Critical Heat Flux in Subcooled Flow Boiling

This chapter reports a survey of recent experimental work performed to obtain parametric trends of critical heat flux (CHF) in water-subcooled flow boiling, with special concern for high liquid velocity and subcooling, small-diameter channels, and low-intermediate pressure, together with a review of current modeling analysis. Under these thermal hydraulic and geometric conditions, the CHF is proved to be very high (up to many tens of MW/m2) such as to match the requirement of fusion reactor high heat flux components for thermal hydraulic design. From the experimental aspect, experiments carried out so far show parametric trends of CHF in terms of subcooling, mass flux, pressure, channel diameter and length, channel orientation, tube wall material and thickness, and peripheral nonuniform heating. Passive techniques for CHF enhancement, such as twisted tapes, helically coiled wires, and the hypervapotron, are also reviewed. Apart from the specific difficulty of properly modeling the CHF phenomenon for a large range of conditions, a major drawback of many existing theories is the use of empirical constants to justify the model with the data, making complex correlations that distort the original idea of the model. Among the many existing models, the superheated layer vapor replenishment and liquid sublayer dryout theories proved capable of predicting CHF under these conditions, extending to uniform and nonuniform heating of the channel and swirl flow as well.