Thetwo phase heat transfer phenomena which includes boiling and condensation ispresent in several thermal systems.
A vapour compression refrigeration cycle isa good example of two phase heat transfer phenomena where boiling and condensationplay an important role in evaporators and condensers. Boiling takes place whena liquid comes into contact with a solid surface whose temperature is higherthan the saturation temperature of the liquid at the given pressure by areasonable amount. An important characteristic of boiling is its extremely highheat transfer coefficient which results in very high heat transfer rates evenat low temperature gradients. The high heat transfer rates that can be achievedin boiling is used to enhance the performance of various components in nuclearpower plants and improve cooling of highpower electronic devices. The phenomenon of boiling is a complicated onebecause of the involvement of large number of variables in the process and thecomplex fluid motion patterns caused by the bubble formation and growth.Generally, the bubbles formed in boiling are not in thermodynamic equilibriumwith the surrounding and the process on a whole does not occur underequilibrium conditions.
Temperature and pressure difference exists between thevapour in the bubbles formed and the liquid. While the pressure difference betweenthe liquid and the vapour is balanced by the surface tension at the interface,the temperature difference provides the driving force for heat transfer betweenthe two phases. If the liquid is at a lower temperature than the bubble, heattransfer takes place from the bubble into the liquid due to which some of thevapour inside the bubble condense and the bubble eventually collapse. When theliquid is at a higher temperature than the bubble, heat transfer is from theliquid to the bubble, leading to the growth in bubble’s size and eventually itrises to the top under the influence of buoyancy.
Boiling is a form ofconvection heat transfer and just like any other convection heat transferphenomena, boiling heat flux from a solid surface to the fluid can be expressedwith the help of Newton’s law of cooling as given below: where iscalled the excess temperature, which represents the temperature excess of thesurface above the saturation temperature of the fluid.