Critical heat flux limit

Heat Transfer Before the Critical Heat Flux Limit... 

The Critical Heat Flux Limit in Pool Boiling... 

As boiling in micro-channel heat sinks is an attractive method for cooling computer CPUs and other high-heat flux devices (such as laser diodes), it is of crucial importance to accurately predict the critical heat flux (CHF) in the small-diameter channels. Critical heat flux or burnout is a limiting value for safe operation of heat dis-... 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

THERMAL EFFECTS ATTENUATION. Shield designs are to also limit exposure of personnel to a critical heat flux value based on the total time of exposure. This value of heat flux is determined by the following equation ... 

In Fig. 23, the highest heat fluxes fhat could be achieved were limited by the high current flow fhrough fhe diode, as in fhe 16 kPa dafa sef, or by the fan speed reaching a maximum, as for the 104 kPa, or by critical heat flux (CHF) conditions, as in the 6.5 kPa case. 

A lower limit for the critical heat flux can be found from the energy balance qdnz = Mcpi [i l(z) — i i] = adirz [do — l(-z)], where 1 is the inlet temperature. From this follows... 

As the wall temperature is significantly greater than the saturation temperature once the critical heat flux has been reached, then the lower limit for the critical heat flux is... 

Even when the critical heat flux is reached not all the liquid is vaporized. Therefore an upper limit for the critical heat flux is obtained, which is necessary for complete evaporation of the liquid. Once again this is found from an energy balance... 

At very high subcoolings, the critical heat flux becomes independent of subcooling, this limit being typically reached when the flux is around 2.5 times for the saturation conditions (Elkss-abgi and Lienhard [156]). This ratio is probably fluid dependent. 

The correlations given here are also limited to well-wetting fluids increase in contact angle gives a decrease in critical heat flux as discussed previously. 

Critical heat flux has attracted a large amount of attention as a result of its importance as a limiting condition in water-cooled water reactors. The literature is vast and could certainly not be dealt with in detail in the present context. Reviews of earlier publications are given by Hewitt [291, 292] and more recent review material is presented in the books of Collier and Thome [3], Carey [4], and Tong and Tang [5], Here the objective is to pick out the most salient points the reader is referred to these earlier reviews for further information. 

Annular flow modeling has been used extensively in predicting critical heat flux phenomena in annular flow. It has been used for prediction of critical heat flux in annuli and rod bundles (see Hewitt [291] for a review) and has also been successfully applied to the prediction of transient critical heat flux and to limiting cases of rewetting of a hot surface (see Hewitt and Govan [298]). 

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