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Nucleate boiling heat transfer coefficient, $h_{\mbox{\protect\scriptsize NcB}}$

Chen uses a nucleate boiling heat transfer coefficient, $h_{\mbox{\protect\scriptsize NcB}}$ [W/( ${\mbox{m}}^2\cdot$K)], which is defined by

$$h_{\mbox{\protect\scriptsize NcB}} = 0.00122 \left[ \frac{ ... ...}} )^{0.24} \Delta p_{\mbox{\protect\scriptsize sat}}^{0.75} S$$ (11.49)

where $\Delta p_{\mbox{\protect\scriptsize sat}}$ [Pa] is defined by

$$\Delta p_{\mbox{\protect\scriptsize sat}} \;\hbox{$=$\kern-... ...ect\scriptsize sat}}(T_w) - p_{\mbox{\protect\scriptsize sat}}$$ (11.50)

where $p_{\mbox{\protect\scriptsize sat}}(T)$ is the saturation pressure [Pa] at temperature T [ ${}^\circ\mbox{C}$].

Note that (11.49) is dimensionally consistent which implies that the unit of $h_{\mbox{\protect\scriptsize NcB}}$ is [W/( ${\mbox{m}}^2\cdot$K)] when the physical quantites on the RHS are in SI-units.

The equation for $h_{\mbox{\protect\scriptsize NcB}}$ is based on the boiling heat transfer coefficient correlation of Forster and Zuber for pool boiling^11.3. The so-called suppression factor, S, not originally in the Forster-Zuber equation account for the steeper temperature gradient in the thermal boundary layer in flow boiling compared to pool boiling, according to Chen.

The dimensionless suppression factor S is a function of the two-phase Reynolds number, ${\mbox{Re}}_{\mbox{\protect\scriptsize TP}}$, used in the definition of the F-factor (11.46). The suppression factor $S = S({\mbox{Re}}_{\mbox{\protect\scriptsize TP}})$ was originally given graphically but in Table 11.2 we give values at some discrete ${\mbox{Re}}_{\mbox{\protect\scriptsize TP}}$ values.

All properties used for the evaluation of $h_{2\phi}$ should be evaluated at the saturation pressure which corresponds to the local pressure.

Note that $h_{2\phi}$ is implicit in the wall temperature T_w, ie we need to apply a numerical non-linear solver in order to obtain T_w.

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