larger and larger until steam blankets the fuel cell walls. Once the fuel cell walls are blanketed with steam, the rate of heat transfer lowers significantly. Heat is not transferred out of the fuel rods as fast as it is being generated, potentially causing a nuclear meltdown. Because of this potential, all nuclear power plants have reactor protection systems that automatically shut down the reactor if the pressure in the primary circuit falls below a safe level, or if the subcooling margin falls below a safe level. Once the reactor is shut down, the rate at which residual heat is generated in the fuel rods is similar to that of an electric kettle, and the fuel rods can be safely cooled just by being submerged in water at normal atmospheric pressure.
  