Critical heat flux

In the study of heat transfer, critical heat flux (CHF) is the heat flux at which boiling ceases to be an effective form of transferring heat from a solid surface to a liquid. Boiling systems are those in which liquid coolant absorbs energy from a heated solid surface and undergoes a change in phase. In flow boiling systems, the saturated fluid progresses through a series of flow regimes as vapor quality is increased. In systems that utilize boiling, the heat transfer rate is significantly higher than if the fluid were a single phase (i.e. all liquid or all vapor). The more efficient heat transfer from the heated surface is due to heat of vaporization and sensible heat. Therefore, boiling heat transfer has played an important role in industrial heat transfer processes such as macroscopic heat transfer exchangers in nuclear and fossil power plants, and in microscopic heat transfer devices such as heat pipes and microchannels for cooling electronic chips. The use of boiling as a means of heat removal is limited by a condition called critical heat flux (CHF). The most serious problem that can occur around CHF is that the temperature of the heated surface may increase dramatically due to significant reduction in heat transfer. In industrial applications such as electronics cooling or instrumentation in space, the sudden increase in temperature may possibly compromise the integrity of the device. The convective heat transfer between a uniformly heated wall and the working fluid is described by Newton's law of cooling: where represents the heat flux, represents the proportionally constant called the heat transfer coefficient, represents the wall temperature and represents the fluid temperature. If decreases significantly due to the occurrence of the CHF condition, will increase for fixed and while will decrease for fixed . The understanding of CHF phenomenon and an accurate prediction of the CHF condition are important for safe and economic design of many heat transfer units including nuclear reactors, fossil fuel boilers, fusion reactors, electronic chips, etc.