Flow boiling in a channel with subcooled inlet

The flow scenario of a fluid which enters a heated channel as subcooled liquid and undergoes boiling is investigated in this section. As pointed out earlier the heat in a BWR is added mainly through a wall heat flux but also by direct energy decomposition in terms of volumetric heat sources. In Figure 6.1 we depict a schematic illustration of the different flow regimes likely to occur in a BWR core.

The fluid enters as subcooled liquid and a thermal boundary layer starts to build up. At point z_n a narrow bubble layer starts to form and we have entered the wall voidage subcooled boiling region. The liquid outside the bubble layer is highly subcooled, ie $\mbox{$<\!{h_\ell}\!>$}_\ell \ll h_f$ which keeps the voids from reaching into the liquid. The void fraction in this region is normally very small and can in many cases be neglected. At point z_d the void departure point we move into the detached voidage region. Since the liquid core is only slightly subcooled, $\mbox{$<\!{h_\ell}\!>$}_\ell < h_f$ , the voids travel further into the liquid core. In the detached region the subcooled void fraction increases considerable and in an accurate BWR analysis^6.1 it is therefore necessary to include a calculation of this subcooled void fraction.

When we reach the point z_bulk the mixture enthalpy corresponds to the saturation enthalpy, ie $\mbox{$<\!{h}\!>$}= h_f$ . If we had neglected subcooled boiling (ie the presence of non-equilibrium) the void profile which we will call the equilibrium void profile would have started here. The z_bulk point divides the channel into a subcooled boiling and a saturated boiling part. Note that due to the non-equilibrium we still have $\mbox{$<\!{h_\ell}\!>$}_\ell < h_f$ !

At the equilibrium point, z_eq, all the liquid has been brought up to saturation, ie $\mbox{$<\!{h_\ell}\!>$}_\ell = h_f$ and we have obtained equilibrium. This point can be characterized by the point at which the actual void profile and the equilibrium void profile merge.

$\begin{figure} % latex2html id marker 24221\rule{\textwidth}{0.2mm} \rule{0cm}... ...\hspace{1em}Flow regimes in a boiling channel with subcooled inlet.}\end{figure}$