9.98. With increased vaporization in a coolant channel, the heated sur­face becomes intermittently exposed to patches of vapor. Since the heat — transfer coefficient decreases markedly when the surface is blanketed with vapor, the wall temperature rises correspondingly. Hence, the wall tem­perature may at first oscillate as the surface is alternately blanketed with vapor or liquid, but will then rise after the wall liquid is completely va­porized. Such behavior, characterized by a marked temperature rise of the heated surface during boiling, as a result of a change in the heat-transfer mechanism, is called a boiling crisis. The foregoing is actually an oversim­plified representation of a complex phenomenon. For example, a wall temperature excursion can occur at a sufficiently high heat flux as a result of bubble-layer blanketing near the surface when the core of the channel is liquid and below the saturation temperature. The term departure from nucleate boiling (or DNB) (§9.91) is often applied to such a case as well as to surface overheating when the bulk of the fluid is at the saturation temperature. Dryout occurs in a channel with a vapor core and annular liquid film when the liquid is evaporated and the heat-transfer coefficient is reduced to that for a vapor in forced convection. Critical heat flux is often used as a generic term to cover all of these boiling-crisis possibilities.

9.99. A key critical heat flux parameter is the quality of the vapor-liquid system; it is defined as the mass fraction of vapor present in the mixture. The quality x may be expressed in thermodynamic terms by

H — Hf X = —77—>

Hfg

where H is the enthalpy of the mixture at the point in the coolant channel of interest, Hfis the enthalpy of the saturated liquid at the applied pressure, and Hfg is the enthalpy of vaporization at this pressure. Application of this equation to the subcooled region yields a negative quality, which is used in some correlations for predicting the critical heat flux under such con­ditions. The local quality for use in correlations must be calculated by an enthalpy balance.

9.100. In PWRs, the water is primarily in the subcooled or low-quality region. Here, a boiling crisis is represented by the DNB condition. It occurs only at a relatively high heat flux. In addition to the heat flux, the onset of DNB depends on such parameters as flow rate, quality of the fluid, system pressure, and increase in enthalpy of the coolant as it passes through the core.

9.101. In BWRs, where the quality is higher and the coolant is saturated, the boiling crisis of concern is dryout. In this case, where a dry patch forms by disruption of the annular liquid film, the wall-temperature excursion tends to be modest and slow when compared with subcooled-system ex­cursions. Since the vapor velocity is high, the heat-transfer coefficient from the surface to steam tends to be large enough to prevent the cladding from being damaged. However, failure could result from thermal cycling as the cladding surface is intermittently wetted and dried.