Critical heat flux

Boiling systems are those in which liquid coolant absorbs energy from a heated solid surface and undergoes a change in phase.

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).

In industrial applications such as electronics cooling or instrumentation in space, the sudden increase in temperature may possibly compromise the integrity of the device.

represents the proportionally constant called the heat transfer coefficient,

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.

Now many aspects of the phenomenon are well understood and several reliable prediction models are available for conditions of common interests.

The use of the term critical heat flux (CHF) is inconsistent among authors.

[3] The United States Nuclear Regulatory Commission has suggested using the term “critical boiling transition” (CBT) to indicate the phenomenon associated with a significant reduction in two-phase heat transfer.

So in general CBT is the result of some degree of liquid deficiency to a local position along a heated surface.

The two mechanisms that result in reaching CBT are: departure from nucleate boiling (DNB) and liquid film dryout.

Shear at the liquid-vapor interface drives the flow of the liquid film along the heated surface.

The process has been shown to occur over many instances of dryout events, which span a finite duration and are local to a position.

[3] The CBT occurs when the fraction of time a local position is subjected to dryout becomes significant.

[3] Post-CHF Post-CHF is used to denote the general heat transfer deterioration in flow boiling process, and liquid could be in the form of dispersed spray of droplets, continuous liquid core, or transition between the former two cases.

Zuber,[6] through a hydrodynamic stability analysis of the problem has developed an expression to approximate this point.

[7] For water at 1 atm, the above equation calculates a critical heat flux of approximately 1000 kW/m2.