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Flow boiling heat transfer in rod bundles

In this text we will not give a detailed survey of the flow boiling process in a boiling channel with subcooled inlet. Instead we will give a quick summary of the different heat transfer modes encountered.

With reference to Figure 6.1 (see p. ) we first have pure single phase turbulent heat transfer. At the point of onset of nucleate boiling, z_n, vapor bubbles start to form and due to the high degree of subcooling bubbles formed collapse almost immediately. This phenomenon results in a high degree of mixing which implies a rapid increase in the heat transfer rate when going from pure single-phase to subcooled boiling.

From the z_n point heat is transferred both by nucleate and convective boiling. Nucleate boiling is vapor bubbles generated at a wall surface due to wall superheat ( $T_w > T_{\mbox{\protect\scriptsize sat}}$) whereas convective boiling is a vaporization which occurs at the liquid-vapor interface.

At the beginning of the boiling process the nucleate boiling is predominant and at the high flow quality condition in the annular flow regime it is the convective boiling which is predominant.

If the flow quality continues to grow when we are in the annular flow regime we reach a point where either the liquid film dries out or is sheared of the wall. At this point, $z_{\mbox{\protect\scriptsize crit}}$, which we will call the boiling transition point a drastic decrease in the heat transfer coefficient of several orders of magnitude is observed which implies an increase of the wall temperature (in a heat flux controlled case). In BWRs the cladding cannot withstand this large temperature increase which justifies that some authors describe the phenomenon with the term burnout.

With reference to the above discussed we identify the following regions of heat transfer:

.

Single-phase turbulent heat transfer: $0 \rightarrow z_n$.

.

Subcooled boiling heat transfer: $z_n \rightarrow z_{\mbox{\protect\scriptsize bulk}}$, where $z_{\mbox{\protect\scriptsize bulk}}$ is characterized by $\mbox{$<\!{x}\!>$}\vert _{z_{\mbox{\protect\scriptsize bulk}}} = 0$.

.

Saturated or bulk boiling heat transfer: $z_{\mbox{\protect\scriptsize bulk}} \rightarrow z_{\mbox{\protect\scriptsize crit}}$.

.

Post-boiling transition heat transfer: $z_{\mbox{\protect\scriptsize crit}} \rightarrow ?$.

We will only consider the three first heat transfer regimes since the post-boiling transition regime is not encountered during normal BWR operation.

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